Electronic structures and optical properties of spinel ZnCr₂O₄

The electronic structures and optical properties of zinc chromite, ZnCr₂O₄, were calculated using the density-functional theory within the local-spin-density approximation (LSDA), the LSDA plus Hubbard-like parameter U, LSDA+U, and the spin-polarized generalized-gradient approximation. ZnCr₂O₄ was found to be a direct-band-gap semiconductor with an optical band gap of 2.9 eV, with the uppermost valence bands (lowermost conduction bands) having mainly Cr-t_2g (Cr-e_g) characteristics. The calculated band gap using LSDA+U showed better agreement with the available experimental data than previous calculations. The optical-conductivity and the reflectivity spectra were also calculated. The calculated reflectivity spectrum exhibited two peaks, one at ~3.5 eV and the other at ~5.8 eV, which were assigned to the electronic interband transitions from the O-2p to the Cr-3d states and from the O-2p derived bands to the Cr e_g bands, respectively.