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

The crystal structure of the U(iv)-phosphate mineral vyacheslavite has been solved from precession electron diffraction tomography (PEDT) data from the natural nano-crystal and further refined using density-functional theory (DFT) calculations. Vyacheslavite is orthorhombic, with the space group Cmca, with a ≈ 6.96 Å, b ≈ 9.07 Å and c ≈ 12.27 Å, V ≈ 775 Å3 (obtained from PEDT data at 100 K), Z = 8. Its structure is a complex heteropolyhedral framework consisting of sheets of UO7(OH) and PO4 polyhedra, running parallel to (001), interconnected by additional PO4 polyhedra. There is an (OH) group associated with the U(iv) polyhedron. The question of H2O presence within the small cavities of the framework has been addressed by the DFT calculations, which have proved that vyacheslavite does not contain any significant amount of H2O at room temperature.

Revealing hydrogen atoms in a highly-absorbing material: an X-ray diffraction study and Torque method calculations for lead-uranyl-oxide mineral curite.

The crystal structure of lead uranyl-oxide hydroxy-hydrate mineral curite, ideally Pb3(H2O)2[(UO2)4O4(OH)3]2, was studied by means of single-crystal X-ray diffraction and theoretical calculations in order to localize positions of hydrogen atoms in the structure. This study has demonstrated that hydrogen atoms can be localized successfully also in materials for which the conventional approach of structure analysis failed, here due to very high absorption of X-rays by the mineral matrix. The theoretical calculations, based on the Torque method, provide a robust, fast real-space method for determining H2O orientations from their rotational equilibrium condition. In line with previous results we found that curite is orthorhombic, with space group Pnma, unit-cell parameters a = 12.5510(10), b = 8.3760(4), c = 13.0107(9) Å, V = 1367.78(16) Å3, and two formula units per unit cell. The structure (R 1 = 3.58% for 1374 reflections with I > 3σI) contains uranyl-hydroxo-oxide sheets of the unique topology among uranyl oxide minerals and compounds and an interlayer space with Pb2+ cations and a single H2O molecule, which is coordinated to the Pb-site. Current results show that curite is slightly non-stoichiometric in Pb content (∼3.02 Pb per unit cell, Z = 2); the charge-balance mechanism is via (OH) ↔ O2 substitution within the sheets of uranyl polyhedra. Disproving earlier predictions, the current study shows that curite contains only one H2O group, with [4]-coordinated oxygen. The hydrogen bonding network maintains the bonding between the sheets in addition to Pb-O bonds; among them, a H-bond is crucial between the OH group on an apical OUranyl atom of an adjacent sheet that stabilizes the entire structure. The results show that the combination of experimental X-ray data and the Torque method can successfully reveal hydrogen bonding especially for complex crystal structures and materials where X-rays fail to provide unambiguous hydrogen positions.