Theoretical studies of the g-shift for Cr4+ ions in GaN crystal from crystal-field and charge-transfer mechanisms

The assignment of the 1.193 eV zero-phonon line of the 3d(2) ion in GaN crystal to the internal luminescence of Cr4+ on the Ga3+ site is analysed. Based on this assignment, the g-shift Ag (=g - g(s)) of the groundstate of this luminescence is calculated by considering not only the conventional contribution due to the crystal-field mechanism, but also the contribution due to charge-transfer mechanism. The calculated result shows good agreement with the observed value, suggesting that the assignment is reasonable. The calculated Ag due to the charge-transfer mechanism is opposite in sign and 62% in magnitude, compared with that due to the crystal-field mechanism, and so it cannot be neglected. It appears that for the 3d(2) (or 3d(n)) ion with a high valence state in crystals (particularly, in the case of the ligand having small optical electronegativity), the reasonable explanation of the g-shift should consider the contributions from both the crystal-field and charge-transfer mechanisms.