OPTICAL PHONONS IN SPIN-PEIERLS COMPOUND NAV2O5

We measured the room temperature far-infrared reflectivity and Raman s cattering spectra of NaV2O5 single crystals. The frequencies of infrar ed active modes are obtained by Kramers-Kronig analysis of reflectivit y data. The assignation of the observed modes is given according to th e lattice dynamical calculation based on the valence shell model. Acco rding to the factor group analysis of the P2(1)mn space group, which a ssumes the existence of the V4+ and V5+ chains, the 15 A(1) and 7 B-1 modes can be expected in both ir and Raman scattering spectra from the (001) plane. Only eight Raman and six infrared modes of A(1) symmetry are clearly seen. In the case of B-1 symmetry, three B-1 modes are ob served both in the Raman and in the ir reflectivity spectra. The frequ encies of these ir and Raman modes differ significantly. Because of th is, we concluded that the space group of the NaV2O5 crystal structure cannot be P2(1)mn (non-centrosymmetric), but the space group which inc ludes the mutual exclusion between Raman and infrared activity (centro symmetric space group). We have shown that the appropriate space group is Pmmn, for which we found our experimental spectra in complete agre ement with factor-group analysis. This means that V atoms are indistin guishable in the unit cell and in a mixed-valence state.