Optical spectra of MgO:Cr³⁺ and MgO:V²⁺ and their pressure-induced shifts

The R line and U band optical spectra of substitutional Cr³⁺ and V²⁺ in MgO have been calculated at normal pressure, and their pressure-induced shifts have been determined by use of the semi-empirical crystal-field (CF) theory and the first-principles SCF embedded-cluster discrete variational (DV-Xα) method. Local lattice relaxations around the impurity ion have been determined to high precision by matching theoretical to experimental spectra. The excellent agreement between crystal-field, SCF, and experimental results verifies the proposed dependence of CF parameters upon pressure, and shows that the SCF approach can be used with confidence in cases where insufficient data are available for semiempirical fitting.