Study of the Wurtzite-Type Binary Compounds. IV. Theory of Double Refraction

Abstract

The Ewald-Born theory of double refraction is applied to wurtzite-type compounds. Except for the small distortion and polarization, these compounds have identical near-neighbor symmetry to zinc blende (cubic) structures, and hence the double refraction is caused primarily by the distant-neighbor interactions. The effects of anisotropy of the Lorentz-Lorenz force and of the Coulomb force are calculated. In the (ionic) approximation of zero cation polarizability the theory predicts a double refraction larger than the measured one by a factor ∼2 for all six compounds that have been measured. In the (covalent) approximation of zero effective ionic charge the theory underestimates the double refraction of AlN, but may be made to agree with the measured values of the other compounds by a suitable choice of atomic polarizabilities. In both approximations the theory predicts n0<ne, in agreement with long-wavelength observations. Anisotropic interaction of polarized light with the solid as a whole, as in exciton transitions, seems to account for the inversion to n0>ne near the absorption edge of CdS.