The Evolution of the IR and Raman Spectra When the Symmetry Reduces—The Case of the LaCoO3 Perovskite

The IR and Raman spectra of the LaCoO3 perovskite (the formal occupancy on Co is d6, low spin, t2g6) are computed by imposing space groups (SG) of decreasing symmetry, from the cubic (SG Pm-3m, N. 221, and Fm-3m, N. 225), to the tetragonal (SG 140), rhombohedral (SG 167) and orthorhombic (SG 62) ones. The total energy differences between these structures, computed at the quantum mechanical level by using an all electron Gaussian type basis set and the full range hybrid B3LYP functional, is extremely small: 0.2 mEh between SG 167 and SG 62, 1.2 mEh between SG 140 and SG 62, and 4.5 mEh between the most stable structure and the cubic, ideal aristotype. These minor differences indicate that the experimentally proposed SG might be one of the (many) possible alternatives. The IR and Raman spectra are very rarely used for the identification of the symmetry of the perovskites at different temperatures. Here we investigate the evolution of the two spectra (IR and Raman) through the various competing space groups, exploring the possibility that they might be used for the identification of the low temperature structure (SG and position of the atoms) of the investigated compounds, and of perovskites in particular.