The pressure-induced shifts of energy spectra of ZnSe : Ni2+

The calculations of the whole energy spectrum of ZnSe:Ni2+ at normal pressu re and pressure-induced shifts of its levels have been carried out on the b asis of the theory of pressure-induced shifts and the diagonalization of th e complete d(8) energy matrix in a regular tetrahedral field. The calculate d results are in very good agreement with experimental data at normal and v arious pressures. By taking into account the pressure-induced shifts of spe ctral bands, the new assignments of bands are given. The comparison between the results of ZnSe:Ni2+ and MgO:Ni2+ shows that the covalency of the bond ing between Ni2+ and ligands (Se2-) in ZnSe:Ni2+ is obviously stronger than the one of the bonding between Ni2+ and ligands (O2-) in MgO:Ni2+; the exp ansion of electron wavefunctions of Ni2+ in ZnSe:Ni2+ under pressure is obv iously larger than that of Ni2+ in MgO:Ni2+ under pressure. On the basis of these results, the physical origins of the remarkable difference between t he pressure-induced shifts of levels of ZnSe:Ni2+ and those of MgO:Ni2+ are explained. It is found that the large expansion of electron wavefunctions and a phase transition of ZnSe:Ni2+ make the pressure-induced shifts of lev els of ZnSe:Ni2+ strongly nonlinear.