Mössbauer study of the lattice dynamics indoped119superconducting and nonsuperconductingYBa2Cu3O7−δ

The temperature dependence of the relative recoil-free fraction of the ${}_{}{}^{119}{}_{}{}^{}\mathrm{Sn}$ Mössbauer absorption has been measured in the orthorhombic superconducting and in the tetragonal nonsuperconducting phase of ${}_{}{}^{119}{}_{}{}^{}\mathrm{Sn}$ doped ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$. For the superconducting ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ phase, the recoil-free fraction shows softening which appears for T<150 K, whereas for the nonsuperconducting ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ phase, it agrees with that obtained using the measured phonon spectrum without any evidence of softening.