Lattice Dynamics at Zone-Center of Sulphide and Selenide Spinels

A rigid-ion model is used to calculate the force constants and effectivedynamical charges of sulphide and selenide spinels. The Raman and infraredphonon modes of normal cubic sulphide spinels MCr₂Se₄(M = Mn, Co, Fe, Hg, Zn, and Cd) and selenide spinels MCr₂Se₄(M = Hg, Zn, and Cd) are calculated at the first Brillouin zone-centre using above model. Thesignificant outcome of the present work is (i) the interatomic interactionbetween Cr-S (Se) dominates over the Cr-S(Se) and S-S(Se-Se) type ofinteratomic interactions, (ii) the effective dynamical charges of thebivalent metal ions are nearly zero, and (iii) the selenide spinels are lessionic than the sulphide spinels and the ionicity decreases asMnCr₂S₄ > FeCr₂S₄ >CoCr₂S₄ > andCdCr₂C₄ >ZnCr₂C₄ >HgCr₂C₄(C = S and Se). The zone-center phonon frequencies, calculated using these parameters, arefound to be in very good agreement with the observed results.