High-resolution neutron powder diffraction study on the phase transitions in BaPbO3

Phase transitions that occurred in perovskite BaPbO3 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 PbO6 octahedra around the two-fold axis (tilt system a0b−b−). The orthorhombic structure shows anisotropic line broadening attributed to the presence of micro twins. At above about 573 K, BaPbO3 undergoes a discontinuous phase transition to a tetragonal structure (space group I4/mcm) with the tilting of the PbO6 octahedra being about the four-fold axis of the cubic aristotype (tilt system a0a0c−). With further increasing the temperature, BaPbO3 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.