Octahedral Volume Research Articles

Crystal Chemistry of Cation Order–Disorder in Pseudobrookite-Type MgTi2O5

Effects of cation order–disorder on the pseudobrookite-type MgTi2O5structure have been studied with single-crystal X-ray diffraction on five synthetic crystals (labeled P600, P700, P800, P1000, and P1400) annealed at 600, 700, 800, 1000, and 1400°C, respectively. The disorder parameters (X=Ti in theM1 site) determined for five samples range from 0.070(5) to 0.485(5). Unit-cell dimensionsaandcincrease withX, whereasbdecreases. The decrease inbwith increasing cation disorder results primarily from the marked reduction in the O3–O3 distance within theM1 octahedron, whereas the variation ofaandcis controlled by the size of theM2 octahedron. From P600 to P1400, the meanM1–O length shortens by 2.35%; in contrast, the meanM2–O distance lengthens by 1.16%. As a consequence, theM1 andM2 sites have a nearly identical octahedral volume in P1400. With increasing cation disorder, the degree of distortion of theM1 octahedron is reduced significantly, while that ofM2 remains essentially unchanged. Cation disorder is also accompanied by straightening of the bands consisting of theM1 andM2 octahedra. There is a close correlation between atomic isotropic displacement factors (Biso) and cation order–disorder:Bisofactors of all atoms in more disordered samples are larger than those in less disordered ones.

An experimental and theoretical investigation of phonons and lattice instabilities in metastable decompressed perovskite

We report detailed Raman and IR spectroscopic measurements for the decompressed high-pressure perovskite phase of . The appearance of a first-order Raman spectrum and slight splittings in the infrared bands suggest that the symmetry of the recovered metastable perovskite phase is lowered from Pm3m. This interpretation is fully supported by first-principles LDA calculations using the LAPW method, which indicate a small tetragonal distortion. The static lattice energy is lowered by 3.3 meV (per formula unit) by allowing rotational relaxation of the octahedra. The calculations permit a reliable assignment of the zone centre phonon modes of perovskite. The calculated pressure dependence of the ferroic IR-active modes is in excellent agreement with our measured data and reveals an incipient soft-mode behaviour in the tension regime. Further calculations of the unit as a function of octahedral volume reveal instabilities to local off-centre displacements as the octahedra are expanded. This provides a detailed understanding of the large dielectric constant for this non-transition-metal-containing decompressed perovskite phase.

Pressure and Temperature Effects on Octahedral-Tetrahedral Equilibria in Pyridine Solutions of Some Cobalt(II) Halides. II

Abstract Temperature and pressure effects on the configuration equilibrium of cobalt(II) iodide in pyridine solution have been studied spectrophotometrically. High temperature favors the tetrahedral species but high pressure favors the octahedral species, the heat and volume changes being 65.3 kJ mol−1 and −6 cm3 mol−1, respectively. Comparisons among three kinds of halide are also made.