Effects of Covalency on the Phonon Dispersion Relations in the NaCl-Type Alkali Halide Crystals. III

抄録

A lattice dynamical model for the NaCl-type crystals are constructed which is a composite of the covalency model and the deformation dipole model. Using this model, the covalency parameter n_c, the number of electrons occupying the outer most s orbitals of an alkali metal atom, is determined from the measured dispersion curves of 13 NaCl-type alkali halides by the least-squares analyses subject to the constraint that the model parameters reproduce the dielectric and elastic constants. The obtained n_c values are contrary to the expectation of the traditional idea (n_c=0): about 0.08 for the fluorides, 0.10 the chlorides, and 0.15 the bromides and iodides.