“Unusual” phase transitions in CeAlO 3

Abstract

High-resolution time-of-flight neutron powder diffraction was carried out to investigate the crystal structures of CeAlO 3 over a wide temperature range between 4.2 and 1423 K. Confirming the recent result of X-ray powder diffraction, the room temperature structure is tetragonal with the space group I4/ mcm (tilt system ( a 0 a 0 c −)). The tetragonal structure persists down to 4.2 K. However, above room temperature CeAlO 3 undergoes three phase transitions: first to the orthorhombic Imma structure (tilt system ( a 0 b − b −)) at, e.g., 373 K, then to the rhombohedral R 3 ¯ c structure (tilt system ( a − a − a −)) at, e.g., 473 K, and finally, to the primitive cubic Pm 3 ¯ m structure which exists above 1373 K. The sequence of phases, Pm 3 ¯ m → R 3 ¯ c → Imma → I 4 / mcm , which occurs in CeAlO 3 is a rare one in oxide perovskites.