Solid solubility in the CeTi₂O₆–CeTiNbO₆ system: A multi-element X-ray spectroscopic study

In order to investigate the limits of solid solubility between Ce-brannerite (CeTi₂O₆) and Ce-aeschynite (CeTiNbO₆), materials in the system CeTi_2–xNb_xO₆ have been produced by a solid state route and characterised by XRD and XANES at the Ce L₃-, Ti K- and Nb K-edges, including Rietveld method refinements and linear combination fitting. Significant solid solubility was observed at the brannerite end, with near-single-phase brannerite observed for x = 0.2, 0.4, and only minor aeschynite observed where x = 0.6 which was identified as exceeding the limit of solubility of Nb. All Nb was present as Nb⁵⁺, with the substitution of Nb⁵⁺ into the brannerite structure permitted by the reduction of the same fraction of Ce⁴⁺ to Ce³⁺. This work expands the crystal chemistry of the titanate brannerites, with Ce-site oxidation states of less than 4+ being possible where sufficient charge-balancing species are available on the Ti-site. Graphical abstract: [Figure not available: see fulltext.].