Lattice dynamics and magnetic ordering in the chalcogenide spinels CdCr2S4 and CdCr2Se4

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Abstract
A detailed analysis is presented of the k = 0 phonons of CdCr2S4 and CdCr2Se4 which is based on a simple force model with four adjustable short range parameters. With the help of the derived dynamical matrices for the different symmetry species, the analysis can be performed easily for any compound with the normal spinel structure. From the measured frequencies of the Raman and infrared active phonons of CdCr2S4 and CdCr2Se4 the force constants and eigenvectors of the different normal modes are calculated. We firstly discuss the derived force fields of these two compounds and secondly the coupling between the internal and external lattice vibrations. Finally, with the help of the calculated normal coordinates, the observed influence of the magnetic ordering on the Raman‐ and infrared active phonons of CdCr2S4 and CdCr2Se4 is studied in detail.