Infrared reflectivity and vibrational structure of superconducting ${\mathrm{Bi}}_2$${\mathrm{Sr}}_2$${\mathrm{CaCu}}_2$${\mathrm{O}}_{8+\mathrm{x}}$

We report the room-temperature infrared (ir) reflectance spectrum and the Kramers-Kronig imaginary dielectric function (ε’ ’) of the ${\mathrm{Bi}}_2$ ${\mathrm{Sr}}_2$ ${\mathrm{CaCu}}_2$ ${\mathrm{O}}_{8+\mathrm{x}}$ (2:2:1:2) superconductor ($T_c$=82 K) in the 250–1000 ${\mathrm{cm}}^{\mathrm{-}1}$ region. ε’ ’ shows phononlike maxima at 276, 316, 347, 361, and 600 ${\mathrm{cm}}^{\mathrm{-}1}$. Several spectral features are similar to those found in 1:2:3-type materials including superconducting M${\mathrm{Ba}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{7\mathrm{-}\mathrm{x}}$ and semiconducting M${\mathrm{Ba}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_6$ (M=Y or some rare-earth element), as expected from their closely related perovskitelike crystal structures. The correlation existing between a factor group analysis of the Γ point phonons in 2:2:1:2 and 1:2:3 compounds affords a tentative assignment of ir-active modes in the bismuth superconductor by comparison with reported data in 1:2:3 materials.