EVIDENCE FOR LOCAL LATTICE INSTABILITY AT T-ASTERISK-SIMILAR-TO-1.4 T-C IN BI2212 BY EXAFS

The temperature dependence of local Cu site conformations in Bi2Sr2CaCu2O8+y (Bi2212) has been determined by EXAFS, a fast (10(-15)s) and local probe. We show the polaronic nature of the one-dimensional structural anharmonic modulation of the CuO2 plane. We have measured the size of the polarons W=(10.6+/-0.6) Angstrom at T