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 Ų. 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 Ų. 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 Ų. 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

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 <i>P</i>2₁/<i>c</i> space group and contains open-branched sechser (six) single silicate chains, whereas K₂(UO₂)­Si₂O₆ crystallizes in the <i>C</i>2/<i>c</i> 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 <b>A⁺</b>–UO₂²⁺–SiO₄^4– phases demonstrates the high polymerization level of silicate groups in the system, which was compared with the family of <b>A⁺</b>–UO₂²⁺–BO₃^3–/BO₄^5– 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 <i>C</i>2/<i>c</i> space group and belongs to the Na₂Si^VISi₂O₇ structure type. Its 3D framework consists of diorthosilicate Si₂O₇ group and ThO₆ octahedra. Noncentrosymmetric K₂ThSi₃O₉ crystallizes in the hexagonal <i>P</i>6₃ 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

Th(As^III₄As^V₄O₁₈): a Mixed-Valent Oxoarsenic(III)/arsenic(V) Actinide Compound Obtained under Extreme Conditions

A high-temperature/high-pressure method was employed to investigate phase formation in the Th­(NO₃)₄·5H₂O–As₂O₃–CsNO₃ system. It was observed that an excess of arsenic­(III) in starting system leads to the formation of Th­(As^III₄As^V₄O₁₈), which is representative of a rare class of mixed-valent arsenic­(III)/arsenic­(V) compounds. This compound was studied with X-ray diffraction, energy-dispersive X-ray, and Raman spectroscopy methods. Crystallographic data show that Th­(As^III₄As^V₄O₁₈) is built from (As^III₄As^V₄O₁₈)^4– layers connected through Th atoms. The arsenic layers are found to be isoreticular to those in previously reported As₂O₃ and As₃O₅(OH), and the geometric differences between them are discussed. Bands in the Raman spectrum are assigned with respect to the presence of AsO₃ and AsO₄ groups

Th(As^III₄As^V₄O₁₈): a Mixed-Valent Oxoarsenic(III)/arsenic(V) Actinide Compound Obtained under Extreme Conditions

A high-temperature/high-pressure method was employed to investigate phase formation in the Th­(NO₃)₄·5H₂O–As₂O₃–CsNO₃ system. It was observed that an excess of arsenic­(III) in starting system leads to the formation of Th­(As^III₄As^V₄O₁₈), which is representative of a rare class of mixed-valent arsenic­(III)/arsenic­(V) compounds. This compound was studied with X-ray diffraction, energy-dispersive X-ray, and Raman spectroscopy methods. Crystallographic data show that Th­(As^III₄As^V₄O₁₈) is built from (As^III₄As^V₄O₁₈)^4– layers connected through Th atoms. The arsenic layers are found to be isoreticular to those in previously reported As₂O₃ and As₃O₅(OH), and the geometric differences between them are discussed. Bands in the Raman spectrum are assigned with respect to the presence of AsO₃ and AsO₄ groups

High-Temperature Phase Transitions, Spectroscopic Properties, and Dimensionality Reduction in Rubidium Thorium Molybdate Family

Four new rubidium thorium molybdates have been synthesized by high-temperature solid-state reactions. The crystal structures of Rb₈Th­(MoO₄)₆, Rb₂Th­(MoO₄)₃, Rb₄Th­(MoO₄)₄, and Rb₄Th₅(MoO₄)₁₂ were determined using single-crystal X-ray diffraction. All these compounds construct from MoO₄ tetrahedra and ThO₈ square antiprisms. The studied compounds adopt the whole range of possible structure dimensionalities from zero-dimensional (0D) to three-dimensional (3D): finite clusters, chains, sheets, and frameworks. Rb₈Th­(MoO₄)₆ crystallizes in 0D containing clusters of [Th­(MoO₄)₆]^8–. The crystal structure of Rb₂Th­(MoO₄)₃ is based upon one-dimensional chains with configuration units of [Th­(MoO₄)₃]^2–. Two-dimensional sheets occur in compound Rb₄Th­(MoO₄)₄, and a 3D framework with channels formed by thorium and molybdate polyhedra has been observed in Rb₄Th₅(MoO₄)₁₂. The Raman and IR spectroscopic properties of these compounds are reported. Temperature-depended phase transition effects were observed in Rb₂Th­(MoO₄)₃ and Rb₄Th­(MoO₄)₄ using thermogravimetry-differential scanning calorimetry analysis and high-temperature powder diffraction methods

Rich Coordination of Nd³⁺ in Mg₂Nd₁₃(BO₃)₈(SiO₄)₄(OH)₃, Derived from High-Pressure/High-Temperature Conditions

A neodymium borosilicate, Mg₂Nd₁₃(BO₃)₈(SiO₄)₄(OH)₃ (<b>MgNdBSi-1</b>), was obtained from a high-temperature (1400 °C), solid-state reaction under high-pressure conditions (4.5 GPa). <b>MgNdBSi-1</b> contains six different types of Nd³⁺ coordination environments with three different ligands: BO₃, SiO₄, and OH groups. Mg²⁺ cations are only bond to BO₃ groups and form porous two-dimensional layers based on 12-membered ring fragments. Surprisingly, the OH groups are retained at high temperature and reside at the center of Mg–BO₃ rings

Synthesis and Study of the First Zeolitic Uranium Borate

A complex, three-dimensional, open-framework, lead uranyl borate, (H₂O)­Pb₃(UO₂)₃­B₁₄O₂₇, denoted as <b>LUBO</b>, was synthesized via a hydrothermal method. <b>LUBO</b> crystallizes in the hexagonal space group <i>P</i>6₃/<i>m</i> and exhibits a zeolite-like anionic borate framework (B₁₄O₂₇)^12‑. The main structural unit of the framework is a tubule consisting of six-membered B₆O₁₈ rings. Each ring is connected to the successive one by three diborate groups, and these tubules propagate along the <i>c</i> axis. The tubules possess six-membered ring (MR) windows in the axial direction and 8-MR windows on its sides. Interconnection of the parallel tubules, which consist exclusively of BO₄ tetrahedra, is provided by triangular BO₃ fragments perpendicular to the axis of the tubules. The framework has large pores as well as channels with 8-MR windows extending along the [100], [010], and [110] directions that are consistent with the overall hexagonal symmetry of the structure. The lead cations occupy 8-MR windows and form [Pb₃(H₂O)] groups with attached water molecules that are located at the center of the tubules. The method of Voronoi–Dirichlet tessellation reveals that the lone pairs of the lead cations are located outside the tubule. Uranyl cations form UO₈ coordination polyhedra in the shape of a hexagonal bipyramid. The thermal stability and vibrational spectroscopy of <b>LUBO</b> are also delineated in this work

High-Temperature Phase Transitions, Spectroscopic Properties, and Dimensionality Reduction in Rubidium Thorium Molybdate Family

Four new rubidium thorium molybdates have been synthesized by high-temperature solid-state reactions. The crystal structures of Rb₈Th­(MoO₄)₆, Rb₂Th­(MoO₄)₃, Rb₄Th­(MoO₄)₄, and Rb₄Th₅(MoO₄)₁₂ were determined using single-crystal X-ray diffraction. All these compounds construct from MoO₄ tetrahedra and ThO₈ square antiprisms. The studied compounds adopt the whole range of possible structure dimensionalities from zero-dimensional (0D) to three-dimensional (3D): finite clusters, chains, sheets, and frameworks. Rb₈Th­(MoO₄)₆ crystallizes in 0D containing clusters of [Th­(MoO₄)₆]^8–. The crystal structure of Rb₂Th­(MoO₄)₃ is based upon one-dimensional chains with configuration units of [Th­(MoO₄)₃]^2–. Two-dimensional sheets occur in compound Rb₄Th­(MoO₄)₄, and a 3D framework with channels formed by thorium and molybdate polyhedra has been observed in Rb₄Th₅(MoO₄)₁₂. The Raman and IR spectroscopic properties of these compounds are reported. Temperature-depended phase transition effects were observed in Rb₂Th­(MoO₄)₃ and Rb₄Th­(MoO₄)₄ using thermogravimetry-differential scanning calorimetry analysis and high-temperature powder diffraction methods

High-Temperature Phase Transitions, Spectroscopic Properties, and Dimensionality Reduction in Rubidium Thorium Molybdate Family

Four new rubidium thorium molybdates have been synthesized by high-temperature solid-state reactions. The crystal structures of Rb₈Th­(MoO₄)₆, Rb₂Th­(MoO₄)₃, Rb₄Th­(MoO₄)₄, and Rb₄Th₅(MoO₄)₁₂ were determined using single-crystal X-ray diffraction. All these compounds construct from MoO₄ tetrahedra and ThO₈ square antiprisms. The studied compounds adopt the whole range of possible structure dimensionalities from zero-dimensional (0D) to three-dimensional (3D): finite clusters, chains, sheets, and frameworks. Rb₈Th­(MoO₄)₆ crystallizes in 0D containing clusters of [Th­(MoO₄)₆]^8–. The crystal structure of Rb₂Th­(MoO₄)₃ is based upon one-dimensional chains with configuration units of [Th­(MoO₄)₃]^2–. Two-dimensional sheets occur in compound Rb₄Th­(MoO₄)₄, and a 3D framework with channels formed by thorium and molybdate polyhedra has been observed in Rb₄Th₅(MoO₄)₁₂. The Raman and IR spectroscopic properties of these compounds are reported. Temperature-depended phase transition effects were observed in Rb₂Th­(MoO₄)₃ and Rb₄Th­(MoO₄)₄ using thermogravimetry-differential scanning calorimetry analysis and high-temperature powder diffraction methods

Rich Coordination of Nd³⁺ in Mg₂Nd₁₃(BO₃)₈(SiO₄)₄(OH)₃, Derived from High-Pressure/High-Temperature Conditions

A neodymium borosilicate, Mg₂Nd₁₃(BO₃)₈(SiO₄)₄(OH)₃ (<b>MgNdBSi-1</b>), was obtained from a high-temperature (1400 °C), solid-state reaction under high-pressure conditions (4.5 GPa). <b>MgNdBSi-1</b> contains six different types of Nd³⁺ coordination environments with three different ligands: BO₃, SiO₄, and OH groups. Mg²⁺ cations are only bond to BO₃ groups and form porous two-dimensional layers based on 12-membered ring fragments. Surprisingly, the OH groups are retained at high temperature and reside at the center of Mg–BO₃ rings