Orientational ordering of three SiO4 tetrahedra in α´L-Ca1.5Sr0.5SiO4 that satisfies bond-valence requirements and avoids O-O repulsion

Abstract

The crystal structure of α´L-Ca1.55(4)Sr0.45(4)SiO4 was analyzed by single-crystal X-ray diffraction. Refinement with isotropic temperature factors yielded R = 5.4%. The occupancies of the M(11), M(12), M(13), M(21), M(22), and M(23) sites by Sr are 0.40(3), 0.37(3), 0.42(3), 0.11(3), 0.04(2), and 0, respectively. The structure contains SiO4 tetrahedra in three different orientations. The orientational ordering of the Si(n)O4 (n = 1-3) tetrahedra accommodates the M(1n)-O (n = 1-3) bond-valence unbalances caused by the ionic radii mismatch and also avoids increasing the O-O electric repulsions between adjacent Si(n)O4 tetrahedra.