Perovskite-like mixed metal ruthenates are of interest owing to their varied electronic and magnetic properties, which are heavily dependent on the ordering of the transition metals. We report the synthesis and structural characterization of the first 1:2 ordered perovskite ruthenate, Sr3CaRu2O9. The structure was determined from a combination of powder X-ray, electron and neutron diffraction data and is characterized by a 1:2 ordering of Ca2+ and Ru5+ over the sixcoordinate B-sites of the perovskite lattice. Sr3CaRu2O9 is the first example of this structure-type to include a majority metal with d electrons (Ru(V), d3). The relationship of this material to the K2NiF4-type Sr1.5Ca0.5RuO4 (i.e., Sr3CaRu2O8) highlights the dramatic effects of the ruthenium valence on the resultant structure. Remarkably, these two structures can be quantitatively interconverted by the appropriate choice of reaction temperature and atmosphere.

The equations of state and axial moduli of the CaBO3 perovskites (B=Zr,Sn,Ti,Ge) and CaFeO2.5 with the brownmillerite structure have been determined using high-pressure, singlecrystal X-ray diffraction. The bulk modulus-specific volume relationship for the Ca-perovskites is nonlinear, with CaSnO3 and CaZrO3 displaying anomalous stiffening (higher bulk moduli) than previously reported and predicted [1,2]. The axial moduli of the a- and c-axes decrease steadily by ∼30% from the least-distorted of the Pbnm perovskites, CaGeO3, to the most distorted, CaZrO3, while the b-axis shows little change. The net result is a threefold increase in the anisotropy of the axial moduli of CaSnO3 and CaZrO3 (∼21%) relative to CaGeO3 and CaTiO3 (∼4-8%). The bulk modulus of CaFeO2.5 falls significantly below the trend for the stoichiometric perovskites. The introduction of 1/6 vacancies on the oxygen positions softens the perovskite structure by 25%.

We report the synthesis of Sr1-xCaxMnO3 and La0.5Ba0.5MnO3 perovskites over extended cation and oxygen composition ranges and describe the dependence of their phase stability on the tolerance factor t = t(x,T,σ) that is a function of composition, temperature, and oxygen content. We show that magnetic transition temperatures depend strongly on the tolerance factor and charge disorder while dependence on the structural disorder is less important. By reducing charge and structural disorder we have significantly increased the Curie and Neel temperatures for perovskite manganites.

A suite of perovskite phases with the compositions NaTixNb1-xO3-0.5x, 0 ≤ x ≤ 0.2, has been synthesized for the first time using the sol-gel method. Rietveld analysis of powder XRD data reveals that with increasing Ti content, the orthorhombic perovskite structure becomes more cubic-like, as evidenced by the smaller differences among its three cell parameters. Enthalpies of formation of the synthesized phases from the constituent oxides and from the elements have been determined by drop solution calorimetry into molten 3Na2O·4MoO3 solvent at 974 K. As Ti4+ substitutes for Nb5+, the formation enthalpies become less exothermic in a nearly linear manner. This behavior suggests that the Ti→Nb substitution destabilizes the perovskite structure, presumably because of the concomitant occurrence of O2- vacancies, which compensate the charge imbalance between Ti4+ and Nb5+ in the structure.

Two perovskite-derived materials, CaCu3Mn4O12, have drawn much recent interest due to their magnetoresistive, dielectric, and mafnetoelectronic characteristics. Here we present initial theoretical insights into each of these points, based on first principles, density functional based calculations. Our results predict CCMO to have a spin-asymmetric energy gap, which leads to distinct temperature- and magnetic field-dependent changes in properties, and helps to account for its observed negative magnetoresistivity. We have studied CCTO primarily to gain insight into the exchange coupling in both these compounds, where the conventional superexchange coupling vanishes by symmetry for both nearest and next nearest Cu-Cu neighbors, a consequence of the structure. In CCTO, it is necessary to go 5th Cu-Cu neighbors to obstain a (superexchange) coupling that can provide the coupling necessary to give three dimensional order. Non-superexchange mechanisms may be necessary to describe the magnetic coupling in this structural clss.

Pulsed laser deposition technique was used to fabricate SrBi2Ta2O9 (SBT) thin films with partial replacement of penta-valent vanadium ion. In this report, we have doped certain concentrations (0, 5, 10, 15 %) of vanadium at Ta-site of SBT thin films and study their influence on the structural and electrical characteristics. Thin films were grown on platinized silicon substrates using an excimer laser (KrF, 248 nm) with an energy density of ∼ 2.5 J/cm2. Xray diffraction studies confirmed the c-axis suppression of the films at lower processing temperature and at higher annealing temperatures. The decrease in the lattice parameter with vanadium doping was attributed to the smaller ionic radii of vanadium in comparison to Ta. Raman modes of SBT thin films shifted to higher frequencies upon vanadium incorporation at Ta-site. The extra Raman mode observed around ∼860 cm-1, was attributed due to the octahedron stretching vibration in the presence of vanadium at the center of octahedral cage. SBT thin films with 5% V doping exhibit enhanced ferroelectric properties, and the ferroelectric properties were degraded for higher vanadium concentration. The SBT thin films show dielectric constant of about ∼258 with tangential loss of 0.02 at a frequency of 100 kHz. Dielectric constant decreases with increase in V-concentration and attributed to the interdiffusion of bismuth into platinum electrode. The leakage current density of the films was also increased upon vanadium incorporation in SBT thin films.

The ferroelectric field effect was demonstrated in epitaxial perovskite LaVO3/(Ba,Sr)TiO3/(Pb,La)(Zr,Ti)O3/(La,Sr)CoO3 heterostructures grown on (001) SrTiO3 single crystal substrates by pulsed laser deposition. A high degree of c-axis orientation and strong in-plane texture revealed by x-ray diffraction indicated the epitaxial crystallographic relationships between the layers. The heterostructure showed optimal ferroelectric hysteresis with remanent polarizations over 30 μC/cm2. The capacitance measurement as a function of voltage coupled with the measurement of channel resistance revealed the formation of a depletion layer in the semiconducting LaVO3. A ratio of ON- to OFF-state channel resistance of up to 2.5 was observed.

The structure, polarization, and zone-center phonons of bulk tetragonal BaTiO3 under compressive epitaxial stress are calculated using density functional theory within the local density approximation. The polarization, computed using the Berry-phase formalism, increases with increasing tetragonality and is found to be enhanced by nearly 70% for the largest compressive misfit strain considered (-2.28%). The results are expected to be useful for the analysis of coherent epitaxial BaTiO3 thin films and heterostructures grown on perovskite substrates having a smaller lattice constant, such as SrTiO3.

High-temperature oxide melt solution calorimetry was used to measure formation enthalpies for several compositions of perovskites of nominal stoichiometry BaPrO3 and BaCeO3. Samples were synthesized from chemical solution methods followed by calcination at 1100-1300°C. PrO2 was synthesized by oxidation of Pr6O11 in an oxygen flow at 280°C. The samples were characterized by microprobe, thermogravimetric and differential thermal analyses. Cell parameters were refined by the Rietveld method. Barium excess in the samples with respect to ideal stoichiometry was detected. Drop solution enthalpies were measured in a Calvet type twin microcalorimeter, using 3Na2O·4MoO3 solvent at 702°C. Preliminary values of the formation enthalpy of BaPrO3 and BaCeO3 from oxides were -70 ±10 kJ/mol and -51 ±10 kJ/mol, respectively. They fall on the normal trend of energetics versus Goldschmidt tolerance factor and do not show any special stabilization of BaPrO3 relative to other MLnO3 perovskites.

Ab initio total energy calculations, combined with the global parametrization method of perovskite structures, are used to investigate the stability of cubic CaSiO3 against octahedral rotations. We propose an equilibrium crystal structure of orthorhombic Pbnm symmetry. The larger compressibility of the SiO6 octahedra relative to the CaO12 polyhedra is reflected in gradual reduction of the orthorhombic distortion with hydrostatic pressure.

A first-principles method, based on density functional perturbation theory, is presented for computing the leading order tunability of high-dielectric-constant materials.

XANES measurements have demonstrated that Nb polarization in (PbZn1/3Nb2/3O3)0.88-(PbTiO3)0.12 is small compared to that in KNbO3. This finding disproves the view of Nb5+ as invariably ferroelectrically active cation, and demonstrates that its polarizability depends on the Asite cations. Our results do not support conventional view of relaxor behavior in Pb(B'1/3Nb2/3)O3 ferroelectrics as a consequence of disruption of the connectivity of polarizable Nb5+ ions by their non-polarizable B' counterparts. Instead, they speak in favor of scenario in which relaxor behavior is caused by randomness of the local direction of Pb polarization due to chemical disorder on the B-sites of the perovskite structure.

The structure and composition of UMo8O26 synthesized by solid state reaction method have been investigated by High Resolution Transmission Electron Microscopy (HRTEM), Selected Area Electron Diffraction, and EDX microanalysis. The ordering of U vacancies results in considerable enlargement of unit cell parameters: an=6.44 nm, bn=1.45 nm, cn=1.6 nm. It is build up of four layers piled up in c direction. Each following layer is shifted relative to previous one by vector bn/4. Eight hexagonal tunnels in each layer are filled by U atoms, while the eight others are vacant (V). Interaction between U cations and vacancies is driving force for ordering. The variation of stoichiometry can be a reason for appearance of incommensurate modulations in these crystals. It seems plausible that this structure might also exhibit superconductivity at low temperatures.

Although epitaxial II:IV perovskite films like BaTiO3 and Pb(Zr,Ti)O3 have been grown by the hydrothermal method, there are hardly any reports on the growth of I:V perovskite thin films by this low temperature technique. In this study, thin films of NaNbO3, KTaO3 and KNbO3 were grown on (100) oriented single crystal SrTiO3 substrates by the hydrothermal method at 200°C and below. X-ray diffraction (XRD) revealed that the as-synthesized films were epitaxial with one of their original cubic axes normal to the substrate (100) plane. Scanning electron microscopy (SEM) showed that the films formed by an island growth mechanism. KTaO3 was also used as a buffer layer in the growth of KNbO3 films in order to investigate the effect of lattice mismatch on surface morphology and epitaxial quality, as determined by SEM and XRD rocking curve measurements, respectively.

At ambient pressure the orthorhombic perovskites R-orthoferrites (R Ξ Lu, Eu, Y, Pr, and La) exhibit very large optical gaps. These large- gap Mott insulators in which the 3d5 high-spin ferric ions carry large local moments and magnetically order at TN > 600 K, undergo a sluggish structural first-order phase transition in the 30-50 GPa range, with the exception of the LuFeO3 which undergoes an isostructural volume reduction resulting from a high to low-spin crossover. High-pressure methods to 170 GPa using Mossbauer spectroscopy, resistance, and synchrotronbased XRD in diamond anvil cells were applied. Following the quasi-isostructural volume reduction (3-5%) the new phase the magnetic-ordering temperature is drastically reduced, to ∼ 100 K, the direct and super-exchange interactions are drastically weakened, and the charge-transfer gap is substantially reduced. The high-pressure (HP) phases of the La and Pr oxides, at their inception, are composed of high- and low-spin Fe3+ magnetic sublattices, the abundance of the latter increasing with pressure but HP phases of the Eu, Y, and Lu oxides consist solely of low-spin Fe3+. Resistance and Mössbauer studies in La and Pr orthoferrites reveal the onset of a metallic state with moments starting at P > 120 GPa. Based on the magnetic and electrical data of the latter species, a Mott phase diagram was established.

In this paper we review our recent achievements in large scale computer simulations of point defects in advanced perovskite crystals. We have calculated the defect migration energies in the KNbO3 cubic phase using quantum chemical method of the Intermediate Neglect of Differential Overlap (INDO) and classical shell model (SM). The migration energies for the O vacancy obtained by means of these two quite different methods are reasonably close (0.68 eV and 0.79 eV, respectively) and also agree with the only experimental estimate available (ca. 1 eV). Atomic relaxations calculated by these two methods also agree quite well. We used INDO method for a large-scale modeling of the atomic and electronic structure of KNbxTa1-xO3 (KTN) perovskite solid solutions. Results for periodic defect model (large unit cell) of 40 and 320 atoms are compared with 135-atom INDO cluster calculations. Periodic Nb impurities in KTaO3 reveal clear off-center displacement beginning with the smallest calculated concentrations, so does an isolated Nb impurity in a cluster INDO calculation. The magnitude of this displacement is close to the EXAFS observation (0.27 a.u.).

The ABO3 perovskite oxides constitute an important family of technologically important ferroelectrics whose relatively simple chemical and crystallographic structures have contributed significantly to our understanding of ferroelectricity. They readily undergo structural phase transitions involving both polar and non-polar distortions from the ideal cubic lattice. This paper focuses on the mixed perovskite system KTa1-xNbxO3, or KTN, which has turned out to be a model system. While the end members KTaO3 and KNbO3 might be expected to be similar, in reality they exhibit very different properties. Their mixed crystals, which can be grown over the whole composition range, exhibit a rich set of phenomena whose study has added greatly to our current understanding of the phase transitions and dielectric properties of these materials. Included among these phenomena are soft mode response, ferroelectric (FE)-to-relaxor (R) crossover, quantum mechanical suppression of the transition, the appearance of a quantum paraelectric state and relaxational effects associated with dipolar impurities. Each of these phenomena is discussed briefly and illustrated. Some emphasis is on the unique role of pressure in elucidating the physics involved.

La1-xSrxCoO3 is distinct because of its unique spin transitions subjecting the system to variable spin-lattice coupling. Pair density function analysis provided strong evidence for local lattice distortions associated with a Jahn-Teller active mode that is directly correlated to the change in the magnetic/conductive states of this system. The presence of Jahn-Teller distortions also serves as an indication for the intermediate spin configuration. Measurements of the dynamic structure function, S(Q, ω), showed that several phonon peaks fluctuate with carrier density and temperature. Such changes were not seen in an otherwise identical structure, LaAlO3. These phonon peaks are quite unusual and are most likely due to localized modes that appear to be dispersionless.

Results of application of the glow rate technique GRT for analysis of the activation energy of thermostimulated annealing of X-ray created F-type color centers in LiBaF3 crystals, pure and containing hetero-valence oxygen centers are presented. It is shown that depending on impurity composition in crystal two alternative mechanisms are involved in annealing of color centers. It is proposed that either the anion vacancy governed migration of F- centers resulting in recombination with complementary defects, or the thermal delocalization of radiation created fluorine (Fi) interstitials captured by anti-structure defects followed by recombination with all kinds of complementary F- type centers are responsible for the recombination of radiation defects above RT.