  We performed a first-principles study of the structural, vibrational,
electronic and magnetic properties of NaMnF3 under applied isotropic pressure.
We found that NaMnF3 undergoes a reconstructive phase transition at 8 GPa from
the Pnma distorted perovskite structure toward the Cmcm post-perovskite
structure. This is confirmed by a sudden change of the Mn-F-Mn bondings where
the crystal goes from corner shared octahedra in the Pnma phase to edge shared
octahedra in the Cmcm phase. The magnetic ordering also changes from a G-type
antiferromagnetic ordering in the Pnma phase to a C-type antiferromagnetic
ordering in the Cmcm phase. Interestingly, we found that the high-spin
d-orbital filling is kept at the phase transition which has never been observed
in the known magnetic post-perovskite structures. We also found a highly
non-collinear magnetic ordering in the Cmcm post-perovskite phase that drives a
large ferromagnetic canting of the spins. We discuss the validity of these
results with respect to the U and J parameter of the GGA+U exchange correlation
functional used in our study and conclude that large spin canting is a
promising property of the post-perovskite fluoride compounds.
