Vibrational Spectra and Effective Charges in Some Spinels

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Abstract
The infrared reflectivity spectra have been observed for sintered MCr2S4(M=Zn, Cd, Hg) near normal incidence in the range from 30 to 900cm-1. The effective charges of Cr and S ions are determined from the frequencies of transverse and longitudinal modes within small error. The Szigetieffective charges e s * per effective valency Z eff for anions show the correlation with the low frequency electronic dielectric constant ε ∞ which has been established for selenide and oxide spinels and expressed as ε ∞-1=2.0/((e s */Z eff e)+(e s */Z eff e)2). By using this empirical relationship, the magnitude of effective charge of cation M is determined within the error in the charge of Cr ion. By the comparison among the obtained effective charges, dielectric constants ε ∞ and the lattice parameters, the increase of ionicity with the decrease of anion radius and a weak influence of chromium ion in comparison with that of cation M to the dielectric constant are shown. From these results, the similarity and the difference of dielectric properties between binary and ternary compounds are discussed.

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