Temperature dependence of crystal structure of CaGeO3 high-pressure perovskite phase and experimental determination of its Debye temperatures studied by low-and hightemperature single-crystal X-ray diffraction

Single-crystal X-ray diffraction study of CaGeO3 perovskite has been conducted over the temperature range of 98 to 1048 K. The crystal begins to deteriorate at a temperature above about 900 K and completely amorphizes by 980 K. The diffraction-intensity distribution and the structure refinements indicate that the Pbnm structure is kept until the occurrence of amorphization. The obtained unit-cell parameters, unit-cell volumes, bond lengths, and displacement parameters increase monotonously with increasing temperature. Thus, no evidence for the existence of the Cmcm high-temperature phase, previously suggested above 520 K, is observed. The Ge-O bond lengths show much smaller thermal expansions than the Ca-O bond lengths; the former ranges between 0.42(2) x 10(-5) K-1 and 0.57(2) x 10(-5) K-1 and the latter between 1.58(4) x 10(-5) K-1 and 3.96(6) x 10(-5) K-1. The Debye temperatures and static disorder components for each constituent atom were determined by applying the Debye model to the temperature dependence of mean square displacements (MSDs) of the atoms. Consequently, no significant static disorder components can be detected in each atom. The Debye temperatures averaged over all directions, obtained from the Debye model fitting to Ue, yield the harmonic one particle potential coefficients of 4.76(2) eV angstrom(-2) for Ca, 11.0(1) eV angstrom(-2) for Ge, 5.02(2) eV angstrom(-2) for O1, and 5.33(5) eV angstrom(-2) for O2. These values become larger in order of Ca < O1 More