Solid State Chemistry

The following sections are included:

• FOREWORD

• A TRIBUTE

• CONTENTS

Solid state chemistry was in its infancy when the author got interested in the subject. In this article, the author outlines the manner in which the subject has grown over the last four decades, citing representative examples from his own contributions to the different facets of the subject. The various aspects covered include synthesis, structure, defects, phase transitions, transition metal oxides, catalysts, superconductors, metal clusters and fullerenes. In an effort to demonstrate the breadth and vitality of the subject, the author shares his own experiences and aspirations and gives expression to the agony and ecstacy in carrying out experimental research in such a frontier area in India.

Please refer to full text.

Chemical methods of synthesis of materials play a crucial role in designing and discovering new materials and also in providing better and less cumbersome methods for preparing known materials. In this article, we shall discuss the chemical synthesis of inorganic solids, in particular oxidic materials. We shall first briefly examine the different classes of chemical reactions generally employed for synthesis and then discuss the various methods used along with several case studies and examples. In addition to the traditional ceramic method, the topics discussed include the combustion method (self-propagating high-temperature synthesis), the precursor method, topochemical routes, intercalation compounds, the ion-exchange method, the sol-gel process, the alkali-flux method, electrochemical methods, the pyrosol process and high pressure methods. The last topic includes hydrothermal synthesis of zeolitic materials. Intergrowth structures and superconducting cuprates are discussed in separate sections. It is hoped that the article will provide a useful survey of chemical methods of synthesis of inorganic materials and will serve as a ready reference to practitioners of the subject.

Lattice imaging technique of high resolution electron microscopy has been employed to examine 4H, 6H and 9R ABO₃ perovskite polytypes. The lattice images can be correlated with the lattice periodicity and the stacking sequence of AO₃ layers and BO₆ octahedra. The study shows the utility and validity of the lattice imaging technique for the study of relatively close-packed systems.

Ferroelectric bismuth oxides of the general formula

(A = Ba, etc., B = Ti or other transition metal) have been examined by high-resolution lattice imaging electron microscopy. The lattice images show dark bands at the positions of the Bi₂O₂ layers, with n–1 lines between them due to the layers of the perovskite A cations or, in favourable circumstances, the fully resolved 0.4 nm square perovskite grid. Dislocations and domain boundaries have been imaged for the first time in ferroelectric crystals. The structure of the dislocations and domain walls is discussed in the light of the microstructural evidence.

Analysis of EXAFS data of complex systems containing more than one phase and one type of coordination, has been discussed. It is shown that a modified treatment of EXAFS function as well as the amplitude ratio plots provide useful means of obtaining valuable structural information. The systems investigated are: biphasic Ni + NiO mixture, NiAl₂O₄ with two coordinations for Ni, NiO + NiAl₂O₄ mixture, CoS + CoO system and Ni dispersed on AL₂O₃. The results obtained with these systems have been most satisfactory and serve to illustrate the utility and the applicability of the innovations described in this paper.

Studies of valence bands and core levels of solids by photoelectron spectroscopy are described at length. Satellite phenomena in the core level spectra have been discussed in some detail and it has been pointed out that the intensity of satellites appearing next to metal and ligand core levels critically depends on the metal–ligand overlap. Use of photoelectron spectroscopy in investigating metal–insulator transitions and spin-state transitions in solids is examined. It is shown that relative intensities of metal Auger lines in transition metal oxides and other systems provide valuable information on the valence bands. Occurrence of interatomic Auger transitions in competition with intraatomic transitions is discussed. Applications of electron energy loss spectroscopy and other techniques of electron spectroscopy in the study of gas-solid interactions are briefly presented.

Electron-energy-loss spectra, recorded from ultramicro quantities (<10⁻¹² g), reveal L_2,3 edges the energy and intensity of which vary systematically with oxidation state in a series of transition metal oxides; the first ever evidence for structure-sensitive features in oxygen K-edges is reported.

Gels of various composition containing SiO₂, Al₂O₃, and P₂O₅ have been investigated by employing high resolution magic-angle-spinning (MAS) ²⁷Al, ²⁹Si, and ³¹P NMR spectroscopy. Changes occurring in the NMR spectra as the gels are progressively heated have been examined to understand the nature of structural changes occurring during the crystallization of the gels. ²⁷Al resonance is sensitive to changes in the coordination number even when the Al concentration is as low as 1 mol%. As the percentage of Al increases , the hydroxyl groups tend to be located on the Al sites while Si remains as SiO_4/2 (Q₄). Mullite is the major phase formed at higher temperature in the aluminosilicate gels. In the case of the silicophosphate gels, Si is present in the form of Q₄ and Q₃ species. There is a change in the coordination of Si from four to six as the gel is heated. The formation of six-coordinated Si is facilitated even at lower temperatures (∼673 K) when the P₂O₅ content is high. The phosphorus atoms present as orthophosphoric acid units in the xerogels change over to metaphosphate-like units as the gel is heated to higher temperatures. In aluminosilicophosphates, Si is present as Q₄ and Q₃ species while P is present as metaphosphate units; Al in these gels seems to be inducted into the tetrahedral network positions.

²⁹Si chemical shifts in a wide variety of silicates in crystalline, glassy and gel states have been related to a parameter, P, which takes into account the electronegativity and the structural description of the silicate units as well as the ionic potential of the modifier cation. The relation, δ(ppm) = 28.4 [1−exp(−P)]−110.5, besides having predictive value, satisfactorily accounts for all the available chemical-shift data on silicates and shows the right kind of limiting behaviour, with δ approaching the Q₀ value at large P.

³¹PNMR spectra of several inorganic phosphates have been examined both in the crystalline and the glassy states. The parameter (Z_eff/r)q clearly demarcates ortho-, pyro- and meta-phosphates in terms of the ³¹P chemical shifts . Based on such a diagram, inorganic phosphate glasses are found to consist essentially of metaphosphate units. NMR resonance of the glasses are generally much broader than those of crystalline phosphates.

After presenting the essentials of the principles and instrumentation involved in photoacoustic spectroscopy, a variety of studies of solids and surfaces carried out by this technique are reported. Some of the important aspects of the spectroscopy examined are signal saturation, intensity enhancement and fluorescence quenching. Applications to the study of amorphous solids, phase transitions and surfaces are discussed.

Please refer to full text.

Please refer to full text.

Spectroscopic methods have provided information of seminal importance in understanding phase transitions in solids. After briefly examining some fundamental concepts related to phase transitions, we shall discuss several case studies particularly involving the use of vibrational (IR and Raman) spectroscopy. Examples will include both order-disorder and displacive transitions. Under the former are included transitions in nitrates, ammonium halides, alkylammonium salts, plastic state of C₆₀ and superionic conductors (specially CsHSO₄). In addition, we shall discuss some aspects of incommensurate phase transitions, the glass transition and electronic phase transitions. Transitions of phosphonitrilic halide tetramers and alkane dicarboxylic acids are also examined.

Several complexes of iron and cobalt transform gradually or abruptly from a low-spin state to a high-spin state with increase in temperature and such transitions have been investigated widely in the past few years. Spin-state transitions in complexes are often accompanied by changes in enthalpy and crystal structure and many of them exhibit characteristics of first-order phase transitions. Transitions in certain complexes are affected by pressure, dopant metal ions and grinding and not infrequently show a plateau in the magnetic susceptibility-temperature plots. Seemingly unknown to coordination chemists, there is a fair body of information on the spin-state transitions in transition metal oxides and other extended solids. The transitions in rare earth cobaltates exhibit a plateau or a maximum in the inverse susceptibility-temperature plots, changes in enthalpy and crystal structure and other characteristics, not unlike those of the complexes. An attempt is made in this article, to bring together the essential features of spin-state transitions in metal complexes and oxides, particularly with respect to the nature of the phase transitions associated with spin cross-over. Models for spin-state transitions are examined and scope for further research indicated.

Pm3m-Fm3m transformations of solid solutions of CsCl with KCl and CsBr exhibit different behaviours. With increasing percentages of KCl, the NaCl structure gets stabilized in the CsCl+KCl system. In the CsCl+CsBr system, the transformation temperature increases with % CsBr and ΔH essentially remains constant. Both these behaviours can be satisfactorily explained in terms of the Born treatment of ionic solids. The Pm3m-Fm3m transformation retains its first-order characteristics in the CsCl+KCl system, but higher-order components seem to be present in the CsCl+CsBr system. Incorporation of vacancies do not affect the transformation of CsCl markedly.

Reversible phase transformations of alkali sulphates, alkali nitrates, and various other inorganic substances have been studied by making use of differential thermal analysis. Thermodynamic and kinetic data on the transformations have been obtained. Thermal hysteresis in reversible transformations has been examined, and the magnitude of hysteresis is shown to be related to the volume changes accompanying the transformations. The origin of hysteresis probably lies in the strain energies associated with the transformations. Approximate strain energies have been estimated from the analysis of the DTA data. On the basis of considerations from the theory of elasticity, it is possible to show that the strain energy is a function of ΔV. Thermodynamic considerations show that the strain energy is related to ΔT × ΔS.

The particle size and crystallite size of anatase increase markedly in the region of the crystal structure transformation. The unit cell of anatase seems to expand prior to the transformation to rutile. This expansion has been attributed to a displacive transformation of the type defined by Buerger. Smaller particle size and larger surface area seem to favour the transformation.

The kinetics of the transformation of anatase prepared by the hydrolysis of titanium sulphate have been studied at different temperatures and are found to be considerably different from the kinetics of the transformation of pure anatase. The transformation becomes immeasurably slow below ∼695 ± 10°C compared to ∼610 ± 10°C for pure anatase. An induction period is observed in the transformation of anatase obtained from sulphate hydrolysis and the duration decreases with increase in temperature. The activation energy is ∼120 kcal/mole, a value higher than that for the pure anatase-rutile transformation. The results have been interpreted in terms of the relative rates of nucleation and propagation processes. The activation energy for the nucleation process seems to be much larger than for the propagation process. The kinetics of the transformation of anatase samples doped with different amounts of sulphate ion impurity have also been studied and the transformation is found to be progressively decelerated with increase in the impurity concentration. The energy of activation for the transformation appears to increase progressively with increase in impurity concentration.

Recent developments in molecular dynamics (MD) and Monte Carlo (MC) methods enable us to fruitfully investigate transformations in solids by employing appropriate potentials. The possibility of varying both the volume and the shape of the simulation cell in these simulation techniques is especially noteworthy. In this article we briefly describe some of the highlights of the recent MD and MC methods and show how they are useful in the study of transitions in monatomic solids, ionic solids, molecular solids (especially orientationally disordered solids), and glasses. The availability of reliable pair potentials will undoubtedly make these methods more and more useful for studying various aspects of condensed matter in the years to come.

Based on an isothermal, isobaric simulation the structure and properties of the plastic crystalline phases of C₆₀ and neopentane have been examined. Instantaneous cooling of the plastic crystalline phases of both C₆₀ and neopentane leads to orientational glassy phases. These are accompanied by significant slowing down of reorientational motion. Constant pressure quench experiments on C₆₀ yield a glass transition temperature of around 80 K.

Monte Carlo simulations of liquid and supercooled liquid states of neohexane, n-hexane, n-pentane, isopentane, neopentane and a model linear molecule are reported. A quantitative measure of the degree of disorder associated with the molecular centre of mass has been obtained from the minimal spanning tree method. The results suggest a strong dependence of the degree of disorder of the centre of mass on the molecular shape. The changes in the degree of disorder on cooling also depend on molecular geometry. There appears to be little difference in the structures as well as the magnitude of positional disorder of the liquid and the glassy states of linear molecules in contrast to those of globular molecules. Based on the results obtained from the simulations, regions have been identified in the m–σ plane, where one may expect plastic crystalline and liquid crystalline phases. This yields fresh insight into the nature of the structural phases diagram for polyatomic systems.

Please refer to full text.

Please refer to full text.

Please refer to full text.

Shell model calculation of defect energies in alkali halides have been carried out using the ion-dependent, crystal-independent potential parameters of Sangster and Atwood (1978). Results indicate that appreciable differences exist between barrier heights for migration of cations and anions. While barrier heights for cations are generally lower than for anions in alkali halides of NaCl structure, the opposite is true in alkali halides of CsCl structure.

Electronic and ionic conductivities of silver selenide crystal (Ag_2+δSe) have been measured over a range of stoichiometry through the α–β transition by using solid state electrochemical techniques. In the high temperature β-phase Ag₂Se shows metallic behaviour of electronic conductivity for high values of δ; with decrease in δ, the conductivity of the material exhibits a transition. The magnitude of change in electronic conductivity at the α–β transition is also determined by stoichiometry. Ionic conductivity of the β-phase does not vary significantly with stoichiometry. A model to explain the observed transport properties has been suggested.

X-ray line widths, surface areas and heats of solution of MgO samples prepared by vacuum dehydration of Mg(OH)₂ at different temperatures in the range 350-1 200°C have been studied. While the surface energy of particles shows a steady decrease with the temperature of preparation, the heats of solution shows an abrupt decrease over a narrow temperature range. The results have been interpreted in terms of pseudocrystallization of the highly disordered oxide followed by sintering; the pseudocrystallization of colloidal oxides appear to be similar to glass transitions.

Isobaric gravimetric studies of PrO_z and TbO_z systems have shown the equilibrium existence of several nonstoicheiometric phases; the transformation temperatures found by differential thermal analysis agree well with the data from gravimetric studies. Oxidation reactions of Pr₂O₃ and Tb₂O₃ to non-stoicheiometric oxides generally show anomalously high entropy changes (ca. −25 e.u./mole of O₂) compared with the value of ca. −44 e.u./mole of O₂ generally found in oxidation of metals. The anomalous entropy changes are shown to arise from configurational factors. Cubic Pr₂O₃ with the defect structure is more readily oxidized than the hexagonal Pr₂O₃. Kinetic studies show that oxidation of Pr₂O₃ is a phase-boundary-controlled reaction.

The solvolytic disproportionation of non-stoichiometric PrO_x and TbO_x in acid solutions to produce higher oxides has been investigated. Some new non-stoichiometric phases have been reported. A number of interesting features of the non-stoichiometric rare earth oxides have been discussed and the need for a satisfactory structural model has been pointed out.

Please refer to full text.

Sesquioxides and nonstoichiometric oxides of rare-earths (Ln) exhibit electrical conductivities in the range 10⁻⁹−10⁻¹ Ω⁻¹ cm⁻¹. The sesquioxides exhibit mixed conduction with some contribution from ionic conductivity and major contribution from electronic conductivity. Seebeck coefficient data as well as the oxygen partial pressure dependence of conductivity indicate that LnO_x compounds are mixed valence semiconductors where oxides with 1.50 ≤ x ≤ 1.75 are p-type semiconductors and oxides with 1.75 ≤ x ≤ 2.00 are n-type semiconductors. The conductivity of LnO_x (Ln = Pr or Tb) goes through a maximum at x ≈ 1.75; Seebeck coefficients are sensibly constant with temperature and approach zero value at x ≈ 1.75. Employing the conductivities and Seebeck coefficients, transport parameters have been calculated. The mechanism of conduction in these oxides can be understood in terms of the hopping model and the small polaron theory. Fully ionised cation vacancies seem to be the predominant defects contributing to the defect structure in rare-earth sesquioxides.

Ti₃O₅ shows a first-order phase transition from the monoclinic structure to the pseudobrookite structure at 448°K, at which temperature a magnetic susceptibility anomaly has been reported earlier in the literature. There is an electrical conductivity discontinuity accompanying the phase transition. Incorporation of Fe stabilizes the high-temperature phase of Ti₃O₅; while with 2% Fe the transition temperature and enthalpy change are lowered, with 5% Fe there is no transition. Mössbauer spectra of 2% Fe-doped Ti₃O₅ are similar below and above the transition temperature and show no evidence for magnetic ordering in the low-temperature phase. These results are compared to the VO₂ transition.

NbO₂, Nb_0.98V_0.02O₂, and Nb_0.95V_0.05O₂ transform from semiconducting to metallic state at temperatures higher than the DTA phase-transition temperatures. Vibrational mode softening and c-axis Nb–Nb pairing appear to be important factors in the mechanism of these transitions. In the solid solutions, c/a ratio is maximum at x = 0.5 since the metal-metal interaction along the c axis becomes minimum; conductivity is minimum at this composition since there are no mobile electrons.

Rare earth ortho-chromites, -manganites and -ferrites are p-type semiconductors with conductivities in the range 10⁻⁴−10⁻¹ ohm⁻¹ cm−1. The conductivity in each series of perovskites decreases with the increasing atomic number of the rare earth. The ionic contribution to conductivity is small in all the three series of solids. None of these solids exhibits intrinsic behavior up to ∼1000°C. The conductivity behaviors of these rare earth compounds reflect the known crystallographic, dielectric and magnetic transitions in these materials. Seebeck coefficients in these compounds are large, typical of narrow-band materials; the Seebeck coefficients show marked changes at temperatures where magnetic and dielectric transitions occur. The electrical transport properties of all the three series of rare earth compounds are essentially controlled by the d-electrons of the transition elements which show localized behavior. This conclusion is in agreement with the results from optical spectra and the predictions of Goodenough. In all these compounds small polarons seem to be responsible for the conduction.

Please refer to full text.

LaMnO_3+δ samples with Mn⁴⁻ content up to 50% have been prepared by different methods. The structure of LaMnO_3+δ changes from orthorhombic to cubic (via rhombohedral) with increase in the Mn⁴⁺ content. LaMnO3+δ samples containing greater than 20% Mn⁴⁺ are ferromagnetic and show resistivity maxima at a temperature Tt which is close to the ferromagnetic Curie temperature. The resistivity maximum is due to the occurrence of a metal-insulator transition. In samples heated to the same temperature, the value of Tt increases with % Mn⁴⁺. For a given sample, Tt increases with the temperature of heat treatment due to the increase in particle size. The onset of ferromagnetism in LaMnO_3+δ accompanied by an insulator–metal transition is similar to that found in La_1−xCa_xMnO₃ and La_1−xSr_xCoO₃.

Single-phase LaNi_1−xMn_xO₃ samples in the compositional range 0 ≤ x ≤ 0.2 were prepared by the decomposition of precursor hydroxide solid solutions, and La₂NiMnO₆ was prepared from a mixture of carbonates. All the samples contained a disordered array of nickel and manganese ions. X-ray absorption and xps measurements indicate the presence of Mn(III) and Ni(III) valence states at room temperature for x = 0.1, 0.2 and 0.5. Magnetic susceptibility data suggest Mn⁴⁺ solute ions and Stoner-enhanced Pauli paramagnetism of the metallic solvent for x = 0.01 with a smooth transition to Mn³⁺ solute ions and a spontaneously magnetised solvent conduction band at x = 0.05. Below 200 K, the x = 0.05 sample forms superparamagnetic clusters, and below 40 K there is evidence for an antiferromagnetic spin-density wave. Comparisons with LaCo_0.95Mn_0.05O₃ and La_0.98Sr_0.02CoO₃ confirm that the long-range magnetic coupling occurs via solvent electrons in a narrow conduction band. The conductivity changes from that of a narrow-band metal for x < 0.01 to that more characteristic of diffusive motion for x > 0.05, but any motional enthalpy appears to remain small (ΔH_m ≃ 0). The x = 0.1 sample exhibits ferrimagnetic spin glass behaviour below 40 K, and the ferromagnetic interactions increase with manganese concentration. The oxide with x = 0.50 is ferromagnetic with a well defined Curie temperature.

There is increasing interest in recent years in the structural chemistry and properties of layered metal oxides possessing the K₂NiF₄ or related structures. Many new oxides of this structure exhibiting novel properties are being reported from time to time in the literature. The crystal chemistry of the oxides of the general formula A₂BO₄ with particular reference to the stability of the K₂NiF₄ structure and the relations between the different structures exhibited by this family of oxides is discussed. Non-stoichiometry in these oxides is another aspect of interest discussed in the article. While K₂NiF₄ itself is a well-known two-dimensional antiferromagnet, oxides of this structure with a variety of magnetic properties are examined in some detail. Besides the ternary A₂BO₄ oxides, the structure and magnetic properties of complex oxides, where the A or/and the B ions are partly substituted by other cations, is discussed. Some of the problems related to this family of oxides that are worth investigating are indicated. Much of the discussion in this article would have relevance in understanding the structure and properties of layered materials.

Electrical and magnetic properties of several oxide systems of K₂NiF₄ structure have been compared to those of the corresponding perovskites. Members of the La_1−xSr_1+xCoO₄ system are all semiconductors with a high activation energy for conduction unlike La_1−xSr_xCoO₃ (x ≥ 0.3) which is metallic; the latter oxides are ferromagnetic. La_0.5Sr_1.5CoO₄ shows a magnetization of 0.5 μ_B at 0 K (compared to 1.5 μ_B of La_0.5Sr_0.5CoO₃), but the high-temperature susceptibilities of the two systems are comparable. In SrO · (La_0.5Sr_0.5MnO₃)_n, both magnetization and electrical conductivity increase with the increase in n approaching the value of the perovskite La_0.5Sr_0.5MnO₃, LaSrMn_0.5SNi_0.5(Co_0.5)O₄ shows no evidence of long-range ferromagnetic ordering unlike the perovskite LaMn_0.5Ni_0.5(Co_0.5)O₃; high-temperature susceptibility behavior of these two insulating systems is, however, similar. LaSr_1−xBa_xNiO₄ exhibits high electrical resistivity with the resistivity increasing proportionately with the magnetic susceptibility (note that LaNiO₃ is a Pauli-paramagnetic metal). High-temperature susceptibility of LaSrNiO₄ and LaNiO₃ are comparable. Susceptibility measurements show no evidence for long-range ordering in LaSrFe_1−xNi_xO₄ unlike in LaFe_1−xNi_xO₃ (x ≤ 0.35) and the electrical resistivity of the former is considerably higher. Electrical resistivity of Sr₂RuO₄ is more than an order of magnitude higher than that of SrRuO₃. Some generalizations of the properties of two- and three-dimensional oxide systems have emerged from these experimental observations.

Density measurements on large single-crystal specimens of La₂NiO_4+δ and Pr₂NiO_4+δ show that oxygen nonstoichiometry arises from the presence of excess lattice oxygen. X-ray photoelectron spectra as well as X-ray absorption edge studies provide no evidence for the existence of Ni³⁺ in these oxygenexcess nickelates under the conditions of the measurements. Transmission electron microscopy has revealed a novel type of exsolution process of the stoichiometric phase out of nonsloichiometric La₂NiO₄ during heating in CO₂ at 870 K for 3 h. An interpretation of the results in terms of the existence of peroxide species within the conducting layers is proposed.

La_2−xSr_xNiO₄ shows a maximum in the c/a ratio around x = 0.6 up to which composition the concentration of holes is equal to that expected theoretically. The material becomes a degenerate semiconductor above a certain temperature up to x = 0.8, but is metallic at room temperature when x ≥ 1.0. All the compositions with x ≥ 0.05 are paramagnetic in the 15 - 300K range. Based on these measurements, an electronic phase diagram is tentatively proposed. It is noteworthy that the antiferromagnetic order disappears at low doping levels in both La_2−xSr_xNiO₄ and La_2−xSr_xCuO₄, but metallicity is seen at 300K only at a much higher x value in the former system.

In order to investigate the factors determining the relative stabilities of layered perovskite and pyrochlore structures of transition metal oxides containing trivalent bismuth, several ternary and quaternary oxides have been investigated. While d⁰ cations stabilize the layered perovskite structure, cations containing partially-filled d orbitals (which suppress ferroelectric distortion of MO₆ octahedra) seem to favor pyrochlore-related structures. Thus, the vanadium analogue of the layered perovskite Bi₄Ti₃O₁₂ cannot be prepared; instead the composition consists of a mixture of pyrochlore-type Bi_1.33V₂O₆, Bi₂O₃, and Bi metal. The distortion of Bi_1.33V₂O₆ to orthorhombic symmetry is probably due to an ordering of anion vacancies in the pyrochlore structure. None of the other pyrochlores investigated, Bi₂NbCrO₇, Bi₂NbFeO₇, TIBiM₂O₇ (M = Nb, Ta), shows evidence for cation ordering in the X-Ray diffraction patterns, as indeed established by structure refinement of TIBiNb₂O₇.

Reaction of bismuth metal with WO₃ in the absence of oxygen yields interesting bronze-like phases. From analytical electron microscopy and X-ray photoelectron spectroscopy, the product phases are found to have the general composition Bi_xWO₃ with bismuth in the 3+ state. Structural investigations made with high resolution electron micrscopy and cognate techniques reveal that when x < 0.02, a perovskite bronze is formed. When x ≥ 0.02, however, intergrowth tungsten bronzes (i.t.b.) containing varying widths of the WO₃ slab are formed, the lattice periodicity being in the range 2.3–5.1 nm in a direction perpendicular to the WO₃ slabs. Image-matching studies indicate that the bismuth atoms are in the tunnels of the hexagonal tungsten bronze (h.t.b.) strips and the h.t.b. strips always remain one-tunnel wide. Annealed samples show a satellite structure around the superlattice spots in the electron diffraction patterns, possibly owing to ordering of the bismuth atoms in the tunnels. The i.t.b. phases show recurrent intergrowths extending up to 100 nm in several crystals. The periodicity varies considerably within the same crystal wherever there is disordered intergrowth, but unit cell dimensions can be assigned from X-ray and electron diffraction patterns. The maximum value of x in the i.t.b. phases is ca. 0.07 and there is no evidence for the i.t.b. phase progressively giving way to the h.t.b. phase with increase in x. Hexagonal tungsten bronzes that contain bismuth with x up to 0.02 can be formed by starting from hexagonal WO₃, but the h.t.b. phase seems to be metastable. Optical, magnetic and electron transport properties of the i.t.b. phases have been measured and it appears that the electrons become itinerant when x > 0.05.

Bi₂VO_5.5 (Bi₄V₂O₁₁), which is the vanadium analog of the first member of the Aurivillius family of oxides of the general formula Bi₂A_n−1B_nO_3n+3, has been prepared and characterized. The vanadate has the expected layered structure and is ferroelectric with a Curie temperature of 720 K. While we have not been able to synthesize the vanadium analog of the n = 2 member of the Aurivillius family, we have examined the structure and properties of a vanadate of the composition Bi₂V₃O₉.

Ferrites of the formula Mo_xFe_3−xO₄, prepared by a soft-chemistry route, show mixed valence states of both iron and molybdenum cations. Mössbauer studies show that Fe²⁺ and Fe³⁺ ions are present on both the A and B sites, giving Fe an average oxidation state between 2+ and 3+. Molybdenum is present in the 3+ and the 4+ states on the B sites. The presence of Mo in the 3 + state has been established by determining the Mo³⁺−O distance (2.2 Å), for the first time, by Mo K-EXAFS. The mixed valence of Fe on both the A and B sites and of Mo on the B sites is responsible for the fast electron transfer between the cations. All the Mössbauer parameters including the line width show a marked change at a composition (x ≈ 0.3) above which the concentration of increases rapidly.

Reduction behaviour of Fe³⁺/AI₂O₃ obtained by the decomposition of the oxalate precursor has been investigated by employing X-ray diffraction (XRD), Mössbauer spectroscopy and electron paramagnetic resonance (EPR) spectroscopy. Calcination of Fe³⁺/AI₂O₃ at or below 1070 K yields mainly a poorly ordered, fine particulate form of η-Al_2−xFe_xO₃. Calcination at or above 1220 K yields α-AI_2−xFe_xO₃. Reduction of Fe³⁺/AI₂O₃ samples calcined at or below 1070 K gives the FeAI₂O₄ spinel on reduction at 870 K; samples calcined at or above 1220 K give AI_2−xFe_xO₃ with a very small proportion of metallic iron. Fe³⁺/AI₂O₃ samples calcined at 1220 K or above yield metallic iron and a very small proportion of the spinel on reduction below 1270 K. In the samples reduced at or above 1270 K, the main product is metallic iron in both ferromagnetic and superparamagnetic forms. The oxalate precursor route yields more metallic iron than the sol–gel route.

⁵⁷Fe Mössbauer measurements show that the T_c of (12)-nm particles of MnFe₂O₄ is higher than that of the (33)-nm particles, which exhibit the bulk T_c value (573 K). The ⁵⁷Fe isomer shift as well as the internal field are, however, independent of the particle size, as is the cation distribution determined by Mn and Fe extended x-ray absorption fine structure and Mn x-ray absorption near-edge structure measurements at different temperatures. The results confirm that the higher T_c, of the (12)-nm particles arises from finite-size scaling.

Please refer to full text.

The high-temperature superconductors are complex oxides, generally containing two-dimensional CuO₂ sheets. Various families of the cuprate superconductors are described, paying special attention to aspects related to oxygen stoichiometry, phase stability, synthesis and chemical manipulation of charge carriers. Other aspects discussed are chemical applications of cuprates, possibly as gas sensors and copperfree oxide superconductors. All but the substituted Nd and Pr cuprates are holesuperconductors. Several families of cuprates show a nearly constant n_h at maximum T_c. Besides this universality, the cuprates exhibit a number of striking common features. Based on Cu(2p) photoemission studies, it is found that the Cu–O charge-transfer energy, Δ, and the Cu(3d)–O(2p) hybridization strength, t_pd, are key factors in the superconductivity of cuprates. The relative intensity of the satellite in the Cu(2p) core-level spectra, the polarizability of the CuO₂ sheets as well as the hole concentration are related to Δ/t_pd. These chemical bonding factors have to be explicitly taken into account in any model for superconductivity of the cuprates.

Superconducting YBa₂Cu₃O_7−δ (0.3 ≤ δ ≤ 0.4) compositions seem to transform to a metastable YBa₂Cu₄O₈ (124)-like phase with a c parameter of 27.2 Å on annealing at 473 K for a few hours.

In YBa₂Cu_3−yGa_yO_7−δ, Ga can be substituted at the Cu(1) site up to y=0.1 without change in structure, but this is accompanied by a slight decrease in the hole concentration and T_c; the same is true when Y is partly substituted by Ca, as in Y_1−xCa_xBa₂Cu_3−yGa_yO_7−δ (0.0 < x ≤ 0.2). When one Ba is replaced by Sr as in YBaSrCu_3−yGa_yO_7−δ, however, Ga can be substituted at the Cu(1) site to a much greater extent (up to y=0.6). In this system , Ga substitution changes the structure from orthorhombic to tetragonal, unlike in YBa₂Cu_3−yGa_yCu₃O_7−δ. Both the hole concentration and T_c decrease with an increase in y and the material becomes non-superconducting for y > 0.2. They y = 0.3 and 0 .4 compositions show metal -semiconductor transitions at 60 and 120 K, respectively, while the y = 0.6 composition is a semiconductor. When Y is partly substituted by Ca as in Y_1−xCa_xBaSrCu_3−yGa_yO_7−δ, the material is superconducting even when y = 0.3. All these Ga-substituted cuprates are in the underdoped region and accordingly T_c increases with increase in hole concentration.

Cuprates of the formula TISr_3−xLn_xCu₂O₇ (Ln = Pr, Nd or Y) derived from the hypothetical TISr₃Cu₂O₇ show superconductivity with T_cs up to 95 K when 0.5 ≲ x ≤ 0.75, the x = 1.0 compositions being insulators. Rietveld analysis of X-ray diffraction profiles has been carried out for two superconducting members of this family. The unit cell a-parameter, and hence the in-plane Cu–O distance, increases with increase in x. The T_c value decreases with increase in x or the in-plane Cu–O distance in all the series of cuprates. Superconductivity in the Tl_1–yPb_ySr_3–xNd_xCu₂O₇ system is found with the highest T_c of 95 K when y = 0.2 and x = 0.5. The in-plane Cu–O distances in all the cuprates studied fall in the range found in the Sr-class of cuprate superconductors.

A systematic study of the Tl_0.5Pb_0.5Sr₂Gd_2−xCe_xCu₂O_9−δ system has revealed the existence of a pure phase in the compositional range 0.0 ≤ x ≤ 0.6 crystallizing in the 1222 structure. It has an intersheet distance of approximately 6 Å, a value much higher than those found in other cuprates with double CuO₂ sheets interleaved by a single fluorite layer. Superconductivity has been observed in the range 0.1 ≤ x ≤ 0.4 with a T_c of 45 K and a superconductive volume fraction up to 20% for the optimal composition. An interesting variation of the superconducting properties of the above system with the composition, i.e. cerium content, has also been noticed. A possible dependence of superconductivity on the coupling between CuO₂ sheets in the layered cuprates has been pointed out to bring out a correlation between structure and properties.

When the Cu(1) site in orthorhombic YBaSrCu₃O_7−δ (T_c = 80 K), is partly substituted by the carbonate ion (upto 50%), the crystal structure becomes tetragonal and the electron diffraction pattern shows evidence for 2a × 2c superstructure; the material however is not superconducting. The same is true when Y is partly replaced by Ca as in YCaBa₂Sr₂Cu₅ (CO3)O_y. When the CO₃ group is partly replaced by NO₃ group as in tie YCaBa₂Sr₂Cu₅ (CO₃)_1−x(NO₃)_xO_y, the structure remains the same but superconductivity is retained. IR spectroscopic studies show that both CO₃ and NO₃ coordinate strongly and are not present as free ions in these oxyanion cuprate derivatives. Cu K-EXAFS studies on the carbonate and carbonato-nitrate derivatives confirm the presence of the oxyanions in the place of CuO₄ units in the Cu-O chains.

While YSr₂Cu₃O₇ cannot be prepared under ambient conditions, partial substitution of the phosphate group for copper, as in YSr₂Cu_2.8(PO₄)_0.2O_y, stabilizes this phase in the orthorhombic structure, but the material is not superconducting. Superconductivity in YSr₂Cu_2.8(PO₄)_0.2O_y, is obtained by increasing the hole concentration through partial substitution of Y by Ca, as in Y_0.7Ca_0.3Sr₂Cu_2.8(PO₄)_0.2O_y (T_c ≈ 40 K). By incorporating the phosphate group in orthorhombic YBaSrCu₃O₇, a stable tetragonal derivative of the formula YBaSrCu_2.8 (PO₄)_0.2O_y (T_c ≈ 47 K) has been prepared; the T_c increases to ∼ 70 K by partial substitution of Y by Ca as in Y_0.7Ca_0.3BaSrCu_2.8(PO₄)_0.2O_y.

The thermopower, α, of many members of the Bi₂Ca_1−xY_xSr₂Cu₂O_8+δ, Bi_2−yPb_yCaSr₂Cu₂O_8+δ, Tl_yCa_1−xY_xBa₂Cu₂O_6+δ TICa_1−xLn_xSr₂Cu₂O_6+δ and T1Sr_2−xLa_xCuO_5+δ systems is positive down to T_c, but is negative in some of them. The slope of the α-T plots above T_c is generally negative independent of the sign of α. The value of α extrapolated to 0 K, α(0), from the hightemperature regime is finite and generally, positive. Compared to the behaviour of YBa₂Cu₃O₇, the data on the Bi and Tl cuprate systems examined here seem to suggest that electrons may be involved in the conduction process as well, at least in systems where both the α and the α-T slope are negative. Of the different systems examined here, Tl_1−xPb_xCaSr₂Cu₂O_6+δ containing Pb in the 4 +state, deserves special mention since it shows negative α over the entire temperature range in addition to a negative slope suggesting that this is likely to be a genuine high-T_c, electron-superconductor. The thermopower of cuprates has been discussed in some detail in terms of entropic and quasiparticle contributions.

The low-field dependence of non resonant microwave absorption (MWA) in polycrystalline samples of YBa₂Cu₃O_7−δ with 0.1 ≤ δ ≤ 0.5 has been measured at T < Tc along with simultaneous monitoring of paramagnetic impurities in the chain copper (Cu-I) sites by EPR spectroscopy. MWA exhibits a cross-over from the normal zero-field minimum to an anomalous zero-field maximum accompanied by the gradual appearance and growth of the EPR signal as δ is changed from 0.1 to 0.5. It is to be noted that in the δ = 0.3 − 0.4 composition range, oxygen vacancies are disordered giving rise to magnetic moments due to isolated Cu²⁺ impurity ions. This correlation of the anomalous MWA with isolated paramagnetic Cu²⁺ impurities is consistent with the presence of π junctions, earlier implicated in the paramagnetic Meissner effect (PME), which in turn is related to the anomalous MWA.

Please refer to full text.

In situ EXAFS and X-ray diffraction investigations of Ni/TiO₂ catalysts show that NiTiO₃ is formed as an intermediate during calcination of catalyst precursors prepared by the wet-impregnation method; the intermediate is not formed when ion-exchange method is used for the preparation. On hydrogen reduction, NiTiO₃ gives rise to Ni particles dispersed in the TiO₂(rutile) matrix. The occurrence of the anatase–rutile transformation of the TiO₂ support, the formation and subsequent decomposition/reduction of NiTiO₃ as well as the unique interface properties of the Ni particles are all factors of importance in giving rise to metal-support interaction. Active TiO₂(anatase) prepared from gel route gives an additional species involving Ni³⁺.

Cu K-absorption edge and EXAFS measurements on binary Cu/ZnO and ternary Cu/ZnO–Al₂O₃ catalysts of varying compositions on reduction with hydrogen at 523 K, show the presence of Cu microclusters and a species of Cu¹⁺ dissolved in ZnO apart from metallic Cu and Cu₂O. The proportions of different phases critically depend on the heating rate especially for catalysts of higher Cu content. Accordingly, hydrogen reduction with a heating rate of 10 K/min predominantly yields the metal species (> 50%), while a slower heating rate of 0.8 K/min enhances the proportion of the Cu¹+ species (∼ 60%). Reduced Cu/ZnO–Al₂O₃ catalysts show the presence of metallic Cu (upto 20 %) mostly in the form of microclusters and Cu¹⁺ in ZnO as the major phase (∼ 60%). The addition of alumina to the Cu/ZnO catalyst seems to favour the formation of Cu¹⁺/ZnO species.

In-situ EXAFS studies of sulphided Mo/TiO₂ catalysts have shown that at low Mo loadings (2–4 wt%), an active species with a short Mo-S distance of 2.25 Å is formed, while on Mo/TiO₂ with high Mo loadings as well as on Mo/γ-Al₂O₃, bulk MoS₂ (Mo-S, 2.42 Å) is formed. The species with the short Mo-S distance has Mo in an oxidation state close to 6+ and is likely to result from the sulphidation of the tetrahedral molybdate species present in the oxidic precursor at low Mo loadings. The calcination temperature of the oxidic precursor appears crucial, a high calcination temperature of 973 K favouring the formation of MoS₃ on sulphidation, and a low calcination temperature of 623 K favouring MoS₂.

Adsorption of nitrogen on Ni/TiO₂, Ni/Al₂O₃, Ni/Al, Ni–Ti alloy, TiO₂, and other surfaces prepared in situ in an electron spectrometer has been investigated by X-ray and UV photoelectron spectroscopy to understand the nature of the strong metal-support interaction (SMSI) state in the annealed Ni/TiO₂ catalyst system. The annealed Ni/TiO₂ surface has a considerable proportion of Ti³⁺ and exhibits dissociative adsorption of N₂ along with weak molecular chemisorption at 80 K just as a Ni–Ti alloy surface. This is in contrast with the nonannealed Ni/TiO₂ surface where there is only molecular chemisorption at 80 K similar to the Ni/Al₂O₃ surface. The adsorption behavior of the annealed Ni/TiO₂ surface is also different from that of TiO₂ (showing only physisorption), TiO_x (reduced surface showing physisorption and dissociation) or Ni/TiO_x (dissociation and stronger chemisorption as on Ni/TiO₂). It is suggested that the annealed Ni/TiO₂ surface may correspond to the SMSI state.

In monometallic Fe/SiO₂ and Ni/SiO₂ catalysts, the transition element undergoes only partial reduction to the metallic state on treatment with hydrogen, unlike in Cu/SiO₂ where the reduction is complete. In-situ Mössbauer and EXAFS investigations of Fe-Ni/SiO₂ catalysts with different Fe/Ni ratios show that the reducibility of both Fe and Ni is greater in the bimetallic catalysts than in the corresponding monometallic catalysts. Reduced bimetallic catalysts show evidence for the formation of FCC or BCC alloy phases that are superparamagnetic or ferromagnetic depending on the composition as well as the heat treatment given to the precursor. In the Ni-rich composition, Fe(25)–Ni(75), a superparamagnetic alloy is formed, while both ferromagnetic and superparamagnetic alloys are formed in the Fe-rich composition, Fe(75)–Ni(25). In the Fe(50)–Ni(50) catalyst, a ferromagnetic alloy, predominantly in the FCC structure, is formed. The presence of Fe in the FCC and Ni in the BCC structure is indeed noteworthy. In the case of Fe-Cu/SiO₂ catalysts, the promotion effect of Cu on the reduction of Fe is marginal and alloy formation is observed mainly in Fe-rich compositions. Alloy formation and the reducibility of Ni in Cu–Ni/SiO₂ catalysts are known to increase with increasing Cu content. It therefore appears that in all these bimetallic mutual promotion of the reducibility of the transition metals and alloy formation go hand in hand.

V₂O₅ supported on ZrO₂ is found to be an excellent sensor for n-propane–butane mixtures at 625 K; in-situ X-ray diffraction studies show that V₂O₅ is reduced to VO₂ with a metastable monoclinic structure on contact with the hydrocarbons and is oxidised back to the parent oxide on exposure to air.

Please refer to full text.

Metallic properties are by no means confined to elemental substances alone. A variety of materials, both inorganic and organic, show metallic properties. Some of these exotic substances exhibit electrical conductivities comparable to those of elemental metals like copper. A large number of systems traverse the transition from the metallic state to the nonmetallic state when there is a change in temperature, pressure or composition. Metal oxides provide a wide range of materials exhibiting metallic behaviour or going through the metal to non-metal (M-NM) transition. Alkali metal-ammonia solutions, with which chemists are all too familiar, probably constitute one of the earliest and most widely studied examples of the M-NM transition. However, a proper recognition of the metallization of ammonia in the context of the variety of solid systems exhibiting M-NM transitions has only been possible recently. Another interesting class of substances is that of expanded metals such as Hg and Cs which become non-metallic when the density is reduced below a critical value. Several organic solids, metal-chain compounds and polymers are not only metallic, but also become superconducting at low temperatures. With such a galaxy of chemical substances exhibiting metallic behaviour, the fundamental, recurring question of vital interest is "what makes a metal?". In this contribution, we shall examine operational criteria as well as criteria derived from models to answer this question. A related question of equal interest to chemists is “how many atoms are necessary to bring about metallic properties?”.

Bremsstrahlung isochromat spectroscopy (BIS) along with ultraviolet and X-ray photoelectron spectroscopy (UPS and XPS) has been employed to investigate the electron states of Pd and Ag deposited on amorphous graphite at different coverages. The metal core level binding energies increase with decreasing cluster size while the UPS valence bands show a decrease in the 4d states at E_F accompanied by a shift in the intensity maximum to higher binding energies. BIS measurements show the emergence of new states closer to E_F with increase in the cluster size. It is pointed out that the observed spectral shifts cannot be accounted for by final-state effects alone and that initial-state effects have a significant role. It therefore appears that a decrease in cluster size is accompanied by a metal–insulator transition.

After microscopic characterization of the size distributions of gold clusters, deposited on carbon substrates by vacuum evaporation or by soft landing, Au(4f) binding energy of the clusters has been measured as a function of the mean cluster size. Similar measurements have been carried out on Au clusters prepared from sols by chemical means and high-nuclearity cluster compounds. In general, small clusters with a mean diameter of ≲2 nm show significantly larger binding energies than the bulk metal value, due to the onset of nonmetallicity. Nonmetallicity manifests itself in terms of a tunneling conductance gap only in clusters of diameter ≲1 nm containing 40 atoms or fewer.

Please refer to full text.

Please refer to full text.