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A series of samples of the system ZnIMgo.8-,N~.2Fe~04 (with x=0,0.2,0.4,0.6 and 0.8), was prepared using the usual ceramic technique. The samples were studied using X-ray diffraction, DTA, TG, TGA and IR spectroscopy. The lattice constant, radius of tetrahedral ion, X-ray density and measured bulk density are found to increase while porosity decreases as zinc ion content increases. Both TG and TGA spectra show two exothermic phases at the magnetic T, and Curie T, transition temperatures while DTA does not indicate any exothermic processes. The transition temperatures were found to increase as Zn ion content decreases. The four IR bands speclfyig femtes are observed whose intensities and positions are composition dependent.

Using recent band-structure results obtained in the local-density scheme, the microscopic longitudinal dielectric function for both the diagonal and the non-diagonal elements is evaluated in the random-phase approximation in the energy range up to 2.5 Ryd.In addition, the influence of exchange and correlation is investigated in a self-consistent way within the time-dependent density-functional approach. We have found that the latter may modify the frequency dependence of the real elements of the dielectric matrix by up to 15%, whereas some imaginary elements are changed by up to 40%. In the long-wave limit, the local-field corrections of the macroscopic dielectric function seem to be negligible for most frequencies. For the diagonal elements of the inverse dielectric matrix with non-zero reciprocal lattice vectors, local-field corrections increase the real parts by up to 10% at low frequencies, whereas at higher frequencies their influence is quite small. The imaginary parts are decreased over the whole frequency range by up to 30%. For all spectra, local-field corrections smooth the structures and the inclusion of exchange and correlation decreases the values of the elements of dielectric matrix. In addition, for realistic wavefunctions, the f sum rule turned out to be badly fulfilled. Plausible arguments for this failure are presented.

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