Thermoelastic Parameters of Aluminate Perovskites from Single Crystal Vibrational Spectroscopy at Ambient and High Pressures
Abstract
Perovskites form a major part of the lower mantle. Raman spectra of perovskites are very weak and all modes for any one orthorhombic perovskite have never been observed. The distribution of modes is important for vibrational modelling of thermoelastic parameters. Effect of varying chemistry and pressure on the spectrum is examined. Room-temperature polarized single-crystal Raman spectra have been measured for both GdAlO3 and YAlO3, which crystallize in the orthorhombic (Pbnm) perovskite structure. Of the 24 possible modes in 4 symmetries, 20 and 17 modes for gadolinium and yttrium aluminates, respectively, were observed. The spectra appear remarkably similar to those of other perovskites regardless of chemistry. Correlation of peaks to structural factors revealed that for the observed modes, lattice parameter plays a dominant role in determining the Raman frequencies while the mass of the A cation plays a minor role. The mass of the B cation plays no obvious role in the Raman spectrum. Comparisons were made to single crystal spectra of 19 other orthorhombic perovskites. Due to their nearly cubic nature, the modes are weak and have not yet all been observed for any one perovskite. Good approximations of the missing modes are given based on the comparisons and symmetry considerations. Raman spectra of yttrium aluminate perovskite have been collected to 24 GPa. These data are used to estimate the effect of pressure on thermoelastic parameters such as heat capacity, entropy, and thermal expansion.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2004
- Bibcode:
- 2004AGUFMMR14A..04C
- Keywords:
-
- 3900 MINERAL PHYSICS;
- 3924 High-pressure behavior;
- 3934 Optical;
- infrared;
- and Raman spectroscopy;
- 3939 Physical thermodynamics;
- 3949 Thermal expansivity