Raman investigation of the orthorhombic-tetragonal phase transition in La2-xSrxCuO4 superconductors

We present polarized Raman spectra from oriented single crystals of La2CuO4 and superconducting La1.85Sr0.15CuO4 that encompass the temperature range above and below the orthorhombic-to-tetragonal phase transition. This transition has a striking effect upon the Raman spectra. As approached from below (the orthorhombic phase), the ?zz spectrum from La2CuO4 shows a strong line at 104 cm-1 that broadens and softens and a weak line at 271 cm-1 that gradually disappears, both lines being unobservable in the tetragonal phase. The soft phonon is not seen in the orthorhombic phase of the Sr-doped sample, but the weak 271 cm-1 line is seen. The oxygen Eg mode of the tetragonal phase is observed to split into B2g and B3g components in the orthorhombic phase.