Mixed Valence and Superconductivity in Perovskite Antimonates

Minu Kim,Hidenori Takagi,Jürgen Nuss,Rainer Pöttgen,Steffen Klenner,Ulrich Wedig,Graham M Mcnally,Reinhard K Kremer,Alexander Yaresko

doi:10.1021/acs.chemmater.1c01362

Abstract

Herein, we present results of the 121Sb Mössbauer spectroscopy in perovskite antimonates Ba1–xKxSbO3, sibling compounds of the well-known high-Tc superconductors Ba1–xKxBiO3. Two Sb valence states SbIII and SbV, 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.

Highlights

Versatile physical and chemical properties of mixed-valence oxides have offered a viable avenue for the continuous development of solid-state research.[1,2] In particular, understanding the effects of mixed valences and their control has been successfully applied to the advent of novel high-Tc superconductors.[3,4] Archetypal examples are superconducting perovskite bismuthates, BaPbxBi1−xO35 and Ba1−xKxBiO3.6,7 The parent compound BaBiO3 is a charge-density-wave (CDW) insulator, which has commonly been attributed to the valence skipping of Bi; formal BiIV with a half-filled 6s orbital disproportionates into BiIII and BiV, either fully filling or emptying the orbital.[8]
Perovskite bismuthates belong to the latter case; in BaBiO3, the actual charge difference between the two Bi sites is revealed to be small,[14,15] and the formation of the CDW order is mostly driven by the oxygen hole states (6s2 + 6s2L2, where L denotes ligand holes).[16]
Bspae1c−txrKoxsScboOpy3,.24F−o3r0 this purpose, we employ 121Sb which allows us to directly probe Sb 5s electrons and reveal the mixed valence of Sb ions; SbIII and SbV, with the nominal electron configurations of [Kr]4d105s25p0 and [Kr]4d105s05p0, are well separated and can safely be distinguished in 121Sb Mössbauer spectra as demonstrated for SbIIISbVO4.24,31 Combined with firstprinciples calculations, our results show that metal s electrons unambiguously contribute to the formation of the CDW order and superconductivity in the antimonates, which may have implications on the general understanding of other high-Tc superconductors including Ba1−xKxBiO3

Summary

INTRODUCTION

Versatile physical and chemical properties of mixed-valence oxides have offered a viable avenue for the continuous development of solid-state research.[1,2] In particular, understanding the effects of mixed valences and their control has been successfully applied to the advent of novel high-Tc superconductors.[3,4] Archetypal examples are superconducting perovskite bismuthates, BaPbxBi1−xO35 and Ba1−xKxBiO3.6,7 The parent compound BaBiO3 is a charge-density-wave (CDW) insulator, which has commonly been attributed to the valence skipping of Bi; formal BiIV with a half-filled 6s orbital disproportionates into BiIII and BiV, either fully filling or emptying the orbital.[8]. In contrast to the bismuthates, ΔCT may be positive for the antimonates; the orbital energy of Sb 5s is anticipated to be higher than that of Bi 6s, possibly due to the reduced relativistic effect of Sb as compared to Bi.[20,21] As a result, Sb 5s electrons may be essential for creating a charge-disproportionation-type CDW order (5s2 + 5s0) in the parent compound as well as novel superconductivity in the potassium-substituted compound. Bspae1c−txrKoxsScboOpy3,.24F−o3r0 this purpose, we employ 121Sb which allows us to directly probe Sb 5s electrons and reveal the mixed valence of Sb ions; SbIII and SbV, with the nominal electron configurations of [Kr]4d105s25p0 and [Kr]4d105s05p0, are well separated and can safely be distinguished in 121Sb Mössbauer spectra as demonstrated for SbIIISbVO4.24,31 Combined with firstprinciples calculations, our results show that metal s electrons unambiguously contribute to the formation of the CDW order and superconductivity in the antimonates, which may have implications on the general understanding of other high-Tc superconductors including Ba1−xKxBiO3

EXPERIMENTAL SECTION

RESULTS AND DISCUSSION

CONCLUSIONS

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