current trends in chemistry of materials...possible directions for future research along with...

Current trends in chemistry of materials

S NATARAJAN and J GOPALAKRISHNAN

Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore 560 012, India.e-mail: [email protected] [email protected]

The many facets of research in the area of solid state materials chemistry have been presented anddiscussed. The article focuses on the present status and broadly concerns dense and porous solids.While many of the materials properties such as ferromagnetism, superconductivity, multiferroicbehaviour and so on have been explored mainly in dense solids, porous solids contribute towardsmany properties in the areas of catalysis, sorption, gas separation including hydrogen storage. Thearticle describes the current status of activity with regard to energy materials as well as creatingmaterials (synthesis and the role of chemistry) and understanding materials (the role of theory).Possible directions for future research along with identifying and highlighting the strengths of theexpertise available in this country are also mentioned.

1. Introduction

Solid state materials chemistry as a distinct disci-pline of chemical sciences has been in existence formany years [1]. The systematic study of correlat-ing the properties with the structures of solid com-pounds during the 60s has been regarded as thebeginning of solid state chemistry. This area nowencompasses a healthy interface between chemi-stry, physics and biology. Conventionally, a studyof synthesis, structure and properties of solid com-pounds constitutes solid state chemistry and therehas been considerable effort in classifying the solidsbased on the observed properties [1,2]. A solidendowed with useful properties becomes a mate-rial that could have practical applications. Thetraditional classification of the solids is based onthe electrical response to the flow of current andhas been in terms of conducting, semi-conducting,insulating and superconducting solids. The dis-covery of transistor behaviour [3], which revo-lutionized the thinking process with regard tothe solids, could be taken as an important stepin this direction. The more recent discovery ofsuperconductivity in compounds based on copperoxides is a landmark event in solid state chemistry

research [4]. This is a remarkable collective electronphenomenon in solids, arising from the formationof pair of electrons that permit the flow of elec-tric current without resistance. Over the years, theobservation of many properties in the same solidnecessitated a change in the classification. In addi-tion, the evolution of another class of solids withlarge open spaces, cavities and channels (poroussolids), which find significant applications in theareas of sorption, separation and catalysis, alsoprovided an impetus for a change in the classifica-tion of solids. Presently, solid materials can broadlybe classified as dense and porous solids.

Tailoring specific property in solids (electri-cal, magnetic, dielectric, etc.) is important fromthe point of view of utilising the solid as amaterial. Recent developments have provided avariety of new materials that include thermo-electric materials [5], superconducting materials[4], ferroelectric materials (multi-ferroic materi-als) [6], materials for hydrogen storage (energymaterials) [7] inorganic-organic hybrid materials[8] and materials for other related properties.In this context, one needs to appreciate the impor-tance of one structural family of dense solids,viz., perovskites, ABO3, (figure 1) for being the

Keywords. Solid materials; dense; porous; energy; synthesis; theory.

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22 S NATARAJAN AND J GOPALAKRISHNAN

Figure 1. The perovskite structure along with someof its layered variants. (a) the perovskite, SrTiO3

(b) the Aurivillius phase, Bi4Ti3O12 {(Bi2O2)[An−1Bn

O3n+1]} (c) the Ruddlesden–Popper phase, Sr4Ti3O10

(A′2[An−1BnO3n+1]) and (d) the Dion–Jaconson phase,

CsCa2Nb3O10 (A′[An−1BnO3n+1]).

platform for realizing many of the materials pro-perties. Perovskite structure can be described asformed by the corner sharing of the BO6 octahedrawith the A ions being accommodated at the cubo-octahedral site [1,2]. The structure can exhibit con-siderable flexibility in terms of dimensionality andaccommodate many elements as part of the struc-ture, which is one of the reasons for the observationof many important properties in this family of com-pounds. In table 1, we list some of the importantproperties exhibited by the compounds that areformed with the perovskite related structures.

In this article, we propose to outline the variousdevelopments that have taken place in the area of

chemistry of materials in recent years. The varioussub-topics have been chosen such that the readeris aware of the current trends and developments inthe area of chemistry of materials. In doing so, wehave highlighted the importance of dense materialsin magnetic, electrical, multiferroic, superconduct-ing and other related properties. Such materialsfind use in communication, data storage, etc. Theporous solids, on the other hand, have been thework-horse for catalysis and in the oil industry,especially the cracking of crude, etc. It has beenshown that the porous solids are excellent materialsfor gas separation, sorption and hydrogen storage.Many of the developments in materials appearto be oriented towards energy as fossil energy isfast depleting and there is an urgent need fordeveloping newer materials. The properties exhi-bited by a solid crucially depend on the synthesisand the role of chemistry in fine tuning the proper-ties is also highlighted. In addition, attempts havebeen made to indicate the major milestones in thedevelopment of the theoretical understanding ofsuch materials as well.

2. Dense solids: Magnetic, electrical,multiferroic and superconducting

materials

In this section, we give the recent developmentsthat have taken place in some of the importantareas of chemistry of dense materials. Conventionalmagnetic materials are mostly dense inorganicsolids. The excitement surrounding the observa-tion of magnetic behaviour in the mineral mag-netite, Fe3O4, by the ancients continues even today.Structure-property correlations in magnetic mate-rials were investigated by many researchers andparticularly by van Vleck, Mott, Hubbard, Stoner,Anderson, Goodenough and Kanamori and manyothers [9]. Based on the principles of crystal chemi-stry, it is now possible to design and preparemany molecular materials with defined magneticbehaviour. The impetus for research in this areais driven by the use of such solids as hard drivesin computers and related purposes. The combina-tion of charge and spin information in a solid andits response to the applied field were importantfor information storage and related purposes. Thisfield is known as spintronics and generally semi-conducting and half-metallic ferromagnetic solidsexhibit this type of behaviour [10]. Based onthe understanding of magnetism in solids, weare now in a position to contribute to this areasignificantly.

Recently, there is much interest in the studyof the interplay of ferromagnetism and ferroelec-tric behaviour in the same solid. The observation

CURRENT TRENDS IN CHEMISTRY OF MATERIALS 23

Table 1. List of the important properties exhibited by the perovskites based solid materials.

S. No. Property/Phenomenon Example

1 Metal (metallic conductor) LaNiO3

2 Superconductivity (Ba,K)BiO3, La2−xSrxCuO4

3 Ferroelectricity BaTiO3, PbTiO3

4 Relaxor ferroelectricity Pb3MgNb2O9

5 High temperature ferroelectricity Bi4Ti3O12

6 Piezoelectric PbTi1−xZrxO3

7 Ferromagnetism SrRuO3

8 Jahn–Teller effect/orbital ordering LaMnO3

9 Colossal magnetoresistance La1−xSrxMnO3

10 Luminescence K2La2Ti3O10: RE

11 Spintronics/half-metallicity/ferromagenetism Sr2FeMoO6

12 Fuel-cell electrode Sr2(Mg.Mn)MoO6

of ferroelectricity, which requires a second orderJahn–Teller effect (off-center displacement ofd◦ cations) and ferromagnetism, which requirespartially filled d shells, appears to have mutualincompatibility. If the two properties need to becombined, one needs to investigate systems con-taining partially filled d-shells of the transitionmetal ion (ferromagnetism) along with a differ-ent mechanism for the observation of ferroelectricbehaviour. In the light of this, the participation anduse of stereoactive lone-pairs in observing ferro-electricity is important. Thus, the combinationof Mn3+/Fe3+ ions and stereoactive lone-pair ofBi3+ has given rise to BiMO3 (M = Mn, Fe) oxidecompounds, which exhibit multiferroic behaviour[11]. One needs to note that the multiferroicmaterials discovered till now do not have boththe large polarizations and the magnetizationssimultaneously above room temperature. The useof f-electron magnetism (rare-earths) along withsecond-order Jahn–Teller effect could provide use-ful avenues for future research.

Materials that exhibit conducting propertiesremain a focus of intense scientific research forboth fundamental as well as for application relatedpurposes. Research on the study of conductingmaterials, especially superconducting ones, hasundergone a major change after the breakthroughof cuprate superconductors with superconductingtransition temperature above 40 K, 20 years ago(figure 2a) [3]. In this light, it may be pointedout that the original theory proposed in 1957 forunderstanding the superconducting behaviour in asolid, the BCS theory [12], does not predict super-conducting transition temperatures above 30 K.Thus, the discovery of cuprate superconductorsis important not only in overcoming the cru-cial barrier in terms of the transition tempera-ture, but also it provided a good opportunity

Table 2. List of new superconductors discovered since 2000along with the Tc.

Year ofS. No. Material discovery Tc (max., K)

1 Sr2RuO4 1994 ∼1K∗

2 MgB2 2001 42

3 NaxCoO2, yH2O 2003 5

4 Y2C3 2004 18

5 CaC6 2005 12

6 LaO1−xFxFeAs 2007 36

∗p-wave (spin-triplet) superconductor [58].

to experiment solid state chemistry with otherrelated phases. This resulted in many differentvariants of structures based on layered copperoxides. Search for new superconducting materialsconstitutes an area of research where solid statechemists can play a major role. This results innew discoveries as well as materials where onecould test the theoretical predictions. In table 2,we list some of the important superconductingmaterials that have been discovered during thelast 10 years. The observation of superconductingbehaviour in materials that have two-dimensionalcharacter (layered solids) suggests that electron-phonon coupling could be one of the reasons andit is believed that superconducting properties insolids can be observed even at 450◦C (figure 2b).This could be a tremendous boost to experimentalresearchers to expand the scope for search of newermaterials via novel synthetic approaches.

Materials that have high dielectric constant(κ) are in constant demand because of theirapplication as capacitors in microcircuits amongothers. In addition, due to the desirability to havesmaller electronic devices, the highly insulatingsolids are employed as interlevel dielectrics (ILD)


Figure 2. Superconductivity then and now. (a) Structure

and superconducting transition (ρ -T curve) of the high Tc

superconductor, YBa2Cu3O7 (b) Structure and supercon-

ducting transition (χ - T ) transition of the newly discovered

superconductor, LaO1−xFxFeAs.

to separate the conducting parts from oneanother. These solids also find applications asgate dielectrics in integrated circuits (IC) andalso in memory elements in microcomputers suchas DRAM and FeRAMs. The recent discovery ofCaCu3Ti4O12 with high κ (>10,000) appears tohold much promise [13].

3. Porous solids

Porous solids have emerged to occupy an impor-tant area of research in chemistry of materials.

The fascination of a ‘boiling stone’ (zeolites) inthe 17th century continues even today. These arespecial solids whose properties arise from theirporous architecture. Porous solids can be broadlyclassified based on their pore sizes. Thus, zeolites(aluminosilicates), aluminophosphates and otherrelated compounds with pore diameters in therange 2–20 A are known as microporous mate-rials, compounds with pore sizes in the range20–100 A are known as mesoporous materials andcompounds with pore sizes >100 A are classified asmacroporous materials [14]. The family of poroussolids now have compounds that include most of


the metals of the periodic table. Of these, thealuminosilicates are the most important and widelystudied family of compounds, as they possess goodthermal stability.

The aluminosilicates are solids possessingregular pore architecture and exhibit a variety ofproperties such as ion-exchange, separation andcatalysis. Many of these properties arise from thesize of the pore window, the accessible void spaceand the dimensionality of the channel system.The empirical formula for an aluminosilicate zeolitecan be expressed as Ax/n[Si1−xAlxO2]. mH2O,where A is a metal cation of valence n, which isgenerally located along with the water moleculesin the channels and can be easily ion-exchanged(figure 3a) [15]. One of the important charac-teristics of these compounds is that they avoidAl ––– O––– Al linkages (known as the Lowenestein’srule). The zeolites have been the main catalystsfor many industrial catalytic processes [16]. Thefunctionalization of the channels within the zeo-lites, preparation of supported oriented thin films,synthesis of polycrystalline membranes, colloidalzeolites with narrow particle size distribution areimportant developments in this area [16]. Of these,the nanozeolites are important as they offer highexternal surface areas and activities, shorter dif-fusion pathways and accessible catalytic sites.Since the particle surface can be functionalized byorganic groups, these can be dispersed easily inorganic medium and also can be used for buildingextended structures. The main problems with theuse of zeolites with small sizes (nano-sizes) are theirlow synthesis yields and inconsistent reproducibi-lity. Chiral zeolites are another important class ofmaterials that are used for chiral catalysis, butthe difficulties in resolving enantiopure zeolites stillpersist.

The discovery of aluminophosphates (AlPO),AlPO4, in 1980s with reasonable thermal stabilityand novel structures is an important developmentin this area [17]. AlPOs offer the possibility of sub-stituting Al3+ by bivalent ions such as Mg, Zn, Co,etc. (MeAlPO) and at the P5+ sites by tetrava-lent Si (SAPO), which can incorporate acidic pro-tons. In addition, the possibility of generatingredox centers in MeAlPOs along with the availa-bility of acidic protons can be utilized for theoxidiation-reduction and the acid catalysis simul-taneously [18]. The oxidation of terminal carbonsin a linear alkane at moderate temperatures andpressures to form the corresponding acids hasbeen demonstrated [18]. This and many otherrelated studies have demonstrated the efficacyof the use of AlPOs in industrially importantprocesses (figure 3b). The lower thermal stabilityalong with reduced recyclability, however, impedesthe progress in the use of AlPOs commercially.

Figure 3. (a) Structure of the most widely used alumi-nosilicate material, Zeolite-Y showing the large pores.(b) Figure shows the many possible chemical reactions onecan perform on transition metal doped aluminophosphates.

This discovery, however, opened up many impor-tant vistas for investigating other related phasesthat lead to an explosive growth of frameworkphosphate structures [19].

In the 1990s, porous metal-organic frameworks(MOF) provided another important developmentin this area [20–23]. These are crystalline solidswith three-dimensionally extended structures inwhich the metal ions or clusters are connectedthrough molecular bridges (figure 4a). These arecompounds that combine the coordination versati-lity of the central metal ions and the functionality


Figure 4. Structures of the mesoporous silica material.(a) MCM-42 and (b) MCM-48. The extra-large pore sizescan be utilised for many important applications (see text).

of the bridging organic molecules. The interest-ing aspect of these structures is that they retainthe integrity of the building blocks, the chemi-cal functionality and rigidity during the synthe-sis. Control of the pore sizes has been achievedby incorporating non-volatile guest species or bycreating interpenetration or interweaving in thestructures. The metal organic frameworks haveweaker bonds, which opens up the possibility ofcreating solvent driven structural transformations.In addition to utilising the coordinatively unsatu-rated metal centers, the MOF compounds alsooffer the possibility of functionalising the bridgingligands. In a sense, the MOFs offer the advantagesof both the organometallic and the organic chemi-stry, which is unique. Some of the MOF struc-tures have been shown to be good adsorbents forhydrogen, which opened up this area further andcreated much interest towards hydrogen storagematerials (figure 4b). The possibility of producingchiral frameworks and chiral structures are chal-lenging and interesting.

The use of surfactants for controlling the for-mation of structures was established in the early1990s. Though the chemistry and physics of micelle

and its properties have been well established, theuse of such arrays for preparing new types of com-pounds is a paradigm shift in synthetic inorganicchemistry. The use of surfactants, block copolymersand liquid crystals as templates for the forma-tion of new forms of silicas with diverse meso-porous structures has opened up this area assuch forms are not possible with zeolite synthesis(figure 5) [24,25]. The important aspect of thesestructures is that the surface walls of many ofthe mesoporous compounds are amorphous as theyare formed by hydrolysing silica esters over themicellar assembly. Many of the mesoporous com-pounds are investigated and characterized usinghigh resolution electron microscopy. Using micel-lar arrays, powders, nanoparticles, self-standingthin films and supported films, monodispersedspheres and monoliths have been prepared. Sur-face functionalisation in mesoporus silicas eitherduring the synthesis or by post-synthesis proce-dures is possible. Since the micellar chemistry is

Figure 5. (a) The structure of metal-organic frame-work structure, [Zn4O(1, 4-bdc)(DMF)8(C6H5Cl)]. (b) Thehydrogen absorption on the above compound.


well understood, the mesoporous compounds havetransitioned from synthesis towards applicationsand commercialization. Some of the mesoporouscompounds find applications in low-k dielectrics,catalysis, separation, optics, magnetics, sensing,etc. The development of periodic mesoporousorganic silicas (PMOs) along with the preparationof non-silicate mesoporous compounds has gener-ated considerable interest in this area. The carbonreplicas of the mesoporous silica are another excit-ing development as they can be used as electrodematerials in secondary lithium batteries, super-capacitors and biosensors [26]. The mesoporouscarbon lacks the ease of surface functionaliza-tion, which needs attention. In addition, an under-standing of mesostructural organization, defects,grain size and nucleation processes are importantfor advanced engineering and utilisation of thesesolids.

The macroporous materials with pore sizesgreater than 100 A are important for theirenhanced mass transport of the guest speciesand reasonable space for pore wall modifications.The pore wall modifications include the inter-nal surface modifications using functional groups,small molecule anchoring and nanoparticle coat-ings. It has been shown that such functionalisa-tion can be utilised for useful catalytic studies.These compounds also offer possibilities for obtain-ing hierarchical structures and multifunctionality.The three-dimensionallly ordered macroporoussolids with wide ranging properties that includeinsulating, semiconducting, conducting (metallic),etc. have been synthesized using the colloidal crys-tal templating technique [27]. The macroporoussolids with length periodicity in the range of visi-ble radiation can function as photonic crystals. Theother applications of the macroporous solids arein the areas of bioglasses, catalysts and batterymaterials. One of the challenges in forming macro-porous compounds is to obtain structures that arenot cubic. In addition, it is also preferable to beable to control the defects so as to use them foroptical studies.

4. Materials for energy conversion andstorage

The problems associated with the anticipateddepletion of the fossil fuels in the near futurehave generated considerable interest in the study ofenergy materials. The need for alternative energysources and efficient ways of generating power isimportant. The vast knowledge gained on the studyof the structure-property correlations in the area ofcatalysts, batteries, fuel cells, thermoelectric and

optical materials would be fruitfully utilized indesigning newer and efficient materials for energy.

4.1 Catalysts

In the area of catalysts, the main challenge isto prepare new materials that would be utilizedfor the selective oxidation of alkanes and also forthe conversion of methane to higher hydrocarbons[28]. The development of new electrocatalysts forfuel cells is also a priority. One of the importantclues for developing new materials for emergingneeds is to fine-tune the ion mobility and electronicdefects in a solid. The oxygen and hydrocarbonadsorption, activation and redox processes dependon the structure and composition of the surface ofthe solid/oxide and our present understanding ofthe surface structures has not reached a state wherewe can predict the performance of the catalysts.This is a very important issue for future research.

4.2 Battery materials

The discovery of redox lithium ion mobility inlayered LiCoO2 is an important landmark in thearea of battery materials. The development andcommercialization of LiCoO2/graphite cell directlyresulted from the contribution of the solid statechemists, which enabled the portable electronicdevices as an integral part of the society. TheLiCoO2 based cell is generally used for low powerapplications. The requirements for high powerapplications such as batteries for electric vehicles,where nickel-metal hydride batteries are used,remains a challenge. The discovery of nanoma-terials and nanocomposites are important devel-opments in this area as the nanocomposites canabsorb stress and strain that develop during thecharge-discharge cycles better [29]. The discover-ies of tin based amorphous oxide anode materials,the development of macroporous electrodes aresignificant. The multi-redox centered compound,LiNi0.5Mn0.5O2 (Ni2+/Ni4+) and LiFePO4 appearsto show much promise as cathode materials [30].The need to increase the capacity and to maintainthe high charge/discharge cycles would requiresystems that are electrochemically active andundergo redox changes that involve more than oneelectron. In the light of this, the Ni2+ → Ni4+,and V3+ → V5+ would offer considerable scope.It is also preferable to investigate other class ofcompounds such as nitrides, phosphides, inter-metallics, etc. The cost of the battery could bean impediment to use many materials, unless thebatteries are meant for specialized needs (medical,space and military related). Under the circum-stances, the role of the materials chemist is impor-tant in this area of research.


4.3 Hydrogen storage materials

For hydrogen to be used as a viable fuel in fuelcells and other related technologies, suitable stor-age materials must be developed. The storage ofhydrogen must take into account the many con-straints such as weight, volume, efficiency, safety,cost, etc. Much focus and emphasis has been givento storing hydrogen in solids and accordingly solidstate chemistry has a significant role in develop-ing such materials. Hydrogen is, generally, takenup using adsorption (physisorption and chemisorp-tion), absorption and chemical reaction mecha-nisms. Materials that possess large surface area(carbon nanotubes) offer excellent opportunitiesvia the adsorption process, while hydride form-ing alloys (hydrogen stored in the interstitial sites)use absorption process and metal hydrides employthe chemical reaction pathways for the storage andrelease of hydrogen.

It has been shown that up to ∼5% hydrogencan be adsorbed on carbon nanotubes at 77 Kand about 1%, at room temperature [31]. Thefavourable van der Waals interactions between theadsorbed hydrogen and the internal surface for easyrelease of hydrogen might work against for adsorb-ing larger quantities at room temperature. Recentstudies have shown that crystalline microporousmetal-organic framework (MOF) compounds com-prising metal ions or metal oxo clusters could pro-vide a reasonable alternative. Hydrogen adsorptionof up to 2–4 wt% has been observed at low temper-ature and high pressures [32].

Intermetallic compounds and alloys for hydro-gen storage employs chemical cleaving of H2 bondat the surface and intercalating the atomic hydro-gen in lattice interstitial sites [33]. The compound,LaNi5, can absorb one atom of hydrogen per metalatom at room temperature and 1–2 atm pressureforming LaNi5H6. Hydrogen can be released byheating the sample to temperatures <100◦C. Theuptake and release of hydrogen can be recycled andgenerally about 2 wt% can be adsorbed in thesecompounds. The main problem in utilising theintermetallics and alloys as hydrogen storage andtransporting agent is their weight and cost. Otherrelated alternatives would be to use the lighter ele-ments (Li, Na, Mg, etc.) as constituents in inter-metallics.

The light weight complex hydrides have beenconsidered as a means of hydrogen storage bychemical reaction pathways. Thus, NaAlH4 andother related borohydrides are being studied.NaAlH4 contains AlH4 anions (alanate ion) withstrong Al ––– H bonds and poses difficulties forthe removal of hydrogen under normal conditions.The hydrogen can be released by heating thesample close to the melting point and recent studies

reveal that the addition of Ti as the catalyst canbring down the temperature of release of hydro-gen to about 180◦C. Though these compounds canbe possible candidates for hydrogen storage, muchwork needs to be carried out to understand andevaluate the dehydrogenation pathways, reducingthe temperature of release of hydrogen etc. [34].

Another class of materials that has been inves-tigated for possible hydrogen storage applicationsare the clathrate hydrates. Clathrate hydrates ofmethane have been studied as they were assumedto be present in vast quantities on the ocean floor,where the high pressures needed to stabilize theclathrates exist [35]. In gas hydrates, the watermolecules usually form cages in which guest mole-cules, such as methane, could be trapped. It wasdiscovered that hydrogen molecules can also beencapsulated in such clathrate structures, albeit atvery high pressures (about 2 kbar). This precludestheir practical application for hydrogen storage.

Recently, it has been shown that the pressure forstoring hydrogen as gas hydrates could be signi-ficantly reduced by having THF (tetrahydrofu-ran) as co-occluding molecule (100 bar) [36]. In theclass-II clathrate, the water molecules form twodifferent sized cavities – the larger one with afree diameter of ∼0.67 nm and the smaller one∼0.5 nm. For a clathrate to be stable, the largecages must be almost completely filled with suit-able guest molecules, while the smaller cages mayeither remain empty or be filled with guest species.It has been shown that the larger cavity canbe filled with tetrahydrofuran (THF) leaving thesmaller sized cavities for storing hydrogen. Thus,with the use of binary clathrate hydrate, onecan store up to ∼4wt% of hydrogen. Clathratehydrates are, thus, comparable to other systemsdiscussed for hydrogen storage such as light metalhydrides and appear to be superior to cryosystemsthat use adsorbents with a large surface area.

4.4 Fuel cells

Fuel cell technologies are important as an alter-native source of energy for the fossil fuel, reduc-ing CO2 emission etc. [37,38]. The H2/O2 fuel cellrequires good conducting membranes for protonand oxygen (electrolyte) and suitable electrodes forthe reaction H2 → 2H+ + e− or O2 + 4e− → 2O2−.In addition a good electronic conductor would alsobe required for the delivery and removal of elec-trons. The direct methanol fuel cells (DMFCs) arenot suitable due to the high cost of the catalyst(Pt) and possible issues related to the reactantsand product crossover. Intemetallics such as PtBiand Ru doped Pt are being studied as alternatecatalysts in DMFC [39]. Further work is neededto look for new type of catalysts which would also


reduce the overpotential at the cathode. Work onproton conductors or oxygen ion conductors arealso being carried out. It is becoming clear thatthere are many interfaces to consider in materialsthat are required for use in fuel cells and also inhigh temperature electrochemical devices.

4.5 Photovoltaics

Polycrystalline thin film solar cells are importantdue to their high efficiencies and ease of prepa-ration [40]. Thus, Cu(InGa)Se2, CdTe and othersemiconductors have been studied for photovoltaicbehaviour with reasonable efficiencies. The highcost of the In and Ga along with the toxicity asso-ciated with Cd are reasons for not utilising themfor harvesting the solar radiation. New solid statematerials are needed that would have bang gaps inthe range 1–1.7 eV. The development of such mate-rials requires a deep understanding of the electronicstructures, the bulk and surface dopant and defectchemistries along with contributions from theoret-ical calculations.

4.6 Luminescent materials

The ever increasing power requirements createopportunities for solid state materials chemiststo investigate newer materials that could lead togreater efficiencies and energy savings in the gen-eration and use of light. Mercury based fluorescentlamps represent one of the most efficient sourcesfor lighting. The use of quantum splitting phos-phors with quantum efficiencies more than 1 canbe considered for lighting purposes [41]. To achievequantum efficiencies of >1, one needs to match theenergy levels of absorption and emission processesalong with observing the emission in the red, greenand blue region of the visible light – a combina-tion of these would give rise to white light. Solidstate lighting is also being developed on the basisof LED-excited inorganic phosphors. Any develop-ment in this area requires a good understanding ofthe local crystal structure and the energy levels ofthe dopant ion.

Considerable effort has been expended on thedevelopment of organic light emitting devices(OLEDs), which function on the basis of favourableelectron-hole combination processes. The develop-ment in this area is hindered by the lack ofbright light and low life times (stability). Thepolycrystalline inorganic solid based light emit-ting devices (ILEDs) that work on the principlesof OLED are being investigated due to the lowercost. A solid state ILED has not been realizedyet due to the high work function (typically 8 eV)and high energy barriers for the hole injectionin inorganic phosphors. Wide band gap inorganic

luminescent materials with work functions of about5 eV provide hope for realising ILEDs [42].

4.7 Transparent conductors

Transparent conductors that have both the opti-cal transparency and electrical conductivity in thesame material, are important for many applica-tions such as automobile and airplane windowsdefrosters and transparent contacts for solar cellsand displays. These are based on n-type oxides andthe first example of transparent thin-film transis-tor (TTFT) is known only recently [43]. The mainchallenge in this area is to produce transparent con-ductors based on p-type materials that can be usedin high performance complementary devices simi-lar to silicon CMOS technology [44].

5. Creating materials – the role ofchemistry [45]

The essence of research in chemistry is the syn-thesis, and it is also true for materials chemistry.If new compounds are not made, then the dis-covery of newer properties also cannot be found.Traditionally solid state materials are prepared byceramic technique – by using the high temperaturesolid state reactions. Exploratory synthesis basedon trial and error approach has yielded numer-ous discoveries and continues to be a valuableand popular method in the chemistry of mate-rials. In this context, it is important to men-tion that India is very well placed due to theavailability of large pool of trained manpower,which is an asset. Directed synthesis is gener-ally employed to target a particular property.Trial and error methods and the directed synthe-sis approaches have provided reasonable advancesin the area of materials science. Combinatorial,high throughput and parallel methods of synthesesallow for optimising experimental conditions andhold considerable promise. These approaches mustbe encouraged in India as they are relatively easywith low input costs.

In general, for the synthesis of a solid, inaddition to the high temperature approach,synthesis from fluxes and melts, hydrothermalmethods and also preparation from solutions havebeen employed. Recently, synthesis of extendedframework compounds by directed assemblyof molecular building blocks, low-temperaturesynthesis of oxides and chalcogenides in thenanometer sizes, are emerging as importantdevelopments. In spite of the considerable progressmade, it is still a challenge to design and predictnew phases a priori. Considerable success has beenachieved in the preparation of homologous series


of compounds – in this approach the fundamentalconcepts that define the basic building units andhow they combine are preserved but the majorproblem of decreasing stability and accessibilityof higher homologues in a series still remains andrequires attention.

5.1 High temperature synthesis

Synthesis at high temperatures involves reactionbetween the constituents at elevated temperaturesand the reaction generally proceeds through thediffusion of the reactants. The low diffusion coef-ficients of the constituents in solid phases (in theorder of 10−12 cm2 s−1) along with the requirementof high thermal activation barriers pose obsta-cles for the completion of the reaction. A priorknowledge of the phase diagram of the reactantswould help in identifying suitable reaction con-ditions. In this process, the reactants, taken instoichiometric quantities, are mixed thoroughly,pressed into a pellet and heated in a furnace atelevated temperatures. A problem associated withthe high temperature synthesis is that monitor-ing of the progress of the reaction is difficult.It would also be difficult to separate phases, ifmixtures of solids are formed during the reaction.To overcome such problems, researchers employ avariety of precursor techniques with the basic ideabeing to take the reactants in one single entity.In spite of all the shortcomings, the importance anduse of the high temperature synthesis in materi-als chemistry cannot be understated. The synthesisof cuprate superconductors, the CMR manganates,dielectric semiconductors, half-metallic ferromag-nets, etc. are examples of the versatility and suc-cess of the high temperature method.

5.2 Chimie douce/soft-chemistry methods [46,47]

High temperatures are often employed in ceramicsynthesis in order to drive the reaction to comple-tion, resulting in thermodynamically stable phasesas products. This, however, is a major limitationbecause many of the interesting inorganic materialsare metastable phases. Also, high temperaturesynthesis precludes realization of the productsin the form of free-standing fine particles, nano-materials and thin films. Synthesis of solids infine particle/nanoparticle morphology as also thinfilms require special strategies that control particlegrowth of products during formation.

Metastable phases are better prepared by alter-nate synthesis routes based on a sound under-standing of the chemistry behind synthesis, viz.,thermodynamics, kinetics and reactivities in thesolid state. The governing principle behind this

approach, which is called chimie douce/soft chemi-stry methods is to have kinetic control ratherthan thermodynamic control, so that metastablephases could be accessed. There are severalways to achieve kinetic control of synthesis. Onecould, for example, prepare a thermodynamicallystable phase by conventional means and subse-quently transform it to a metastable phase throughappropriate soft chemical reaction. For example,NASICON-framework phosphate, V2(PO4)3, wasprepared from Na3V2(PO4)3 by redox deinterca-lation of sodium using Cl2/Br2 in nonaqueousmedium. It is also possible to obtain a metastablephase by carrying out the synthesis at conditions(e.g., high pressure, high temperature) at which thephase is thermodynamically stable and quenchingit to laboratory conditions. The recent synthesis offerromagnetic-ferroelectric Bi2NiMnO6 is an exam-ple. Accessing metastable phases through controlof nucleation and growth kinetics in the presence ofsuitable templates under hydrothermal conditionsis another well-known strategy that is commonlyemployed for the synthesis of zeolitic and microp-orous solids.

Among the several chimie douce approaches thathave been developed to overcome the limitationsof the conventional ceramic method, metathesis(chemical double exchange) is noteworthy [48].Although metathesis reactions involving exchangeof atomic/ionic entities between reaction partnersfor example, Fe2O3 + Al → Al2O3 + Fe (Thermitereaction) has long been known and even finds appli-cation (e.g. rail track welding), use of reactionsof this kind for synthesis of inorganic materialsin general is a development of recent times. Theprime characteristic of such metathesis reactionsis that they are highly exothermic, evolving alarge amount of heat almost instantaneously. Someof the typical examples of metathesis reactionsemployed in the synthesis of non-oxide materialsare:

MoCl5 + 5/2(Na2S) → MoS2 + 5NaCl + 1/2S;

ΔH = −890 kJ mol−1

GaI3 + Na3As → GaAs + 3NaI;

ΔH = −489 kJ mol−1

TiCl3 + Li3N → TiN + 3LiCl;

ΔH = −670 kJ mol−1

GaCl3 + Li3N → GaN + 3LiCl;

ΔH = −645 kJ mol−1

The large reaction enthalpies result inhigh adiabatic reaction temperatures generally


Figure 6. The structure of (a) K2La2Ti3O10 and the metathetic modifications by the reaction with (b) BiOCl to form(c) Bi2La2Ti3O12 phase.

>1000◦C, making the product formation almostinstantaneous. While the high temperaturesgenerated favour the formation of thermodynami-cally stable phases, the short reaction durationcould in some cases stabilize metastable phases(e.g. cubic zirconia) in synthesis based on thisapproach.

A different kind of metathesis developed by oneof us (JG) enables structural control during synthe-sis enabling formation of pre-determined products.A typical example is

K2La2Ti3O10 + 2BiOCl

→ [Bi2O2][La2Ti3O10] + 2KCl

where a Ruddlesden–Popper phase is topochemi-cally transformed into the corresponding Aurvil-lious phase. Here, the layered perovskite entity,[La2Ti3O10]2− and the layered [Bi2O2]3+ unit,which are exchanged retain their structuralidentity in the product (figure 6). Similarmetatheses have been carried out with VOSO4

and Sr2Cu2O2Cl2, instead of BiOCl, yieldingnovel metastable materials, [VO][La2Ti3O10] and[Sr2CuO2][La2Ti3O10], which are otherwise inac-cessible by conventional means.

A major difference between metathesis reactionsof this kind and those listed earlier is that these

reactions are not self-propagating and requireslonger duration. The formation of the co-producedionic salt with a high lattice energy is likely thedriving force for such reactions, but since the reac-tion enthalpies are not likely to be high, metathe-sis of this kind requires longer duration whilestabilizing metastable products at relatively lowtemperatures. The combination of oxidizing andreducing agents in an appropriate manner to formcontrollable flame at relatively low temperaturesthrough combustion has been fruitfully employedfor the preparation of a variety of oxide materialswith nanocrystallinity [49]. Though this approachis fast, the resulting product needs to be furtherprocessed at elevated temperatures to obtain goodcrystallinity.

6. Understanding materials – theoreticaldevelopments

The development of several methodologies duringthe last two decades along with the availabilityof powerful computers has contributed signifi-cantly towards the improvements in the theoreticalinvestigations. The early electronic structurecalculations based on local spin density approxi-mation (LSDA) methods generally fails when


employed for strongly correlated systems such astransition metal oxides [50]. The realistic descrip-tion of the electronic structure of magnetic systemscontaining strongly correlated localized electronscan be treated by a modified approach of usinga combination of LSDA and Hubbard ‘U’ method(LSDA + U) [51] and the self-interaction correctedpseudo-potential method (pseudo-SIC) [52]. Thetwo methods, though different, yield band gaps andband structures, magnetic moments and crystalstructures that are in reasonable agreement withthe experimental findings.

Recently, computer intensive methods, such asthe LDA + DMFT and the GW, have been develo-ped for more rigorous physical foundation. Thedevelopments of the first-principles description ofstrongly correlated materials along with the emer-gence of ‘theory of polarizations’ have helpedin the conceptual understanding of spontaneouspolarizations in bulk crystals (complicated by theperiodicity of the crystal lattice) within the densityfunctional framework [53]. These developmentshave helped in the calculations of ferroelectricpolarizations, dielectric constants, piezoelectriccoefficients and the electric field responses in asolid, routinely. The development of first princi-ples method for calculating metal-insulator het-erostructures is another important development.The advancements in the ab initio methods will bevery beneficial for understanding the electronic andstructural study of solids.

The quantum density functional theoreti-cal (DFT) methods, generally, are limited to1000 atoms and ∼10−12 s time scales and thetight binding methods are limited to <105 atoms.The use of classical interatomic potential help insimulating about million times larger size both innumber (space) and time as such large systemsare required to study the problems of interest insolid state chemistry. The growing use of molecu-lar modeling (MM) in combination with the quan-tum mechanics (QM) could help in arriving atbetter results. The QM methods can be appliedfor studying the chemical reactions, the changeof coordination numbers whereas MM methodscan be applied for classical interactions. Thus, theQM-MM approaches would require careful consi-deration in the incorporation of QM methodologywithin the MM surroundings. By combining mole-cular dynamics (MD) with Monte-Carlo techniquesemploying the bond order dependent potentials,one is in a position to investigate the many issuesassociated with materials chemistry [54].

The theoretical advancements over the yearshave been employed in the areas of synthe-sis, in understanding phase stabilities in multi-component systems, kinetic studies, magneticbehavior, spin polarizations in complex solids,

thermoelectrics and in multi-functional solids. Thetremendous advancement in utilizing the densityfunctional methodology along with the computa-tional power, the electronic structure theory hasnow become an integral part of research in solidstate chemistry. The electronic structure theoryhelps not only in the interpretation and rationali-zation of experimental results, but also in ourunderstanding of the crystal structure-propertyrelationships. It is important that the theoreti-cal methods need to be developed from beingan analytical tool into a more predictive instru-ment to make computationally designed materi-als. The DFT methods used for determining bandgaps, elastic and other related properties maybe improved to study a combination of proper-ties via high throughput methods. Computationalmethods may be developed to study competingproperties such as multiferroics and employed tosuggest target compositions and structures forfuture synthesis. It is important to utilize theknowledge garnered from the synthesis along withthe advancements obtained from computationalapproaches gainfully to meet the demands of thedeveloping area of materials chemistry.

The need for more accurate band structuremethods necessitates the development of moreefficient algorithms [55]. The importance of interfa-cial interactions observed in quantum dots or nano-thermoelectrics with larger length scales requireadvancements in the DFT methods. The inabilityof the DFT to handle the van der Waals interac-tions and hydrogen bonds in solids is another areawhere developments are required. The non-localfunctionals, which are computationally intense,need improvements to describe many of the pro-perties of layered solids such as graphite, MoS2, etc.Theoretical understanding of the electron trans-fer processes have not been developed in greatdetail and this is another area which requiresattention. A major hurdle in the utilization oftheoretical methodologies in solid state chemistryresearch is the lack of proper literature describ-ing the usefulness of DFT and other techniques ina language accessible to non-experts. For a moreelaborate account, the article by Kanatzidis andPoeppelmeier is recommended, which provides anexcellent survey of the current research scenario(both experiment and theory) of this field with aninternational perspective [56].

7. Concluding remarks

A brief overview of the current scenario of mate-rials chemistry presented here, albeit constrainedby the limitations of the authors, brings out theimmense research opportunity that exists both in


experiment as well as theory, which in our opinionshould go hand in hand to make tangible progress.As part of this article, we have also pointed outsome of the fruitful directions of research in thisfield that could be pursued profitably by youngresearchers continuing research in this area. Whilethe practice of theory in this field has largely beena posteriori, what is required is theory with possi-ble predictability. Of course, we do realize that itis a tall order for the field of chemistry [57], moreso in the field of materials [58].

Acknowledgements

The authors thank the Department of Scienceand Technology (DST) Government of India, forthe award of Ramanna fellowship. JG also thanksthe INSA for the award of a senior scientistposition.

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