The zero-resistance temperature of the high-${\mathit{T}}_{\mathit{c}}$ oxide superconductor ${\mathrm{Bi}}_2$${\mathrm{CaSr}}_2$${\mathrm{Cu}}_2$${\mathrm{O}}_{8+\mathrm{\delta }}$ (2:1:2:2) can be significantly increased by annealing at 500 °C in an oxygen-reduced environment. Raman spectroscopy of high-density sintered samples shows that processing the material in this way increases the strength of the Raman peak of highest frequency (650 ${\mathrm{cm}}^{\mathrm{-}1}$), leaving the strengths of the other oxygen peaks unchanged. The Raman data confirm that the mobile oxygen atoms are located in the bismuth layer and support those models of oxygen structure in which excess oxygen is incorporated interstitially. They provide evidence that the incommensurate-to-commensurate transition observed in electron diffraction in the vicinity of 500 °C is associated with a loss of oxygen from the bismuth layer.

• Received 10 May 1991

DOI:https://doi.org/10.1103/PhysRevB.45.370