Localization vs. delocalization of orbitals in the quasi-one-dimensional conductor Na 0.33V 2O 5.

Abstract

The beta phase of sodium vanadium oxide bronzes Na xV 2O 5 extends from x=0.22 to fx=0.4. Well inside the stability range, at x=0.33, the electrical conductivity undergoes abrupt change and the material becomes a quasi-one-dimensional conductor. We present a study of the electronic structure and excitation properties of Na 0.33V 2O 5 by x-ray photoelectron spectroscopy (XPS) and appearance potential spectroscopy (APS). While the difference between the Fermi level of the sample material (as determined by XPS) and the edge of the vanaium L 3 APS spectrum indicates strong final state many-body effects due to the localized character of the excited orbitals, the presence of a negative undershoot in the APS spectrum suggests a rather efficient screening (therefore an extended charge mobility) at the site of the excited atom. These apparently conflicting data support and illustrate the idea of domain formation in the sodium vanadium oxide bronze Na 0.33V 2O 5.