Sodium-tungsten bronze thin films: I. Optical properties of dilute bronzes

Abstract

Transmission and reflection spectra in the range 0·4 to 4·1 eV have been obtained for Na_xWO₃ thin films as a function of x up to 0·2, and the influence of grain size has been investigated. The data were refined to yield the absorption coefficient and the real part of the dependence of refractive index on photon energy. A model was constructed which is in accord both with the present experimental and with previously published data. The inserted metal atoms (sodium in the present case) give rise to electron states substantially above the conduction band (c.b.) minimum of the host (WO₃). The main absorption edge is the result of an indirect transition which increases with x because the c.b. minimum starts to fill with electrons. The absorption band below the main edge (the colour band) is a composite band because of transitions between the c.b. minimum and a sub-c.b., and between the minimum and the empty Na⁺ states.