Effects of lattice distortion on the deep Cu center in ZnS

Self-consistent linear combination of atomic orbitals cluster calculations are performed for the substitutional copper impurity in ZnS, showing the electronic changes induced by the impurity. The new bonding forces lead to a relaxation of the neighboring atoms. Total-energy calculations yield a new energy minimum at an outward breathing distortion of 0.21 Å. Consequently the $e$ and $t_2$ levels are shifted in the gap, and the crystal-field splitting parameter $10Dq=\epsilon \left(t_2\right)-\epsilon \mathrm{}\left(e\right)\mathrm{}$ is calculated to be 0.68 eV for the relaxed cluster compared to 0.77 eV from experiment.