Rb1.66cs1.34tb[Si5.43ge0.57o15]Center Dot H2o, A New Member Of The Od-Family Of Natural And Synthetic Layered Silicates: Topology-Symmetry Analysis And Structure Prediction

Crystals of new silicate-germanate Rb1.66Cs1.34Tb[Si5.43Ge0.57O15]center dot H2O have been synthesized hydrothermally in a multi-component system TbCl3:GeO2:SiO2 = 1:1:5 at T = 280 degrees 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) angstrom, sp. gr. Pbam. New Rb,Cs,Tb-silicate-germanate consists of a [Si-5.Ge-43(0).O-57(15)](infinity infinity) corrugated tetrahedral layer combined by isolated TbO6 octahedra into the mixed microporous framework as in synthetic K3Nd[Si6O15]center dot 2H(2)O, K3Nd[Si6O15] and K3Eu[Si6O15]center dot 2H(2)O 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.