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Modern Methods in Heterogenous Catalysis Research:
Structure Determination by Neutron Diffraction
Elisabeth IrranInstitut für ChemieTechnische Universität Berlin
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History of neutron diffraction
1932 Chadwick discovers the neutron
1936 Mitchell & Powers confirm wave property of neutron
1945 first nuclear reactor: Oak Ridge
1946 Shull & Wollan: first neutron diffraction experiment
1969 Rietveld: Neutron powder diffraction refinement
1994 Nobel prize for Shull and Brockhouse
„for pioneering contributions to the development of neutron
scattering techniques for studies of condensed matter...“
Clifford Shull (1915 – 2001)
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Properties of the neutron
Mass 1.674928(1)∙10-27 kg
Radius ~ 0.7 fm
Lifetime (free particel) 887 ± 2 sec (~ 15 min)
Spin ½
Charge 0
Magnetic moment -9.6491783(18)∙10-27 J T-1
-1.913 μN (nuclear magneton)
Quark structure udd
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deBroglie wavelength
λ =
as gas: Maxwell-Boltzmann distribution
E = mv2 = kT
λ2 =
273 K (thermal neutrons) → 1.55A
can be used for diffraction experiments
Properties of the neutron
23
mvh
21
3mkTh2
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Neutron sources
nuclear fission:
n + 235U → n + n + smaller nuclei
spallation:
protons injected onto heavy element target
yields 20 - 30 neutrons per proton
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Nuclear fission as neutron source
High flux reactor of ILL, Grenoble
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Experimental hall of ILL/Grenonle
Nuclear fission as neutron source
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after emission neutrons have energies of several
MeV, slowed down to thermal energy (room
temperature) at a moderator: light or heavy
water, graphite
total reflection of neutrons:
neutrons extracted from the moderator by beam
tubes or by neutron guides to the experiment
Nuclear fission as neutron source
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diffraction experiments:
continous source, single wavelength is extracted from
the Maxwellian distribution by a monochromator crystal
constant wavelength diffraction, angle dispersive
Nuclear fission as neutron source
Experimental hall of HMI/Berlin
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Spallation as neutron source
ISIS, Rutherford Appleton Laboratory, Chilton/UK
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pulsed neutrons → time-of-flight (TOF) experiments
one scattering angle, usually at angles 2θ > 90°
neutrons sort itselves accordig to their velocity und
wavelength, therefore time of flight T is proportional to
the wavelength:
L total flight path
Spallation as neutron source
hLmλ=T
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Neutron reactorsILL Institut Laue-Langevin Grenoble /F 1971
BENSC Hahn-Meitner-Institut Berlin /D 1992
FRG-1 Forschungszentrum Geesthacht near Hamburg /D 1958
FRM-II TU München Garching /D 2004
LLB Laboratoire Léon Brillouin Gif-sur-Yvette /F 1980
HFIR Oak Ridge National Laboratory Oak Ridge /USA
Spallation SourcesISIS Rutherford Appleton Laboratory Oxford /UK 1985
SINQ Paul Scherrer Institut Villigen /CH 1996
LANSCE Los Alamos National Laboratory Los Alamos /USA
IPNS Argonne National Laboratory Argonne /USA
KENS High Energy Tsukuba /Jpn.
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Detection of neutrons
proportional counter:10BF3 gas tube: 1n + 10B → 7Li + 4He + γ3He gas tube: 1n + 3He → 3H + 1H + γ
scintillator or NIP (neutron imaging plate)
converter: 1n + 6Li → (7Li) → 3He + 4He + γ
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Neutron (non-magnetic) diffraction
X-ray structure factor Fhkl
neutron structure factor Fhkl
Intensity IhklLP Lorentz-polarisation factorA absorption
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Neutron (non-magnetic) diffraction
Scattering lengths b for neutron diffractionAtomic form factors f for X-ray diffraction
Interaction of X-rays with electrons Interaction of neutrons with nuclei
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Scattering length b
Properties Consequences
- interaction with the nucleus (very small no decrease of intensity with 2θ
compared to neutron wavelength) temperature factors more accurate
- scattering length independent of atomic neighbouring elements can be
number distinguished
- b in the same order of magnitude light elements can be detected besides heavy
elements
- b independent of wavelength
- every isotope has its own b
- dimension of b: 1 fm = 10-15 m
- b positive or negative
- scattering power much smaller than X-rays larger sample necessary
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Absorption cross sections σabs
Absorption: usually small sample environment (furnace,...) no problem
but: resonance absorber: high absorption
(3He, 6Li, 10B, 113Cd, 149Sm, 157Gd) used as absorber, avoided in diffraction
dependent on cross section σabs
dimension of σ : 1 barn = 10-24 cm2
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Scattering lengths b for neutron diffraction
http://www.ati.ac.at/~neutropt/scattering/table.html
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Neutron powder diffraction
Vanadium sample cans neutron powder diffraction
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Neutron powder diffraction
Portion of the first powder diffraction pattern of NaCl taken at Oak Ridge by Shull and Wollan
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Localization of light elements: hydrogen (deuterium)
after C.G. Shull, E.O. Wollan, G.A. Morton, and W.L. Davidson, Phys. Rev. 73, 482 – 487 (1948)
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Localization of light elements: deuterium
after E.O. Wollan, W.L. Davidson, and C.G. Shull,, Phys. Rev. 75, 1348 – 1352 (1949)
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Localization of light elements: oxygen in ZrV2O7
25 °C470 °C
after N. Khosrovani, A.W. Sleight, and T. Vogt, J. Solid State Chem. 123, 355 – 360 (1997)
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Differentiation between neighbouring elements
Cu8[P12N18O6]Cl2 sodalite
N/O ordered or not?
after N. Stock, E. Irran, and W. Schnick, Chem. Eur. J. 4, 1822 (1998)
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Differentiation between neighbouring elements
Neutron powder diffraction patternX-ray powder diffraction pattern
Rietveld refinement of neutron powder data D2B at ILL/Grenoble:
N/O statistically distributed
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Neutron magnetic diffraction
interaction with electrons of the atomic shellmagnetic form factor is angle dependent
results:- magnetic ordering - orientation of the electron spins
(experiments with polarized neutrons)- valence distribution (magnetic momentum)
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Ordering of the spins
spins:
a) disordered: paramagneticb) parallel: ferromagneticc) antiparallel: Néel type antiferromagneticd) uncompensated: Néel type ferrimagnetice) triangular antiparallel: ferrimagneticf) helical spiral: compensated antiferromagnetic
or uncompensated ferrimagneticg) canted: weak ferromagnetich) canted: compensated antiferromagnetic
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Magnetic ordering in ferromagnets
Schematic phase diagram of bulk Holmium
a) simple spiralb) conical spiral c) complex spiral
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Magnetic ordering in the antiferromagnet MnO
after C.G. Shull and J. S. Smart, Phys. Rev. 76, 1256 - 1256 (1949)
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Magnetic ordering in the antiferromagnet MnF2
after R. A. Erickson, Phys. Rev. 90, 779 – 785 (1953)
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Magnetic diffraction with polarised neutrons
DyFe4Al8complex cycloidal magnetic configuration
UFe4Al8U: weak ferromagnetism
Fe: almost antiferromagnetic
J.A. Paixao, P.J. Brown, B. Lebech, and G.H. Lander in: Exploring Matter with neutrons, ILL, 2000.
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Conclusion
Neutron diffraction - is complementary to X-ray diffraction
- allows us to detect light elements besides heavy elements
- neighbouring elements can be distinguished
- reliable temperature factors
- magnetic ordering determined