High-temperature phase transitions in SrHfO₃

The crystal structure of SrHfO₃ has been studied at high temperatures using powder neutron diffraction and the Rietveld method. From 300 K to approximately 670 K the structure of SrHfO₃ is orthorhombic (Pnma). By 870 K the material adopts a second orthorhombic structure (Cmcm). The material then undergoes a further phase transition and is tetragonal (I4/mcm) from 1000 to 1353 K. At higher temperatures (> 1360 K) the structure is the ideal cubic perovskite (Pm3⁻m). The angle of rotation of the oxygen octahedron in the tetragonal phase is taken as the order parameter and its temperature variation is consistent with a second order phase transition.