Anisotropic effect and negative dielectric permittivity in crystals

Anisotropy effects on the negative dielectric permittivity (DP) formation in the frequency region near the phonon lattice vibrations are considered using three crystalline modifications of boron nitride. Quantitative data on frequency intervals and negative DP values are presented for all anisotropic modifications. It is shown that anomalous dispersion of the refractive index falls to 0 <; n <; 1 in the presence of negative DP, but always leaves n >; 1 in the absence of negative DP. The significant role of the polar vibrations damping to determination of the real limits of negative DP region is clarified, especially for crystals with symmetry lower than cubic.