Structure and phase transition in BaThO3: A combined neutron and synchrotron X-ray diffraction study

The structure of BaThO3, obtained by solid state synthesis, was refined for the first time by the Rietveld method using a combination of synchrotron X-ray and neutron powder diffraction data. BaThO3 has an orthorhombic structure at room temperature, in space group Pbnm with a alpha = 6.3491(5), b = 6.3796(4) and c = 8.9907(7) angstrom. Heating BaThO3 to above 700 degrees C results in a continuous transition to a second orthorhombic structure, in space group Ibmm, demonstrated by both in situ neutron and synchrotron X-ray powder diffraction measurements. The coefficient of volumetric thermal expansion for BaThO3 is determined to be 1.04 x 10(-5) degrees C-1 from 50 to 625 degrees C (Pbnm phase), and 9.43 x 10(-6) degrees C-1 from 800 to 1000 degrees C (Ibmm phase). BaThO3 was found to decompose upon exposure to atmospheric moisture resulting in the formation of ThO2. The thermal expansion of ThO2, which invariably co-exists with BaThO3, is also described. (C) 2017 Elsevier B.V. All rights reserved.