Optical phonons of Ca1.8Sr0.2CuO3 with the Cu-O chain structure

Abstract

Raman and ir spectra of ${\mathrm{Ca}}_{1.8}$ ${\mathrm{Sr}}_{0.2}$ ${\mathrm{CuO}}_3$ with the Cu-O chain structure have been investigated. Scattering peaks due to Raman-allowed ${\mathit{A}}_{\mathit{g}}$-mode phonons have been observed at 306 and 530 ${\mathrm{cm}}^{\mathrm{-}1}$ at room temperature for the polarization of light along the a and c axes. On the other hand, when the polarization is along the b axis (chain direction), Raman-forbidden peaks have been observed at 235, 440, 500, and 690 ${\mathrm{cm}}^{\mathrm{-}1}$ with strong second-order features, which show strong resonance enhancement at the charge-transfer gap energy of 2.0 eV. They are ascribed to the zone-boundary phonons, which appear through the strong coupling of the electron-hole pair excited on the Cu-O chain with phonons associated with the motion of atoms in the chain.