Raman spectroscopic evidence of site-selective isotopic substitution of oxygen inBi2Sr2Ca0.8Y0.2Cu2O8+δand mode assignments of oxygen vibrations

The site-selective substitution of ${}_{}{}^{18}{}_{}{}^{}\mathrm{O}$ for ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$ in ${\mathrm{Bi}}_2$ ${\mathrm{Sr}}_2$ ${\mathrm{Ca}}_{0.8}$ ${\mathrm{Y}}_{0.2}$ ${\mathrm{Cu}}_2$ ${\mathrm{O}}_{8+\mathrm{\delta }}$ has been investigated by Raman spectroscopy. Three prominent Raman bands are observed at 297, 464, and 623 ${\mathrm{cm}}^{\mathrm{-}1}$. They all shift to lower wave numbers as the ${}_{}{}^{18}{}_{}{}^{}\mathrm{O}$ content increases. The bands at 297 and 464 ${\mathrm{cm}}^{\mathrm{-}1}$ shift at similar rates but the band at 623 ${\mathrm{cm}}^{\mathrm{-}1}$ shifts at a slower rate. These results show that all three bands are due to the vibrations involving oxygen atoms, and that the rate of oxygen substitution is site dependent. Based on these experimental results, tentative mode assignments have been made assuming a tetragonal symmetry: 623 ${\mathrm{cm}}^{\mathrm{-}1}$→${\mathrm{O}}_{\mathrm{pl}}$(${\mathit{A}}_{\mathit{g}}$), 464 ${\mathrm{cm}}^{\mathrm{-}1}$→${\mathrm{O}}_{\mathrm{ap}}$, and 297 ${\mathrm{cm}}^{\mathrm{-}1}$→${\mathrm{O}}_{\mathrm{Bi}}$.