Pressure Effects on 3dⁿ (n=4, 9) Insulating Compounds: Long Axis Switch in Na₃ MnF₆ not Due to the Jahn-Teller Effect

The pressure-induced switch of the long axis of MnF₆ ^3- units in the monoclinic Na₃ MnF₆ compound and Mn³⁺ -doped Na₃ FeF₆ 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 Na₃ MnF₆ the switch arises from the anisotropic response of the low symmetry lattice to hydrostatic pressure. Indeed, while the long axis of a MnF₆ ^3- 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 Na₃ MnF₆ and CrF₂ at ambient pressure is also discussed together with changes induced by pressure in Na₃ MnF₆ . The present work opens a window for understanding the pressure effects upon low symmetry insulating compounds containing d⁴ or d⁹ ions.