Single-Crystal Synthesis and Structures of Rb-Boroleucite Rb(BSi2)O6 and Boropollucite Cs(BSi2)O6 at 293 and 120 K

Abstract

The structures of Rb-boroleucite Rb_1.0(B_0.333Si_0.667)₃O₆ and boropollucite Cs_0.87(B_0.290Si_0.710)₃O₆ were studied using single crystals, which were prepared under hydrothermal conditions. In the zeolite-like framework structures, Rb and Cs atoms are located in large window cavities formed by four, six, and eight tetrahedra. The B and Si atom randomly occupy general positions. Cubic space groups of both phases (acentric sp. gr. I\(\bar {4}\)3d for Rb and centrosymmetric holohedral sp. gr. Ia\(\bar {3}\)d for Cs) remain unchanged at room temperature and down to 120 K. Rubidium boroleucite has a stoichiometric formula. Boropollucite is characterized by a deviation from the stoichiometric composition, which is manifested in the predominance of Si atoms in tetrahedra and the defect occupation of the Cs site. The size of alkali metal cations plays a crucial role in the existence of a particular modification, which is consistent with earlier results.