The sodium tungsten bronzes: a study of the changes in electronic structure with composition using high-resolution electron spectroscopy

Abstract

The variation with composition of the electronic structure of polycrystalline sodium tungsten bronzes NaxWO3(0<x<1) has been studied using a combination of high-resolution electron spectroscopic techniques. Samples on either side of the metal-nonmetal transition (x approximately 0.25 from conductivity data) were used. Low-energy electron energy-loss spectra (LEELS) agree well with those predicted in previous optical studies. In particular the effective electron mass (m*) shows the expected variation with bulk composition. Ultraviolet photoelectron spectroscopy (UPS) has been used to study the valence and conduction bands. Measurements of work function ( phi ); density of states at the Fermi level (g(EF)); and effective electron mass at the Fermi level (mEF*) all show linear variations with bulk composition, in agreement with the findings of magnetic susceptibility and low-temperature specific heat capacity studies. A linear variation of the ratio of the intensities of the conduction and valence bands with bulk composition is also observed across the whole composition range. The variation in shape of the conduction band with composition is consistent with the formation of an impurity band which overlaps a nearly-free-electron-like conduction band that narrows with increasing sodium content. The data obtained in this study are consistent with a mechanism for the metal-nonmetal transition involving the formation of localised small polarons when the electron concentration falls below a critical value.