Optical phonons in spin - Peierls compound NaV2O5

We measured the room temperature far-infrared reflectivity and Raman scattering spectra of NaV2O5 single crystals. The frequencies of infrared active modes are obtained by Kramers - Kronig analysis of reflectivity data. The assignation of the observed modes is given according to the lattice dynamical calculation based on the valence shell model. According to the factor group analysis of the P21mn space group, which assumes the existence of the V4+ and V5+ chains, the 15 A1 and 7 B1 modes can be expected in both ir and Raman scattering spectra from the (001) plane. Only eight Raman and six infrared modes of A1 symmetry are clearly seen. In the case of B1 symmetry, three B1 modes are observed both in the Raman and in the ir reflectivity spectra. The frequencies of these ir and Raman modes differ significantly. Because of this, we concluded that the space group of the NaV2O5 crystal structure cannot be P21mn (non-centrosymmetric), but the space group which includes the mutual exclusion between Raman and infrared activity (centrosymmetric space group). We have shown that the appropriate space group is Pmmn, for which we found our experimental spectra in complete agreement with factor-group analysis. This means that V atoms are indistinguishable in the unit cell and in a mixed-valence state. (author). Letter-to-the-editor