High-pressure crystal chemistry and phase transition of RbTi2(PO4)3

Abstract

Unit-cell parameters of RbTi2(PO4)3, a framework structure (space group R3c) with alternating corner-linked Ti octahedra and P tetrahedra that define six-coordinated alkali cation sites, were determined at eight pressures from 0.0 to 6.2 GPa, and crystal structure refinements were obtained at 0.0 and 6.2 GPa. The structure remains dimensionally rhombohedral at all pressures, with the a axis more than four times more compressible than c. Compressibilities of both axes increase abruptly above about 1.7 GPa; the bulk modulus decreases from 104(3) GPa (K'=4) to approximately 60 GPa. Furthermore, the appearance of reflections that violate the c glide-plane symmetry above 1.7 GPa signals the occurrence of a reversible phase transition involving distortion of the framework. From the pressure dependence of the intensities of critical X-ray reflections, the phase transition is revealed as being continuous and close to second order in character. Structure refinements of the 6.2 GPa data reveal R3 symmetry, an acentric space group, for the high-pressure form. Average Ti-O-P angles decrease from 151.5 to 140 degrees between room pressure and 6.2 GPa. These changes result in a coordination change for Rb from one centric six-coordinated site (Rb-O=2.855 AA) at room pressure, to two symmetrically distinct acentric 12-coordinated sites (mean Rb-O approximately=3.01 AA) at 6.2 GPa.