Philip Kegler

Hydrothermal Synthesis, Study, and Classification of Microporous Uranium Silicates and Germanates

Four novel uranyl silicates and germanates with framework structures, K4Na2(UO2)3(Si2O7)2·3H2O, K4Na2(UO2)3(Ge2O7)2·3H2O, H3O(UO2)2(HGe2O7)·2H2O, and Na2(UO2)GeO4, have been synthesized by means of the hydrothermal method. The structures of the title compounds were refined by single-crystal X-ray diffraction and characterized by Raman spectroscopy. We used the method of secondary building units (SBUs) for a crystal chemical analysis of the 3D framework and their topologies. The framework of the K4Na2(UO2)3(T2O7)2·3H2O (T = Si, Ge) series exhibits large 14-membered rings and smaller 8-membered rings which are built upon [UT4] pentamers. The internal size of the largest pores is approximately 12.39 × 3.33 Å2. H3O(UO2)2(HGe2O7)·2H2O is based on 10-membered rings with intermediate sized pores. They are built upon [U2Ge2] tetramers with 7-fold-coordinated U. The internal dimension of the pores in H3O(UO2)2(HGe2O7)·2H2O is smaller compared to the K4Na2(UO2)3(T2O7)2·3H2O (T = Si, Ge) series with ∼5.91 × 5.33 Å2. Its topology is similar to several uranium germanate synthetic phases and silicate minerals, especially α- and β-uranophane which are constructed from similar building units. A novel 3D framework type of Na2(UO2)GeO4 with 8-membered rings demonstrates the smallest free volume in the family of porous uranium germanates. It crystallizes in tetragonal symmetry and is built upon corner sharing of [UGe4] pentamers. The size of the channels is ∼6.76 × 4.27 Å2. The vibrational bands in Raman spectra were associated with pyro-(Si2O7)6- and -(Ge2O7)6- groups, with the Ge-OH bond and with H3O+ cations, confirming the results of the X-ray crystallographic structural characterization. We systemized existing uranyl silicates and germanates based on their building units and chemical composition. We found a simple structural dependence between synthetic conditions and chemical composition.

Investigation of reactivity and structure formation in a K–Te–U oxo-system under high-temperature/high-pressure conditions

The high-temperature/high-pressure treatment of the K-Te-U oxo-family at 1100 °C and 3.5 GPa results in the crystallization of a series of novel uranyl tellurium compounds, K2[(UO2)3(TeIVO3)4], K2[(UO2)TeO14], α-K2[(UO2)TeVIO5] and β-K2[(UO2)TeVIO5]. In contrast to most of the reported uranyl compounds which are favorable in layered structures, we found that under extreme conditions, the potassium uranyl oxo-tellurium compounds preferably crystallized in three-dimensional (3D) framework structures with complex topologies. Anion topology analysis indicates that the 3D uranyl tellurite anionic framework observed in K2[(UO2)3(TeIVO3)4] is attributable to the additional linkages of TeO3 polyhedra connecting with TeO4 disphenoids from the neighboring U-Te layers. The structure of K2[(UO2)TeO14] can be described based on [UTe6O26]22- clusters, where six TeO5 polyhedra enclose a hexagonal cavity in which a UO8 polyhedron is located. The [UTe6O26]22- clusters are further linked by TeO5 square pyramids to form the 3D network. Similar to uranyl tellurates, both α-K2[(UO2)TeVIO5] and β-K2[(UO2)TeVIO5] contain TeO6 octahedra which share a common face to form a dimeric Te2O10 unit. However, in α-K2[(UO2)TeVIO5], these Te2O10 units connect with UO6 tetragonal bipyramids to form a 3D structural framework, while in β-K2[(UO2)TeVIO5], the same Te2O10 dimers are observed to link with UO7 pentagonal bipyramids, forming 2D layers. Raman measurements were carried out and the vibration bands related to TeIV-O, TeVI-O and UVI-O bonds are discussed.

Giant Volume Change and Topological Gaps in Temperature‐ and Pressure‐Induced Phase Transitions: Experimental and Computational Study of ThMo2O8

By applying high temperature (1270u2005K) and high pressure (3.5u2005GPa), significant changes occur in the structural volume and crystal topology of ThMo2 O8 , allowing the formation of an unexpected new ThMo2 O8 polymorph (high-temperature/high-pressure (HT/HP) orthorhombic ThMo2 O8 ). Compared with the other three ThMo2 O8 polymorphs prepared at the ambient pressure (monoclinic, orthorhombic, and hexagonal phases), the molar volume for the quenched HT/HP-orthorhombic ThMo2 O8 is decreased by almost 20u2009%. As a result of such a dramatic structural transformation, a permanent high-pressure quenchable state is able to be sustained when the pressure is released. The crystal structures of the three ambient ThMo2 O8 phases are based on three-dimensional (3D) frameworks constructed from corner-sharing ThOx (x=6, 8, or 9) polyhedra and MoO4 tetrahedra. The HT/HP-orthorhombic ThMo2 O8 , however, crystallizes in a novel structural topology, exhibiting very dense arrangements of ThO11 and MoO4+1 polyhedra connecting along the crystallographic c axis. The phase transitions among all four of these ThMo2 O8 polymorphs are unveiled and fully characterized with regard to the structural transformation, thermal stability, and vibrational properties. The complementary first principles calculations of Gibbs free energies reveal the underlying energetics of the phase transition, which support the experimental findings.

Comparison of Uranium(VI) and Thorium(IV) Silicates Synthesized via Mixed Fluxes Techniques

Two uranium and two thorium silicates were obtained using high temperature mixed fluxes methods. K14(UO2)3Si10O30 crystallizes in the P21/ c space group and contains open-branched sechser (six) single silicate chains, whereas K2(UO2)Si2O6 crystallizes in the C2/ c space group and is built of unbranched achter (eight) silicate chains. The crystals of K14(UO2)3Si10O30 and K2(UO2)Si2O6 are related by increasing U/Si molar ratios, and both structures contain the same secondary building units (SBUs), [USi6] heptamers. The triangle diagram for all known A+-UO22+-SiO44- phases demonstrates the high polymerization level of silicate groups in the system, which was compared with the family of A+-UO22+-BO33-/BO45- compounds. For both thorium silicates, the transformation of K2ThSi2O7 to K2ThSi3O9 was found to be a factor of the reaction time. K2ThSi2O7 crystallizes in the C2/ c space group and belongs to the Na2SiVISi2O7 structure type. Its 3D framework consists of diorthosilicate Si2O7 group and ThO6 octahedra. Noncentrosymmetric K2ThSi3O9 crystallizes in the hexagonal P63 space group and adopts mineral wadeite-type structure based upon triorthosilicate Si3O9 rings and ThO6 octahedra. The coordination environment of thorium for all existing oxo-anion compounds including B, Si/Ge, P/As, Cr/Mo/W, and S/Se/Te are summarized and analyzed. Additionally, spectroscopic properties of all novel materials have been studied.

High-Pressure Synthesis, Structural, and Spectroscopic Studies of the Ni–U–O System

The first comprehensive structural study of the Ni-U-O system is reported. Single crystals of α-NiUO4, β-NiUO4, and NiU3O10 were synthesized, and their structures were refined-using synchrotron single-crystal X-ray diffraction data supported by X-ray absorption spectroscopic measurements. α-NiUO4 adopts an orthorhombic structure in space group Pbcn and is isostructural to CrUO4 containing corrugated two-dimensional (2D) layers of corner-sharing UO6 polyhedra and edge-sharing one-dimensional (1D) zigzag α-PbO2 rutile-like chains of NiO6 polyhedra in the [001] direction. β-NiUO4 is isostructural to MgUO4 and has an orthorhombic structure in space group Ibmm, which contains alternating 1D chains of edge-sharing UO6 and NiO6 polyhedra in the [001] direction as in regular TiO2 rutile. NiU3O10 forms a triclinic structure in space group P1̅ and is isostructural with CuU3O10, where it forms a three-dimensional (3D) framework structure built through a mixture of UO6 and UO7 polyhedra in which the NiO6 polyhedra sit isolated within the framework. X-ray absorption near-edge structure (XANES) measurements, conducted using XANES mapping of single crystals, support the presence of hexavalent uranium in the three structures. The polymorphs of NiUO4 were found to only form under high-pressure and high-temperature conditions (≥4 GPa and 700 °C). It is argued that this is a consequence of the relative size difference between the Ni2+ and U6+ cations, where the Ni2+ cation is effectively too small for the Ibmm structure and too large for the Pbcn structure to form under ambient pressure conditions. This does not appear to be an issue for NiU3O10, which forms under ambient pressure conditions, where NiO6 polyhedra sit isolated within the framework of 3D connected UO6/UO7 polyhedra. Synthesis conditions indicate that β-NiUO4 is the preferred higher-pressure phase and that the transformation to this occurs irreversibly at a temperature between 950 and 1000 °C, when P = 4 GPa. The routes toward the synthesis of the oxides and the associated structural and spectroscopic results are described with respect to the structural chemistry of the Ni-U-O system, the larger AUO4 family of oxides (A = divalent or trivalent cation), and also their relation to the rutile-related family of oxides.

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

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.