New isoformula potassium borosilicates with different zeolite frameworks: Monoclinic centrosymmetric boroleucite K(BSi2)O6 and chiral KBSi2O6

Abstract

The structures of new isoformula potassium borosilicates, obtained under hydrothermal conditions, namely, the monoclinic modification of the cubic boroleucite K(BSi2)O6 (space group P21/a), which was previously unknown, and chiral KBSi2O6 (space group P212121)) are studied. A framework is formed both in the monoclinic and orthorhombic new structure in which each two tetrahedra share a vertex, as is typical of silicate frameworks. Since the two compounds have the same chemical formula, they could be spoken of as polymorphic modifications, but they are essentially different in structure: in the orthorhombic phase, the B and Si atoms are distributed over the tetrahedra in an ordered fashion, whereas B together with Si occupy statistically all tetrahedra in the monoclinic modification of boroleucite. The frameworks of both compounds are similar, although the framework is slightly denser in the orthorhombic borosilicate. Large rings consisting of six, seven, or eight tetrahedra determine the zeolite type of the frameworks, whose voids accommodate K atoms.