  We report pressure-dependent reflection and transmission measurements on
ZnCr$_2$Se$_4$, HgCr$_2$S$_4$, and CdCr$_2$O$_4$ single crystals at room
temperature over a broad spectral range 200-24000 cm$^{-1}$. The pressure
dependence of the phonon modes and the high-frequency electronic excitations
indicates that all three compounds undergo a pressure-induced structural phase
transition with the critical pressure 15 GPa, 12 GPa, and 10 GPa for
CdCr$_2$O$_4$, HgCr$_2$S$_4$, and ZnCr$_2$Se$_4$, respectively. The
eigenfrequencies of the electronic transitions are very close to the expected
values for chromium crystal-field transitions. In the case of the chalcogenides
pressure induces a red shift of the electronic excitation which indicates a
strong hybridization of the Cr d-bands with the chalcogenide bands.

Materials Science