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

Haijian Li; Eike M Langer; Philip Kegler; Giuseppe Modolo; Evgeny V Alekseev

doi:10.1021/acs.inorgchem.8b01781

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

Inorg Chem. 2018 Sep 4;57(17):11201-11216. doi: 10.1021/acs.inorgchem.8b01781. Epub 2018 Aug 20.

Authors

Haijian Li¹, Eike M Langer¹, Philip Kegler¹, Giuseppe Modolo¹, Evgeny V Alekseev^{1

2}

Affiliations

¹ Institute of Energy and Climate Research (IEK-6) , Forschungszentrum Jülich GmbH , 52428 Jülich , Germany.
² Institut für Kristallographie , RWTH Aachen University , 52066 Aachen , Germany.

PMID: 30125085
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.