For calculating the properties of ${\mathrm{Ni}}^{2+}$ centers in cubic ZnS, CdS, and ZnSe a molecular complex (cluster) of 17 atoms with the impurity atom at the center has been modeled using a self-consistent-field (SCF) closed-shell formalism in the complete-neglect-of-differential-overlap (CNDO) approximation. The effect of the surrounding crystal upon the cluster is taken into consideration by suitable boundary conditions. On the basis of the closed-shell results for the impurity clusters ${\mathrm{NiS}}_4$${\mathrm{Zn}}_{12}$, ${\mathrm{NiS}}_4$${\mathrm{Cd}}_{12}$, and ${\mathrm{NiSe}}_4$${\mathrm{Zn}}_{12}$, double-ionization potentials ($V_{\mathrm{DI}}$) for determining the energy difference E(${}^3$$A_2$)-E(${}^3$$T_2$) of the ${\mathrm{Ni}}^{2+}$ impurity spectrum were calculated. The calculated values for E(${}^3$$A_2$)-E(${}^3$$T_2$)=$V_{\mathrm{DI}}$(${}^3$$A_2$) -$V_{\mathrm{DI}}$(${}^3$$T_2$) are in satisfactory agreement with the experimental values. In the case of ${\mathrm{Ni}}^{2+}$ centers in cubic CdS, E(${}^3$$A_2$)-E(${}^3$$T_2$) is computed to be 0.47 eV compared with the experimental value of 0.51 eV.

• Received 15 March 1985

DOI:https://doi.org/10.1103/PhysRevB.32.5273