Pb₈K_1.68Na_0.32[(Ge_0.65Si_0.35)₂O₇]₃—a Silicogermanate Analogue of the Mineral Nasonite

Abstract

Crystals of new silicogermanate Pb₈K_1.68Na_0.32[(Ge_0.65Si_0.35)₂O₇]₃ were synthesized under hydrothermal conditions in the PbO–SiO₂–GeO₂ system. The new compound is a synthetic variety of the diorthosilicate mineral nasonite Pb₆Ca₄[Si₂O₇]₃Cl₂ related to apatite. The hexagonal unit cell parameters are a = 10.1822(3) Å, c = 13.4088(5) Å, sp. gr. P6₃/m. The crystal structure was determined by direct methods using the SHELX program package and refined to R = 0.0647 based on 975 reflections with I ≥ 1.96σ(I). The new compound is characterized by wide isomorphism. The Ge and Si atoms occupy a tetrahedral site, and the K and Na atoms share a large-cation site. The main difference between the new compound and nasonite is that all Ca sites in the new compound are occupied by Pb, K, and (K,Na) atoms, and the tetrahedral site is mainly occupied by Ge. In nasonite the channels are filled by Cl anions that compensate the positive charge, whereas these channels are empty in the new synthetic variety. The presence of a large lead cation at the site occupied by Ca in nasonite causes structure distortion and changes in the coordination polyhedra. The umbrella-like coordination of the lead atom at the Pb1 site provides additional space for the lone pair of the Pb atom in the empty channel of the crystal structure; however, the crystals are centrosymmetric and, consequently, exhibit no nonlinear-optical properties.