Raman study of pure and doped (Li,Sr) single crystals ofLa2CuO4−y

Raman spectra are presented of pristine and doped (Li,Sr) single crystals of ${\mathrm{La}}_2\mathrm{Cu}{\mathrm{O}}_{4-y}$. Based on polarization measurements, we assign the 424-${\mathrm{cm}}^{-1\mathrm{}}$ line to the symmetric $A_{1g}$ vibration of the out-of-plane oxygen atoms along the tetragonal axis. The line at 231 ${\mathrm{cm}}^{-1\mathrm{}}$ is identified as the $A_{1g}$ vibration of the La atoms along the tetragonal axis and the line at 238 ${\mathrm{cm}}^{-1\mathrm{}}$ as the $E_g$ vibration of the out-of-plane oxygen atoms parallel to the Cu-O layers. A symmetry-forbidden Raman line is observed at 680 ${\mathrm{cm}}^{-1\mathrm{}}$ which, by comparison with infrared measurements, can be tentatively assigned to an in-plane oxygen vibration. The dependence of the Raman spectra upon Li and Sr doping is presented and discussed. Especially noteworthy are two strong and sharp Raman lines (at 757 and 773 ${\mathrm{cm}}^{-1\mathrm{}}$ at room temperature) in the Sr-doped crystals, because of their higher frequencies, polarization sensitivities, and unusual temperature dependences.