INFRARED VIBRATIONAL-MODES AND ANISOTROPY OF THE EFFECTIVE IONIC CHARGE IN CUALSE2, CUALS2, AND CUGASE2 CRYSTALS

Reflectivity spectra of CuAlS2, CuAlSe2, and CuGaSe2 crystals have been investigated in the wave number range 50 to 600 cm(-1) for the polarizations E parallel to c and E perpendicular to c. The fundamental phonon parameters, the limiting dielectric constants epsilon(infinity) and epsilon(s) and the reflectivity spectra contours have been calculated by using classical dispersion relations for both E parallel to c and E perpendicular to c configurations. The Szigeti effective charges and the relative ion charges of Cu, Al, Ga, Se, S anions and cations have been calculated in dependence on the incident light polarization.