Pressure Effects on 3dn (n=4, 9) Insulating Compounds: Long Axis Switch in Na3MnF6 not Due to the Jahn‐Teller Effect

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. By means of first principles calculations, this work shows that the usual explanation for the long axis switch induced by pressure in low symmetry compounds containing 3dn (n=4, 9) ions, i. e., the Jahn‐Teller effect, cannot be considered since in the parent phase the symmetry constraints are not fulfilled. However, the reported results for Na3MnF6 reveal that the pressure‐induced switch arises from the anisotropic response of the low symmetry lattice to hydrostatic pressure, leading to a change of the electronic wavefunction.