Compression of cadmium orthosilicate, Cd2SiO4: a high-pressure single-crystal diffraction study

Abstract The compression of cadmium orthosilicate, Cd 2 SiO 4 , was studied by high-pressure single-crystal diffraction carried out in a diamond-anvil cell. The equation of state at room temperature (third-order Birch-Murnaghan: V 0 = 695.71(3) Å 3 , K 0.298 = 119.2(5) GPa, K ′ = = 6.17(4)) was determined from unit-cell volume data to 9.51 GPa. Compression of the orthorhombic structure ( Fddd , Z = 8) was found to be anisotropic with relative compressibilities of 2.10:1.00:4.77 for the a , b , and c axes. The variation of the lattice parameters with pressure is described by: a / a 0 = 0.99989(7) − 3.10(3) × × 10 − GPa −1 × P + 8.2(3) × 10 −5 GPa −2 × P 2 ; b / b 0 = = 1.00007(8) − 8.8(9) × l0 −2 GPa −1 × P ; c / c 0 = 1.00001(5) − 4.16(3) × 10 −3 GPa −1 × P + 6.5(2) × 10 −5 GPa −2 × P 2 . Crystal structures were determined at pressures of 3.01 GPa, 5.44 GPa, and 8.69 GPa. The anisotropy of the unit-cell compression is controlled by the distortion of the CdO 6 and SiO 4 polyhedra due to their unusual interconnection through shared edges. Cation-cation repulsion between Cd and Si atoms, which results in short and long Cd–O bonds, was found also to be responsible for unusual compressional behaviour related to the strength of the bond: the shorter the Cd–O bond the higher its compressibility. The large uniaxial distortion of the silicate tetrahedron is responsible for the relatively higher compressibility along the c axis. The compressional differences between the a and b axis result from the compressional anisotropy of the CdO 6 octahedron.