Cr³⁺-Doped Fluorides and Oxides: Role of Internal Fields and Limitations of the Tanabe–Sugano Approach

Abstract

This work is aimed at clarifying the changes on optical spectra of Cr(3+) impurities due to either a host lattice variation or a hydrostatic pressure, which can hardly be understood by means of the usual Tanabe-Sugano (TS) approach assuming that the Racah parameter, B, grows when covalency decreases. For achieving this goal, the optical properties of Cr(3+)-doped LiBaF(3) and KMgF(3) model systems have been explored by means of high level ab initio calculations on CrF(6)(3-) units subject to the electric field, E(R)(r), created by the rest of the lattice ions. These calculations, which reproduce available experimental data, indicate that the energy, E((2)E), of the (2)E(t(2g)(3)) → (4)A(2)(t(2g)(3)) emission transition is nearly independent of the host lattice. By contrast, the energy difference corresponding to (4)A(2)(t(2g)(3)) → (4)T(1)(t(2g)(2)e(g)(1)) and (4)A(2)(t(2g)(3)) → (4)T(2)(t(2g)(2)e(g)(1)) excitations, Δ((4)T(1); (4)T(2)), is shown to increase on passing from the normal to the inverted perovskite host lattice despite the increase in covalency, a fact which cannot be accounted for through the usual TS model. Similarly, when the Cr(3+)-F(-) distance, R, is reduced both Δ((4)T(1); (4)T(2)) and the covalency are found to increase. By analyzing the limitations of the usual model, we found surprising results that are shown to arise from the deformation of both 3d(Cr) and ligand orbitals in the antibonding e(g) orbital, which has a σ character and is more extended than the π t(2g) orbital. By contrast, because of the higher stiffness of the t(2g) orbital, the dependence of E((2)E) with R basically follows the corresponding variation of covalency in that level. Bearing in mind the similarities of the optical properties displayed by Cr(3+) impurities in oxides and fluorides, the present results can be useful for understanding experimental data on Cr(3+)-based gemstones where the local symmetry is lower than cubic.

Internal electric fields and color shift in Cr3+-based gemstones

“…Indeed the variations of this quantity along the series of oxides or fluorides have been shown to reflect the changes induced by the internal field on the electronic density. 34 It is worth noting now the similarities between the present problem and the static Jahn-Teller effect taking place, for instance, for d 9 ions under an initial coordination which is perfectly octahedral. In such cases the values of axial and equatorial metal-ligand distances of the equilibrium tetragonal geometry and the involved Jahn-Teller energy, E JT , can also be understood only through the frozen wave functions corresponding to the initial high-symmetry configuration.…”

“…As it has been discussed in the previous section, the great localization of the electrons of the impurity makes that this simple CrO 6 9− unit, but subject to the electric field E R (r), it is enough to unveil the microscopic origin of the different optical properties displayed by Cr 3+ in a series of oxides or fluorides. 11,12,17,34 Calculations have been performed by means of the ADF 2010.02 code. 50 The generalized gradient approximation (GGA) exchange-correlation energy was computed according to the Perdew-Wang-91 functional.…”

“…32,33 Bearing in mind these facts, recent results on Cr 3+ -doped different fluoride and oxide lattices have shown that the observed differences in absorption and emission spectra can all be understood by just considering the internal electric field, E R (r), created by the rest of lattice ions upon the CrF 6 3− or CrO 6 9− complex, respectively, where active electrons are confined. 34 This internal field is generated by the corresponding electrostatic potential, V R (r), through E R (r) = − ∇V R (r). Obviously, covalency in the complex is influenced by such an internal field, although unpaired electrons are not flowing outside the complex.…”

“…Also the distinct optical and magnetic properties exhibited by transition-metal impurities in the normal perovskite KMgF 3 and in the inverted perovskite LiBaF 3 have been accounted for through the same mechanism. 32,34 Furthermore, it has been shown that E R (r) plays a key role for understanding the local structure and the associated spectroscopic properties of Cu 2+ -and Cr 3+ -doped layered perovskites such as K 2 AF 4 (A = Mg, Zn). 39 Despite that the mere addition of the internal electric field E R (r) to the CrO 6 9− complex allows one to understand an important number of experimental facts corresponding to the Cr 3+ -based gemstones, there is, however, a fundamental question which needs to be answered.…”

Direct Observation of Cr3+ 3d States in Ruby: Toward Experimental Mechanistic Evidence of Metal Chemistry

“…The DFT study by Verger et al confirms the suggestion from previous observations , that the 10Dq value includes effects of the nature of the (second nearest neighbor) cationic environment. Moreno and co-workers have systematically worked along these lines, as they have shown by many DFT calculations ,,,− that the inclusion of a nonlocal electric field E­(r) of the host lattice, acting on the localized electrons of the chromium ions, is causing nonzero additions to the local 10Dq, depending on how the additional electric field works along different spatial directions of the ion. Inclusion of such E­(r) helps for example to explain why the spinel MgAl 2 O 4 :Cr 3+ and ruby appear red and emerald green. , …”