Rb1.66Cs1.34Tb[Si5.43Ge0.57O15]·H2O, a New Member of the OD-Family of Natural and Synthetic Layered Silicates: Topology-Symmetry Analysis and Structure Prediction

Anastasiia Topnikova,Dina Deyneko,Anatoly Volkov,Elena Belokoneva,Olga Dimitrova

doi:10.3390/min11040395

Anastasiia Topnikova, Dina Deyneko + Show 3 more

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https://doi.org/10.3390/min11040395

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Journal: Minerals	Publication Date: Apr 9, 2021
Citations: 4	License type: CC BY 4.0

Affiliation: Lomonosov Moscow State University

Abstract

Crystals of new silicate-germanate Rb1.66Cs1.34Tb[Si5.43Ge0.57O15]·H2O have been synthesized hydrothermally in a multi-component system TbCl3:GeO2:SiO2 = 1:1:5 at T = 280 °C and P = 100 atm. K2CO3, Rb2CO3 and Cs2CO3 were added to the solution as mineralizers. The crystal structure was solved using single crystal X-ray data: a = 15.9429(3), b = 14.8407(3), c = 7.2781(1) Å, sp. gr. Pbam. New Rb,Cs,Tb-silicate-germanate consists of a [Si5.43Ge0.57O15]∞∞ corrugated tetrahedral layer combined by isolated TbO6 octahedra into the mixed microporous framework as in synthetic K3Nd[Si6O15]·2H2O, K3Nd[Si6O15] and K3Eu[Si6O15]·2H2O with the cavities occupied by Cs, Rb atoms and water molecules. Luminescence spectrum on new crystals was obtained and analysed. A comparison with the other representatives of related layered natural and synthetic silicates was carried out based on the topology-symmetry analysis by the OD (order-disorder) approach. The wollastonite chain was selected as the initial structural unit. Three symmetrical ways of forming ribbon from such a chain and three ways of further connecting ribbons to each other into the layer were revealed and described with symmetry groupoids. Hypothetical structural variants of the layers and ribbons in this family were predicted.

Highlights

Published: 9 April 2021Si and Ge elements have equal formal charge 4+ and tetrahedral coordination
Si and Ge occur together only as the isomorphic substitution Si-Ge in tetrahedra
K3Nd[Si6O15]·2H2O [44] were analyzed, emphasizing the presence of a xonotlite-like ribPolytypic relations were derived for the compounds and the crucial role of large K cations bon

Summary

Introduction

Si and Ge elements have equal formal charge 4+ and tetrahedral coordination. Si and Ge occur together only as the isomorphic substitution Si-Ge in tetrahedra. Both elements are present isomorphically in the minerals sanbornite, milarite, albite, perrierite, farmakosiderite, garnet, titanite, and zeolite analogues [1]. Lone pair heavy metals act together with different anionic units containing mixing components. Such an approach is used in materials design to result in promising properties. There are no silicate-germanates with Tb in the nature or synthetic compounds according to [1,2]

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