Assessing the oxidation states and structural stability of the Ce analogue of brannerite

Abstract

The Ce-containing analogue of brannerite (ie, UTi 2 O 6 ) was previously considered to be stoichiometric (ie, CeTi 2 O 6 ); however, it has recently been determined that the material is O deficient.This oxygen-deficient material has been suggested to be charged balanced by the presence of a minor concentration of Ce 3+ or by the A-site being cation deficient with the Ce oxidation state being 4+. A variety of Ti-containing oxides (including brannerite) have been investigated as potential nuclear wasteforms, and it is necessary to understand the electronic structure of a proposed nuclear wasteform material as well as how the structure responds to radiation from incorporated waste elements. The radiation resistance of a material can be simulated by ion implantation. The objective of this study was to confirm the Ce oxidation state in the cationand oxygen-deficient material (ie, Ce 0.94 Ti 2 O 6 − δ ) and to determine how radiation damage affects this material. X-ray photoelectron spectroscopy (XPS) and X-ray absorption near-edge spectroscopy were used to study Ce 0.94 Ti 2 O 6 − δ before and after being implanted with 2 MeV Au − ions.Analysis of the Ce 3d XPS spectra from the as-synthesized samples by using a previously developed fitting method has unequivocally shown that Ce adopts both 4+ (major) and 3+ (minor) oxidation states, which was confirmed by examination of magnetic susceptibility data. Analysis of XPS and X-ray absorption near-edge spectroscopy spectra from ion-implanted materials showed that both Ce and Ti were reduced because of radiation damage and that the local coordination environments of the cations are greatly affected by radiation damage.

The Effect of Temperature on the Stability and Cerium Oxidation State of CeTi2O6 in Inert and Oxidizing Atmospheres

“…This is in contrast to literature reports of Ce 3+ fractions of 30−41%, as measured by XPS and measurements of magnetic susceptibility. 19 This extent of reduction and the corresponding amount of O vacancies is in good agreement with the recently reported DFT calculations, which predicted a relatively high upper bound of stability in the system Ce (4−2δ)+ Ti 2 O 6−δ , when compared to other Ce 4+ oxides. 20 When the reported energies of O vacancy formation are compared to each other, both DFT 20 and Mott-Littleton 16 methods predict that the vacancies formed will be preferentially located at the O sites coordinated with at least one Ce cation.…”

“…Aluri et al compared the Ce oxidation states of as-quenched material and material that had been annealed at 800 °C or slow-cooled in air using Ce L 3 -edge XANES, Ce 3d X-ray photoelectron spectroscopy (XPS), and measurements of the magnetic susceptibility. The differences in the Ce L 3 -edge XANES between the as-quenched and slow-cooled brannerite agree with the earlier findings of Huynh et al The XPS determined that Ce 3+ contents were significantly greater than was determined in the present work: 41% Ce 3+ for the material quenched from 1325 °C and 32% Ce 3+ for the material slow-cooled from 1325 °C, with an estimation of 38% Ce 3+ for a material annealed at 800 °C determined by magnetic susceptibility measurements …”

“…The differences in the Ce L 3 -edge XANES between the as-quenched and slow-cooled brannerite agree with the earlier findings of Huynh et al The XPS determined that Ce 3+ contents were significantly greater than was determined in the present work: 41% Ce 3+ for the material quenched from 1325 °C and 32% Ce 3+ for the material slowcooled from 1325 °C, with an estimation of 38% Ce 3+ for a material annealed at 800 °C determined by magnetic susceptibility measurements. 19 A recent report 20 of DFT computed oxygen vacancy formation energies (E vf O ) suggests that, in comparison to many other Ce 4+ oxides, CeTi 2 O 6 has a relatively low E vf O and can support a high fraction of O vacancies when the mechanism of charge balancing is reduction of Ce 4+ to Ce 3+ . Defect formation energies for Ce-brannerite have also been reported by Stennett et al, where the mechanism of charge balancing was taken to be the formation of Ce vacancies (e.g., the formation of O−Ce−O vacancy clusters), with no reduction of Ce.…”

A one-step synthesis of rare-earth phosphate–borosilicate glass composites

“…47 Overlap of the 4d and 5p orbitals increases when the coordination number decreases, which increases the dipolar character of the pre-edge transition. [48][49][50] Fig. 7 shows the Y K-edge XANES spectra from the glass-ceramic composites having a 20 and 40 wt% ceramic loading and made by the 1 and 2-step synthesis methods and are compared to the spectrum from pure YPO 4 .…”

X-Ray Photoelectron Spectroscopy of Murataite Ceramics Containing Lanthanides

“…Therefore, XPS gives a more correct estimate of the contents of cations of diff erent valences. The study of Ce-brannerite showed [36] that the discrepancy between the XPS data for the sample surface and the XAS data for the volume is relatively small. Note that data on the composition and valence of elements in the surface layer is very im- portant, since this region of the matrix is exposed to the solution.…”