First evidence for charge ordering in NaV$_2$O$_5$ from Raman spectroscopy

TL;DR

This study provides the first spectroscopic evidence that NaV$_2$O$_5$ undergoes a charge ordering phase transition at 34 K, involving a redistribution of vanadium charges and a structural change.

Contribution

It presents the first evidence from Raman spectroscopy for charge ordering in NaV$_2$O$_5$, identifying the transition and associated charge redistribution and structural arrangement.

Findings

Charge ordering occurs at T_c=34 K.

Vanadium ions change from uniform V$^{4.5+}$ to V$^{4+}$ and V$^{5+}$.

Low-temperature phase has a zig-zag ladder structure.

Abstract

We argue on the basis of symmetry selection rules and Raman scattering spectra that NaV$_2$O$_5$ undergoes a charge ordering phase transition at T$_c$=34 K. Such a transition is characterized by the redistribution of the charges at the phase transition and corresponds to the change of the vanadium ions, from uniform V$^{4.5+}$ to two different V$^{4+}$ and V$^{5+}$ states. In the low temperature phase the V$^{4+}$ ions are forming a "zig-zag" ladder structure along the {\bf b}-axis, consistent with the symmetry of the P2/b space group.