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Polar phonons of HgCr2S4 and CdCr2S4 are studied by far-infrared spectroscopyas a function of temperature and external magnetic field. Eigenfrequencies,damping constants, effective plasma frequencies and Lyddane-Sachs-Tellerrelations, and effective charges are determined. Ferromagnetic CdCr2S4 andantiferromagnetic HgCr2S4 behave rather similar. Both compounds are dominatedby ferromagnetic exchange and although HgCr2S4 is an antiferromagnet, no phononsplitting can be observed at the magnetic phase transition. Temperature andmagnetic field dependence of the eigenfrequencies show no anomalies indicatingdisplacive polar soft mode behavior. However, significant effects are detectedin the temperature dependence of the plasma frequencies indicating changes inthe nature of the bonds and significant charge transfer. In HgCr2S4 we provideexperimental evidence that the magnetic field dependence of specific polarmodes reveal shifts exactly correlated with the magnetization showingsignificant magneto-dielectric effects even at infrared frequencies.

Polar phonons and spin-phonon coupling in HgCr2S4 and CdCr2S4