HIGH-PRESSURE BEHAVIOR OF GDFEO3-TYPE PEROVSKITES

The GdFeO₃-type perovskites, with orthorhombic symmetry (Pbnm), show deviations from the ideal cubic structure (Pm3m) via the tilting and distortion of the octahedral and dodecahedral sites (Sasaki, Prewitt, and Liebermann, 1983). The relative compressibilities of the octahedral (BO6) and dodecahedral (AO12) sites play an important role in the evolution of distortion and tilting of octahedra under high pressure. In order to understand the high-pressure behavior of GdFeO₃-type perovskites, it is necessary to be able to measure atomic-level compressibilities among the bonds.

High-pressure single-crystal X-ray diffraction has proven to be very effective for characterizing atomic-level compression processes and structural changes in condensed systems under high pressure. Recent improvements have reduced uncertainties in structure refinements of crystals at high pressure in a diamond anvil cell to the level approaching that obtained in air. Based on these improvements, the structural changes of several GdFeO₃-type perovskites have been investigated up to pressures of ~8 GPa at 298K. The results show that the compression behavior varies markedly between different perovskites. In CaSnO₃, for example, the SnO6 octahedra display anisotropic compression and the distortion of SnO6 increases with pressure. In the dodecahedral CaO12 site, the 8 shortest Ca-O bond lengths are more compressible than SnO6 site, but the four longer Ca-O separations are less compressible. As a result, the interoctahedral angles Sn-O1-Sn and Sn-O2-Sn decrease with pressure, indicating that octahedral titling increases with increasing pressure. YAlO₃ displays the inverse trend with increasing pressure. The compressibility of YO12 site is strongly anisotropic and consequently the distortion decreases with pressure due to the fact that the longer Y-O bond lengths are more compressible than the shorter Y-O bond lengths. The AlO6 octahedra undergo a nearly isotropic compression and are more compressible than the YO12 site. Pressure induces an increase of the interoctahedral angles Al-O1-Al and Al-O2-Al, indicating the octahedral tilting decreases with increasing pressure, which is equivalent to the movement of oxygen atoms (4c) along the direction of unit cell a-axis. A model is proposed to explain these different trends.