NaCl structure lattice vibrations

Raman spectra are treated in the section on lattice vibrations, see p. 329, and only some supplemental data are given below. The first-order spectra are symmetry forbidden in the cubic NaCl structure, but possibly permitted by lattice defects. 

Chapter 3 is devoted to dipole dispersion laws for collective excitations on various planar lattices. For several orientationally inequivalent molecules in the unit cell of a two-dimensional lattice, a corresponding number of colective excitation bands arise and hence Davydov-split spectral lines are observed. Constructing the theory for these phenomena, we exemplify it by simple chain-like orientational structures on planar lattices and by the system CO2/NaCl(100). The latter is characterized by Davydov-split asymmetric stretching vibrations and two bending modes. An analytical theoretical analysis of vibrational frequencies and integrated absorptions for six spectral lines observed in the spectrum of this system provides an excellent agreement between calculated and measured data. 

Only the NaCl region has been measured for magnesium and calcium orthovanadates (57) thus no data are available for either of the bending vibrations. Previously the IR spectrum of Ca3(V04)2 had been discussed with incorrect crystallographic data. An X-ray structural analysis has only recently been reported 54) which shows that in the lattice crystallographicaUy non-equivalent VO4 groups are present. Also in the case of Mg3(V04)2 it was not previously possible to differentiate between two possible site symmetries, C2 or Cj 51). A recently reported crystallographic study shows that the VO4 ions sit on C, sites (55). 

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