Polar phonons and spin-phonon coupling in HgCr2S4 and CdCr2S4 studied with far-infrared spectroscopy

Polar phonons of HgCr(2)S(4) and CdCr(2)S(4) are studied by far-infrared spectroscopy as a function of temperature and external magnetic field. Eigenfrequencies, damping constants, effective plasma frequencies, Lyddane-Sachs-Teller relations, and effective charges are determined. Ferromagnetic CdCr(2)S(4) and antiferromagnetic HgCr(2)S(4) behave rather similar. Both compounds are dominated by ferromagnetic exchange, and although HgCr(2)S(4) is an antiferromagnet, no phonon splitting can be observed at the magnetic phase transition. Temperature and magnetic-field dependencies of the eigenfrequencies show no anomalies, indicating displacive polar soft-mode behavior. However, significant effects are detected in the temperature dependence of the plasma frequencies, indicating changes in the nature of the bonds and significant charge transfer. In HgCr(2)S(4), we provide experimental evidence that the magnetic-field dependence of specific polar modes reveals shifts exactly correlated with the magnetization, showing significant magnetodielectric effects even at infrared frequencies.