COMPRESSION OF CADMIUM ORTHOSILICATE, CD2SIO4 - A HIGH-PRESSURE SINGLE-CRYSTAL DIFFRACTION STUDY

The compression of cadmium orthosilicate, Cd2SiO4, was studied by high -pressure single-crystal diffraction carried out in a diamond-anvil ce ll. The equation of state at room temperature (third-order Birch-Murna ghan: V-0 = 695.71(3) Angstrom(3), K-0.298 = 119.2(5) GPa, K' = = 6.17 (4)) was determined from unit-cell volume data to 9.51 GPa. Compressio n of the orthorhombic structure (Fddd, Z = 8) was found to be anisotro pic with relative compressibilities of 2.10:1.00:4.77 for the a, b, an d c axes. The variation of the lattice parameters with pressure is des cribed by: a/a(o) = 0.99989(7) -3.10(3) x x 10(-3) GPa(-1) x P + 8.2(3 ) x 10(-5) GPa(-2) x P-2; b/b(o) = = 1.00007(8)-8.8(9) x 10(-2) GPa(-1 ) x P; c/c(o) = 1.00001(5) -4.16(3) x 10(-3) GPa(-1) x P + 6.5(2) x 10 (-5) GPa(-2) x 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 com pression is controlled by the distortion of the CdO6 and SiO4 polyhedr a due to their unusual interconnection through shared edges. Cation-ca tion repulsion between Cd and Si atoms, which results in short and lon g Cd-O bonds, was found also to be responsible for unusual compression al 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 com pressibility along the c axis. The compressional differences between t he a and b axis result from the compressional anisotropy of the CdO6 o ctahedron.