  We present a detailed study of the dielectric and charge transport properties
of the antiferromagnetic cubic spinel HgCr2S4. Similar to the findings in
ferromagnetic CdCr2S4, the dielectric constant of HgCr2S4 becomes strongly
enhanced in the region below 60 - 80 K, which can be ascribed to polar
relaxational dynamics triggered by the onset of ferromagnetic correlations. In
addition, the observation of polarization hysteresis curves indicates the
development of ferroelectric order below about 70 K. Moreover, our
investigations in external magnetic fields up to 5 T reveal the simultaneous
occurrence of magnetocapacitance and magnetoresistance of truly colossal
magnitudes in this material.
