Orientational ordering of three SiO₄ tetrahedra in α´_L-Ca_1.5Sr_0.5SiO₄ that satisfies bond-valence requirements and avoids O-O repulsion

Abstract

The crystal structure of α´_L-Ca_1.55(4)Sr_0.45(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.