Abstract
Usually the Jahn-Teller effect of Cu2+ in a regular octahedral host lattice site leads to a tetragonal elongation with a small o-rhombic component. If the relevant crystallographic site is slightly tetragonally compressed however in the host lattice structure already – as in Ba2ZnF6 – the Jahn-Teller effect may stabilize this distortion symmetry as well. This can be deduced from the EPR-spectroscopic gn -values of 1.99 for the mixed crystals Ba2Zn1-xCuxF6 up to x = 0.3. The angular overlap parameters obtained from the ligand field spectra suggest in addition, that the tetragonal compression of the CuF6 polyhedra is considerable, with CuF distances of about 1.90 Å (2 x) and 2.14 Å (4 x), respectively. Increasing cooperative elastic interactions between the CuF6 polyhedra in the (001) planes at larger Cu2+-concentrations induce tetragonally elongated octahedra (with a small o-rhombic component) in antiferrodistortive order at x ≥ 0.6. This cooperative Jahn-Teller order, which is characteristic for Ba2CuF6, can be verified simply by acentric displacements of the F-.ligands connecting two Cu2+-ions in the (001) planes.
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