LAUSR

Abstract The triclinic phase of the bismuth orthotantalate (β-BiTaO4) exhibited strong anisotropy of thermal expansion in the temperature range of 25–750 °C. The character of the thermal expansion of β-BiTaO4 changed radically near 200 °C, which was not observed in its isomorphic analog (β-BiNbO4). In the same temperature range the electrical properties also exhibited the local extremum. Crystal structure of β-BiNbO4 was refined at 25, 200, 400 and 600 °C from powder X-ray diffraction data. The role of bismuth lone-electron pair is discussed. β-BiTaO4 has a potential to be used as a thermal expansion material.