Local lattice distortions and Raman spectra in the ${\mathrm{La}}_{1-x}{\mathrm{Ca}}_x{\mathrm{MnO}}_3$ system

Raman spectra were measured for the full range of hole doping of the orthomanganites of the form ${\mathrm{La}}_{1-x}{\mathrm{Ca}}_x{\mathrm{MnO}}_3$ $(0\mathrm{}<~x<~1)$ from ambient temperature to 75 K. Two high-frequency phonons that we assign to stretching modes of the oxygen atoms in the ${\mathrm{MnO}}_6$ octahedra soften below the transition temperature to ferromagnetic insulator and disappear at a temperature that depends on the Ca concentration. The correlation of their properties with the lattice distortions is examined. At room temperature, and for $x\sim 0.35$ and $x\sim 0.70,$ phonons attributed to the relative rotations of the octahedra show a strong correlation with the octahedral distortion and the reported low-temperature modifications of the magnetic properties of these compounds.