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

Abstract

The crystal structure of a synthetic two-layer polytype of CaSiO3 pseudowollastonite was determined using single-crystal X-ray diffraction data. It is monoclinic with space group C2/c and unit-cell parameters a = 11.8322(6), b = 6.8624(8), c = 10.5297(5) Å, β = 111.245(8)°, and V = 796.9(1) Å3. The material is isostructural with two-layer SrSiO3 and SrGeO3 and has basic structural features similar to those found in four-layer CaSiO3 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 β(a):β(b):β(c) = 2.25:2.36:1.00 and the bulk modulus, K(T), is 86(1) GPa with K1 = partial differentialK(T)/partial differentialP = 3.8(4).