Transition Metal Complexes Coupled to Vacancies in Oxides: Origin of Different Properties of Cr~(3+) in MgO Bounded to a 100 or 110 Mg~(2+) Vacancy

摘要

Despite the importance of vacancies over the properties of insulating oxides its influence on neighboring transition metal ions is far from being understood. This work is devoted to find the origin of various up to now unexplained properties of chromium bounded either to a <100> or a <110> Mg~(2+) vacancy in MgO. In these model systems particular attention is paid to understand, by means of ab initio calculations, why the cubic field splitting parameter, 10Dq, is surprisingly 1600 cm~(-1) higher for a <100> than for a <110> vacancy, a fact behind the suppression of the sharp ~2E → ~4A_2 luminescence in the latter case. Our calculations, which reproduce the main experimental facts, prove that the average Cr~(3+)?O~(2?) distance is the same within 0.8% for both systems, and thus, the low 10Dq value for a <110> vacancy is shown to be due mainly to the electrostatic potential from the missing Mg~(2+) ion, which increases the energy of antibonding t_(2g) (～xy, xz, yz) levels. By contrast, for a <100> Mg~(2+) vacancy that potential provides a supplementary increase of the e_g (～x~2 ? y~2, 3z~2 ? r~2) level energy and thus of 10Dq. The existence of the ~2E → ~4A_2 luminescence for Cr~(3+)-doped MgO under perfect cubic symmetry or with a <100> vacancy is shown to be greatly helped by the internal electric field created by the rest of the lattice ions on the CrO_6~(9?) unit, whose importance is usually ignored. The present results underline the role of ab initio calculations for unveiling the subtle effects induced by a close vacancy on the properties of transition metal ions in oxides. At the same time they stress the failure of the empirical superposition model for deriving the equilibrium geometry of C_(4v) and C_(2v) centers in MgO:Cr~(3+).