Colour due to Cr3+ ions in oxides: a study of the model system MgO:Cr3+

Chemical Bonding, 10Dq Parameter and Superexchange in the Model Compound KNiF3

The cubic field splitting parameter, 10Dq, plays a central role in the ligand field theory on insulating transition metal compounds. Experimental data obtained in the last 50 years prove that 10Dq is highly dependent on changes of the metal-ligand distance, R, induced by chemical or applied pressures. Despite this fact has important consequences on optical and magnetic properties of such compounds, its actual origin is still controversial. Seeking to clarify that crucial issue, this work is focused on KNiF3, a reference system among insulating transition metal compounds. By means of first principles calculations we show that, contrary to what is usually thought, the R-dependence of 10Dq arises neither from the crystal field contribution nor from the covalent admixture of 3d(Ni) with valence 2p(F) orbitals. Indeed, we prove that it is mainly due to the residual covalency with deep 2s(F) orbitals, highly sensitive to R variations. As a salient feature the present calculations show that the 3d-2pσ and 3d-2pπ admixtures raise practically equal the energy of antibonding eg and t2g orbitals of NiF6 4- units in KNiF3 thus leading to a null contribution to 10Dq. This conclusion is not significantly altered when considering the change of covalency on passing from the ground state 3A2(t2g 6eg 2) to the excited state 3T2(t2g 5eg 3). The different influence of chemical bonding on the superexchange constant, J, and 10Dq is also discussed in a second step. It is pointed out that the strong dependence of J upon R can hardly be explained through the behavior of the 3d-2pσ covalency derived for a single NiF6 4- unit. For the sake of clarity, the meaning of 10Dq is also briefly analyzed.

Red Shift in Optical Excitations on Layered Copper Perovskites under Pressure: Role of the Orthorhombic Instability

The red shift under pressure in optical transitions of layered compounds with CuCl₆ ^4- units is explored through first-principles calculations and the analysis of available experimental data. The results on Cu²⁺ -doped (C₂ H₅ NH₃ )₂ CdCl₄ , that is taken as a guide, show the existence of a highly anisotropic response to pressure related to a structural instability, driven by a negative force constant, that leads to an orthorhombic geometry of CuCl₆ ^4- units but with a hole displaying a dominant 3z² -r² character (z being the direction perpendicular to the layer plane). As a result of such an instability, a pressure of only 3 GPa reduces by 0.21 Å the longest Cu²⁺ -Cl^- distance, lying in the layer plane, while leaving unmodified the two other metal-ligand distances. Owing to this fact, it is shown that the lowest d-d transition would experience a red shift of 0.34 eV while the first allowed charge transfer transition is also found to be red shifted but only by 0.11 eV that reasonably concurs with the experimental value. The parallel study on Jahn-Teller systems CdCl₂ :Cu²⁺ and NaCl:Cu²⁺ involving tetragonal elongated CuCl₆ ^4- units shows that the reduction of the long axis by a pressure of 3 GPa is three times smaller than that for the layered (C₂ H₅ NH₃ )₂ CdCl₄ :Cu²⁺ compound. Accordingly, the optical transitions of such systems, which involve a positive force constant, are much less sensitive to pressure than in layered compounds. The origin of the red shift under pressure undergone by the lowest d-d and charge transfer transitions of (C₂ H₅ NH₃ )₂ CdCl₄ :Cu²⁺ is discussed in detail.

Pressure Effects on 3d n (n=4, 9) Insulating Compounds: Long Axis Switch in Na 3 MnF 6 not Due to the Jahn‐Teller Effect

The pressure‐induced switch of the long axis of MnF₆³⁻ units in the monoclinic Na₃MnF₆ compound and Mn³⁺‐doped Na₃FeF₆ is explored with the help of first principles calculations. Although the switch phenomenon is usually related to the Jahn‐Teller effect, we show that, due to symmetry reasons, it cannot take place in 3dⁿ (n=4, 9) systems displaying a static Jahn‐Teller effect. By contrast, we prove that in Na₃MnF₆ the switch arises from the anisotropic response of the low symmetry lattice to hydrostatic pressure. Indeed, while the long axis of a MnF₆³⁻ unit at ambient pressure corresponds to the Mn³⁺−F₃⁻ direction, close to the crystal c axis, at 2.79 GPa the c axis is reduced by 0.29 Å while b is unmodified. This fact is shown to force a change of the HOMO wavefunction favoring that the long axis becomes the Mn³⁺−F₂⁻ direction, not far from crystal b axis, after the subsequent relaxation process. The origin of the different d‐d transitions observed for Na₃MnF₆ and CrF₂ at ambient pressure is also discussed together with changes induced by pressure in Na₃MnF₆. The present work opens a window for understanding the pressure effects upon low symmetry insulating compounds containing d⁴ or d⁹ ions.

Ground State and Optical Excitations in Compounds with Tetragonal CuF64- Units: Insight into KAlCuF6 and CuFAsF6

It has been argued that AAlCuF₆ (A = K, Cs) and CuFAsF₆ are the only known crystals that exhibit compressed CuF₆^4- units due to the Jahn-Teller effect. However, no grounds for this singular behavior have yet been reported. By means of first-principles calculations on such compounds and the isomorphous compounds involving Zn²⁺ ions instead of Cu²⁺, we prove that neither the ground state nor the equilibrium geometry of CuF₆^4- complexes in KAlCuF₆ and CuFAsF₆ is the result of a Jahn-Teller effect. In contrast, it is shown that the internal electric field, E_R(r), created by the rest of the lattice ions upon the localized electrons in the complex, plays an important role in understanding this matter as well as the d-d transitions of these two compounds. The energy of an optical transition is shown to involve two contributions: the intrinsic contribution derived for the isolated CuF₆^4- unit at equilibrium and the extrinsic contribution coming from the E_R(r) field. Aside from reproduction of the experimental d-d transitions observed for KAlCuF₆, it is found that in CuFAsF₆ the b_1g(x² - y²) → a_1g(3z² - r²) transition is not the lowest one due to the stronger effects from the internal field. Interestingly, the intrinsic contribution corresponding to that transition can simply be written as β(R_eq - R_ax) where R_eq and R_ax are the equatorial and axial Cu²⁺-F^- distances and β = 2.7 eV/Å is the same for all systems involving tetragonal CuF₆^4- units and an average metal-ligand distance close to 2.03 Å. This shows the existence of a common point shared by the Jahn-Teller system KZnF₃:Cu²⁺ and other non-Jahn-Teller systems such as KAlCuF₆, CuFAsF₆, K₂ZnF₄:Cu²⁺, and Ba₂ZnF₆:Cu²⁺. Although most Jahn-Teller systems display an elongated geometry, there are however many Cu²⁺ compounds with a compressed geometry but hidden by an additional orthorhombic instability. The lack of that instability in KAlCuF₆ and CuFAsF₆ is also discussed.

Understanding the Structure and Ground State of the Prototype CuF2 Compound Not Due to the Jahn-Teller Effect

Insulating CuF₂ is considered a prototype compound displaying a Jahn-Teller effect (JTE) which gives rise to elongated CuF₆^4- units. By means of first-principles calculations together with an analysis of experimental data of both CuF₂ and Cu²⁺-doped ZnF₂, we demonstrate that such an idea is not correct. For ZnF₂:Cu²⁺, we find that CuF₆^4- units are compressed always along the Z local axis with a hole essentially in a 3 z²- r² antibonding orbital, in agreement with experimental EPR data that already underline the absence of a JTE. The structure of the monoclinic CuF₂ crystal also comes from compressed CuF₆^4- complexes, although hidden by an additional orthorhombic instability due to a negative force constant of b_2g and b_3g local modes. The associated distortion, similar to that involved in K₂CuF₄ and other layered Cu²⁺ compounds, is also shown to be developed for ZnF₂:Cu²⁺ upon increasing the copper concentration. The origin of this cooperative effect is discussed together with the differences between non-Jahn-Teller systems like ZnF₂:Cu²⁺ and CuF₂ and true Jahn-Teller systems like KZnF₃:Cu²⁺. From present results and those on layered compounds, the usual assumption of a JTE for explaining the properties of d⁹ ions in low-symmetry lattices can hardly be right.

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

The role of transition metals in chemical reactions is often derived from probing the metal 3d states. However, the relation between metal site geometry and 3d electronic states, arising from multielectronic effects, makes the spectral data interpretation and modeling of these optical excited states a challenge. Here we show, using the well-known case of red ruby, that unique insights into the density of transition metal 3d excited states can be gained with 2p3d resonant inelastic X-ray scattering (RIXS). We compare the experimental determination of the 3d excited states of Cr³⁺ impurities in Al₂O₃ with 190 meV resolution 2p3d RIXS to optical absorption spectroscopy and to simulations. Using the crystal field multiplet theory, we calculate jointly for the first time the Cr³⁺ multielectronic states, RIXS, and optical spectra based on a unique set of parameters. We demonstrate that (i) anisotropic 3d multielectronic interactions causes different scaling of Slater integrals, and (ii) a previously not observed doublet excited state exists around 3.35 eV. These results allow to discuss the influence of interferences in the RIXS intermediate state, of core–hole lifetime broadenings, and of selection rules on the RIXS intensities. Finally, our results demonstrate that using an intermediate excitation energy between L₃ and L₂ edges allows measurement of the density of 3d excited states as a fingerprint of the metal local structure. This opens up a new direction to pump-before-destroy investigations of transition metal complex structures and reaction mechanisms.

Large Differences in the Optical Spectrum Associated with the Same Complex: The Effect of the Anisotropy of the Embedding Lattice

Transition-metal complexes with a well-defined geometry are usually considered to display almost the same properties independently of the system where they are embedded. Here we show that the above statement is not true depending on the anisotropy of the host lattice, which is revealed in the form of the electric field created by the rest of lattice ions over the complex. To illustrate this concept we analyze the origin of the surprisingly large differences in the d-d optical transitions of two systems containing square-planar CuF₄^2- complexes, CaCuF₄, and center II in Cu²⁺-doped Ba₂ZnF₆, even though the Cu²⁺-F^-distance difference is just found to be 1%. Using a minimalist first-principles model we show that the different morphology of the host lattices creates an anisotropic field that red-shifts the in vacuo complex transitions to the 1.25-1.70 eV range in CaCuF₄, while it blue-shifts them to the 1.70-3.0 eV region in Ba₂ZnF₆:Cu²⁺. This particular example shows how the lattice anisotropy strongly alters the optical properties of a given transition-metal complex. This knowledge opens a new path to tune the spectra of this large family of systems.