Effect of structural and magnetic transitions in ${\mathrm{La}}_{1-x}{M}_x{\mathrm{MnO}}_3$ ($M=\mathrm{Sr},$ Ca) single crystals in Raman scattering

Polarized Raman spectra of undoped and doped ${\mathrm{La}}_{1-x}{M}_x{\mathrm{MnO}}_3$ single crystals have been studied in the temperature range from 5 to 423 K ($M=\mathrm{Sr},$ Ca). The spectral intensity is mainly related to scattering by optical phonons at $q\sim 0$ (for $x\approx 0$) and the density of vibrational states for $x>\mathrm{}\mathrm{0.1.}\mathrm{}$ Raman spectra from the undoped ${\mathrm{LaMnO}}_3$ structure distorted by the static Jahn-Teller effect are consistent with the orthorhombic $D_{2h}^{16}$ space group. Raman spectra of the doped ${\mathrm{La}}_{1-x}{M}_x{\mathrm{MnO}}_3$ crystals have weakly resolved features and intensities comparable to those of the second-order spectra in cubic media. The anomalies in the phonon spectra of doped crystals were explained by the doping dependence of the ionic radius of a La/Sr site. The most intense $B_{1g}$ mode in ${\mathrm{LaMnO}}_3$ crystal has an abrupt frequency shift near $T_N=140\mathrm{}\mathrm{K},$ which corresponds to a paramagnetic to canted-antiferromagnetic phase transition.