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.

Temperature dependence of the octahedral tilting angles in BaPbO3. The continuous line in tetragonal phase region is the fit to the expression: ϕ=A(Tc−T)β with the fitted values of Tc=658(1) K, β=0.36(2) and A=1.1(1). The shaded area indicates the possible two-phase region.Figure optionsDownload as PowerPoint slide