Nuclear-resonance investigation of the low-mobility region in cubic metallic sodium tungsten bronze,NaxWO3

Nuclear-resonance measurements of linewidth, Knight shift, nuclear spin-lattice relaxation time $T_1$, and spin-echo parameters are reported for ${}_{}{}^{23}{}_{}{}^{}\mathrm{Na}$ in a series of pseudocubic sodium tungsten bronzes ${\mathrm{Na}}_x{\mathrm{WO}}_3$, where $x$ varies from 0.22 up to 0.57. The measurements presented here cover the critical low-mobility region on, and possibly just below, the metallic side of the metal-insulator transition. Structural information on these alloys of low-$x$ value is deduced from x-ray and resonance measurements, arguments are presented for discounting the relevance of percolation theories in understanding the conduction processes in the bronzes of high-$x$ value, the experimental fact that the density of states $g\left(E_F\right)$ is directly proportional to $x$ is correlated with recent band-structure calculations, and the strong enhancements of $T_1^{-1\mathrm{}}$ in the dilute regime are discussed in the context of the proximity of the metal-insulator transition.