Mixed Valence and Superconductivity in Perovskite Antimonates

Herein, we present results of the ¹²¹Sb Mössbauer spectroscopy in perovskite antimonates Ba_1–xK_xSbO₃, sibling compounds of the well-known high-T_c superconductors Ba_1–xK_xBiO₃. Two Sb valence states Sb^III and Sb^V, forming a charge-density-wave (CDW) order, are unambiguously revealed in un- and under-doped phases at x = 0–0.5. As the CDW order is suppressed at x = 0.65, the compound becomes superconducting and all of the Sb sites become equivalent with the intermediate valence of +4.65, consistent with that anticipated from the potassium content. These results provide the direct evidence of metal s electrons participating in the formation of the CDW order and superconductivity in the material, and thus they have important implications for clarifying the underlying mechanism related to mixed valence and potential valence fluctuations of metal ions. The use of Mössbauer active nuclei offers a deeper insight into the structural properties and superconductivity in mixed-valence main-group oxides.