Influence of extreme conditions on the formation and structures of caesium uranium(VI) arsenates

Periodic trends in trivalent actinide halides, phosphates, and arsenates

Due to the limited abundance of the actinide elements, computational methods, for now, remain an exclusive avenue to investigate the periodic trends across the actinide series. As every actinide element can exhibit a +3-oxidation state, we have explored model systems of gas-phase actinide trihalides, phosphates, and arsenates across the series to capture the periodic trends. By doing so, we were able to capture the periodic trends down the halogen series as well, and for the first time we are reporting a study on actinide astatides. Using scalar and spin-orbit relativistic Density Functional Theory (DFT) calculations, we have explored the variations in bond lengths, bond angles, and the charges on actinides (An). Despite the use of different sets of ligands, the trends remain similar. The properties of trivalent Pa, U, Np, and Pu are nearly identical; similar ionic radii could be the reason. The actinide elements show a tendency to exhibit a pre-Pu and a post-Cm behaviour, with Am acting as a switch. This could be due to the change in the behaviour from d-f-type to f-filling/d-type at around Pu-Cm in the actinides as already proposed in the previous literature. Bond lengths in the AnX3 increase down the halide series, and the atomic charges decrease on the actinide elements.

Advances and perspectives of actinide chemistry from ex situ high pressure and high temperature chemical studies

High pressure high temperature (HP/HT) studies of actinide compounds allow the chemistry and bonding of among the most exotic elements in the periodic table to be examined under the conditions often only found in the severest environments of nature. Peering into this realm of physical extremity, chemists have extracted detailed knowledge of the fundamental chemistry of actinide elements and how they contribute to bonding, structure formation and intricate properties in compounds under such conditions. The last decade has resulted in some of the most significant contributions to actinide chemical science and this holds true for ex situ chemical studies of actinides resulting from HP/HT conditions of over 1 GPa and elevated temperature. Often conducted in tandem with ab initio calculations, HP/HT studies of actinides have further helped guide and develop theoretical modelling approaches and uncovered associated difficulties. Accordingly, this perspective article is devoted to reviewing the latest advancements made in actinide HP/HT ex situ chemical studies over the last decade, the state-of-the-art, challenges and discussing potential future directions of the science. The discussion is given with emphasis on thorium and uranium compounds due to the prevalence of their investigation but also highlights some of the latest advancements in high pressure chemical studies of transuranium compounds. The perspective also describes technical aspects involved in HP/HT investigation of actinide compounds.

Effect of solution acidity on the crystallization of polychromates in uranyl-bearing systems: synthesis and crystal structures of Rb2[(UO2)(Cr2O7)(NO3)2] and two new polymorphs of Rb2Cr3O10

Three new rubidium polychromates, Rb2[(UO2)(Cr2O7)(NO3)2] (1), γ-Rb2Cr3O10 (2) and δ-Rb2Cr3O10 (3) were prepared by combination of hydrothermal treatment at 220 °C and evaporation of aqueous solutions under ambient conditions. Compound 1 is monoclinic, P 2 1 / c $P{2}_{1}/c$ , a = 13.6542(19), b = 19.698(3), c = 11.6984(17) Å, β = 114.326(2)°, V = 2867.0(7) Å3, R 1 = 0.040; 2 is hexagonal, P 6 3 / m $P{6}_{3}/m$ , a = 11.991(2), c = 12.828(3) Å, γ = 120°, V = 1597.3(5) Å3, R 1 = 0.031; 3 is monoclinic, P 2 1 / n $P{2}_{1}/n$ , a = 7.446(3), b = 18.194(6), c = 7.848(3) Å, β = 99.953(9)°, V = 1047.3(7) Å3, R 1 = 0.037. In the crystal structure of 1, UO8 bipyramids and NO3 groups share edges to form [(UO2)(NO3)2] species which share common corners with dichromate Cr2O7 groups producing novel type of uranyl dichromate chains [(UO2)(Cr2O7)(NO3)2]2−. In the structures of new Rb2Cr3O10 polymorphs, CrO4 tetrahedra share vertices to form Cr3O10 2− species. The trichromate groups are aligned along the 63 screw axis forming channels running in the ab plane in the structure of 2. The Rb cations reside between the channels and in their centers completing the structure. The trichromate anions are linked by the Rb+ cations into a 3D framework in the structure of 3. Effect of solution acidity on the crystallization of polychromates in uranyl-bearing systems is discussed.

Crystal growth of novel 3D skeleton uranyl germanium complexes: influence of synthetic conditions on crystal structures

Five centrosymmetric uranyl germanate compounds, K₈BrF(UO₂)₃(Ge₂O₇)₂, Rb₆(UO₂)₃(Ge₂O₇)₂·0.5H₂O, Cs₆(UO₂)₂Ge₈O₂₁ and A⁺₂(UO₂)₃(GeO₄)₂ (A⁺ = Rb⁺, Cs⁺), were synthesized in this work. K₈BrF(UO₂)₃(Ge₂O₇)₂ and Rb₆(UO₂)₃(Ge₂O₇)₂·0.5H₂O were obtained under mixed KF-KBr flux and hydrothermal conditions, respectively. Both structures crystallized in the triclinic P1[combining macron] space group and have similar anionic frameworks featuring novel hexagon shaped 12-membered channels. The condensation of two different types of SBU [UGe₄] pentamers (A) and (A2) results in the formation of K₈BrF(UO₂)₃(Ge₂O₇)₂ and Rb₆(UO₂)₃(Ge₂O₇)₂·0.5H₂O frameworks. Cs₆(UO₂)₂Ge₈O₂₁ was obtained from a CsF-CsCl high temperature flux, and it also crystallized in the centrosymmetric triclinic P1[combining macron] space group. The structure of Cs₆(UO₂)₂Ge₈O₂₁ has a novel oxo-germanate layer composed of germanate tetrahedra and trigonal bipyramids. Two new SBU types, (4²·5²-A2) and (5⁴-A2) [UGe₄] pentamers, were found in the structure of Cs₆(UO₂)₂Ge₈O₂₁. A⁺₂(UO₂)₃(GeO₄)₂ (A⁺ = Rb⁺, Cs⁺) were synthesized by a high temperature/high pressure (HT/HP) technique, and both structures with oval-shaped 12-membered channels crystallized in the centrosymmetric orthorhombic Pnma space group. The extreme conditions led to the formation of [U₂Ge₂] tetramers (E), which consist of 7-coordinated U and 5-coordinated Ge. Different synthetic methods of uranyl germanate compounds resulted in a distinct coordination environment of the uranyl cations and a variety of U[double bond, length as m-dash]O and U-O bond lengths, further affecting the dimensionality and types of uranyl units and SBUs. The Raman and IR spectra of the five new phases were collected and analyzed.

Structural and Spectroscopic Investigation of Novel 2D and 3D Uranium Oxo-Silicates/Germanates and Some Statistical Aspects of Uranyl Coordination in Oxo-Salts

Synthesis, structural and spectroscopic characterization, and topological analysis of five novel uranyl-based silicates and germanates have been performed. The open-framework K₄(UO₂)₂Si₈O₂₀·4H₂O has been synthesized under hydrothermal conditions and is based upon [USi₆] heptamers interconnected via edge-sharing. Its structure is composed of sechser silicate layers with 4-, 8-, and 16-membered rings. The largest 16-membered rings have an average dimension of ∼8.93 × 9.42 Ų. β-K₂(UO₂)Si₄O₁₀ has been obtained by the high-temperature flux growth method. Its 3D framework contains a loop-branched sechser single layer with 4- and 8-membered rings and consists of the same [USi₆] heptamers as observed in K₄(UO₂)₂Si₈O₂₀·4H₂O. Na₆(UO₂)₃(Si₂O₇)₂ has also been synthesized from melted fluxes and represents a 2D layer structure composed by [USi₄] pentamers. Two iso-structural compounds A⁺(UO₂)(HGeO₄)·H₂O (A⁺ = Rb⁺, Cs⁺) were synthesized via the hydrothermal method, and their structures are of the α-uranophane type. The 2D layers consist of [U₂Ge₂] tetramer secondary building units (SBUs). The Raman spectra of all novel phases were collected, and bands were assigned according to the existing oxo-silicate rings and oxo-germanium units. Additionally, we performed a statistical investigation of the local coordination of uranyl ions in all known inorganic structures with different oxo-anions (TO_x, T = B³⁺, Si/Ge⁴⁺, P/As⁵⁺, S/Se/Te⁶⁺, Cr/Mo/W⁶⁺, P/As³⁺, and Se/Te⁴⁺). We found a direct correlation between the ionic potential of the central cations T in oxo-anions in their higher oxidation states and the coordination number of uranyl groups.

Detection and identification of solids, surfaces, and solutions of uranium using vibrational spectroscopy

The purpose of this review is to provide an overview of uranium speciation using vibrational spectroscopy methods including Raman and IR. Uranium is a naturally occurring, radioactive element that is utilized in the nuclear energy and national security sectors. Fundamental uranium chemistry is also an active area of investigation due to ongoing questions regarding the participation of 5f orbitals in bonding, variation in oxidation states and coordination environments, and unique chemical and physical properties. Importantly, uranium speciation affects fate and transportation in the environment, influences bioavailability and toxicity to human health, controls separation processes for nuclear waste, and impacts isotopic partitioning and geochronological dating. This review article provides a thorough discussion of the vibrational modes for U(IV), U(V), and U(VI) and applications of infrared absorption and Raman scattering spectroscopies in the identification and detection of both naturally occurring and synthetic uranium species in solid and solution states. The vibrational frequencies of the uranyl moiety, including both symmetric and asymmetric stretches are sensitive to the coordinating ligands and used to identify individual species in water, organic solvents, and ionic liquids or on the surface of materials. Additionally, vibrational spectroscopy allows for the in situ detection and real-time monitoring of chemical reactions involving uranium. Finally, techniques to enhance uranium species signals with vibrational modes are discussed to expand the application of vibrational spectroscopy to biological, environmental, inorganic, and materials scientists and engineers.