Direct observation of Jahn-Teller critical dynamics at a charge-order Verwey transition

By means of diffuse x-ray scattering (DS) and inelastic x-ray scattering (IXS), we probe directly the chargeordering (CO) dynamics in the Verwey system (NaMn3)Mn4O12, where a peculiar quadruple perovskite structure with no oxygen disorder stabilizes a nearly full Mn3+/Mn4+ static charge order at TCO = 175 K concomitant to a commensurate structural modulation with propagation vector qCO = (21, 12, 0). At TCO, the IXS spectra unveil a softening of a 35.3-meV phonon at qCO. Lattice dynamical calculations enable us to attribute this soft phonon to an Ag mode whose polarization matches the Jahn-Teller-like distortion pattern of the structural modulation. This result demonstrates that the Jahn-Teller instability is the driving force of the CO Verwey transition in (NaMn3)Mn4O12, thus elucidating a long-standing controversy regarding the mechanism of this transition observed in other mixed-valence systems like magnetite.