Mixed anion oxides, such as oxyhydrides and oxyhalides, showing exotic transport properties, are of interest in the research field of solid state ionics^[1,2]. The anion-doping effects have also attracted much attention for perovskite-type proton conductors^[3,4]. Meanwhile, unlike conventional cation-doping techniques, it is challenging for anion doping to precisely control and quantify the doping concentration, which is essential for interpreting their defect equilibrium. In this study, anion-doped Ba-Zr-based proton conductors were prepared by a topochemical reaction at high temperatures, and their defect equilibrium was clarified based on exact anion content analyzed by solid-state NMR spectroscopy and DFT calculations. The impacts of anion doping on the transport properties were also investigated.
    Sc-doped, Co-doped, and (Sc, Co)-co-doped BaZrO₃ were prepared as a single-phase host oxide by a solid-state reaction. BaF₂, an F source, was mixed with the barium zirconate powders to have a molar ratio of 1:1. The topochemical reaction was carried out at 1200 °C for 10 h. The excess BaF₂ that is unreacted can be removed by hot water after the reaction. ¹⁹F MAS NMR was conducted to identify the occupation site and amount of F ions introduced into the perovskite-type phase combined with shielding coefficients derived from DFT calculations.
    ¹⁹F MAS NMR spectra of undoped, Sc-, Co- and (Sc, Co)-doped BaZrO₃ were taken after the topochemical reaction at 1200 °C. In addition to a BaF₂ peak (-14.7 ppm), a new peak was observed at around -90 ppm for Sc-containing BaZrO₃. The peak intensity increases with increasing Sc content. The DFT-calculated chemical shift of 2-coordinated F (Zr-F-Zr) in BaZrO₃ was also found to be around -90 ppm, depending on local structures. Meanwhile, undoped BaZrO₃ and Co-doped BaZrO₃ show no extra peak other than BaF₂. This NMR analysis implies that F can fully occupy O-sites of BaZrO₃ to compensate for the effective charge of Sc, i.e., [Sc_Zr’] = [F_o^•], if enough F source is available at high temperatures. The F content in 5, 10, and 20%Sc-doped BaZrO₃ was determined as 4.9, 10.2, and 17.1 mol%, respectively. Even though proton concentration in the F-doped samples was reduced due to the presence of F_o^•, its impact on proton and hole conductivities strongly depended on the F content and the presence of Co. In addition to these F-doped samples, the defect equilibrium in hydride-ion(H^-)-doped Ba-Zr-based oxides prepared by the topochemical reaction will also be discussed.