Structural and electronic properties of d1 transition-metal compounds a synchrotron study of A2BTiF6 double Perovskites and the charge ordering in Na0.33V2O5

Materials containing (d1) ions, such as Ti3+ and V4+, have attracted significant attention due to their electronic and magnetic properties, offering potential applications in renewable energy technology [1] [2]. This research employed high-resolution synchrotron X-ray diffraction (XRD) to resolve the structural and electronic transitions in two d1 mixed-metal systems. Firstly, a previous study on A2BTiF6 (A, B: Na, K, Cs, Rb A≠B) double perovskite (DP) [3] reported multiple structural transitions (e.g., cubic to tetragonal in Rb2NaTiF6 and successive transitions in K2NaTiF6), suggesting the presence of Jahn-Teller (JT) distortion based on laboratory techniques, this analysis was conducted to investigate using basic laboratory techniques. To the best of our knowledge, this is the only reported study eon these compounds. To provide an accurate structural analysis, four A2BTiF6 DP compounds were re-synthesised and analysed with variable-temperature synchrotron XRD. Our analysis suggests that, among the compounds studied, Rb2KTiF6 shows the clearest evidence of a full JT transition, undergoing successive structural changes from cubic to tetragonal to orthorhombic upon cooling. The remaining compounds were found to retain their high-symmetry cubic phase, suggesting that cation size influences the structural susceptibility to JT distortion. Secondly, the mixed-metal oxide Na0.33V2O5 has recently attracted attention due to its complex low-temperature (LT) behaviour, including successive supercell transformations (a b c to a 2b c to a 6b c), an antiferromagnetic anomaly near 25 K, and a nearby superconducting phase [4] [5]. To map the underlying electronic order, the structural evolution was investigated using combined Rietveld refinement and Maximum Entropy Method (MEM). The results support charge differentiation among the vanadium sites, consistent with a charge ordered LT state. Tentative oxidation-state assignments were proposed by combining MEM-derived integrated electron densities with structural information.