Microscopic origin of pressure-induced isosymmetric transitions in fluoromanganate cryolites

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 $P{2}_1/n$ variant over the other.

arXiv comment10 pages, 9 figures

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