Negative ionic states of tin in the oxide superconductor <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mi mathvariant="normal">Sr</mml:mi><mml:mrow><mml:mn>3</mml:mn><mml:mo>−</mml:mo><mml:mi>x</mml:mi></mml:mrow></mml:msub><mml:mi>SnO</mml:mi></mml:math> revealed by Mössbauer spectroscopy

We report the temperature variation of the $^{119}\mathrm{Sn}$-M\"ossbauer spectra of the antiperovskite (inverse perovskite) oxide superconductor ${\mathrm{Sr}}_{3\ensuremath{-}x}\mathrm{SnO}$. Both superconductive (Sr deficient) and nonsuperconductive (nearly stoichiometric) samples exhibit major $\ensuremath{\gamma}$-ray absorption with an isomer shift similar to that of ${\mathrm{Mg}}_{2}\mathrm{Sn}$. This fact shows that ${\mathrm{Sr}}_{3\ensuremath{-}x}\mathrm{SnO}$ contains the metallic anion ${\mathrm{Sn}}^{4\ensuremath{-}}$, which is rare, especially among oxides. In both samples, we observed another $\ensuremath{\gamma}$-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 $\ensuremath{\gamma}$-ray absorptions originate from the Sn atoms neighboring the Sr deficiency.

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