Evolution of optical properties of chromium spinels CdCr${}_2$O${}_4$, HgCr${}_2$S${}_4$, and ZnCr${}_2$Se${}_4$ under high pressure

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.