Lattice vibrations of alpha '-NaV2O5

We have measured far infrared reflectance and transmittance spectra as well as Raman scattering spectra of alpha(')-NaV2O5 single crystals for all the principal polarizations. The temperature range above the phase transition temperature T-c=35 K was investigated, mainly. On the basis of this experim ental study and of the lattice dynamics calculations we conclude that the s ymmetry of NaV2O5 in the high-temperature phase is described by the centros ymmetric D-2h(13) space group. This conclusion leads to important physical consequences concerning the interpretation of one-dimensional magnetic prop erties of NaV2O5 and of the phase transition at 35 K considered earlier to be an ordinary spin-Peierls transition. The assignment of the observed phon ons is given. Values of dielectric constants are obtained from the infrared data. Asymmetric shapes of several infrared lines and higher-order infrare d vibrational spectra are discussed. The crystal field energy levels of the 3d electron localized at the V4+ site have been calculated in the framewor k of the exchange charge model using the values of effective charges obtain ed from the lattice dynamics calculations. According to the results of thes e calculations, the broad optical bands observed earlier in the vinicity of 1 eV can be interpreted as phonon assisted d-d transitions. (C) 1999 Ameri can Institute of Physics. [S1063-7761(99)02006-5].