High-resolution neutron powder diffraction study on the phase transitions in BaPbO 3

Abstract

Phase transitions that occurred in perovskite BaPbO 3 have been investigated using high-resolution time-of-flight neutron powder diffraction. The structure at room temperature is orthorhombic (space group Imma), which is derived from the cubic aristotype by tilting the PbO 6 octahedra around the two-fold axis (tilt system a 0 b − b −). The orthorhombic structure shows anisotropic line broadening attributed to the presence of micro twins. At above about 573 K, BaPbO 3 undergoes a discontinuous phase transition to a tetragonal structure (space group I4 /mcm) with the tilting of the PbO 6 octahedra being about the four-fold axis of the cubic aristotype (tilt system a 0 a 0 c −). With further increasing the temperature, BaPbO 3 experiences a continuous phase transition to a simple cubic structure (space group Pm3¯ m) at above about 673 K. The later phase transition is characterised by a critical exponent of β=0.36, depicted by the three-dimensional Heisenberg universality class. The earlier reported Imma→ I2/ m phase transition above room temperature has not been observed.