Na3Tb3[Si6O18] · H2O, a synthetic analogue of microporous mineral gerenite

Abstract

Crystals of a new silicate, Na₃Tb₃[Si₆O₁₈] · H₂O, space group \(P\bar 1\), are obtained under hydrothermal conditions. The formula of the compound is determined in the course of structure solution. The silicate is a synthetic analogue of the gerenite mineral (Ca_1.21Na_0.57)(Y_2.24Dy_0.68)Si₆O₁₈ · 2H₂O, whose structure contains six-membered rings formed by SiO₄ tetrahedra. The [Si₆O₁₈] rings are connected by TbO₆ octahedra into a mixed microporous framework with voids filled by Na atoms and water molecules. The new silicate differs from gerenite by the occupation of the Ca position by Na atoms and population of the pores sandwiched between six-membered rings. By virtue of conditions of hydrothermal synthesis in the absence of Ca and excess of Na in the system, an additional Na position appears in the void. It is populated statistically, and in gerenite it was occupied by water molecules only. In the new structure, the position of water is split into two statistically populated positions. The inclusion of Na atoms in additional positions in framework pores and their high thermal vibrations are indicative of ion-exchange properties of the structure. Possible paths of ion exchange are discussed.