Understanding the Structure and Ground State of the Prototype CuF 2 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 b2g and b3g local modes. The associated distortion, similar to that involved in K2 CuF4 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.