Comment on the cluster model of alkali-metal tungsten bronzes

In a recent paper Webman, Jortner, and Cohen proposed that the alkali metal $M$ tungsten bronzes comprise a two-phase system composed of regions of metallic $M\mathrm{W}{\mathrm{O}}_3$, surrounded by insulating W${\mathrm{O}}_3$. In this comment we point out that thermodynamic measurements on ${\mathrm{Na}}_x\mathrm{W}{\mathrm{O}}_3$ and ${\mathrm{H}}_x\mathrm{W}{\mathrm{O}}_3$ are inconsistent with this model and that these bronzes should be thought of as nonstoichiometric but homogeneous compounds with no tendency to form a two-phase system.