Crystal structure of novel high-pressure perovskite K2/3Th1/3TiO3, a possible host for Th in the upper mantle

The high-pressure perovskite K2/3Th1/3TiO3 was synthesized at P = 6 GPa and T = 1200 degreesC. This compound does not form at ambient pressures. as bo th solid-state reaction and synthesis from the melt yield a mixture of thor ianite (ThO2) and jeppeite (K2Ti6O13). K2/3Th1/3TiO3 is a partially ordered derivative of the ideal perovskite structure, which crystallizes with tetr agonal symmetry, in space group P4/mmm. a = 3.9007(2), c = 7.8099(7) Angstr om, V = 118.83(2) Angstrom (3), Z = 2. The structure of this compound was r efined by the Rietveld method from the X-ray diffraction powder data. The d egree of disorder calculated from the refined cation occupancies of the la and lb sites is 58%. The K1+ cations preferentially enter the la site, wher eas most Th4+ is accommodated in the comparatively smaller 1b site (polyhed ral volumes are 53 and 46 Angstrom (3), respectively). In response to this two-dimensional (planar) ordering, the Ti4+ cations are displaced by about 0.1 Angstrom toward the planes populated by the lower-charged cations. K2/3 Th1/3TiO3 and related structures may be a viable repository for Th in Ti-ri ch alkali metasomatites in the lithospheric upper mantle.