STRUCTURES AND PHASE-TRANSITIONS OF PRBGEO5 IN THE TEMPERATURE-RANGE 20-800-DEGREES-C

High-resolution neutron powder diffractometry has been used to charact erize the structural phase transitions which occur in PrBGeO5 at 550 a nd 750 degrees C. The change in lattice parameters of the trigonal uni t cell as a function of temperature, observed in the range from ambien t to 800 degrees C, shows that the 550 degrees C transition is associa ted with an abrupt contraction of the unit cell volume. The crystal st ructure stable in the temperature interval between 550 and 750 degrees C is isomorphous with the stillwellite structure of LaBGeO5 in its po lar variant stable up to 530 degrees C. The phase transition at 750 de grees C to a non-polar variant of the structure mimics the 530 degrees C transition in LaBGeO5. The structural parameters of these two PrBGe O5 phases have been refined from data taken at 650, 770 and 800 degree s C. The structure of the PrBGeO5 phase stable at ambient temperature gives rise to extra reflections not given by the stillwellite structur e. These can be indexed on a triple cell with a' approximate to root 3 a, c' approximate to c and the structure has been determined and refin ed from a combination of neutron powder data and an ambient temperatur e single crystal x-ray diffraction study.