RAMAN-SPECTROSCOPIC EVIDENCE OF SITE-SELECTIVE ISOTOPIC-SUBSTITUTION OF OXYGEN IN BI2SR2CA0.8Y0.2CU2O8+DELTA AND MODE ASSIGNMENTS OF OXYGENVIBRATIONS

The site-selective substitution of O-18 for O-16 in Bi2Sr2Ca0.8Y0.2Cu2 O8+delta has been investigated by Raman spectroscopy. Three prominent Raman bands are observed at 297, 464, and 623 cm-1. They all shift to lower wave numbers as the O-18 content increases. The bands at 297 and 464 cm-1 shift at similar rates but the band at 623 cm-1 shifts at a slower rate. These results show that all three bands are due to the vi brations involving oxygen atoms, and that the rate of oxygen substitut ion is site dependent. Based on these experimental results, tentative mode assignments have been made assuming a tetragonal symmetry: 623 cm -1 --> O(pl)(A(g)), 464 cm-1 --> O(ap), and 297 cm-1 --> O(Bi).