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

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.

Explaining the optical spectrum of CrF2 and CuF2 model materials: role of the tetragonal to monoclinic instability

The properties of MF2 (M = Cr, Cu) model compounds are usually interpreted assuming a Jahn-Teller effect leading to elongated MF64- units. By means of the analysis of experimental data and first-principles calculations on both the monoclinic P21/c structure and the parent rutile structure (tetragonal P42/mnm space group), we prove that such an assumption is not correct. It is shown that in MF2 compounds, the MF64- complexes are actually compressed in the parent phase but along a different direction, a situation that is however hidden by an additional orthorhombic instability due to a negative force constant of b2g and b3g modes of the cell. This distortion plays a key role in understanding the high experimental value of the lowest d-d transition energy, E1 = 1.23 and 0.93 eV for CrF2 and CuF2, respectively, when compared to the value E1 = 0.40 eV derived for the Jahn-Teller system of KZnF3:Cu2+. Aside from reproducing reasonably the experimental values of spin allowed d-d transitions of both compounds, our first-principles calculations show the existence of an accidental degeneracy involving the yz and xy levels in the final P21/c structure. Moreover, the internal electric field of CrF2 and CuF2 is found to be much less anisotropic than in layered compounds like K2CuF4 and thus it has little influence on the d-d transition energy. The influence of the (3z2 - r2) - (x2 - y2) hybridization, caused by the orthorhombic distortion, on the electronic density and the magnetic coupling between layers is also briefly discussed. The present results stress that the interpretation of experimental data using simple parameterized models can lead to wrong conclusions.