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 ${\mathrm{Na}}_3{\mathrm{MnF}}_6$. 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 ${\mathrm{MnF}}_6$ octahedral units. We identify a significant (70%) change in the orbital occupancy of the $e_g$ manifold of the $3d^4$ Mn(III) to be responsible for the transition, stabilizing one monoclinic $P{2}_1/n$ variant over the other. The orbital reconstruction as a driving force for the transition is confirmed by analogous calculations of isostructural $3d^0$ ${\mathrm{Na}}_3{\mathrm{ScF}}_6$, which shows no evidence of an IPT up to 6.82 GPa.

• Received 14 May 2014
• Revised 8 August 2014

DOI:https://doi.org/10.1103/PhysRevB.90.094114