Microscopic origin of pressure-induced isosymmetric transitions in fluoromanganate cryolites

Using first-principles density functional theory calculations, we investigate the hydrostatic pressure-induced reorientation of the Mn-F Jahn-Teller bond axis in the fluoride cryolite Na3MnF6. We find that a first-order isosymmetric phase transition (IPT) occurs between crystallographically equivalent monoclinic structures at approximately 2.15 GPa, consistent with earlier experimental studies. Mode-crystallography analyses of the pressure-dependent structures in the vicinity of the transition reveal a clear evolution of the Jahn-Teller bond distortions in cooperation with an asymmetrical stretching of the equatorial fluorine atoms in the MnF6 octahedral units. We identify a significant (70%) change in the orbital occupancy of the eg manifold of the 3d4 Mn(III) to be responsible for the transition, stabilizing one monoclinic P21/n variant over the other. The orbital reconstruction as a driving force for the transition is confirmed by analogous calculations of isostructural 3d0 Na3ScF6, which shows no evidence of an IPT up to 6.82 GPa.