Polyhedral control of the rhombohedral to cubic phase transition in LaAlO3 perovskite

The pressure-induced structural changes of LaAlO 3 perovskite, a rhombohedral perovskite with symmetry, have been investigated up to 8.6 GPa in a diamond-anvil cell at room temperature using single-crystal x-ray diffraction. A fit of a third-order Birch–Murnaghan equation of state to the P – V data yields values of K T 0 = 177(4) GPa and . The evolution of the structure with pressure shows that compression of the LaO 12 site is strongly anisotropic, with the three longest La–O bonds being more compressible than the other nine shorter La–O bond lengths. Consequently the distortion of the LaO 12 site decreases with increasing pressure. The rotation angle around the threefold axis of the AlO 6 octahedra is totally determined by the relative compressibilities of the La and Al sites, and decreases significantly with pressure. This variation leads to the rhombohedral to cubic phase transition. A new model, introduced to predict the high-pressure behaviour of the GdFeO 3 ( Pbnm )-type perovskites, is extended to rhombohedral perovskites in this paper. The model correctly predicts that the AlO 6 site is more compressible than the LaO 12 site in LaAlO 3 , and that the rhombohedral to cubic phase transition is controlled by the relative compressibilities of the two cation sites.