Theory of the color change of Nax WO3 as a function of Na-charge doping

We report theoretical investigations of the coloration of WO3 upon charge insertion using sodium tungsten bronze (Nax WO3) as a model system. Our results explain well the systematic color change of Nax WO3 from dark blue to violet, red-orange, and finally to golden yellow as sodium concentration x increases from 0.3 to unity. Proper accounts for both the interband and the intraband contributions to the optical response are found to be very important for a detailed understanding of the coloration mechanism in this system.