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Long wavelength optical phonon frequencies for mixed chromite spinels

A.K.Kushwaha

A detailed study of long wavelength optical phonon frequencies for mixed chromite spinels CdCr2Se4(1-x)S4x, Zn1-xCdxCr2S4, CdxZn1-xCr2Se4 and HgxZn1-xCr2Se4 are presented using rigid-ion model. In this model, the short-range part is evaluated by the expansion of potential energy in the harmonic approximation using TaylorÂ’s expansion and the long-range part is evaluated by screened Coulomb potential. Using the above proposed model, we have calculated the composition dependent zone-centre optical phonon frequencies for mixed chromite spinels. We have found an overall good agreement between experimental and theoretical results. We have also found that the ionicity of spinel structure compounds decreases as CdCr2X4 > ZnCr2X4 > HgCr2Se4, (X = S and Se) and sulphide spinels are more ionic than the selenide spinels.