Crystal structure of vyacheslavite, U(PO(4))(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 Å(3) (obtained from PEDT data at 100 K), Z = 8. Its structure is a complex heteropolyhedral framework consisting of sheets of UO(7)(OH) and PO(4) polyhedra, running parallel to (001), interconnected by additional PO(4) polyhedra. There is an (OH) group associated with the U(iv) polyhedron. The question of H(2)O 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 H(2)O at room temperature.