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

Abstract

The O 1s 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 O 2− species and structural data of the uranyl minerals are used to resolve the fine structure of the O 1s envelope. The resolution of the O 1s spectra includes, for the first time, different O 2− 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 1s 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 O 2− in the equatorial plane of the uranyl polyhedra linking uranyl polyhedra with ( TO n ) 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 (H 2O) groups in the interstitial complex of the uranyl minerals ( H 2 O interst ) occur at 533.0–533.8 eV and bands of physisorbed (H 2O) groups on the surface of uranyl minerals ( H 2 O adsorb ) 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 H 2 O interst : O U O between the bulk and surface structures is larger for uranyl minerals with higher percentages of H 2 O interst as well as, with a higher number of interstitial H 2O groups that are not bonded to interstitial cations, resulting in easier dehydration of interstitial H 2O groups in uranyl minerals during exposure to a vacuum.