Dielectric properties of crystals with rutile structure. II. Experimental results (neutron-Raman) and model discussion

To carry out a study of the lattice dynamics of a crystal with a rather complex structure (MgF 2 ) a whole set of complementary experimental results and, particularly, absolute measurements of first-order Raman polarisabilities have been obtained. Applying a convenient phenomenological model (shell model) to the case of MgF 2 a quite satisfactory description of the strictly dynamical properties (eigenvalues of the phonon spectrum and to some extent eigenvectors) are found. Going further in the application of this model the dielectric properties of the crystal (dielectric constant, electronic polarisabilities of ions, strengths of infrared modes and Raman polarisabilities) are calculated. Although the orders of magnitude are correctly reproduced, complete agreement between the model and experiment is far from being achieved for these latter properties.