Polarized Raman spectra of single and polycrystalline $R_{1-x}{A}_x{\mathrm{MnO}}_3\hspace{0.5em}(R=\mathrm{L}\mathrm{a},\mathrm{P}\mathrm{r};A=\mathrm{C}\mathrm{a},\mathrm{S}\mathrm{r})$ ceramic samples were studied as a function of temperature. For the rhombohedral ${\mathrm{LaMnO}}_{3.1}$ and ${\mathrm{La}}_{0.7}{\mathrm{Sr}}_{0.3}{\mathrm{MnO}}_3,$ the observed Raman peaks were associated with modes arising from the folding of the Brillouin zone under lattice deformation. For the orthorhombic ${\mathrm{LaMnO}}_{3.0},$ the Raman spectra are consistent with the $\mathrm{Pnma}$ structure and show an anomalous softening of the 494 and 604 ${\mathrm{cm}}^{-1}$ modes below the antiferromagnetic ordering temperature $T_N\simeq 140$ K. Polycrystalline samples of ${\mathrm{La}}_{0.5}{\mathrm{Ca}}_{0.5}{\mathrm{MnO}}_3$ show a dramatic change of the Raman spectra between 100 and 160 K, which was associated with the increase of the orthorhombic distortion observed by others for $T\lesssim 240$ K. Other $R_{1-x}{A}_x{\mathrm{MnO}}_3$ single crystals, with small orthorhombic distortions, show Raman spectra which are similar to those observed in the rhombohedral samples.

• Received 24 February 1998

DOI:https://doi.org/10.1103/PhysRevB.58.11435