In situ powder diffraction study of titanite (CaTiOSiO4) at high pressure and high temperature

A set of powder diffraction data was collected for synthetic titanite (CaTi OSiO4) at simultaneously high pressures and high temperatures in a P-T fiel d between 275 K to 650 K and room pressure to 4.9 GPa, respectively. With t hese data it was possible to relate the A2/n high-pressure phase at >3.5 GP a (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 dat a also allowed the extraction of P-V-T equations of state with the followin g parameters: far P2(1)/a: K-298.0 = 113.4(3), (partial derivative K-T,K-0/ partial derivative T)(P) = -0.061(3) GPa/K, V-298.0 = 369.04(2) Angstrom(3) , alpha(0) = 2.07(5)/10(5) K. For A2/a: K-298.0 = 135.2(2), (partial deriva tive K-T,K-0/partial derivative T)(P) = -0.073(1) GPa/K, V-298.0 = 367.12(2 ) Angstrom(3), alpha(0) = 2.8(2) / 10(5) K, where K is bulk modulus, V is v olume, and ex is thermal expansivity. A structural analysis based on Rietve ld refinements revealed that the polymerized CaO7 polyhedra dominantly affe ct the structural response to changing pressure and temperature. The TiO6 o ctahedra rotate almost rigidly in response to the compression of the CaO7 p olyhedra with which they share edges. The SiO4 tetrahedra show a strong ang ular distortion with only little change in bond lengths.