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

First documented on magnetite (Fe$_{3}$O$_{4}$) [1], the Verwey transition is a drastic manifestation of non-adiabatic system. A structural change, accompanied by a sudden rise in electric resistivity, is caused by the coupling of electronic and lattice degrees of freedom. Verwey’s original charge-ordering explanation for the phenomenon is shown to be too simple for the complex structural distortions and lattice dynamics found in Verwey-like systems, and a complete explanation remains controversial [2-3]. Verwey-like transitions are observed in several mixed-valence transition metal oxides but the coexistence of charge-ordered and -disordered phases, incommensurate structural modulations like stripe phases and electronic phase separation make the study of Verwey-like transition dynamics challenging [4-5].
The model system (NaMn$_3$)Mn$_4$O$_{12}$ presents several characteristics that are helpful in the overcoming of such challenges, since the quadruple perovskite structure (AA'$_3$)B$_4$O$_{12}$ permit half doping of the octahedral Mn B-sites free from chemical substitutions and hinder the formation of oxygen defects. As a result (NaMn$_3$)Mn$_4$O$_{12}$ exhibits an almost full charge order of the Mn$^{3+}$ and Mn$^{4+}$ ions in these sites at $T_{\rm CO}$=175 K. At the same temperature, a commensurate structural modulation with propagation vector ${\bf q}_{\rm CO}=\left(\frac{1}{2},\frac{1}{2},0\right)$, much simpler than the one found in magnetite and other systems, is established [6].
These favorable conditions allowed the probing of the dynamics of the charge-ordering transitions by means of diffuse and inelastic x-ray scattering. These dynamics are manifested by the softening of an Ag phonon at ${\bf q}_{\rm CO}$. At the same time, this softening is seem to disrupt a different incipient distortion unveiled by diffuse x-ray scattering experiments. Lattice dynamical calculations allowed to identify the polarization of this soft phonon, which matches the Jahn-Teller-like distortion pattern of the commensurate structural modulation, demonstrating the Jahn-Teller instability to be the driving force of the charge-order Verwey transition.

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