Incipient multiferroicity in Pnma fluoroperovskite NaMnF3

Recent theoretical studies predict the suppressed ferroelectric instability in orthorhombic Pnma perovskites and experimental evidence is due. We observed significant softening at cooling of the lowest-frequency polar phonon at the Brillouin zone center in the Pnma antiferromagnetic fluoroperovskite NaMnF3 that is the direct proof of the theoretically predicted ferroelectric instability. In contrast to oxides where the hybridization plays the dominant role, the effective ionic charges in fluoroperovskites are close to their nominal valencies that confirms the geometric origin of the observed incipient ferroelectricity. Furthermore, below the Néel temperature, the softening phonon clearly shows a strong coupling with the magnetic subsystem as a result of dynamical modulation of the superexchange interaction. Our findings clarify microscopic mechanisms of the incipient multiferroicity in the Pnma fluoroperovskites and reveal still unexplored opportunities of this class of materials for further research and potential applications.