Comment on “Local Lattice Structure Study of the Octahedral (CrO₆)⁹⁻ Clusters for Cr³⁺ Ion Doping in a Variety of Oxide Crystals by Simulating the Corresponding EPR and Optical Spectra”

Abstract

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 ...