Structures and phase transitions of in the temperature range

Abstract

High-resolution neutron powder diffractometry has been used to characterize the structural phase transitions which occur in at 550 and . The change in lattice parameters of the trigonal unit cell as a function of temperature, observed in the range from ambient to , shows that the transition is associated with an abrupt contraction of the unit cell volume. The crystal structure stable in the temperature interval between 550 and is isomorphous with the stillwellite structure of in its polar variant stable up to . The phase transition at to a non-polar variant of the structure mimics the transition in . The structural parameters of these two phases have been refined from data taken at 650, 770 and . The structure of the phase stable at ambient temperature gives rise to extra reflections not given by the stillwellite structure. These can be indexed on a triple cell with and the structure has been determined and refined from a combination of neutron powder data and an ambient temperature single crystal x-ray diffraction study.