In situ powder diffraction study of titanite (CaTiOSiO (sub 4) ) at high pressure and high temperature

A set of powder diffraction data was collected for synthetic titanite (CaTiOSiO (sub 4) ) at simultaneously high pressures and high temperatures in a P-T field between 275 K to 650 K and room pressure to 4.9 GPa, respectively. With these data it was possible to relate the A2/a high-pressure phase at >3.5 GPa (room temperature) to the A2/a high temperature phase observed above 825 K (room pressure). The slope of the phase transition is -180 K/GPa. The data also allowed the extraction of P-V-T equations of state with the following parameters: for P2 (sub 1) /a: K (sub 298,0) = 113.4(3), (theta K (sub T,0) /theta T) (sub P) = -0.061(3) GPa/K, V (sub 298,0) = 369.04(2) Aa (super 3) , alpha (sub 0) = 2.07(5)/10 (super 5) K. For A2/a: K (sub 298,0) = 135.2(2), (theta K (sub T,0) /theta T) (sub P) = -0.073(1) GPa/K, V (sub 298,0) = 367.12(2) Aa (super 3) , alpha (sub 0) = 2.8(2)/10 (super 5) K, where K is bulk modulus, V is volume, and alpha is thermal expansivity. A structural analysis based on Rietveld refinements revealed that the polymerized CaO (sub 7) polyhedra dominantly affect the structural response to changing pressure and temperature. The TiO (sub 6) octahedra rotate almost rigidly in response to the compression of the CaO (sub 7) polyhedra with which they share edges. The SiO (sub 4) tetrahedra show a strong angular distortion with only little change in bond lengths.