HIGH-TEMPERATURE CRYSTALLOGRAPHIC STUDY OF MG-DOPED TBALO3 AND YALO3

The structures of Mg-doped TbAlO3 and YA1O(3) have been examined by ti me-of-flight powder neutron diffraction at room temperature and at 573 , 773, 1023 and 1223 K. For both compounds, at all temperatures, struc tural and anisotropic thermal parameters were refined in space group P nma (No. 62) by full-profile Rietveld methods to weighted-profile R va lues of less than 0.042. The changes in difference nuclear densities, Delta rho, due to changes in temperature are illustrated by difference -density maps around the atoms. The difference-density maps are constr ucted from F-obs, phased by F-calc, and the high-temperature data are corrected by an isotropic temperature factor obtained locally around t he atom in question. Delta rho p is computed as Delta rho(low temperat ure) Delta rho(high temperature) with the coordinates of the atoms tra nslated to the origin (0, 0, 0). The observed difference-density distr ibutions are described by mainly zeroth- and second-order spherical ha rmonics (quadrupolar functions), the natures of which vary between ato mic sites. The difference-density maps provide a direct picture of the average in space and time of atomic thermal vibrations. These observa tions are less biased by the least-squares method than refined paramet ers. The nonspherical components to the atomic thermal vibrations are similar in Mg-doped TbAlO3 and YAlO3 but the absolute magnitude is sig nificantly higher in TbAlO3.