Infrared properties of W-doped charge-density-wave material   K$_{0.3}$MoO$_3$
  The optical conductivity spectra of quasi-one dimensional compounds K$_{0.3}$Mo$_{1-x}$W$_x$O$_3$ (x=0, 0.03 and 0.15) have been studied over broad frequencies. While the dc resistivity measurements indicate no sign of CDW transition in heavily W-doped blue bronze, the optical conductivity spectra still show a single particle gap at around 0.2 eV for \textbf{E} parallel to the chain direction. Such impurity effect challenges our understanding about the occurrence of the optical gap with the CDW transition. 
T. Feng, N. L. Wang, P. Zheng, Z. J. Chen, M. L. Tian