In this study, the crystal structure evolution of solid solutions between giant-tetragonal (GT) perovskite-type PbVO3 and BiCoO3, namely, (1 – x)­PbVO3–xBiCoO3, is reported. The all-proportional solid solutions are successfully synthesized using a high-pressure and high-temperature method. A region of cubic phase is observed in the 0.4 ≤ x ≤ 0.75 range. The tetragonal distortion (c/a ratio) systematically decreases toward the cubic phase region, and these phases coexist in boundary regions. Magnetic susceptibility and soft X-ray absorption spectroscopy measurements reveal that 0.5PbVO3–0.5BiCoO3 possesses a valence state of V5+ and Co2+, which is different from the end members, V4+ and Co3+. The crystal structure changes stem from a self-doping effect induced by intermetallic charge transfer between the V and Co ions on the B-site (V4+ + Co3+ → V5+ + Co2+). This finding shows that tuning an electronic structure by making a solid solution is an effective way to control the stability of the GT structure.

Electronic states in solid-solution transition metal oxides may differ from those in their parent compounds, and this results in interesting electronic properties. In this study, the valence states and electronic properties of solid solutions of (1 − x)PbVO 3 − xBiCrO 3 are reported. The solid solutions are successfully synthesized under high-pressure and high-temperature conditions of 5 GPa or 7 GPa and 1273 K, respectively. A change in the crystal structure from centrosymmetric monoclinic (C2/c), as in BiCrO 3 , to polar tetragonal (P4mm), as in PbVO 3 , is observed. A tetragonal-to-cubic phase transition, which results in a negative thermal expansion, was observed in 7/8PbVO 3 -1/8BiCrO 3 at approximately 700 K. X-ray absorption spectroscopy and magnetic studies reveal that the Cr and V atoms are in the valence states 3+ and 4+, respectively, which are the same as those of the parent compounds. Thus, it was concluded that the valence state of V 4+ /Cr 3+ and the electron localization in the (1 − x)PbVO 3 − xBiCrO 3 solid solutions are considerably robust.

Chemical substitution is an effective way to improve electrocatalytic properties in transition metal oxides. We investigate the synergistic effect between Fe 4+ and Co 4+ ions on the catalytic activity for oxygen evolution reaction (OER) in the Fe-Co-mixed perovskite oxide CaFe 1¹x Co x O 3 . The OER activity of CaFe 1¹x Co x O 3 is substantially increased by small amounts of Co (Fe) doping into CaFeO 3 (CaCoO 3 ), leading to the superiority compared to the pure Fe and Co perovskite oxides. The x dependences of the OER overpotential and specific activity for CaFe 1¹x Co x O 3 (0.05 ¼ x ¼ 0.95) are expressed by constant offset from the weighted average between CaFeO 3 and CaCoO 3 , which can be interpreted to be the synergistic effect between Fe 4+ and Co 4+ ions on OER activity. The absence of the optimum x for the highest activity for CaFe 1¹x Co x O 3 contrasts with the volcano-like plots reported in various mixed-metal oxides. First-principle calculations using the special quasirandom structure models on CaFe 1¹x Co x O 3 (x = 0.03-0.5) demonstrate that about half the amount of Fe 4+ is electronically activated to possess smaller charge-transfer energies, corroborating the enhancement of catalytic activity in CaFe 1¹x Co x O 3 . These findings provide new insight into the synergistic effects in complex transition metal oxide catalysts.