Electronic structure of Cr impurity in Al2O3 from first-principle calculation

We have carried out the first-principle calculation of the electronic structure of ruby (Cr impurity in Al2O3). Our results showed that the crystal lattice distortion lifted degeneracy of 3d levels of Cr ions. When the Coulomb interaction between 3d-electrons of Cr ions was taken into account via the LDA+U calculation scheme, the energy gap increased. The obtained value of the first excitation from the valence band to the first empty impurity state agreed well with the optical spectroscopic data