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

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.

Polytypism of Compounds with the General Formula Cs{Al2[TP6O20]} (T = B, Al): OD (Order-Disorder) Description, Topological Features, and DFT-Calculations

“…The heteropolyhedral MT-frameworks with similar stoichiometry (3) have been found in compounds with the general formula Rb{ [6] M 3+ 2[ [4] T 3+ P6O20]}, where M = Al, Ga; T = Al, Ga [26,56] 8) indicating the possible OD-character as was previously shown for compounds with tetrameric [57] and pentameric [20] borophosphate FBUs, as well as for the silicate layers [58,59]; 2.…”

“…The first one corresponds to a layer with the symmetry P2(2)2 1 consisting of tetrahedral chains. The tetrahedral layer in Cs{Al 2 [TP 6 O 20 ]} and tetrahedral pseudolayer in Rb{M 2 [TP 6 O 20 ] are formed by the same FBU and demonstrate the symmetrical relationship (Figure8) indicating the possible OD-character as was previously shown for compounds with tetrameric[57] and pentameric[20] borophosphate FBUs, as well as for the silicate layers[58,59]; 2.The second one consists of an octahedral layer with the symmetry P2 1 (2)2 1 similar to that observed in Cs{Al 2 [TP 6 O 20 ]} (T = Al, B) (the layer group P2 1 22 1 is a subgroup of the layer group Pcam). To date, there are no other polytypes of this type of framework, however they may be found later.…”

Symmetry Analysis of the Complex Polytypism of Layered Rare-Earth Tellurites and Related Selenites: The Case of Introducing Transition Metals

“…The symmetry analysis of such disordered compounds is based on the concepts of OD theory [22,23] and the symmetry of the OD structures is described by groupoids (instead of crystallographic space groups), which can contain the non-crystallographic symmetry operations [24][25][26]. This approach allows to determine the symmetry relations for the whole family of known polytypes and predict the hypothetical ones [27], which is important for the searching of new materials [28][29][30][31][32][33][34]. The OD structures with the different polytypes have been also described for the tellurium compounds, in particular, for the hydrous magnesium orthotellurate (VI) Mg [37].…”

Editorial for the Special Issue “Modularity and Twinning in Mineral Crystal Structures”

“…The use of the OD theory is also central in the articles by Aksenov et al [7] and Topnikova et al [8]. In [7], the crystal structures of compounds with the general formula Cs{ [6] Al 2 [ [4] TP 6 O 20 ]} (T = Al, B) are analyzed.…”

“…Article [8] reports the crystal structure of a new silicate-germanate with formula Rb 1.66 Cs 1.34 Tb[Si 5.43 Ge 0.57 O 15 ]•H 2 O. However, for this special issue, the important part of the article is represented by the comparison of the investigated structure with those of other related layered natural and synthetic silicates.…”