Patrick M. Woodward

Octahedral Tilting in Perovskites. I. Geometrical Considerations

The 23 Glazer tilt systems describing octahedral tilting in perovskites have been investigated. It is shown that in tilt systems a+a+a−, a+b+b−, a+a+c−, a+b+c−, a0b+b− and a0b+c− it is not possible to link together a three-dimensional network of perfectly rigid octahedra. In these tilt systems small distortions of the octahedra must occur. The magnitude of the distortions in the a+a+a− and a0b+b− tilt systems are estimated. A table of predicted space groups for ordered perovskites, A2MM′O6, for all 23 tilt systems is also given.

Prediction of the crystal structures of perovskites using the software program SPuDS

The software program SPuDS has been developed to predict the crystal structures of perovskites, including those distorted by tilting of the octahedra. The user inputs the composition and SPuDS calculates the optimal structure in ten different Glazer tilt systems. This is performed by distorting the structure to minimize the global instability index, while maintaining rigid octahedra. The location of the A-site cation is chosen so as to maximize the symmetry of its coordination environment. In its current form SPuDS can handle up to four different A-site cations in the same structure, but only one octahedral ion. Structures predicted by SPuDS are compared with a number of previously determined structures to illustrate the accuracy of this approach. SPuDS is also used to examine the prospects for synthesizing new compounds in tilt systems with multiple A-site coordination geometries (a(+)a(+)a(+), a(0)b(+)b(+), a(0)b(-)c(+)).

Octahedral Tilting in Perovskites. II. Structure Stabilizing Forces

The 23 Glazer tilt systems describing octahedral tilting in perovskites have been investigated. The various tilt systems have been compared in terms of their A-cation coordination and it is shown that those tilt systems in which all the A-cation sites remain crystallographically equivalent are strongly favored, when all the A sites are occupied by the same ion. Calculations based on both ionic and covalent models have been performed to compare the seven equivalent A-site tilt systems. Both methods predict that when the tilt angles become large, the orthorhombic a+b−b− tilt system will result in the lowest energy structure. This tilt system gives the lowest energy structure because it maximizes the number of short A—O interactions. The rhombohedral a−a−a− tilt system gives a structure with a slightly lower Madelung energy, but increased ion–ion repulsions destabilize this structure as the tilt angles increase. Consequently, it is stabilized by highly charged A cations and small to moderate tilt angles. The ideal cubic a0a0a0 tilt system is only observed when stabilized by oversized A cations and/or M—O π-bonding. Tilt systems with nonequivalent A-site environments are observed when at least two A cations with different sizes and/or bonding preferences are present. In these compounds the ratio of large-to-small cations dictates the most stable tilt system.

Investigations of the electronic structure of d0 transition metal oxides belonging to the perovskite family

Abstract Computational and experimental studies using linear muffin tin orbital methods and UV-visible diffuse reflectance spectroscopy, respectively, were performed to quantitatively probe the relationships between composition, crystal structure and the electronic structure of oxides containing octahedrally coordinated d0 transition metal ions. The ions investigated in this study (Ti4+, Nb5+, Ta5+, Mo6+, and W6+) were examined primarily in perovskite and perovskite-related structures. In these compounds the top of the valence band is primarily oxygen 2p non-bonding in character, while the conduction band arises from the π ∗ interaction between the transition metal t2g orbitals and oxygen. For isostructural compounds the band gap increases as the effective electronegativity of the transition metal ion decreases. The effective electronegativity decreases in the following order: Mo6+>W6+>Nb5+∼Ti4+>Ta5+. The band gap is also sensitive to changes in the conduction band width, which is maximized for structures possessing linear M–O–M bonds, such as the cubic perovskite structure. As this bond angle decreases (e.g., via octahedral tilting distortions) the conduction band narrows and the band gap increases. Decreasing the dimensionality from 3-D (e.g., the cubic perovskite structure) to 2-D (e.g., the K2NiF4 structure) does not significantly alter the band gap, whereas completely isolating the MO6 octahedra (e.g., the ordered double perovskite structure) narrows the conduction band width dramatically and leads to a significant increase in the band gap. Inductive effects due to the presence of electropositive “spectator” cations (alkali, alkaline earth, and rare-earth cations) tend to be small and can generally be neglected.

Cation ordering in perovskites

Although both A- and B-site cations have the same simple cubic topology in the perovskite structure they typically adopt different patterns of chemical order. As a general rule B-site cations order more readily than A-site cations. When cation ordering does occur, rock salt ordering of B/B′ cations is favored in A2BB′X6 perovskites, whereas layered ordering of A/A′ cations is favored in AA′B2X6 and AA′BB′X6 perovskites. The unexpected tendency for A-site cations to order into layers stems from the bond strains that would result at the anion site if A and A′ cations of different size were to order with a rock salt arrangement. The bonding instabilities that are created by layered ordering are generally offset either by anion vacancies or second order Jahn–Teller distortions of a B-site cation. Novel types of A-site cation ordering can be stabilized by a+a+a+ or a+a+c− tilting of the octahedra.

Structure refinement of triclinic tungsten trioxide

Abstract The thermodynamically stable form of WO 3 at room temperature is triclinic. The structure of this form of WO 3 was for the first time refined from neutron diffraction data. The data were obtained on a new high resolution powder diffractometer. The structure obtained appears to be somewhat more accurate than that obtained from single crystal X-ray diffraction data. Rietveld approach was used to refine the neutron powder diffraction data. Results using the software packages GSAS, Rietan and PROFIL are compared. Using GSAS, an R wp of 7.97 and an R p of 5.87 were obtained. Neutron diffraction data were also collected and analysed on a sample which was a mixture of the monoclinic and triclinic forms of WO 3 ; Rietveld refinements of this data were stable and yielded reasonable results.

Characterization of electronic structure and defect states of thin epitaxial BiFeO3 films by UV-visible absorption and cathodoluminescence spectroscopies

UV-visible absorption and cathodoluminescence spectra of phase-pure epitaxial BiFeO3 thin films grown on SrTiO3(001) substrates by ultrahigh vacuum sputtering reveal a bandgap of 2.69–2.73eV for highly strained ∼70nm thick BiFeO3 films. This bandgap value agrees with theoretical calculations and recent experimental results of epitaxial BiFeO3 films, demonstrating only minimal bandgap change with lattice distortion. Both absorption and cathodoluminescence spectra show defect transitions at 2.20 and 2.45eV, of which the latter can be attributed to defect states due to oxygen vacancies.

Epitaxial thin films of the giant-dielectric-constant material CaCu3Ti4O12 grown by pulsed-laser deposition

Pulsed-laser deposition has been used to grow epitaxial thin films of the giant-dielectric-constant material CaCu3Ti4O12 on LaAlO3 and SrTiO3 substrates with or without various conducting buffer layers. The latter include YBa2Cu3O7, La1.85Sr0.15CuO4+δ, and LaNiO3. Above 100–150 K, the thin films have a temperature independent dielectric constant as do crystals. The value of the dielectric constant is of the order of 1500 over a wide temperature region, potentially making it a good candidate for many applications. The frequency dependence of its dielectric properties below 100–150 K indicates an activated relaxation process.

Order-disorder in A2M3+M5+O6 perovskites

Using x-ray and neutron diffraction data, the degree of order of the octahedral site cations has been determined for the perovskites Sr 2 AlNbO 6 and Sr 2 AlTaO 6 , which have been prepared by several different methods and annealed at temperatures up to 1690 °C. The degree of order generally increases with increasing synthesis temperature. The amount of cation ordering is, therefore, primarily controlled by kinetic processes and not by thermodynamic equilibrium considerations. Increased order obtained with increased heating time confirms this general kinetic limitation on the degree of order. However, annealing Sr 2 AlNbO 6 in the highest temperature region resulted in some decrease in order, presumably due to thermodynamic considerations. The cubic edge of both compounds decreases significantly with increasing order. Ordered domains are separated by antiphase boundaries which occur in high concentrations. The cubic cell edge within the ordered domains is significantly smaller than the overall cell edge when the concentration of antiphase boundaries is high. The antiphase boundaries cause significant lattice strain which generally decreases as the concentration of antiphase boundaries decreases. Results on other A 2 M 3+ M 5+ O 6 systems are briefly presented.

PRESSURE-INDUCED INTERMEDIATE-TO-LOW SPIN STATE TRANSITION IN LACOO3

Synchrotron x-ray powder-diffraction experiments reveal that the transition from a magnetic intermediate spin (IS) state t 5 2ge 1 g to a nonmagnetic low-spin ground state t 6 2g in LaCoO 3 normally observed when cooling manifests itself under pressure by an anomalously low bulk modulus of 150(2) GPa and an initially very large Co-O bond compressibility of 4.8×10 - 3 GPa - 1 which levels off near 4 GPa. The continuous depopulation of the IS state is driven by an increased crystal-field splitting resulting in an effective reduction of the size of the Co 3 + cation.