Microscopic origin of pressure-induced isosymmetric transitions in fluoromanganate cryolites

TL;DR

This study uses first-principles calculations to reveal the microscopic mechanism behind pressure-induced isosymmetric transitions in fluoromanganate cryolites, highlighting the role of Jahn-Teller distortions and orbital occupancy changes.

Contribution

It uncovers the microscopic origin of isosymmetric transitions in Mn-based cryolites under pressure, emphasizing Jahn-Teller effects and orbital reconfiguration as key factors.

Findings

First-order isosymmetric transition at ~2.15 GPa in Na₃MnF₆

No transition observed in Na₃ScF₆ up to 6.82 GPa

Jahn-Teller distortion evolution linked to orbital occupancy change

Abstract

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 Na$_3$MnF$_6$. We find a first-order isosymmetric transition occurs between crystallographically equivalent monoclinic structures at approximately 2.15 GPa, consistent with earlier experimental studies. Analogous calculations for isostructural $3d^0$ Na$_3$ScF$_6$ show no evidence of a transition up to 6.82 GPa. Mode crystallography analyses of the pressure-dependent structures in the vicinity of the transition reveals a clear evolution of the Jahn-Teller bond distortions in cooperation with an asymmetrical stretching of the equatorial fluorine atoms in the MnF$_6$ octahedral units. We identify a change in orbital occupancy of the $e_g$ manifold in the $3d^4$ Jahn-Teller active Mn(III) to be responsible for the transition, which stabilizes one monoclinic $P2_1/n$ variant over the other.