XPS spectra of uranyl minerals and synthetic uranyl compounds. II: The O 1s spectrum

The O Is spectrum is examined for 19 uranyl minerals of different composition and structure. Spectra from single crystals were measured with a Kratos Axis Ultra X-ray Photoelectron Spectrometer with a magnetic-confinement charge-compensation system. Well-resolved spectra with distinct maxima, shoulders and inflections points, in combination with reported and measured binding energies for specific O2- species and structural data of the uranyl minerals are used to resolve the fine structure of the O 1 s envelope. The resolution of the O 1 s spectra includes, for the first time, different O2- bands, which are assigned to O atoms linking uranyl with uranyl polyhedra (U-O-U) and O atoms of uranyl groups (O=U =O). The resolved bands in the O 1 s spectrum occur at distinct ranges in binding energy: bands for (U-O-U) occur at 529.6-530.4 eV, bands for (O=U =O) occur at 530.6-531.4 eV, bands for O2- in the equatorial plane of the uranyl polyhedra linking uranyl polyhedra with (TOn) groups (T= Si, S, C, P, Se) (T-O) occur at 530.9-532.2 eV; bands for (OH) groups in the equatorial plane of the uranyl polyhedra (OH) occur at 532.0-532.5 eV, bands of (H2O) groups in the interstitial complex of the uranyl minerals (H2Ointerst) occur at 533.0-533.8 eV and bands of physisorbed (H2O) groups on the surface of uranyl minerals (H2Oadsorb) occur at 534.8-535.2 eV. Treatment of uranyl minerals with acidic solutions results in a decrease in U-O-U and an increase in OH. Differences in the ratio of OH: O=U=O between the surface and bulk structure is larger for uranyl minerals with a high number of U-O-U and T-O species in the bulk structure which is explained by protonation of underbonded U-O, U-O-U and T-O terminations on the surface. The difference in the ratio of H2Ointerst: O=U=O between the bulk and surface structures is larger for uranyl minerals with higher percentages of H2Ointerst as well as, with a higher number of interstitial H2O groups that are not bonded to interstitial cations, resulting in easier dehydration of interstitial H2O groups in uranyl minerals during exposure to a vacuum. (C) 2009 Elsevier Ltd. All rights reserved.