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

摘要

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 a(0)b(-)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 a(0)a(0)c(-)). With further increasing the temperature, BaPbO3 experiences a continuous phase transition to a simple cubic structure (space group Pm3m) at above about 673 K. The later phase transition is characterised by a critical exponent of beta = 0.36, depicted by the three-dimensional Heisenberg universality class. The earlier reported lmma-12/m phase transition above room temperature has not been observed. (C) 2007 Elsevier Inc. All rights reserved.