Transition Metal Complexes Coupled to Vacancies in Oxides: Origin of Different Properties of Cr³⁺in MgO Bounded to a ⟨100⟩ or ⟨110⟩ Mg²⁺Vacancy

Abstract

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 (2)E → (4)A2 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 (2)E → (4)A2 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 CrO6(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 C4v and C2v centers in MgO:Cr(3+).

Effect of Oxygen Vacancies on the Mechanoluminescence Response of Magnesium Oxide

“…The crystallite sizes of the MgO­(SCS) powders are in agreement with previously reported data on MgO produced by a similar methodology . Furthermore, very small changes in lattice parameters for all the materials are noted (see Table ), suggesting the absence of significant substitutional impurities in the materials, which can potentially result in lattice parameter variations. , The lattice parameter values are in close agreement with those reported by Janet et al . for MgO nanopowders produced at different calcination temperatures.…”

“…39 Furthermore, very small changes in lattice parameters for all the materials are noted (see Table 1), suggesting the absence of significant substitutional impurities in the materials, which can potentially result in lattice parameter variations. 40,41 The lattice parameter values are in close agreement with those reported by Janet et al 12 for MgO nanopowders produced at different calcination temperatures. The scanning electron micrographs obtained for the MgO powders exhibited powder agglomerates of varying dimensions (Figure S1).…”

Off-centre motion in doped cubic oxides: A general view on the instability

“…This conclusion is consistent with EPR data on SrO:Cr 3+ [36] showing the existence of a centre with perfect cubic symmetry. This situation is thus similar to that encountered for MgO:Cr 3+ where, aside from the cubic centre, involving a remote charge compensation, other centres with close-by magnesium vacancies in h1 0 0i or h1 1 0i directions are also observed [44][45][46]. Bearing in mind that R 0 = 2.58 Å for SrO, the obtained value of the Cr 3+ -O 2À distance for the cubic centre in SrO:Cr 3+ (R = 2.197 Å) is, as expected, higher than R = 2.032 Å previously derived for the same centre in MgO:Cr 3+ where R 0 = 2.106 Å [46].…”

“…This situation is thus similar to that encountered for MgO:Cr 3+ where, aside from the cubic centre, involving a remote charge compensation, other centres with close-by magnesium vacancies in h1 0 0i or h1 1 0i directions are also observed [44][45][46]. Bearing in mind that R 0 = 2.58 Å for SrO, the obtained value of the Cr 3+ -O 2À distance for the cubic centre in SrO:Cr 3+ (R = 2.197 Å) is, as expected, higher than R = 2.032 Å previously derived for the same centre in MgO:Cr 3+ where R 0 = 2.106 Å [46]. These results are qualitatively consistent with the increase undergone by the g-shift of the cubic centre, g 0 -g, on going from MgO:Cr 3+ (g 0 Àg = 0.022) to SrO:Cr 3+ (g 0 Àg = 0.034) [36].…”

Electron paramagnetic resonance of Fe3+in gallium borate: Superposition model analysis

“…, in particular, that contributions from different metal‐to‐ligand bonds are axially symmetric and can be treated independently and that the characteristics of an individual bond are transferable across different crystals. These assumptions have been shown not to hold in the case of covalent bonding where changing metal‐to‐ligand distances or introducing a vacancy can modify the electronic density of the whole complex . Meanwhile, in iron‐doped GaBO 3 crystal, all ligands are located at the same distance from Fe 3+ ion; besides, no structure defects are present in the close vicinity of the paramagnetic ion.…”