Metastability in $\mathrm{Eu}{\mathrm{Ba}}_2{\left({\mathrm{Cu}}_{1-x}{\mathrm{Sn}}_x\right)}_3{\mathrm{O}}_{7-y}$ studied by ${}_{}{}^{119}{}_{}{}^{}\mathrm{Sn}$ and ${}_{}{}^{151}{}_{}{}^{}\mathrm{Eu}$ Mössbauer spectroscopy

The metallic oxide compound system $\mathrm{Eu}{\mathrm{Ba}}_2{\left({\mathrm{Cu}}_{1-x}{\mathrm{Sn}}_x\right)}_3{\mathrm{O}}_{7-y}$ ($x=0.05, 0.10, 0.20$) was prepared for investigation with ${}_{}{}^{151}{}_{}{}^{}\mathrm{Eu}$ and ${}_{}{}^{119}{}_{}{}^{}\mathrm{Sn}$ Mössbauer spectroscopy. The best fits of the ${}_{}{}^{119}{}_{}{}^{}\mathrm{Sn}$ Mössbauer lines reveal at least two different ${\mathrm{Sn}}^{\mathrm{IV}}$ sites. The ${}_{}{}^{151}{}_{}{}^{}\mathrm{Eu}$ spectra are characteristic for ${\mathrm{Eu}}^{\mathrm{III}}$ state in these compounds. We have found a considerable difference between the shapes of ${}_{}{}^{119}{}_{}{}^{}\mathrm{Sn}$ Mössbauer spectra recorded at room temperature and those recorded at the temperature of liquid nitrogen in the case of the sample having the highest tin content. Nevertheless, there is no evidence that this difference has a direct connection with the superconducting behavior. A time-dependent splitting was found in the ${}_{}{}^{119}{}_{}{}^{}\mathrm{Sn}$ spectra of $\mathrm{Eu}{\mathrm{Ba}}_2{\left({\mathrm{Cu}}_{0.8}{\mathrm{Sn}}_{0.2}\right)}_3{\mathrm{O}}_{7-y}$ at 78 K. It had a maximum at about 350 h after the synthesis of the sample. Some anomalous temperature dependences were also found in the ${}_{}{}^{151}{}_{}{}^{}\mathrm{Eu}$ and ${}_{}{}^{119}{}_{}{}^{}\mathrm{Sn}$ Mössbauer parameters. Our findings can be associated with different Sn sites as well as low-temperature phase transformation in these oxide compounds.