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

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