Crystal structure and compressibility of a two-layer polytype of pseudowollastonite (CaSiO3)

The crystal structure of a synthetic two-layer polytype of CaSiO 3 pseudowollastonite was determined using single-crystal X-ray diffraction data. It is monoclinic with space group C2/c and unitcell parameters a = 11.8322(6), b = 6.8624(8), c = 10.5297(5) A, β = 111.245(8)°, and V = 796.9(1) A 3 . The material is isostructural with two-layer SrSiO 3 and SrGeO3 and has basic structural features similar to those found in four-layer CaSiO 3 pseudowollastonite, except for the stacking sequence of layers of CaO6 octahedra and Si3O9 ternary rings. The compressibility of the structure was measured up to 9.94 GPa and no phase transition was observed. With increasing pressure, all unit-cell parameters decrease nonlinearly with a positive curvature. The axial compression ratios at room pressure are βa:βb:βc = 2.25:2.36:1.00 and the bulk modulus, KT, is 86(1) GPa with K' = ∂KT/ ∂P = 3.8(4).