The bismuth orthotantalate with high anisotropic thermal expansion

The triclinic phase of the bismuth orthotantalate (beta-BiTao(4)) exhibited strong anisotropy of thermal expansion in the temperature range of 25-750 degrees C. The character of the thermal expansion of beta-BiTao(4) changed radically near 200 degrees C, which was not observed in its isomorphic analog (beta-BiNbo(4)). In the same temperature range the electrical properties also exhibited the local extremum. Crystal structure of beta-BiNbo(4) was refined at 25, 200, 400 and 600 degrees C from powder X-ray diffraction data. The role of bismuth lone-electron pair is discussed. beta-BiTaO4 has a potential to be used as a thermal expansion material. (C) 2019 Acta Materialia Inc Published by Elsevier Ltd. All rights reserved.