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.