Evolution of the 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 ZnCr2Se4, HgCr2S4, and CdCr2O4 single crystals at room temperature over a broad spectral range 200–24 000 cm−1. The pressure dependence of the phonon modes and the high-frequency electronic excitations indicate that all three compounds undergo a pressure-induced structural phase transition with critical pressures of 15, 12, and 10 GPa for CdCr2O4, HgCr2S4, and ZnCr2Se4, 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 redshift of the electronic excitations, which indicates a strong hybridization of the Cr d bands with the chalcogenide bands.