Solid Solubility in the CeTi₂O₆ – CeTiNbO₆ System—A Multi-Element Laboratory X-Ray Spectroscopic Study

Following separation of Pu and/or the minor actinides from spent nuclear fuel, there are currently three possible routes of management: storage above ground (temporarily or indefinitely), reuse within nuclear reactors for further power generation (<i>e.g.</i> as MOX fuels or within inert matrix fuels), or disposal in a tailored wasteform material. Many studies have examined materials for the effective immobilisation of Pu, but the minor actinides have received comparatively little attention. Mineralogical brannerites and aeschynites have both been observed to have become metamict over time, yet samples still retain a significant fraction of their U/Th inventory, suggesting that both phases are significantly durable with respect to aqueous attack, even after partial or full amorphisation. The titanate family of synthetic brannerites includes the prototypical end-members CeTi₂O₆, UTi₂O₆, ThTi₂O₆ and PuTi₂O₆, with only Ce-brannerite being non-radioactive. In this work the Ce-brannerite – Ce-aeschynite system has been examined (following an expected substitution of Ce⁴⁺_1-xCe³⁺_xTi_2-xNb_xO₆, with x = 0, 0.2, ... , 0.8, 1.0), with Ce acting as a surrogate for both Pu³⁺/Pu⁴⁺ and the trivalent minor actinides.<br/><br/>Materials were produced following a solid state route (starting from CeO₂, TiO₂, and Nb₂O₅), and reacted at 1350 °C for 48 hrs in an air atmosphere. Materials were analysed using XRD, SEM-EDX, and Raman spectroscopy. Ce L₃-edge and Nb K-edge XANES were collected using a laboratory X-ray spectrometer, and utilised to examine the Ce and Nb oxidation states, with further measurements planned at the Ti K-edge to examine the Ti coordination environment in these materials.<br/><br/>The system exhibits significant solid solubility towards the Ce-brannerite endmember, with no aeschynite observed until the target stoichiometry reached CeTi_1.4Nb_0.6O₆ (x = 0.6), where it was observed as a minor phase (approx. 7.5 wgt.%). Preliminary XANES measurement at the Ce L₃-edge and Nb K-edge show that Nb is likely incorporated into the brannerite structure as Nb⁵⁺, with an equivalent fraction of Ce⁴⁺ reduced to Ce³⁺ to retain charge balance. Rietveld refinements show that the Ce-brannerite structure expands significantly to accomodate the larger cations (Ce³⁺, 1.15 Å; Ce⁴⁺, 1.01 Å; Nb⁵⁺, 0.78 Å; Ti⁴⁺ 0.745 Å), with changes in the unit cell volume of up to approx. 5.5% with increasing Nb substitution. This work further expands the crystal chemistry of the brannerite structure, with significant structural flexibility displayed by the inforporation of up to 0.4 f.u. Nb⁵⁺/Ce³⁺ (<i>i.e.</i> Ce^3.6+Ti_1.6Nb_0.4O₆).