Pressure Effects on 3d n (n=4, 9) Insulating Compounds: Long Axis Switch in Na 3 MnF 6 not Due to the Jahn-Teller Effect.

The pressure-induced switch of the long axis of MnF 6 3- units in the monoclinic Na 3 MnF 6 compound and Mn 3+ -doped Na 3 FeF 6 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 3d n (n=4, 9) systems displaying a static Jahn-Teller effect. By contrast, we prove that in Na 3 MnF 6 the switch arises from the anisotropic response of the low symmetry lattice to hydrostatic pressure. Indeed, while the long axis of a MnF 6 3- unit at ambient pressure corresponds to the Mn 3+ -F 3 - 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 Mn 3+ -F 2 - direction, not far from crystal b axis, after the subsequent relaxation process. The origin of the different d-d transitions observed for Na 3 MnF 6 and CrF 2 at ambient pressure is also discussed together with changes induced by pressure in Na 3 MnF 6 . The present work opens a window for understanding the pressure effects upon low symmetry insulating compounds containing d 4 or d 9 ions.