Abstract: The applicability of the Jahn-Teller (JT) framework to 6-fold coordinated d9 ions whose local symmetry is not strictly octahedral is explored by means of first principle calculations. Our results contradict much of the existing literature where these systems are analyzed within the quasi-JT regime which assumes the usual JT description with a small splitting between b1g(?x2-y2) and a1g(?3z2-r2) orbitals and also the existence of three nearly equivalent distortions. To clarify this issue we investigate the equilibrium geometry (equatorial, Req, and axial, Rax, Cu2+-F- distances) and optical transitions for CuF64- units formed in Cu2+-doped the tetragonal Ba2ZnF6 host lattice. While the experimental d-d transitions cannot be reproduced through the isolated CuF64- unit at the equilibrium geometry, a reasonable agreement is reached adding in the calculation the internal electric field, ER(r), created by the rest of lattice ions on the electrons confined in the complex. It is shown that this tetragonal field, ER(r), already produces a gap ?0 ? 0.35 eV between b1g(?x2-y2) and a1g(?3z2-r2) orbitals of Ba2ZnF6:Cu2+ when Rax = Req. Nevertheless, as this internal field leads to a ?0 value higher than typical JT barriers it drastically modifies the characteristic pattern of a JT effect. In particular, it prevents the existence of three equivalent distortions as confirmed by experimental EPR data. Furthermore, ER(r) is shown to be the main physical reason behind an unusual compressed ground state with the hole in the a1g(?3z2-r2) level while it is always placed in the b1g(?x2-y2) level for MX6 complexes (M = Cu2+, Ag2+, Ni+; X = F-, Cl-) in cubic lattices displaying a static JT effect. While the experimental results of CuF64- in Ba2ZnF6 cannot be understood within the JT framework it is pointed out that a quasi-JT situation can however happen for a d9 ion in a cubic lattice under a strain of ?10-3 in agreement with experimental data. The present results stress the key role played by the internal electric fields for a quantitative understanding of compounds with transition metal cations. Moreover, they also demonstrate that in the interpretation of experimental data the use of a simple model should be avoided unless all its assumptions are well justified.

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