“Unusual” phase transitions in CeAlO3

High-resolution time-of-flight neutron powder diffraction was carried out to investigate the crystal structures of CeAlO3 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 (a0a0c−)). The tetragonal structure persists down to 4.2 K. However, above room temperature CeAlO3 undergoes three phase transitions: first to the orthorhombic Imma structure (tilt system (a0b−b−)) at, e.g., 373 K, then to the rhombohedral R3¯c structure (tilt system (a−a−a−)) at, e.g., 473 K, and finally, to the primitive cubic Pm3¯m structure which exists above 1373 K. The sequence of phases, Pm3¯m→R3¯c→Imma→I4/mcm, which occurs in CeAlO3 is a rare one in oxide perovskites.

Temperature dependence of the octahedral tilting angles in CeAlO3. The continuous line in rhombohedral phase region is the fit to the expression: ϕ=A(Tc−T)β with fitted values of Tc=1371 K, β=0.35 and A=0.52. Note that the lines that separate the I4/mcm and Imma phases and the Imma   and R3¯c phases are not experimentally determined transition temperatures but for visualisation purpose.Figure optionsDownload as PowerPoint slide