Abstract
We measured the room temperature far-infrared reflectivity and Raman scattering spectra of 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 space group, which assumes the existence of the and chains, the 15 and 7 modes can be expected in both ir and Raman scattering spectra from the (001) plane. Only eight Raman and six infrared modes of symmetry are clearly seen. In the case of symmetry, three 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 crystal structure cannot be (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.
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