Infrared study of the phonon modes in PrMnO3 and CaMnO3

The infrared (IR) reflectivity spectra of orthorhombic manganese perovskites PrMnO3 and CaMnO3 are studied in the frequency range of optical phonon modes at temperatures varying from 300 to 4K. The IR phonon spectra of these two materials are analyzed by a fitting procedure based on a Lorentz model, and assigned to definite vibrational modes of Pnma structures by comparison with the results of lattice dynamical calculations. The calculations have been performed in the framework of a shell model using short range Born–Mayer–Buckingham and long range Coulomb potentials, whose parameters have been optimized in order that the calculated Raman and IR active phonon frequencies, and lattice parameters match with their experimental values. We find a close correspondence between the values of the IR phonon frequencies of PrMnO3 and CaMnO3, which shows that the substitution of the Pr3+ ions with Ca2+ results in a reduction of the frequency of medium- and high-energy IR phonons, and an increase of the frequency of those of low-energy. Nevertheless, the experimentally obtained IR phonon amplitudes of the two materials appear to be unrelated. A comparative study of the vibrational patterns of these modes reveals that most of them correspond to complex atomic vibrations significantly different from PrMnO3 to CaMnO3 which cannot be assigned only to a given type of vibration (external, bending, or stretching modes). In particular, these results confirm that the structure of CaMnO3 is quite far from the ideal (cubic) perovskite structure.