xrde-bookx-ray diffraction is a powerful tool for characterization of nano- materials, bulk...
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XRD e-Book
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XRD e-Book
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Lab-Training.com
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XRD e-Book
By
Dr. Saurabh Arora
Founder : Lab-Training.com
Director : Auriga Research Pvt. Ltd., Arbro Pharmaceuticals Pvt. Ltd.
E-mail : [email protected]
Dr. Deepak Bhanot
Vice President : Training & Development
E-mail : [email protected]
Auriga Research Pvt. Ltd.
Division of
Arbro Pharmaceuticals Pvt. Ltd.
Analytical Division,
4/9 Kirti Nagar Industrial Area, New Delhi - 110015 (INDIA)
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Author’s Profile
Dr Saurabh Arora
He is a trained pharmacist with Master’s and Doctorate degrees in pharmaceutics
from reputed Indian institutes NPIER and Jamia Hamdard University respectively.
Managing Director, Auriga Research Ltd.
He has setup 2 contract laboratories and a clinical research company along with
managing and growing the existing business. His organization has grown multiple
folds and he has been fortunate to spearhead the growth initiatives backed by a team
of over 350 employees..
Specialties: Formulation Development, Analytical Development, Chromatography,
Mass spectroscopy, GMP. GLP, GCP, Laboratory designing, Residue analysis,
Project management, International business and all that goes into growing and
managing a business
Founder, Lab-Training.Com
Lab-Training.Com is developing and offering a series of free and paid E-Learning
courses on various analytical and laboratory techniques. He is responsible for the
course concepts, course content creation and review and course execution.
Founder, Food Safety Helpline
Food Safety Helpline has been established to help Food Business Operators
implement the Food Safety and Standards Act
Dr Deepak Bhanot
He is a seasoned professional having over 30 years expertise beginning from sales and
product support of analytical instruments. After completing his graduation and post
graduation from Delhi University and IIT Delhi he went on to Loughborough
University of Technology, UK for doctorate research in analytical chemistry. His
mission is to develop training programs on analytical techniques and share his
experiences with broad spectrum of users ranging from professionals engaged in
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analytical development and research as well as young enthusiasts fresh from
academics who wish to embark upon a career in analytical industry.
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XRD e-Book
Powder X-Ray Diffraction (XRD) available inARBRO Analytical Divisio, New Delhi
We are proud to inform that we have added X-ray diffraction (XRD) system in our
laboratory. XRD is a valuable tool for analysis and testing of materials used in
pharmaceuticals, food, cosmetics, studies on nano materials, forensics, geological
grading of minerals, electronic materials etc. In fact the scope for application is so
vast that it is difficult to mention them all here. However, our esteemed customers can
now take full advantage of analysis conducted on XRD as the analysis can establish
the purity of the testing material at various stages. Not only this but the technique
used for XRD analysis in laboratories has a distinct advantage because it affords non
destructive testing.
XRD Analysis in Pharmaceutical Industry
In the pharmaceutical industry, XRD analysis plays an important role in the
development of new drugs, as it helps to characterise active materials as well as tests
material at different stages of manufacturing so that quality control is effectively
maintained. All these procedures help to improve the formulation, analyse the
compatibility of excipients, detect bioavailability and improve on factors of stability
in different environments.
XRD Analysis in Food Industry
In the food industry several analytical techniques are used to monitor food additives
in the manufacturing process. XRD provides information on polymorphism
(occurrence in several forms), degree of crystallinity which influences hardness,
density, transparency and diffusion of additives and amorphism (being available
without crystallisation even in the minute particles). These three parameters are tested
in foods so as to control the texture and stability of foods under different storage
conditions. XRD is particularly useful in conducting studies on food ingredients to
develop quality products and especially tests starches, fats, chocolates or candies.
Quality is particularly important for development of foods so that they have the exact
flavour, texture and taste and should not be over sweet, salty, sour or bitter.
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XRD Analysis in Cosmetics Industry
Cosmetics are composed of a number of nano-particles to improve cosmetic features
especially in lotions, soaps, toothpaste and conditioners. These cosmetics are popular
but the compounds used in them can penetrate the skin and cause allergies and toxins.
Therefore, XRD is used for testing nano particles for the safe development of
cosmetic products so that their cosmetic features remain intact but products remain
safe.
Arbro Pharmaceuticals Pvt. Ltd has state-of-the art laboratory equipment and well
trained technicians to carry out testing procedures with utmost accuracy. If you would
like to use our testing services please feel free to contact us through the contact form
or call us now on +91-11-45754575. We will be happy to provide you a proposal for
your testing requirements using XRD.
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Table of Contents
1. Introduction to XRD course and its objectives
2. Wealth of information provided by XRD technique
3. Material characterization by XRD studies
4. Understanding nature of matter through knowledge of crystallography
5. Single Crystal and Powder XRD
6. How is X-ray diffraction different from X-ray fluorescence
7. Component parts of an X-ray diffractometer
8. Global manufacturers of XRD systems
9. Applications of powder XRD studies
10. Top 10 interview questions on XRD technique
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Introduction to XRD Course and its Objectives
“It is possible to fly without motors, but not without knowledge and skill”
— Wilbur Wright
We understand that everyone has busy work schedules and today’s hectic life style
leaves you little or no time to refer voluminous books to learn any technique.
However, for sustained growth learning has to be adopted as a lifelong habit. In an
effort to make your learning task easy we embarked upon the e-Book which
comprises of 10 chapters. Each chapter will provide a brief introduction to various
apects of XRD technique.
Reading a chapter and understanding it will not take more than about 10 minutes and
you will get ample time to assimilate the content before you move to the next
chapter. The free programme is designed to give an insight into the technique and
once your interest is captivated you can opt for full time advanced online or contact
programmes. Such programmes will offer additional benefits of practical exposure
and interaction with our technical experts. XRD has emerged as a major analytical
technique in diverse fields such as pharmaceuticals, foods, forensics, cosmetics,
minerals & geological studies.
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Wealth of information provided by XRDanalysis technique
Internal atomic arrangements in complex molecules
What are X-Rays?
X-rays constitute a small fragment of the electromagnetic spectrum having
wavelengths ranging from 0.01 to 10 nm. This region falls between gamma and UV
regions. In other words their wavelength is longer than gamma rays but smaller than
UV rays. Depending on their energy levels x-rays are classified as hard x-rays with
energies ranging from 5eV to 10keVand soft x-rays with energies in the range of
100eV to 5keV.
X-rays have a unique property of penetrating seemingly opaque objects such as tissue
but are absorbed by bones and other metallic objects which serves to make them a
useful diagnostic tool in medical applications.
Discovery of X-Rays
Wilhelm Conrad Roentgen of Germany is credited with the discovery of x-rays in
1895. He observed a strange fluorescence glow on some crystalline material kept
near a cathode ray tube. The glow persisted even when the crystals were covered with
a dark paper. The unknown radiation emerging from the cathode ray tube was termed
x-rays by Roentgen as x is commonly used alphabet for an unknown factor in
mathematics. Roentgen was awarded the Nobel Prize in physics for this discovery in
1901.
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X-rays present a unique ability to the analytical chemist to understand the structure of
materials. X-rays simply do not get reflected by reflecting surfaces superficially but
they have the ability to penetrate and get diffracted from internal atomic layers within
crystals. This possibility arises as their wavelengths are in the range of atomic
dimensions of crystals that constitute most of the materials under examination.
XRD analysis has not only contributed to our understanding of arrangement of atoms
within crystalline materials but has contributed immensely to establish purity of
materials and identification of different phases. Such studies yield valuable details on
materials ranging from polymers, metallurgical samples, pharmaceuticals, electronic
components, minerals and nano materials. The scope of applications is vast and in this
article some of the application areas of XRD analysis are briefly discussed.
Single crystal and Powder XRD analysis are two common techniques used in XRD
laboratories. Both the techniques have a distinct advantage that they are
non-destructive and it is possible to handle small amount of samples that are generally
available. Single crystal studies provide the geometrical arrangement of atoms in the
crystal lattice including critical information on bond lengths and angles. However,
such studies take 2 to 3 days for data collection and subsequent analysis. On the other
hand powder XRD analysis can be concluded in about half an hour. In addition
sample preparation is simple and the information made available can be used for both
qualitative and quantitative purposes.
Applications of X-rays
Pharmaceuticals
XRD analysis is a valuable tool for development of new drugs, characterization of
active materials and excipients, testing at different stages of manufacture for effective
quality control. Such information helps to improve quality of formulations, bio
-availability and improvement of stability characteristics.
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Geological Applications
Powder XRD is a powerful technique for characterization and grading of minerals.
Each mineral has a well defined crystal geometry and yields a unique x-ray diffraction
pattern which helps in taking commercial decisions before undertaking mining
operations.
Stability Studies
Changes in environmental parameters such as temperature and humidity can lead to
certain solid phase transitions which can be conveniently monitored using powder
XRD. Such information provides useful clues on the stability of final finished
products and can make contributions to evaluate their stability behaviour.
Studies on Nano Materials
The properties of materials depend largely on crystal size. This feature can provide
useful information on nano particles which are the building blocks of pharmaceuticals,
polymers and composites.
Electronics
Micro Electronics is contributing significantly to advances in computer and electronic
consumer products. Silicon and gallium arsenide are two popular materials used in
integrated circuits production. XRD has been used in such applications to provide
useful information on crystal structures and defects in manufacturing of
microelectronic components.
Forensics
Powder XRD plays a significant role in criminal investigations. Samples collected
from scene of crime such as broken glass, paint chips, hair and powders provide
unique x-ray diffraction patterns which can help trace their origin and ultimately to
lead to the prime suspects.
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Medical diagnosis
X-rays can penetrate soft tissues but not bones or other foreign objects embedded in
the body. This feature is put to use with for diagnosis of own bone fractures and
deformities, foreign bodies deformities and tumor growths. The photographic plates
also called radiograms and provide useful information for treatment of such
conditions. X-rays can damage healthy tissue so excessive exposure should be
avoided by using protective shields.
Industrial applications
X-rays find several applications in nondestructive testing for cracks and fatigue in
manufactured components. X-ray lithography uses precise X-ray beams to embed
miniature circuit patterns on silicon chips.
Radiation Therapy
High energy x-ray beams are used for destruction of cancerous cells. However, as the
beams do not differentiate between normal and cancerous cells their use requires
monitoring by medical experts.
Baggage Security
Baggage materials like leather and plastics are transparent to x-rays but concealed
guns and knives made of metal are opaque to them. This feature serves as a quick
means for screening of baggage by security personnel at airports and railway
stations.Structural arrangement of atoms and molecules
X-rays have found use in study of crystal geometries and polymorphism in materials.
This is possible because the wavelength range of x-rays is similar to size of atoms in
such substances. Both Single crystal and Powder systems are available for such
studies and there are several commercial manufacturers to provide such instruments.
Astronomical Research
X-rays are also present in interstellar space. Main sources being neutron stars and
black holes at centre of spiral galaxies which gobble up stellar matter in
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their neighborhood and in the process emit x-rays. Based on such phenomena
astronomers have postulated studies on origins of the universe. Typically such studies
are conducted with the help of orbiting telescopes.
XRD alone or in combination with other techniques such as chromatography, FT-IR
or DSC provides valuable confirmatory information on nature and characteristics of
materials.
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Material Characterization by X-ray DiffractionStudies
Symmetric Faced Crystal
X-ray diffraction is a valuable tool in the hands of the analytical chemist for material
characterization which is based on arrangement of atoms in the crystal lattices. The
biggest advantage is that it is a non-destructive technique and the sample can be
reused for further investigations by other means. Samples can range from
symmetrically ordered atomic arrangements to random arrangements as in amorphous
substances. The common applications of x-ray diffraction studies include crystal
lattice dimensional analysis, grain size, crystal defects and residual strain.
X-rays interact with solid materials to generate diffraction patterns. Data on
diffraction patterns resulting from interaction of x-rays with inorganic and organic
solids shows definite patterns which can be used as fingerprints in identification of
such materials. Diffraction data base is maintained by the International Centre of
Diffraction data (ICDD) which was formerly known as Joint Committee on Powder
Diffraction Standards (JCPDS). Such reference data can be purchased direct from
ICDD or through x-ray diffraction instrument suppliers.
X-ray diffraction studies easily distinguish between single crystal orderly
arrangements of atoms to polycrystalline arrangements. The atomic planes of crystal
lattices responsible for scattering of x-rays are their reflective surfaces. Scattered
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beams when in phase interact constructively and intensities are maximum at particular
angles. Such reflecting patterns on reaching the detector will generate a response
which can be matched with the pattern from standard materials.
It is interesting to note that under the influence of heat small particles anneal to form
larger aggregates. This becomes apparent as peak intensities get enlarged and help
distinguish nano-particles from larger aggregates of particles.
Applications of XRD
X-ray diffraction is a powerful tool for characterization of nano- materials, bulk
materials and thin polymeric films.
Phase Studies
Powder crystallographic studies help characterization on the basis of chemical
composition of materials. Such studies provide valuable details on phase
transformations resulting from subjecting materials to extremes of temperatures.
Degree of Crystallization
Polymeric materials often exhibit mixed behaviour as they can be partially crystalline
and partially amorphous. The degree of amorphous content can vary with the
conditions used during their processing. The more the amorphous content the greater
will be the peak broadening so the ratio between the peaks of pure crystalline
standards and polymeric materials will give an idea on the degree of crystallinity in
the polymeric sample.
Residual Stress
Stress is defined as force acting on a material per unit area and any deformation per
unit length is referred to as strain. Residual stress is the stress that remains in the
material when the force responsible for it is removed. In synthetic materials residual
stress results from material treatment processes such as machining, welding, heat
quenching, etc. On the other hand in geological samples such stress could be the result
from natural rock dynamics under the earth's surface. X-ray diffraction is helpful in
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studies on residual stress introduced in materials through artificial or natural
processes.
X-ray diffraction also helps study the dominant or preferred orientation of
polycrystalline aggregates. Such information is beneficial in relating orientations of
aggregates or texture to the desirable properties of materials.
In conclusion it can be said that x-ray diffraction studies provide valuable information
which can help characterize both manufactured as well as naturally occurring
materials.
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Understanding nature of matter throughknowledge of Crystallography
Scientist with crystal structure lattice model
All matter is composed of atoms and their clusters called molecules. The arrangement
of atoms and molecules characterize a substance and assign its unique identity.
Superficially it is difficult to appreciate such arrangements but if you were to have a
peek down to the atomic and molecular levels you will be amazed to see that many of
the commonly occurring materials display a highly symmetric and orderly
arrangement of atoms and molecules. X-ray diffraction or XRD is tool that helps you
gain such an insight into structure of matter at the atomic level. An understanding of
the science of crystallography will be necessary to fully exploit the potential of the
XRD technique.
What is Crystallography?
X-ray Crystal diffraction
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Bragg’s law is the foundation stone on which the edifice of x-ray diffraction stands. It
holds same significance in crystal structure determinations as the Beer- Lambert
law(link) holds in light absorbance measurements. The law was established by Sir
W.H Bragg and his son Sir W.L Bragg in 1913 to explain the diffraction of x-rays
from atomic planes of crystals of sodium chloride, zinc sulphide and diamond.The
Nobel prize for Physics was awarded to the Bragg duo in 1915 for their contribution
in the field of crystallography.
A beam of x-rays incident to a crystal face gets partially scattered by the atoms of the
crystal. The fraction that is not scattered reaches the next atomic layer where another
part is scattered and remainder passes across to the next layer and so on. As a result
the diffraction pattern is generated from the constructive and destructive interference
of X-rays diffracted from each plane. The diffracted beams interact constructively if
the beams are in phase or destructively if they are out of phase. A basic requirement
for x-rays to diffract is that the sample exhibit crystallinity and the spacing between
the atomic layers must lie in the wavelength range of x-ray radiation.
Diffraction pattern on film
The Bragg’s law can be expressed mathematically as
nλ = 2d Sinθ
where,
λ is the wavelength of x-ray beam
θ the angle of incidence
d the spacing between the atomic planes
n is an integer
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Diffracted beam from several randomly oriented crystals in the sample progresses in a
conical shape from the crystal face and the diffraction pattern can be displayed on a
photographic film as a series of concentric rings.
Based on XRD measurements you can record the distance between the atomic layers
of the crystal lattice, estimate bond lengths and angles, and confirm the identity of the
unknown materials by correlating their crystal lattice structure with standard reference
materials.
The law though developed to observe scattering of X-rays by crystals is applicable
to determine the structure using different beams such as electrons, ions, neutrons or
protons with wavelengths in the range of distances between the atoms or molecules in
the crystal.
Crystallography is a science dealing with study of crystals. It is an established fact
that pure compounds have a definite arrangement of atoms which are responsible for
their characteristic properties irrespective of the source of the material. Examples are
common salt which has a cubic arrangement of sodium and chlorine atoms, diamond a
pure form of carbon has tetrahedral atomic arrangement of carbon atoms whereas
graphite has hexagonal ring structures.
It is obvious that crystals can exist in solid state only. Gases or liquids do not exhibit
crystallinity. However, solids can exist both in crystalline or non-crystalline states.
The present article discusses some of the crystalline states and characteristic features
of materials that are common in nature.
Crystal lattice
Crystals comprise of well organized array of atoms, molecules or ions. A unit cell or a
crystal lattice is the smallest structure of the compound or element which repeats itself
by translation throughout the crystal. It can be said that a lattice is theoretically an
infinite array of atoms, molecules or ions which repeats itself to constitute a crystal. A
lattice made up of same atoms as in case of elements is called monoatomic and if it
has more than one type of atoms it is called a polyatomic lattice. There are different
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types of lattices which are classified according to the geometrical arrangements of
atoms which will be taken in a subsequent article.
Seven Crystal System Shapes
Crystals are three-dimensional symmetric arrangements of atoms, molecules or ions.
Such arrangements repeat themselves at regular intervals keeping the same relative
orientation to one another. This is a unique property of crystalline materials which are
specific to different crystalline compounds irrespective of the source of origin be it
natural or synthetic.
If you consider each atom, molecule or ion as a point then such an arrangement is in
translational symmetry and the outline of such arrangement is called a crystal
lattice .A unit cell comprising of single type of atoms is monoatomic whereas one
comprising of more than one type of atoms is called polyatomic cell.
Crystals can be considered as planes joining groups of atoms with fixed distances and
angles .These dimensional constants are characteristic features of different crystalline
materials.
Crystal Geometries
Axis Axis Angles
1 Cubic A= B = C α = β = ƴ = 90
2 Tetragonal A = B ≠ C α = β = ƴ = 90
3 Orthorhombic A ≠ B ≠ C α = β = ƴ = 90
4a Hexagonal A = B ≠ C α = β = ƴ = 90, ƴ = 120
4b Rhomboheral A= B = C α = β = ƴ ≠ 90, ƴ < 120
5 Monoclinic A ≠ B ≠ C α = ƴ + 90, β > 90
6 Triclinic A ≠ B ≠ C α ≠ β ≠ ƴ ≠ 90
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Crystal Systems
A crystal system is a group of crystal structures used to describe the axial arrangement
of crystals. There are seven basic crystal shapes
Cubic
This arrangement consists of three axis perpendicular to each other with all sides
equal in length. The cubic system has a lattice point at each of its eight corners and
has six faces.
Hexagonal
The hexagonal arrangement has four axes. Three of these are horizontal at 120° to
each other and the fourth axes is perpendicular to the three horizontal axes. It
comprises of eight faces
Tetragonal
A tetragonal system has a square base and top like in cubic arrangement but has an
extended vertical height. It has three axes at 90° to each other and a total of six faces
Rhombohedral
The rhombohedral is similar in shape to a cube but is inclined in one direction.Its
three axes are perpendicular to each other with two horizontal and one vertical. It has
six faces.
Orthogonal
Orthogonal crystals consist of three axes perpendicular to each but of different
lengths.It has six faces.
Monoclinic
Monoclinic crystal has three unequal axes. The front face axes are oblique to each
other and the third axes is perpendicular to the other two. The system has six faces
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Triclinic
The structure has three unequal crystallographic axes which intersect one another
obliquely. It has six faces.
Bravais in 1848 postulated that seven crystal systems can exist in 14 distinct types of
configurations. The unit cells of Bravais lattice are
Cubic – 3 (simple cubic, body centred cubic, face centred cubic)
Tetragonal – 2 (simple, body centred )
Orthorhombic – 4 (simple, body centred, base centred, face centred)
Hexagonal-1 (simple)
Rhombohedral – 1(simple)
Monoclinic – 2(simple, base centred)
Triclinic – 1 (simple)
Miller indices
The atomic orientations in crystals are responsible for their shapes. It often becomes
necessary to define different planes within a lattice mathematically. Physical
properties of materials such as electrical conductivity, thermal conductivity,
deformation under loads,etc are dependent on orientations in some crystals. Such
behaviour is referred to as anisotropy.
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To understand Miller induces it is important to understand the commonly used
expressions x, y, z are axes passing through origin a,b,c are unit cell lengths along the
three axes Miller induces express planes as (hkl) where h, k and l are integers.
Gen Convention for assigning Miller indices:
Determine the intersection of the plane along the three axes-a,b and c.
Suppose a plane intersects x axis at a/2, y axis at the end and c axis at c/3.
These are expressed as 1/2,1and 1/3.
The reciprocals of these become 2,1, 3.
The Miller indice of this plane is expressed as (213), ie, in brackets and
without commas.
In a cubic system planes having same indices regardless of order and sign are
equivalent but the same will not be true for other geometries.
Types of crystals and their properties
Single Crystals
Single crystals are large enough in size and their outer boundaries display distinct
geometrical shapes. Some of these are found naturally but for others the shapes can be
cut artificially to make them more attractive. Typical examples are gemstones which
can be cut to different shapes to improve their shapes and light reflection properties.
Ionic Crystals
The atoms constituting ionic crystals have different electro- negatives and are held
together in their positions by electrostatic forces. Such crystals are generally hard and
have high melting points
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Covalent Crystals
Covalent crystals are characterized by sharing of electrons between atoms of the
crystal. Strong bonding makes the crystals extremely hard and give them high melting
points. A typical example of such crystals is diamond whose hardness is difficult to
match.
Metallic Crystals
Metallic crystals are constituted of metal atoms positioned on the lattice sites. The
outer shell electrons of metal atoms are free to move around throughout the lattice.
This property makes such crystals good conductors of heat and electricity and also
show high melting points.
Molecular Crystals
Molecular crystals comprise of individual molecules at the lattice points. Such
molecules are held together by weak non-covalent forces such as van der waals forces
or hydrogen bonding. Such interactions render molecular crystals with softness and
low melting points. Examples of molecular crystals are proteins which exhibit
crystalline features.
Polycrystalline Materials
Polycrystalline materials are clusters of several crystallites or grains having different
sizes and orientations. The orientations can be random in nature and influenced by
their growth and processing during formation.
Allotropes or Polymorphs
Allotropes or polymorphs are different forms of an element which are made up of the
atoms bound together in different geometrical arrangements. A common example is
diamond which can be present in a tetragonal arrangement in diamond but in
hexagonal ring arrangement in graphite. Such polymorphs show different physical
properties.
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Amorphous Solids
Amorphous or non-crystalline solids do not exhibit long-range order characteristic of
a particular crystalline material. Such materials often exhibit only a short-range order.
Examples are glass, polymer films and gels
Anisotropy
Anisotropy refers to the difference in properties when measured along different axial
directions in a crystal. Such physical properties include absorbance of light, reflective
index, conductivity, tensile strength, etc. Anisotropy generally results from distortion
and elongation of grains in some direction during the formation of crystals.
Single Crystal and Powder XRD
The objective of XRD studies is to gain insight into the structural arrangement in
crystal lattices and to characterize materials on the basis of their crystallinity and
phase compositions. Such information is gained by adopting either single crystal or
powder XRD techniques.
Single Crystal XRD
Single crystal XRD technique is applied when information is required on atomic
arrangements within crystal lattices such as location of atoms, crystal lattice
arrangement, bond angles and lengths and interplanar distances.
The technique requires a single crystal of homogeneous composition with optically
clear faces. The size of the crystal ideally should be between 0. 1 – 0.2mm. Before
mounting on the goniometer head the uniformity of the crystal is examined with a
microscope fitted with a polarizing attachment.
Single crystal XRD is a nondestructive technique requiring no sample preparation
except selection of a suitable crystal. The crystal is attached to the tip of a thin glass
fibre using an epoxy or in a loop which fits into the goniometer head.
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The main disadvantage of the technique is that it is time consuming and may require
several hours to days for a single study.
Powder XRD
Single crystal studies are time consuming and require a very high degree of
homogeneity of the crystalline specimen. A simpler option is available through
powder XRD where instead of a single crystal studies are made on powder specimens.
The technique requires minimal sample preparation and there is significant time
saving as a single run gets completed in 15 – 20 minutes.
The sample rotates in the path of the x-ray beam at an angle θ whereas the detector
mounted on an outer circular arm rotates correspondingly 2θ to collect the diffracted
beam. The constant relation between the angles of rotation is maintained by the
goniometer. Generally,for most studies, data are collected at 2θ angles from 5 degrees
to 70 degrees.
The sample is ground finely so as to obtain required homogeneity and offset any
preferred orientations of crystal faces in the path of the incident x-ray beam. Particle
size ideally should be between 1 – 10μm (200 mesh size). A preferred approach is to
make a slurry of the powder in a drop or two of viscous liquid before applying to the
slide and allowing the liquid to dry out.This helps in getting a uniform random
orientation powder film of uniform thickness.
Care should be exercised to prevent over grinding of sample as this can complicate
the diffractogram due to strain broadening through introduction of lattice
deformations.
Powder XRD is a popular technique for studies on materials such as pharmaceuticals,
cosmetics, foods, rocks, minerals, soils, etc for common features such as degree of
crystallinity, polymorphism, phase transitions due to temperature and humidity
changes and stability behavior of different products.
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Interpretation of Powder XRD data requires standard reference data base for desired
applications such as pharmaceuticals, minerals, etc. Such a database can be created
in-house or can be obtained from recognized standard bodies.
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How are X-Ray Diffraction (XRD) and X-RayFluorescence (XRF) different?
Portable hand held XRF analyser
Both XRD and XRF are material characterization techniques which have gained
popularity in the past several decades.
In simple layman terms X-ray fluorescence is a technique for determination of the
elemental composition of the sample without differentiating between the different
chemical compounds that are present in the sample. On the other hand x-ray
diffraction provides information on the sample composition in terms of compounds
present, degree of crystallinity and its amorphous content.. In a way it can be said that
both the techniques complement one another and give a total picture of sample
composition. The attractive feature common to both the techniques is that they are
non-destructive and the sample can be recovered for frequent confirmatory tests.
In order to understand the differences between the two techniques it is important to
have clarity on the basic principles of the two techniques
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XRD
All matter is composed of atoms which show a degree of periodicity in their structural
arrangements. The scattering of a homogeneous x-ray beam by collision with
electrons of atoms result in diffraction which is dependent on the wavelength of
x-rays and the distance between the planes of the atoms arranged in such arrays.
The study provides valuable details on structural arrangements of crystals in unit cells
or lattices. Qualitative confirmation of compounds present in samples can be arrived
at by matching the diffraction patterns with library collection of standard diffraction
patterns. The technique finds potential applications in:
identification of chemical composition of minerals and industrial products in
terms of number of phases, degree of crystallinity and amorphous content.
phase transformations and structural changes due to changes in temperature,
stress or gas phase environment
Texture analysis of thin films
XRF
XRF provides the elemental composition of sample in percentages but will not
differentiate between the compounds in which a particular element exists or the phase
of the compounds.
The bombardment of x-rays results in the eviction of electrons from inner shells. The
vacancies created tend to be filled up by the electrons from outer shells. In the process
the energy is released as x-rays which are characteristic for each element present in
the sample. XRF provides elemental composition in diverse samples such as minerals,
cements, petroleum products, polymers, plastics and paints. XRF permits
quantification of both metallic and nonmetallic elements of the periodic table from
fluorine (atomic number 9 upwards). Sensitivities of up to fractions of a percent to
ppm levels are commonly achieved.
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Combined XRD and XRF instruments
The wealth of information provided by both XRD and XRF techniques individually
has propelled development of combined technology instruments. A single sample can
be used to provide a saving of time and space in addition to information on elemental
and phase composition. Such instruments provide comprehensive analysis
information on metals, alloys, sinters, minerals, cements and refractory materials.
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Component parts of an X-ray Diffractometer
Schematic arrangement of XRD Diffractometer components
X-ray studies are mainly carried out in two basic configurations, namely, Single
crystal and Powder XRD. However, the component parts of the x-ray spectrometer are
in general common and comprise of:
Source of x-rays
Sample stage
Detector
The article will provide basic details on the component parts of the x-ray
diffractometer.
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Source of x-rays
Schematic diagram of X-ray tube
X-ray tube is a common source of x-rays. It comprises of an evacuated tube which
contains a copper block anode bearing a metal target made of any of the metals such
as molybdenum, tungsten, copper, rhodium, silver or cobalt .The cathode is a tungsten
filament .On passage of electric current through the filament electrons are generated
which move towards the anode under the highly accelerated voltage typically 30 –
150 kV. The accelerating electrons on striking the metal surface knock out electrons
from the inner shells and the vacancies created are filled by electrons from the outer
shells. In the process metal atoms emit x-rays. However, this involves heating of the
metal block and x-rays constitute only a small fraction of the total energy liberated.
The emitted x-rays exit the tube through a berylium window. The copper block
needs to be cooled with a supply of water to dissipate the excessive heat generated.
The Be window helps transmit a monochromatic beam of x-rays. Further
monochromatization can be achieved by making use of a zirconium filter when using
molybdenum as metal target. It absorbs the unwanted emissions while allowing the
desired wavelengths to transmit.
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Sample stage
Sample stage is also known as sample holder or a goniometer. Single crystal
diffractometers make use of 4 circle goniometers. These circles help position the
crystal planes for optimum x-ray diffraction settings. The sample stage can be a
simple needle that holds the crystal in place or glass plate or fiber on which the crystal
is mounted using an epoxy resin. Only sufficient quantity of epoxy resin is used so
that the crystal is clearly mounted and not embedded in the resin. The fiber is
mounted on a brass mounting pin and then inserted into the goniometer head. The
sample is then centred with an optical arrangement such as a microscope or video
camera and making adjustments along X, Y and Z directions to achieve optimum
centering under the crosshairs of the viewer.
Detectors
In earlier days photographic films were used for recording the absorption pattern of
diffracted beams. With the advances in detection technology more sensitive detector
options were incorporated in advanced instruments. Such detectors include gas filled
transducers, scintillation counters and semiconductor transducers. Solid state detectors
offer highest levels of sensitivity and speed of analysis.
Global manufacturers of XRD systems
X-ray diffraction offers a rapid and non-destructive approach for identification and
characterization of unknown materials. The range of applications includes metals,
minerals, plastics and polymers, pharmaceuticals, protective thin film coatings,
cosmetics, confectionary and synthetic foods, semiconductors, forensic specimens, etc.
The diverse range of applications provide an impetus to analytical instrument
manufacturers to manufacture affordable XRD systems for use in research
laboratories and industrial establishments.
Several global reputed manufacturers provide state of art XRD systems with advanced
control, data acquisition and data processing features.The number of manufacturers
are over a dozen and some of them are listed here.
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1. Rigaku Corporation
2. PANalytical
3. Bruker
4. Anton Paar GmbH
5. Agilent Technologies
6. Shimadzu corporation
7. Olympus
8. Angstrom Advanced Inc
9. Stoe and Cie, GmbH
10. Proto manufacturing.
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Applications of Powder XRD Studies
Pharmaceuticals
Applications of XRD
Pharmaceutical formulations are available as tablets, capsules, ointments, pills, oral
liquids, aerosol sprays, etc. Even a single drug can be found in several different
formulations which impart it different characteristics as per administration
requirements .In case of solid formulations X-ray diffraction has been used as a
non-destructive technique for testing of inconsistencies in manufactured batches, new
drug development, and detection of impurities, degree of crystallinity and presence of
polymorphism. In other words XRD serves as a fingerprinting technique for
pharmaceutical formulations. It helps in optimization of manufacturing parameters,
compatibility of excipients with active ingredients and controlling properties of
dosage forms such as dissolutuion rate of active ingredients and stability behaviour
under different environmental conditions.
The article discusses some common applications of the technique in drug manufacture
and quality control .It is important to mention that XRD is a sensitive technique and
errors in sample preparation can lead to wrong inferences. The sample collected
should be representative of the material and with uniform distribution of particles.
During sample collection and preparation the material should not be subjected to
unwanted stress or other environmental conditions as such factors can result in
adverse effect on the material characteristics.
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Identification
Conventionally the analysis is carried out by crushing the tablet or capsule gently in a
mortar and pestle or a grinding mill. Vigorous grinding can introduce mechanical
stress leading to phase transitions. For small sample quantities capillary mounts can
be used. Today modern diffractometers have provision to correct for the curvature
when examining tablets or capsules with curved surfaces. XRD analysis can help
determine the uniform distribution of tablet ingredients. Such distribution can
establish the preferential treatment of surface modifying agents such as pigments or
lubricants which can influence shelf life of a formulation, its dissolution rate and
disintegration behaviour. Accessories are also available to conduct studies under
variable temperature and humidity.
Degree of hydration or solvation
Hydrated or solvated molecules have different characteristics. Thermal techniques
such as TGA and DSC are being used to determine the number of molecules of
hydration or solvation but XRD helps easy correlation of such states with crystal
structure. Variable temperature accessories further provide an idea of temperature or
humidity on associated properties of such materials.
Amorphous content
A drug substance can contain excipients which are amorphous in nature. In
comparison with crystalline materials an amorphous material shows instability and
transforms to crystalline phase over time due to environmental conditions during
storage. Solubility of a drug increases with increasing amorphous content so it
becomes necessary to quantify the amorphous content. X-ray diffraction offers a
useful tool for quantifying the amorphous content. This is achieved by comparison of
the intensity ratio between the peak of crystalline component with the hollow intensity
of the amorphous content.
Polymorphic impurity profiling
A formulation can contain polymorphic impurities, products of degradation and
excipients which can have a bearing on the functionality, stability and effectiveness of
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the active ingredient. Different polymorphs of the same compound exhibit different
properties. Compression forces during tablet production can cause changes in
composition of such compounds. X-ray diffraction is an established method for
estimation of such substances
Pharmaceutical analysis poses quite a few challenges due to presence of a broad range
of materials, their orientations and mixed crystallinity. However, XRD provides
useful information and in combination with other contemporary techniques such as
TGA, FT-IR, DSC, NMR,etc can provide useful details on pharmaceutical
formulations.
Foods
Applications of XRD
The growth of our civilization has seen a parallel growth in the food industry. In
pre-historic ages humanity was dependent mainly on vegetables, fruits and meat for
its daily food intake. However, over the years there has been an increasing demand for
processed synthetic foods to cater to the needs of a growing population and to meet
scarcities in different regions having harsh climatic conditions.
Processed foods need to have some basic characteristic properties which make them
acceptable to the consumer. Some of these features are:
Colour which can be introduced with a blend of natural and artificial colouring agents
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Flavours need to be added to make foods appealing or suppress some inherent bad
odours
Preservatives are needed to improve the shelf life for long time storage and use
Stabilisers are added to improve their texture. This means modification of the
crystalline behaviour and characteristics of the phases of different constituents
Anti- oxidants are added to prevent decay of food by oxidation
Emulsifying agents help in facilitating uniform distribution of fats and oils in
aqueous media
Buffering agents control the acidity or pH of the food over storage period
The number of additives added to foods are growing by the day to match consumer
tastes and lead to a corresponding increase in market potential.
Several analytical techniques are routinely used to monitor the d presence of such
food additives in manufacturing processes. The role played by XRD cannot be
overlooked. X-ray diffraction provides us information on polymorphism, degree of
crystallinity and amorphism. Such parameters help control the texture and stability of
foods under different storage conditions.
X-ray diffraction has helped conduct studies on following common food ingredients
such as
Starches
Fats
Candies
Starches
X-ray diffraction complements the DSC studies in starch gelatinization in foods.
Gelatinization results when starch crystals melt with increasing temperature. It has
been observed that the melting temperature increases with the decrease in moisture
content. Gelatinization decreases the degree of crystallinity of starch granules and
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increases the non-crystalline amorphous content. This results in apparent changes in
optical and rheological flow properties of foods.
Fats
X-ray diffraction has been extensively used in studying the polymorphism (different
molecular packing arrangements in crystals) of fats such as margarines. The three
common polymorphs of fats are α, ß and ß’. Packing arrangement in α form is
hexagonal, triclinic in ß and orthorhombic in ß’. The ß form is the most stable form
whereas α form is the least stable of the three. Polymorphic differences are of
significance in food industry as such differences bring about changes in physical
properties such as texture. Such inter- conversions can result from transitions from ß’
to the more stable ß form under different processing conditions.
Candies
Candies and chocolates are very popular among children. X-ray diffraction plays a
useful role in manufacturing products of consistent quality in terms of sweetness,
texture and flavor. Such characteristics depend on the balance between crystalline and
amorphous proportions of constituents which can be easily monitored with the help of
the X-ray diffraction technique.
The scope of X-ray diffraction in the food industry is increasing as consumers
demand consistency of quality and new ingredients are finding their way into new
products which are developed at an amazing pace.
Cosmetics
XRD Cosmetics
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Cosmetics have been in use since time immemorial. In earlier ages several potentially
toxic inorganic pigments such as malachite, mercuric sulphide or white lead were
used in cosmetic preparations. In contemporary products several organic components
such as parabens, alkylphenols, etc have also been included. It was a common concept
that cosmetic products were only meant for external applications and therefore were
safe as skin was believed to be impermeable to external applications. However, it is
now established that skin is not an impermeable barrier which can prevent penetration
of chemical compounds present in cosmetics. A common user is primarily concerned
with irritations and allergies only due to use of such products but it has been
established that systematic absorption can cause chronic toxicity resulting in harmful
consequences.
It is also a common belief that cosmetic products are safe as the cosmetics industry is
under regulation of US Food and Drug Administration. On the contrary this is a self
regulated industry. Undoubtedly there are several established global brands which
exercise strict quality control during manufacturing but it is also true that in the grey
markets, especially in third world countries, counterfeit products find a flourishing
market. Such fake products are not manufactured under any controls worth
mentioning and their use can do more harm than good.
Cosmetics come in various forms such as lipsticks, soaps, toothpastes, body
deodorants, face powders, nail polishes, shaving creams, face creams and anti- ageing
creams, sunshades, moisturizers, shampoos, hair oils, etc. The list is virtually endless
and new products are continuously being added to keep pace with growing consumer
demands. The formulations are available as powders, creams, gels, lotions and liquid
suspensions containing un- dissolved solids. Several techniques find use in analysis of
cosmetic products some of which are Transmission Electron Microscopy, Atomic
Force Microscopy, X-Ray Fluorescence, Laser Desorption Ionization Mass
Spectroscopy, Inductively Coupled Optical Emission Spectroscopy and X Ray
Diffraction analysis. Most of the mentioned techniques involve high initial
investments so become unaffordable by common testing laboratories. Out of these
XRD is affordable and does provide valuable information on components which
exhibit crystalline or semi-crystalline properties. However, the role of expensive
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instruments cannot be underplayed and should be restored to when specific details on
sample constituents are required.
X-ray diffraction reveals the proportion of crystalline and amorphous content of a
product. Sharp peaks result from the crystalline content and amorphous content results
in broad humps. The identity of crystalline compound and can be established through
a match with the data base of known crystalline materials.
XRD is a non- destructive technique which also does not require sample pretreatment
or dilutions. It makes use of a collimated beam of x-rays that is directed onto the
sample holder,tube or a powder smeared evenly on a glass slide. The diffraction
pattern results from constructive interference of beams scattered by the faces of
crystal planes of the sample. The x-ray incident beam scans the sample over a range
of degrees by rotating the sample stage at a predetermined angular scan rate
Nanotechnology has brought about a revolution in fields of materials science,
electronics, pharmaceuticals, foods, etc. Cosmetics have not been left out and nano
particles (having diameters below 100nm) are being increasingly used to improve the
desirable features of cosmetic products. Nano emulsion in the form of lotions and
conditioners are commonly available. Studies are reported on nano emulsions used in
sunscreens comprising of TiO2 nano particles. Similarly silver nano particles find use
in some soaps, toothpastes and face creams and act as bactericides. Similarly other
nano particles have shown promise in other commonly used cosmetic products.
X-ray diffraction has played a significant role in studies on such particles as their
properties achieve the desirable cosmetic features and also their uptake by the skin.
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Geology and Mining
Copper bearing ore on mine conveyor
X-ray diffraction or XRD has made valuable contributions in the field of geology and
mining. Geology is the scientific study on structure of planet earth, formation of rocks,
sediments and minerals. Mining is an applied science which concerns with extraction
of ores, fossil fuels for energy needs and minerals for commercial processing.
Minerals are inorganic materials majority of which exhibit a degree of crystallinity.
Each mineral generates its unique x-ray diffraction pattern which serves to identify it.
The diffraction pattern can be matched with a library of over 17,000 patterns for
different minerals offered by the International Centre for Diffraction Data
(ICDD).Using the data base helps in both qualitative and quantitative estimation of
minerals. In addition x-ray diffraction provides details on degree of crystallinity,
amorphous content, grain size, lattice deformations due to strain and can help trace
the origins of minerals.
XRD Analysis complements information obtained from other analytical techniques
such as polarized light microscopy and scanning electron microscopy. However, the
lower cost of XRD makes it the preferred choice for studies directed towards
evaluation of fine grain minerals and mixtures or intergrowths at the microscopic
level. However, for elemental composition and texture analysis one has to resort to
scanning probe microscopy and scanning electron microscopy.
Minerals exist in the Earth's crust under different forms such as rocks, clays and
sediments or as nodules on the sea bed. Some typical practices for sampling of
geological specimens are outlined in the article.
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Rocks
Rocks can vary in size as well as in uniformity of composition. The surface
composition can differ from the interior due to weathering under natural
environmental surroundings. Sampling plays a crucial role in rock analysis. The
sample size should be at least 5 – 10 g. It should comprise of different layer cuttings
or drillings at different surface points which after fine grinding in a pulverising mill is
dried at around 60°C overnight and quartered to get a uniform sample.The sample
thus obtained is mounted onto a surface mount to obtain a uniform surface for
analysis.
Clays
Like rocks 10- 15 g of clay sample is taken and any rock bits are sieved out. The fine
clay mix is ground in a mortar and pestle to obtain stone free specimen. The powder is
transferred to a centrifuge tube topped up with Calgon solution and after shaking is
centrifuged to remove the course sediments. Centrifugation is repeated to permit
sedimentation of fine solids. A small portion of the clear sediment is spread on a glass
slide and allowed to dry. The slide is then mounted onto the sample stage of
goniometer and subjected to the typical XRD analysis. In addition to mineralogical
analysis it also gives useful information on particle size.
Weathering of Rocks and minerals
The composition of minerals undergoes continual changes due to environmental
factors such as acid mine drainage, hydrological changes, changes in temperature and
pressure due to rock dynamics. Such changes can be monitored using XRD analysis
and can help establish the evolution of mineral resources in different mineral rich
areas. The data collected from remote sensing can be correlated with the XRD
analysis data to reach commercial decisions concerning mining activities.
Gem and Jewellery
XRD is a useful tool for grading of minerals in terms of their origin, degree of
crystallinity and purity. Such details are essential for jewellery makers to manufacture
items with features in demand in the consumer market.
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Forensics
Searching for evidence from articles collected from scene of crime
Forensic analysis is a scientific study which assists criminal investigations. Whenever
any crime takes place there is exchange of material between the accused and the
victim such as textile fibers, blood stains, gunshot residues, broken glass, saliva,
semen, etc. Chemical and biological tests on such materials can provide undisputed
evidence to nail down the suspect. However, forensic analysis poses a big challenge
to the analyst. Firstly, the amount of sample collected from the scene of crime is often
limited and it is just not possible to replenish it. Secondly, the results of such analysis
can lead to conviction so findings need to be reported with extreme care so that
innocent persons do not suffer. For this reason nondestructive techniques are preferred
as the sample can subsequently be tested for further confirmation.
X-ray diffraction or XRD is one such technique which is nondestructive and the
sample requires minimum sample preparation prior to analysis. The only requirement
is that the sample should be homogeneous in nature so as to provide uniform analysis
results even if a small portion is analysed from a bulk quantity. Samples commonly
received for forensic testing commonly include:
Building materials such as cement, concrete, steel rods, bricks,etc
Drugs of abuse and other banned substances
Residues from site of arson such as kerosene oil, gasoline or cotton lints
Explosive residues and splinters from sites
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Accident scene samples such as blood spots, paint chips,etc
Assault samples such as torn garments, broken glass,cosmetic marks, semen
stains,etc
Theft and robbery site samples which include gunshot residues, tools, murder
weapons, forged documents, blood residues,etc.
The samples can be analysed by XRD if it exhibits a degree of crystallinity even if the
remaining content is amorphous. The technique helps establish the presence or
absence of a particular material through comparison against a reference XRD data
base. The present article discusses some of the XRD applications of common samples
picked up from scene of crime.
Drugs and pharmaceuticals
Drug and pharmaceutical samples are commonly seized from smugglers, carriers and
from raids conducted on rave parties. Such substances can pose a challenge as their
identity is seldom known. It can be a pure compound or even an engineered product
produced by illegal laboratories to conceal their real identity. XRD helps identify one
seized material from another or to identify the seized powder with known materials
such as cocaine, morphine, heroin and amphetamines or different compounds
introduced to conceal the real identity. Other analytical techniques such as visible
light is microscopy, GC – MS, HPLC, NMR and FT-IR also help complement the
findings of XRD
Explosives
Explosives are commonly recovered as powdered mixes, gels and liquids. Such
materials are often recovered from hijack suspects or at post-blast sites. While it is
simple to identify the pure material the analysis of residues collected from a blast site
can prove to be a daunting task. It can contain a mix of both organic and inorganic
material such as human blood, flesh, pellets, broken glass and residues of other
damaged material. At times the concentration of explosive in the residue could be so
low that analysis and detection can require separation and extraction prior to analysis.
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Paints and pigments
Samples of paints and pigments are generally recovered as minute flakes from clothes
of accident victims or vehicles involved in accidents. The collected specimens are
compared for preliminary colour matching by optical studies followed by presence of
crystalline components existing as pigments such as titanium dioxide, lead oxide or
red lead. The more the number of crystalline components present the better is the
degree of match. A final confirmation can be reached by ascertaining the organic
composition by pyrolysis GC analysis.
Textile fibers
It is common to recover pieces of cloth as evidence from scene of some crimes.
Textile fibers are a composite of crystalline and amorphous molecules. The extent of
similarity between the recovered material and accused person’s clothing can be useful
evidence. XRD helps establish the crystallinity of such textile specimens. Textiles
such as cotton, polyester and nylon show significant crystallinity whereas wool fibers
are predominantly amorphous.
Documents
Documents examination becomes necessary in cases involving forgeries, ransom
notes, counterfeit currency, hate mails and wrapping used for seized drugs. XRD
provides useful information on the crystallinity,match of crystallinity of cellulose and
inorganic fillers. An inexpensive copying paper generally shows absence of fillers like
titanium dioxide or anatase and is predominantly cellulose.
X-ray diffraction is a useful tool in the hands of the forensic analyst as it serves to
complement the findings of other analytical techniques. In recent years the technique
has also found widespread application in the rapid screening of passenger baggage at
major airports across the world. Specially designed screening systems help screen
passenger baggage for explosives and narcotics without the need for sniffer dogs or
manual searches. Such systems offer significant assistance in prevention of drug
smuggling and acts of terrorism without deputing additional manpower or
inconveniencing other passengers.
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10 interview questions on XRD technique
Q1. What are x-rays and what are their general applications?
Answer :
X-rays constitute a small portion of the electromagnetic spectrum. These are high
energy radiations ranging from 0.01 to 10 nm wavelengths. These are capable of
penetrating most common solid materials and find use in nondestructive diagnosis.
X-rays are classified as hard x-rays with energies ranging from 5eV- 10keV and soft
x-rays with energies in the range of 100eV to 50 keV.
The most common application that comes to mind is in medical diagnosis where
x-rays are used to get images of broken or cracked bones and foreign objects lodged
inside the body. Besides medical diagnosis x-rays find common use in destruction of
cancerous cells, non-– destructive testing of defects in mass produced materials,
screening of passenger baggage for prohibited items and studies on crystallinity and
polymorphism in materials.
Q 2. What do you understand by x-ray diffractometry?
Answer :
A. X-ray diffractometry is a versatile technique for characterization of crystalline and
semi-crystalline materials. It provides a wealth of information on crystal structures,
phases, texture, crystal defects and strain .It is based on constructive interference of a
beam of x-rays diffracted at different angles from each set of atomic planes within the
crystal lattice. The technique serves to fingerprint a material on the basis of periodic
arrangement of atoms within the crystal lattice.
Q 3. What is the basic law governing diffraction of x-rays?
Answer :
The diffraction of x-rays results in the constructive interference when conditions of
Braggs law are fulfilled. The law states
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nλ =2d Sinθ
where n is an integer, λ is the wavelength of incident x-ray beam, d is the interplanar
separation between the atomic layers of the crystal and θ is the angle of diffraction.
The diffracted rays are led to the detector to form a pattern or a diffractogram.
Q 4. How many basic crystal lattice structures are there. Can you name the seven
basic structures?
Answer :
Crystalline materials show systematic atomic arrangements which repeat themselves
at regular intervals in three dimensions keeping the same relative orientation to one
another.
The regular arrangements are called crystal geometries.There are seven basic crystal
lattice arrangements and Bravais postulated that the seven crystal systems can exist in
14 distinct crystal configurations which are called Bravais lattices. The 7 basic
configurations are cubic, hexagonal, tetragonal, rhombohedral, orthogonal,
monoclinic and triclinic.
Q5. Explain the basic configuration of an XRD system
Answer :
X-ray diffractometer consists of three basic components – an x-ray source or tube,
sample holder and an x-ray detector.
X-rays are generated by targeting a metal cathode block with electrons having
sufficient energy from a heated filament.The electrons having sufficient energy
dislodge inner shell electrons of the target material to produce characteristic x-rays.
These rays are collimated and directed to the sample stage which holds the specimen
sample. The diffracted rays are directed to the detector which converts the x-ray
signal to counts which are recorded electronically. In earlier days photographic films
were used to record the x-ray signals but today solid-state electronic devices are used
as detectors.
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Q6. List out some advantages of XRD technique
Answer :
XRD is a material characterization technique which provides a wealth of information
on material properties and serves as a fingerprint for identification of the material
under investigation. The specific advantages of the technique are that it is
non-destructive and sample can be reused for other confirmatory tests. It is a rapid
technique which takes about 15 to 20 minutes to run a diffractogram and requires
minimum sample preparation and offers ease of comparison of results with available
data bases.
Q 7. What are the disadvantages of the technique
Answer :
The advantages far outweigh the disadvantages. However, some of the disadvantages
are that for identification purpose homogeneous and single phase materials are
required for identification. You have to purchase separately or create your own data
base of reference materials for confirmatory studies. The sample needs to be carefully
prepared as over grinding can introduce stress in the lattices.
Q 8. What information is available from single crystal and Powder XRD
techniques
Answer :
X-ray diffraction is broadly speaking divided into single crystal and powder
diffraction studies. Single crystal studies are carried out on a single homogeneous
crystal with optically clear faces. The size of the crystal selected should ideally be
between 0.1 to 0.2mm across. The crystal is examined for uniformity with the help of
a microscope with polarising attachment before mounting on the goniometer head.
Single crystal provides details on unit cells of crystals in terms of location of atoms,
crystal systems, bond angles and distances between atomic planes. The technique is
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non-destructive in nature but takes a long time which may extend from several hours
to days to complete a single study.
Powder XRD on the other hand makes use of a finely ground powder instead of a
single crystal which is mounted on the sample slide.It is a rapid technique which takes
usually 15 – 20 minutes to complete a single scan and requires minimum sample
preparation
The information available from powder XRD includes degree of crystallinity,
amorphous content, phase composition and influence of environmental changes such
as humidity and temperature on phase transitions.
Q9 . What common precautions are necessary for preparation of samples for
powdered XRD analysis
Answer :
Sample homogeneity is an important consideration. The sample should be finely
ground to offset any preferred orientation of crystal faces. The powder particles
should ideally be between 1– 10 μm i.e. 200 mesh size. A suggested approach is to
make a slurry in a viscous liquid and allow the liquid to dry out after mounting on the
slide. This helps preserve random orientation of the material on the slide. Care should
be taken to avoid over grinding as this can introduce strain broadening due to
introduction of lattice deformations.
Q10. What are the common application areas of Powder XRD?
Answer :
Powder XRD has proved to be a versatile tool for rapid and non-destructive testing of
materials and has found applications in several areas such as pharmaceuticals, foods,
geological samples and minerals, cosmetics, forensic investigation samples,
archaeological artifacts, polymeric films and corrosion product studies. The technique
is finding application in new fields with advancements in research activities and
industrial materials.
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Conclusion
We believe that you enjoyed the free e- course on XRD. The course provided you an
insight into the components of XRD system and their individual contribution towards
the overall accuracy and precision of your results. Apart from a general introduction
the course was designed keeping in mind the requirements of the XRD user. Without
going into mathematical treatment of the subject an attempt has been made to convey
the basics concepts and offer practical tips on effective utilization of the XRD system.
The last chapter of the course provides answers to 10 common questions that you may
be faced with as you move up your career ladder. However, learning is a lifelong
process and there will be several unanswered questions and queries which will be
coming up in your mind from time to time. Our suggestion to you would be to post
such queries or comments on the site and we shall try to offer clarifications to the best
of our ability based on our expertise and experience.
Once again we take the opportunity to thank you for your interest. Please feel free to
participate actively by contributing articles in areas off your interest and offer your
valuable comments and suggestions.
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