The dispersion curves along the directions [100] and [001] of have been measured by inelastic neutron scattering. The results are analysed with different rigid-ion models: Born - von Karman and valence force field models, which give a good agreement with experiments. Additional measurements of the acoustical branches allow us to determine elastic stiffness constants which are () , , , , , . The compressibility calculated from these constants is in accordance with the experimental value.

3862THE CRYSTAL STRUCTURE OF MAGNESIUM DIMOLYBDATE this figure, the unshared O atoms are in the staggered conformation, whereas (b) and (c) represent two possible eclipsed forms. Further classification of the discussed group of compounds is based on the value of the X-O-X interbond angle which, in theory, can vary from 102 to 180 ° (Clark, 1972). Usually it falls within the range 110-155 o. The Mo2 O2-ion described in the present paper is close to the configuration in Fig. 3(c) Phase I of CBr 4 (above 47°C) is cubic, space group Fm3m, with a = 8.82 A, Z = 4. The molecules are orientationally disordered and the structure is described by the use of symmetry-adapted functions. The orientational probability is discussed in terms of these functions.

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The structure and lattice dynamics of CuInSe 2 were studied using ab initio calculations. The phonon dispersion relations and phonon density of states were calculated using the direct method. The results are in good agreement with recently obtained inelastic neutron scattering data.

Various known parameters of the atom–atom potential for the bromine atoms interaction have been used to model the experimental structure and the cohesive energy of the ordered phase II of CBr4. The set of parameters corresponding to krypton has turned out to give the best agreement with the experimental observations. The frequencies of the lattice vibrations at the zone center and the elastic constants have then been calculated and shown to be in a good quantitative agreement with the existing experimental data. The validity of the rigid molecule approach has been briefly discussed.