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

Abstract

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) ų, and two formula units per unit cell. The structure (R₁ = 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 O_Uranyl 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.