New borate LaB5O8(OH)2 · 1.5H2O with a {4[3T + Δ]∞∞ + Δ}∞∞∞ complex framework. Its place in the structural system based on symmetry and topology analysis in terms of the OD theory

Abstract

Crystals of a new hydrous rare-earth borate LaB5O8(OH)2 · 1.5H2O(1), space group P21/n, are prepared hydrothermally. The crystal structure is solved using the heavy-atom method without a priori knowledge of the chemical formula. The complex anionic radical is built of pleated layers, which are structurally and topologically related to layered pentaborates but do not have the fifth boron polyhedron in the building block; these layers are linked into the {4[3T + Δ]∞∞ + [Δ]}∞∞∞ framework through BO3 triangles. Lanthanum atoms reside in valleys of the layers; framework voids are occupied by water molecules. The framework of the new borate in its topology and symmetry is an analogue of the centrosymmetric variety of lead borate (hilgardite): in both, the layers are apolar and are type I in terms of the OD theory.