Molten salts have traditionally been used as recrystallizing solvents for the crystal growth of mixed metal oxides. In recent years, many examples of the direct preparation of mixed metal oxides from molten salt solutions have been reported. We have been exploring the use of chloride melts as a reaction media to prepare complex early transition metal oxides. Specific examples of new synthetic routes to interesting titanates and niobates will be presented. In one case, we have prepared a series of layered niobate perovskite solid acids from molten salts at temperatures well below those traditionally used in solid state syntheses. In the second case, we have discovered a new synthetic route to a poorly characterized reduced calcium titanate that is otherwise very difficult to make. The synthesis and characterization of these two classes of compounds will be described.

New forms of Sr-Co oxide with one-dimensional structures related to mixed-layer hexagonal perovskites have been synthesized and characterized by TEM. Crystals of Sr5Co2O12 grown from molten KOH flux and oxidized under slow cooling have structures based on a 3/2 ratio of mixed [Sr3CoO6] and [Sr3O9] layers. This is the first known structure with layers of this type stacked in a non-integer ratio, yielding chains of face-sharing octahedral and trigonal prismatic Co-sites with a 3 + 1 sequence along the c-axis. In oxygen-deficient Sr5Co2O12−x, the {110} structural modulation of the stoichiometric 5:4 phase becomes rotated to an irrational orientation, possibly in association with vacancy ordering, forming an incommensurate superstructure. For the compound Sr6Co5O15, previously known to have a 1/1 ratio of [Sr3CoO6] and [Sr3O9] layers with a 4 + 1 octahedral/trigonal prismatic site sequence, introduction of excess oxygen atoms leads to formation of a commensurate rhombohedral superlattice in which c is doubled relative to the stoichiometric 6:5 phase.

Phase formation in the system Li3SbO4-CuO has been studied by solid state chemistry reaction, at temperatures between 650° and 1150°C. Two new complex oxides were found and characterized by X-ray powder diffraction (XRD), Li3CuSbO5 (O) and Li3Cu2SbO6 (S). The latter exhibits a new partially ordered rock salt structure and shows a low level of electronic conductivity [1]. The crystal structure of Li3SbO4was determined [2].

We present some experimental results of the family of compounds with a general formula GaSr2Can−1CunO2n+3(n = 2,3, 4, 5). This system has been synthesized using the solid-state reaction method at ambient pressure. We have found that the reaction temperature for these compounds is 930±3°C; the samples melt just above this temperature. The powder diffraction data were analyzed assuming a single-phase and the obtained structure is different to the one prepared at high pressure [1,2]. However, comparing the powder patterns we found that the compounds are isostructural to the Al — Sr — Ca — Cu — O system prepared at high pressure [3]. For n = 2,3,4 and 5 a shift of the (105) peak has been observed, which is due to the increases in the number of Ca and Cu — O2 planes in the crystalline structure. Notice that to obtain a single-phase for the above compounds, high pressure is not necessary.

In-situ high temperature electrical conductivity and thermopower have been measured simultaneously on a number of ordered perovskite-like oxides containing double CUO4/2 sheets. Equilibrium measurements have been conducted as a function of oxygen partial pressure, temperature and chemical substitution in order to understand the relationships between the chemical architecture and the transport and defect properties. Data for LaBa2Cu2NbO8 and LaCa2Cu2GaO7 are presented and compared with those of known triple perovskite superconductors, Y1−xCaxSr2Cu2GaO7 and YBa2Cu3O7−δ, and several quadruple perovskites, Ln′Ln″Ba2Cu2M2O11 (Ln = Lanthanide, Y; M = Sn, Ti). These materials belong to a general family of superconductors which are constructed from similar ‘active’ layers (double perovskite blocks of square-pyramidal copper-oxygen sheets), and interleaved with fixed valence cations in perovskite-like ‘conditioning’ layers. Similarities in the transport properties of the non-superconducting and superconducting materials at elevated temperatures are illustrated, and the amount and types of defects, including carrier concentrations, are correlated with the internal chemistry and inner architecture of each material.

We have prepared a wide Ba1−xSrxCl1+yF1−y solid solution (0<x<0.25, −0.20<y<0.20) that has the BaClF structure without any sign of lattice distortion or ordering of the cations or of the anions other than that already present in BaClF. Furthermore, 119Sn Mossbauer spectroscopy shows that tin(ll) is present in this material in the form of Sn2+ ions instead of being covalently bonded Sn(ll) with a stereoactive lone pair of electrons, which is nearly always the case when the anions are small and strongly electronegative, like for F and Cl. The tin situation is similar to that observed in SnCl2.

The arrangement of atoms in a crystal is determined by two factors, the bonding preferences of individual atoms (giving rise to short range order), and the translational symmetry operations of the space group (giving rise to long range order). Chemical rules can be used to determine which atoms are bonded, and hence the maximum possible symmetry of the formula unit. Space group theory is then used to find the space groups that are compatible with this symmetry. In favourable cases, using the principle of maximum symmetry, the structure can be completely determined, but in all cases the analysis yields insights into the restrictions that determine what crystal structures might be possible.

The local orbital extension of the Linearized Augmented Planewave (LAPW+LO) method within the local density and general gradient approximations was used to optimize internal coordinates and calculate total energies of Pb(Zri1/2Ti1/2)O3 (PZT) superlattices with B site cations ordered along the [001] and [111] directions. Ferroelectric structures and bond-length distributions similar to those obtained from experimental data were found for all three investigated chemically ordered phases. The Ti atom sits in an off-center position of a slightly distorted TiO6 octahedron, the structure of which is mostly independent of the chemical ordering pattern. Coupling between the Ti and Zr containing octahedra results in highly distorted ZrO6 units. This relatively high energy part of the structure could decrease the stability of these perovskite compounds against zone-boundary rotations of the BO6 octahedra. Polar, zone-center only distortions result in lower energy [111] ordered superstructures when compared to the [001] ordered structure. The lowest energy chemically ordered PZT is the one with B-site cations ordered along the [111] direction and has 14mm symmetry. Our total energy results and a simple statistical model predict a wide miscibility gap PZT at the morphotropic composition is therefore likely a multiphase or metastable material which may be responsible for its sensitivity to synthesis and annealing conditions.

There exists a long-standing controversy concerning the nature of the dominant point defect mechanism in rutile TiO2. Previous classical shell model calculations by Catlow el al [1] find a strong preference for Schottky as opposed to Frenkel-type defects, lending support for oxygen vacancy rather than titanium interstitial compensation in reduced rutile. However, reviews of experimental studies [2], show that many conflicting conclusions have been reached. Ab initio total-energy calculations have been performed on a parallel computer to help resolve this controversy. First results indicate a Schottky formation energy (of the bound Schottky trio) consistent with the Mott-Littleton values of Catlow et al [1]. A first attempt is made at calculating the heat of reduction through determination of the formation energy of a neutral oxygen atom vacancy. As a. result some interesting insight is gained into the redox chemistry of reduced rutile.

We have recently developed a novel computational method for molecular dynamics simulations of metal oxide ceramics. This approach explicitly includes variable charge transfer between anions and cations. This method has been used to model the structural properties of bulk and surface alumina and aluminum systems including tensile failure of the bulk systems, as well as to model the rupture under tensile stress of an interface between a (0001) face of a-alumina and a (111) face of aluminum. We have applied this method to perform atomic-scale simulations of nanoindentation of ceramic and model rigid tips onto metal and ceramic substrates.

AgGaS2 is a technologically important semiconductor for its large birefringence coefficient. In this work we compare the theoretical ab-initio all-electron FLAPW results with very refined experimental data obtained with accurate X-ray analysis. In particular we focus our attention to the electronic distribution along the significative bonding directions connecting the three different atoms. Furthermore, the charge density contours around Ag provide a clear evidence of the contribution of its d orbitals to the chemical bond.

A study of cubic dihydrides TiH2 and ZrH2 and of H in cubic Laves-phase compounds by means of ab-initio total-energy calculations in the local density-functional approximation is presented. First, for optic vibrational modes in the two dihydrides the calculated local potentials and excitation energies are compared to experimental results obtained by inelastic neutron scattering. Second, the relative stabilities of H on three types of interstitial sites in the cubic Laves-phase compounds, denoted by b, e and g, are investigated. Theoretically, a preference of g sites is found for M=V, Cr, and of e sites for M=Fe, Co. These results are discussed with respect to experimental observations and to empirical criteria for H accomodation in transition-metal compounds.

Both experiments and band-structure calculations indicate that shear strains lead to closure of energy band gaps in periodic solids, and closure of the LUMO-HOMO gaps in molecules (this is also expected for non-metallic glasses). It allows athermal and ultrafast mechano-chemical reactions to occur. Simple models account for the phenomena. For periodic solids, shear shortens one axis and extends another. In contrast, hydrostatic compression causes both axes to contract. Thus the former shifts the band gaps oppositely for the two directions, causing a reduction in the minimum gap. For molecules, shear tends to increase the bonding orbitals while decreasing the antibonding orbitals, thereby reducing the LUMO-HOMO gaps. The models are consistent with band structure models for diamond.

Shear-induced gap reductions account for the athermal chemical reactions observed by Bridgman, Enikolopyan, Boldyrev, and others. They account for the ultra-fast reactions that occur in explosives.

With the aid of today's powerful computers, quantum mechanical methods are becoming accurate enough to determine the properties of materials before going into laborious and expensive synthesis processes. These “computer experiments” do not require any input other than the atomic numbers of the constituent species. In addition, they allow full control over the “experimental” parameters. In this way the effect of any parameter on the materials properties can be investigated, which is usually not possible in traditional experiments.

We used the ab initio pseudopotential method to study the effects of chemistry and structure on the voltage obtained from intercalating Li ions into transition metal oxides. The effect of transition metal and anion chemistry was systematically studied by changing M in LiMO2, (M=Ti, V, Mn, Co, Ni, Cu and Zn) and X in LiCoX2 (X=Se, S and O), keeping the structure fixed at α-NaFeO2 layered structure. In addition, the effect of structure was studied by performing computer experiments on LiCoO2 and LiMnO2 in α-NaFeO2, LiScO2 and Al2MgO4(spinel) structures.

The computed voltages are in good agreement with experimental values. We found that, the electronic band structure and the extent of charge transfer between Li and O ions is correlated to the output voltage. We will explain how the output voltage is affected by the chemistry of the transition metal.

The thermophysical properties of diamond, a metastable material at room temperature, are difficult to measure at high temperatures. These properties are of interest for testing equation of state and interatomic potential models. Here we utilize a geometrical lattice transformation, one dimensional lattice dynamical theory, and the principle of corresponding states to calculate the elastic constants of diamond over an extended temperature range.

Structural and spectroscopie characteristics of a series of Eu 2+-doped Sr and Ba borates have been studied to establish simple correlations among emission color, excitation energies, Stokes shifts, and O atom environments.

As part of an ongoing investigation to characterize the properties and structure of zinc halide-tellurium oxide glasses, we report preliminary measurements of the optical properties of several Nd- and Er-doped tellurites. Measurements include florescence lifetimes and estimates of the theoretical radiative lifetimes (as obtained by traditional Judd-Ofelt analysis of optical absorption spectra) as well as phonon sideband studies sensitive to vibrational characteristics near the rare earth ion. The response of these optical features to the substitution of alternative halides is examined.

Epitaxial potassium niobate films have been deposited by metalorganic chemical vapor deposition (MOCVD). X-ray diffraction of the films deposited on (100) spinel substrates shows (110) and (001) reflections, indicating a multidomain structure. The electro-optic properties of the thin films were measured using a transmission technique. Effective electro-optic coefficients as large as 470 pm/V were observed. The observed electro-optic response was dominated by domain reorientation effects.

Phase formation and crystal growth conditions for different combinations of cations corresponding to chemical formula of KTP were studied. Preparation of new Ti-less arsenates which arc isostructural with KTP is reported. In the single crystals grown Ti4+ cations were completely substituted with M2+ + Nb5+ (M = Mg, Zn) or M3+ + Nb5+ (M = Al, Cr, Ga, Fe, In) cation couples. The crystals were prepared by spontaneous crystallization using flux method. The structure analysis was made by X-ray powder diffraction technique. The crystals composition was measured by electron probe microanalysis (EPMA). Second harmonic generation (SHG) effect was studied by the powder technique with a pulsed YAG:Nd laser (1064 nm). SHG signal of the crystals was found to be slightly greater than that of KTiOAsO4 (KTA).

Optical second harmonic generation has been observed in transparent tellurite glass-ceramics of Ba0-TiO2-TeO2 system. BaTiO3 and unidentified crystalline phase precipitate from 15BaO·15TiO2·7OTeO2 glass. The temperature of maximum rate of nucleation for these crystalline phases is estimated to be 380°C. The BaTiO3 crystal precipitated is a cubic phase with lattice parameter of 0.4045 nm, which is slightly larger than the value reported in the JCPDS cards. The optical second harmonic intensity of the present transparent tellurite glass-ceramics is comparable to the intensity of poled tellurite glasses although the BaTiO3 phase precipitated is cubic and the crystal face is randomly oriented in the glass-ceramics. For instance, the second harmonic intensity of the present transparent tellurite glass-ceramics is about twice as large as that of 20WO3·80TeO2 glass poled at 280°C under the external electric field of 3 kV, the second-order nonlinear coefficient, d33, of which is 0.10 pm/V.