RAMAN-SPECTROSCOPIC STUDY OF AN APPARENT PHASE-TRANSITION AT 234K IN THE HIGH-TCSUPERCONDUCTOR YBA2CU3-XVXO7-Y

Raman spectroscopy of the high-T//c family YBa//2Cu//3// minus //xV//xO//7// minus //y with y equals 0. 1 and x from 0 to 0. 6 shows the emergence of a mode at 644 cm** minus **1 as temperature decreases below T//C equals 234 K. This mode was observed by Macfarlane, Rosen and Seki at low temperatures and correctly interpreted, we believe, as a nominally forbidden odd-parity LO phonon by Liu et al. However, its temperature dependence has not been analyzed before. We show that it correlates strongly with sound velocity anomalies (3% decrease) and ultrasonic attention divergences reported near 234 K by Wang Yening et al. We interpret this as a continuous or nearly continuous phase transition at 234 K, probably from D//2//h to C// 2//v point group symmetry (Pmmm yields Pmm2) involving oxygen octahedron rotation and the consequent loss of a mirror plane. There is independent evidence for such a C//2//y structure in the TEM study by Beyers at al.