Mechanisms responsible for the local geometry around Jahn-Teller impurities in K2NiF4 type lattices are shown to be different from those generating the warping in cubic crystals. The present density functional theory calculations reveal that the elastic anisotropy of the host lattice (visible for closed shell impurities) and the electric field created by the rest of lattice ions upon active electrons make it possible to have d(9) ions in an elongated geometry but with a A(1g) ground state. The puzzling difference between equilibrium geometries for Cu2+ and Ni+ in layered perovskites can reasonably be understood.
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+).
The microscopic origin of optical and magnetic properties displayed by Cr 3+ impurities in oxide lattices is a subject of current interest as it plays a key role for understanding the color exhibited by gemstones like ruby (Al 2 O 3 :Cr 3+ ), emerald (Be 3 Si 6 Al 2 O 18 :Cr 3+ ) or alexandrite (BeAl 2 O 4 :Cr 3+ ). In a recent paper Kuang et al. claim 1 that reasonable values of the mean equilibrium Cr 3+ -O 2-distance, R (and also of the small distortion undergone by the oxygen octahedron), can be obtained from an analysis of experimental values of optical transitions and EPR parameters through a parametrized crystal-field model. In essence such an analysis is founded on two main assumptions:(1) Electronic properties due to Cr 3+ impurities in oxides can be understood considering only the CrO 6 9-complex at the right equilibrium geometry. (2) Accordingly, changes in the cubic field splitting parameter, 10 Dq, reflect necessarily variations of R following the law 10 Dq ) KR -n , where K is a constant and the exponent n would be close to 5.As the color of insulating oxides doped with Cr 3+ essentially depends on the energy of the first spin allowed 4 A 2g (t 2g 3 ) f 4 T 2g (t 2g 2 e g ) transition, 2 which is just equal to 10 Dq, such assumptions mean that a change in color implies a variation of the average equilibrium Cr 3+ -O 2-distance. By virtue of this fact in the analysis carried out by Kuang et al. the experimental 10 Dq values measured for emerald (16130 cm -1 ) and ruby (18070 cm -1 ) are explained by stating 1 that mean equilibrium Cr 3+ -O 2-distances are R ) 202 and 195.4 pm for emerald and ruby, respectively. However, this conclusion is against experimental EXAFS data, 3-5 which unambiguously prove that emerald and ruby have the same R value (equal to 197 pm) within the experimental uncertainty (1 pm). Along this line a nearly equal value R ) 198 ( 1 pm has recently been measured 6 for the spinel MgAl 2 O 4 :Cr 3+ . It is certainly surprising that these relevant experimental facts are not taken into consideration in the paper 1 by Kuang et al. In fact, information on the actual R values for ruby and emerald is given in refs 3 and 7, which are also quoted in the paper 1 by Kuang et al.From a fundamental point of view the wrong conclusion reached by Kuang et al. is thus not different from that reached by Orgel 50 years ago. 8 This author already showed that, in the framework of the traditional ligand field theory, the different color displayed by ruby and emerald can only be explained by assuming that R(emerald) -R(ruby) should be around 5 pm.It has recently been shown that this failure of the traditional ligand field theory obeys the fact that properties of a transition metal impurity, M, in an insulating lattice are assumed to be explained only through the MX N complex (formed with the N ligands) in vacuo. 7,9 However, although active electrons are usually confined in the complex region they are also subject to the internal electric field, E R , due to all ions of the insulating host lattice lying ...
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
customersupport@researchsolutions.com
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.