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.
We present the results of pressure-dependent far-infrared reflectivity measurements on the multiferroic perovskite BiFeO3 at room temperature. The observed behavior of the infrared-active phonon modes as a funtion of pressure clearly reveals two stru
ctural phase transitions around 3.0 and 7.5 GPa, supporting the results of recent Raman and x-ray diffraction studies under pressure. Based on the pressure-dependent frequency shifts of the infrared-active phonon modes we discuss the possible character of the phase transitions.
