Assignment of optical phonons at the zone center of distorted orthorhombic RCrO3 (R = La, Pr, Nd, Sm, Eu) perovskites using force-field lattice model

摘要

The assignment of the optical phonons to the specific active modes of an arbitrary crystal structure provides important keys to establish a proper correlation between the structural changes, microstructural characterization and the properties of the material. A short-range force-field model is, therefore, applied to investigate the optical phonon distributions and complex behavior of atomic motions in distorted orthorhombic (space group: Pbnm) perovskites RCrO3 (R = La, Pr, Nd, Sm, Eu). The lattice dynamics calculations are performed taking account of the nine bond-stretching and seven angle-bending force constants within the framework of normal coordinate analysis. The model calculations provide good agreement between our theoretical and the previously observed Raman and infrared modes of orthochromites RCrO3. All of the total forty nine optical modes of these perovskites are assigned to the specific modes at Gamma-point of the Brillouin zone simultaneously. The potential energy distribution (PED) coefficients have also been evaluated to investigate the contribution of each force constant to the calculated optical phonon modes. Depending on the bond strengths and PED of various coupling between ions forming bonds, complex behavior of several low-wave number Raman modes, namely, mode mixing and mode crossing, in the light of atomic movements are addressed for this series of orthochromites.