Crystal structure of vyacheslavite, U(PO4)(OH), solved from natural nanocrystal: a precession electron diffraction tomography (PEDT) study and DFT calculations

Molybdenum Disorder in Hydrated Sedovite, Ideally U(MoO4)2·nH2O, a Microporous Nanocrystalline Mineral Characterized by Three-Dimensional Electron Diffraction, Density Functional Theory Computations, and Complexity Analysis

Sedovite, U⁴⁺(Mo⁶⁺O₄)₂·nH₂O, is reported as being one of the earliest supergene minerals formed of the secondary zone. The difficulty of isolating enough pure material limits studies to techniques that can access the nanoscale combined with theoretical analyses. The crystal structure of sedovite has been solved and refined using the dynamical approach from three-dimensional electron diffraction data collected on natural nanocrystals found among iriginite. At 100 K, sedovite is monoclinic a ≈ 6.96 Å, b ≈ 9.07 Å, c ≈ 12.27 Å, and V ≈ 775 ų with space group C2/c. The microporous structure presents a characteristic framework built from uranium polyhedra and disordered Mo pyramids creating pore hosting water molecules. To confirm the formula U⁴⁺(Mo⁶⁺O₄)₂·nH₂O, the possible presence of a hydroxyl group that would promote Mo⁵⁺ was tested with density functional theory (DFT) computations at the ambient temperature. DFT predicts that sedovite is a ferromagnetic insulator with a fundamental bandgap of E_g ∼ 1.7 eV with its chemical and physical properties dominated by U⁴⁺ rather than Mo⁶⁺. The structural complexity, I_G,tot, of sedovite was evaluated in order to get indirect information about the missing formation conditions.

Hydrogen disorder in kaatialaite Fe[AsO2(OH)2]5H2O from Jáchymov, Czech Republic: determination from low-temperature 3D electron diffraction

Kaatialaite mineral Fe[AsO2(OH)2]5H2O from Jáchymov, Czech Republic forms white aggregates of needle-shaped crystals with micrometric size. Its structure at ambient temperature has already been reported but hydrogen atoms could not be identified from single-crystal X-ray diffraction. An analysis using 3D electron diffraction at low temperature brings to light the hydrogen positions and the existence of hydrogen disorder. At 100 K, kaatialaite is described in a monoclinic unit cell of a = 15.46, b = 19.996, c = 4.808 Å, β = 91.64° and V = 1485.64 Å3 with space group P21/n. The hydrogen sites were revealed after refinements both considering the dynamical effects and ignoring them. The possibility to access most of the hydrogen positions, including partially occupied ones among heavy atoms, from the kinematical refinement is due to the recent developments in the analysis of 3D electron data. The hydrogen bonding observed in kaatialaite provides examples of H2O configurations that have not been observed before in the structures of oxysalts with the presence of unusual inverse transformer H2O groups.

Meet the Cerium(IV) Phosphate Sisters: CeIV (OH)PO4 and CeIV 2 O(PO4 )2

Two new cerium(IV) phosphates were obtained: cerium(IV) hydroxidophosphate, Ce(OH)PO₄ , and cerium(IV) oxidophosphate, Ce₂ O(PO₄ )₂ , which were shown to complement the classes of isostructural compounds M(OH)PO₄ and R₂ O(PO₄ )₂ , where M=Th, U and R=Th, U, Np, Zr. Ce₂ O(PO₄ )₂ oxidophosphate is formed by elimination of H₂ O from the crystal structure of Ce(OH)PO₄ during its thermal decomposition. The structures of Ce(OH)PO₄ and Ce₂ O(PO₄ )₂ are related to each other with the same Cmce space group and similar unit cell parameters (a=6.9691(3) Å, b=9.0655(4) Å, c=12.2214(4) Å, V=772.13(8) ų , Z=8; a=7.0220(4) Å, b=8.9894(5) Å, c=12.544(1) Å, V=791.8(1) ų , Z=4, respectively).

Hydrogen bonding in the crystal structure of phurcalite, Ca2[(UO2)3O2(PO4)2]·7H2O: single-crystal X-ray study and TORQUE calculations

The crystal structure of phurcalite, Ca₂[(UO₂)₃O₂(PO₄)₂]·7H₂O, orthorhombic, a = 17.3785 (9) Å, b = 15.9864 (8) Å, c = 13.5477 (10) Å, V = 3763.8 (4) ų, space group Pbca, Z = 8 has been refined from single-crystal XRD data to R = 0.042 for 3182 unique [I > 3σ(I)] reflections and the hydrogen-bonding scheme has been refined by theoretical calculations based on the TORQUE method. The phurcalite structure is layered, with uranyl phosphate sheets of the phosphuranylite topology which are linked by extensive hydrogen bonds across the interlayer occupied by Ca²⁺ cations and H₂O groups. In contrast to previous studies the approach here reveals five transformer H₂O groups (compared to three expected by a previous study) and two non-transformer H₂O groups. One of the transformer H₂O groups is, nevertheless, not linked to any metal cation, which is a less frequent type of H₂O bonding in solid state compounds and minerals. The structural formula of phurcalite has been therefore redefined as {Ca₂(H₂^[3]O)₅(H₂^[4]O)₂}[(UO₂)₃O₂(PO₄)₂], Z = 8.