Abstract: The pressure-induced switch of the long axis of MnF63- units in the monoclinic Na3MnF6 compound and Mn3+-doped Na3FeF6 is explored with the help of first principles calculations. Although the switch phenomenon is usually related to the Jahn-Teller effect, we show that, due to symmetry reasons, it cannot take place in 3dn (n=4, 9) systems displaying a static Jahn-Teller effect. By contrast, we prove that in Na3MnF6 the switch arises from the anisotropic response of the low symmetry lattice to hydrostatic pressure. Indeed, while the long axis of a MnF63- unit at ambient pressure corresponds to the Mn3+-F3- direction, close to the crystal c axis, at 2.79 GPa the c axis is reduced by 0.29 Å while b is unmodified. This fact is shown to force a change of the HOMO wavefunction favoring that the long axis becomes the Mn3+-F2- direction, not far from crystal b axis, after the subsequent relaxation process. The origin of the different d-d transitions observed for Na3MnF6 and CrF2 at ambient pressure is also discussed together with changes induced by pressure in Na3MnF6. The present work opens a window for understanding the pressure effects upon low symmetry insulating compounds containing d4 or d9 ions.

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