Polar Germanosilicate K1.46Pb1.54Ca[(Ge0.23Si0.77)3O9](OH)0.54 ⋅ 0.46H2O with a Wollastonite Chain and Wide Isomorphism

Abstract

Crystals of new polar germanosilicate with a general formula K_1.46Pb_1.54Ca [(Ge_0.23Si_0.77)₃O₉](OH)_0.54 · 0.46H₂O were synthesized under hydrothermal conditions. The orthorhombic unit cell parameters are a = 14.0879(3) Å, b = 5.9202(1) Å, c = 13.0465(3) Å, sp. gr. Pmn2₁. The crystal structure was determined by direct methods using the SHELX program package and refined to R = 0.0388 based on 3643 reflections with I ≥ 1.96σ(I). The new compound is characterized by a wide isomorphism in the sites occupied by Ge and Si, K and Pb, (OH) and H₂O, only three of the ten cation sites in the structure being occupied by Pb, K, and Ca atoms without impurities. The anionic radical comprises wollastonite three-membered tetrahedral chains linked by Pb1 octahedra to form a mixed framework known for K,Sc silicate and the high-temperature modification of K,Ho silicate. The umbrella-like coordination of the lead atom in the site Pb2(Pb_0.64K_0.36) (the fourth coordination bond is formed with a water molecule or an OH group in the channel of the structure) reflects the fact that the lone pair of Pb atoms is inactive. Although the crystals are acentric, they apparently do not have good nonlinear-optical properties.