Abstract: We have studied the Mossbauer effect of YBa_2Cu_(2.88)Fe_(0.12)O_(7-x) superconductive oxide from 78 to 273K. Two minima of recoilless fraction and Debye temperature were found near 110K and 220K. It means that lattice softening happened near these two temperatures. We also observed minima of isomer shift near 110K and quadrupole splitting increase when temperature is lower than 110K. We suggest that lattice softening would lead to the displacement of oxygen atoms. This may be relative to superconductive transition.

Key words: Y-Ba-Cu-Fe-O superconductor, Mossbauer effect, lattice softening