T-dependent Sn119 Mossbauer Debye-Waller factor f(T) in EuBa2Cu2.98Sn0.02O7- displays evidence of a substantial softening in lattice vibrations at T110 K. In a previous Eu151 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-Tc oxide superconductors.
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