Softening of Cu-O vibrational modes as a precursor to onset of superconductivity in ${\mathrm{EuBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$

T-dependent ${}_{}{}^{119}{}_{}{}^{}\mathrm{Sn}$ Mossbauer Debye-Waller factor f(T) in ${\mathrm{EuBa}}_2$ ${\mathrm{Cu}}_{2.98}$ ${\mathrm{Sn}}_{0.02}$ ${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ displays evidence of a substantial softening in lattice vibrations at T≤110 K. In a previous ${}_{}{}^{151}{}_{}{}^{}\mathrm{Eu}$ study, no detectable softening in f(T) was found at the rare-earth site. These results taken together show that Sn atoms as a dilute impurity replace predominantly Cu(1) sites in the Cu-O chains of the orthorhombic structure. It appears that a softening in vibrational modes localized onto the Cu-O linear chains occurs as a precursor to the onset of superconductivity. These results show that phonons do appear to play an important role in electron pairing in these high-$T_c$ oxide superconductors.