Negative ionic states of tin in the oxide superconductor Sr3−xSnO revealed by Mössbauer spectroscopy

Abstract

We report the temperature variation of the ¹¹⁹Sn-Mössbauer spectra of the antiperovskite (inverse perovskite) oxide superconductor Sr₃₋ₓ SnO. Both superconductive (Sr deficient) and nonsuperconductive (nearly stoichiometric) samples exhibit major γ-ray absorption with an isomer shift similar to that of Mg₂ Sn. This fact shows that Sr₃-ₓ SnO contains the metallic anion Sn⁴⁻, which is rare, especially among oxides. In both samples, we observed another γ-ray absorption with a larger isomer shift, indicating that there is another ionic state of Sn with a higher oxidation number. The temperature dependence of the absorption intensities reveals that the Sn ions exhibiting larger isomer shifts have a lower energy of the local vibration. The larger isomer shift and lower vibration energy are consistent with the values estimated from the first-principles calculations for hypothetical structures with various Sr-deficiency arrangements. Therefore, we conclude that the additional γ-ray absorptions originate from the Sn atoms neighboring the Sr deficiency.