We have measured the infrared reflectivity spectra of ${\mathrm{La}}_{1-y}\left[\mathrm{Sr}\right(\mathrm{Ba}\left){]}_y{\mathrm{Mn}}_{1-x}\right[\mathrm{Cu}(\mathrm{Zn},\mathrm{Sc}){]}_x{\mathrm{O}}_3$ $(0<y<~0.3,0<~x<~0.10)$ manganites, in a wide frequency $(100\mathrm{}–4000{\mathrm{cm}}^{-1})$ and temperature (80 K–300 K) range. The reflectivity spectra were analyzed by a fitting procedure based on a model that includes Drude, midinfrared electronic, and phonon oscillator contributions to the dielectric constant. Six infrared active ${(3A}_u$ and ${3E}_u)$ modes of rhombohedral symmetry are clearly observed. We assigned all observed modes according to existing lattice dynamical calculations for the rhombohedral structure of ${\mathrm{LaMnO}}_3.$ A decrease of the rhombohedral distortion below the paramagnetic-ferromagnetic transition is manifested through a lowering or complete removal of the frequency difference between the $A_u$ and $E_u$ modes.

• Received 10 December 2002

DOI:https://doi.org/10.1103/PhysRevB.68.064302