Indium silicate with an imandrite-type structure

Abstract

This work reports the synthesis and characterization of novel zeolite-like indium silicate MS-2 (Minho-Sofia, solid number 2). The structure of this material is analogous to that of the mineral imandrite (Na₆Ca_1.5FeSi₆O₁₈), with In instead of Fe in the octahedral position. MS-2 is the first structurally confirmed indium silicate prepared under mild hydrothermal conditions and the only synthetic indium silicate related to the lovozerite mineral group. MS-2 (Na_6.23Ca_1.62In_0.68Si₆O₁₈) exhibits significant indium deficiency in the octahedral position thus having the highest Si/In (8.8) ratio among the known indium silicates. The framework consists of occupationally disordered InO₆ octahedra interconnected by 6-membered rings of [Si₆O₁₈] tetrahedra. The three-dimensional (3D) tunnel system is occupied by Na⁺ and Ca²⁺ charge-balancing ions. The low framework density (16.2 FC/1000 ų) and high thermal stability (up to 900 °C) are comparable to other molecular sieves.