Structural phase transitions in BaPrO3

The crystal structures adopted by BaPrO3 at and above ambient temperature have been examined using a combination of synchrotron X-ray and neutron diffraction. BaPrO3 has been established to undergo a series of phase transitions from P b n m     orthorhombic → I b m m     orthorhombic → R 3 ¯ c     rhombohedral → P m 3 ¯ m cubic. BaPrO3 is the second A2+B4+O3 perovskite found to adopt rhombohedral symmetry in preference to the I4/mcm tetragonal structure. Analysis of the octahedral tilting through the rhombohedral to cubic phase transition indicates that this transformation is continuous and tricritical in nature. The tricritical behaviour of this transition is likely to be a result of the competition between tetragonal and rhombohedral structures to be the preferred phase, with the rhombohedral symmetry adopted by BaPrO3 being stabilised by the unusually large B-site cation.