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

Abstract

By means of diffuse and inelastic x-ray scattering (DS,IXS), we probe directly the charge-ordering (CO) dynamics in the Verwey system (NaMn${}_3$)Mn${}_4$O${}_{12}$, where a peculiar quadruple perovskite structure with no oxygen disorder stabilizes a nearly full Mn${}^{3+}$/Mn${}^{4+}$ static charge order at $T_{\mathrm{CO}}$=175 K concomitant to a commensurate structural modulation with propagation vector ${\mathbf{q}}_{\mathrm{CO}}=(\frac{1}{2},\frac{1}{2},0)$. At $T_{\mathrm{CO}}$, the IXS spectra unveil a softening of a 35.3 meV phonon at ${\mathbf{q}}_{\mathrm{CO}}$. Lattice dynamical calculations enable us to attribute this soft phonon to a A${}_g$ 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 (NaMn${}_3$)Mn${}_4$O${}_{12}$, thus elucidating a long-standing controversy regarding the mechanism of this transition observed in other mixed-valence systems like magnetite.

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