W0159

Perovskites at High Pressure. Nancy Ross, Jing Zhao, Ross Angel, Crystallography Laboratory, Dept. Geological Sciences, Virginia Tech, Blacksburg, VA 24060 USA.

Perovskites are probably the single most technologically-important group of compounds because of their electrical and elastic properties. Despite their simple structure of corner-linked cation-anion octahedra with void space occupied by large charge-balancing cations, a predictive model for their physical properties and structural phase transitions has yet to be developed. For example, different theories predict that these perovskites should become more or less with increasing hydrostatic pressure.

We have therefore determined the changes in distortion of a number of orthorhombic perovskites with Pbnm symmetry by single-crystal X-ray diffraction to pressures of ~9 GPa. Unit-cell parameters were determined on a Huber four-circle diffractometer customised for precise unit-cell parameter determination. Intensity data was collected on an Xcalibur diffractometer equipped with both a point detector and a CCD detector. High pressures were generated with transmission-geometry diamond-anvil cells of the ETH-design with 4:1 methanol:ethanol mixture as hydrostatic pressure medium. Data integration and reduction was performed with the WinIntegrStp and Absorb programs (www.crystal.vt.edu).

We find that some orthorhombic perovskites such as CaSnO₃ exhibit a small but significant increase in distrortion with increasing pressure while CaTiO₃ shows no significant change. By contrast the tilt angles of the AlO₆ octahedra in YAlO₃, for example, decrease with increasing pressure corresponding to a decrease in orthorhombic distortion. Clearly these results demonstrate that even the compressional behaviour of orthorhombic perovskites is determined by a subtle balance of competing forces.