Evidence of Structural Instability Near T*~250 K in the Resistivity of Bi_2Sr_2CaCu_2O_8 Whiskers

We report anomalous features in the normal-state resistivity of single crystalline Bi_2Sr_2CaCu_2O_8 whiskers near some characteristic temperature T* \~ 250 K. From these varied oxygen-doping samples, the resistance (R) was found to deviate {\it upward} from the linear-T relation for T< T*, while R keeps decreasing with T. The deviation, $\Delta R \equiv R - (a + bT)$, reaches a maximum in a temperature range of ~ 25 K just before R-T follows the well-known descent associated with the pseudogap opening in the underdoped samples. A second kink was also found in some samples immediately following the first one, resulting in an S-shaped feature in the R-T curve. The resistance then resumes nearly linear-T decrease below the anomalies in contrast to the first case. We interpret these features as being related to crystal structure transformation (or lattice distortion), as was evidenced in thermal and elastic property measurements. These structural instabilities seem to be connected to the subsequent dynamics as T is further lowered.