Formation of Open Framework Uranium Germanates: The Influence of Mixed Molten Flux and Charge Density Dependence in U-Silicate and U-Germanate Families

Li, Haijian; Alekseev, Evgeny; Kegler, Philip; Langer, Eike; Modolo, Giuseppe

doi:10.1021/acs.inorgchem.8b01781

Journal Article

FZJ-2018-05463

Formation of Open Framework Uranium Germanates: The Influence of Mixed Molten Flux and Charge Density Dependence in U-Silicate and U-Germanate Families

Li, H.FZJ* ; Langer, E.FZJ* ; Kegler, P.FZJ* ; Modolo, G.FZJ* ; Alekseev, E. (Corresponding author)FZJ*

2018
American Chemical Society Washington, DC

Inorganic chemistry 57(17), 11201 - 11216 (2018) [10.1021/acs.inorgchem.8b01781]

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Please use a persistent id in citations: doi:10.1021/acs.inorgchem.8b01781

Abstract: Seven novel open-framework uranyl germanates, K2(UO2)GeO4, K6(UO2)3Ge8O22, α-Cs2(UO2)Ge2O6, β-Cs2(UO2)Ge2O6, Cs2(UO2)GeO4, and A(UO2)3(Ge2O7)2 (A = [NaK6Cl]6+, [Na2Cs6Cl2]6+), were grown from different mixed molten fluxes. The three-dimensional (3D) structure of K2(UO2)GeO4 with 8-ring channels can be built upon [UGe4] pentamer secondary building units (SBUs). The 3D framework of K6(UO2)3Ge8O22 with trapezoid (Ge8O22)12– clusters consists of two types of [UGe4] pentamers. The 3D framework of α-Cs2(UO2)Ge2O6 with 10-ring channels, crystallizing in the P21/n space group, is constructed by [UGe4] pentamers. The structure of β-Cs2(UO2)Ge2O6 contains achter (eight) single germanate chains and is composed of [UGe6] heptamers and [UGe4] pentamers. The structure of Cs2(UO2)GeO4 with hexagonal 10-ring channels is composed of [U3Ge4] heptamers and twisting five-fold GeO4 tetrahedra in four-membered Ge4O12 rings occur. 3D frameworks of NaK6Cl(UO2)3(Ge2O7)2 (space group Pnnm) and Na2Cs6Cl2(UO2)3(Ge2O7)2 (P21/c) can be constructed from the same SBUs [UGe4] pentamers. Thermal stability of salt-inclusions was studied by TG and PXRD analysis. Analysis of charge density for the U–Si–O system indicates that the polymerization of silicate units reduces the cross-links of the 3D frameworks. The concept of SBUs combined with the cutting and gluing strategy was applied to understand and analyze the distinct 8-, 10-, 12-, and 14- membered channels for the uranyl germanate family. The charge density of all known 3D U–Si/Ge–O frameworks has been investigated, which shows strong correlations with chemical composition of corresponding phases. The increase of Si/O (Ge/O) ratios in silicate units results in the decrease of negative charge density. Moreover, the charge density increases with decreasing countercation size within the same Si/O ratio. The correlations can be used to predict inclusion phase formation within U–Si/Ge–O families. Raman spectra of the studied uranyl germanates were measured, and bands were assigned on the basis of structural features.

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