Nuclear-magnetic-resonance study of the cubic alkali tungsten bronzes

NMR measurements of the cubic alkali tungsten bronzes ${\mathrm{Na}}_x\mathrm{W}{\mathrm{O}}_3$ ($0.22\lesssim x\lesssim 0.84$) and ${\mathrm{Li}}_x\mathrm{W}{\mathrm{O}}_3$ ($0.27\lesssim x\lesssim 0.49$) are presented. Knight shifts, relaxation times, and line shapes of ${}_{}{}^{183}{}_{}{}^{}\mathrm{W}$ were studied over the temperature range 1.35-4.2 K. $T_1$ and $K$ of ${}_{}{}^7{}_{}{}^{}\mathrm{Li}$ at 77 and 298 K were also measured. These results coupled with previous ${}_{}{}^{23}{}_{}{}^{}\mathrm{Na}$ measurements are correlated to possible conduction-band models. The results for $x>\mathrm{}0.40\mathrm{}$ are consistent with a $\mathrm{W}5d\left(t_{2g}\right)$ conduction band analogous to the Re $5d\left(t_{2g}\right)$ band in Re ${\mathrm{O}}_3$ band-structure calculations. For $x<\mathrm{}0.40\mathrm{}$ the data suggest admixture of Na and W $6s$ states in the ${\mathrm{Na}}_x\mathrm{W}{\mathrm{O}}_3$ conduction band. Regarding the metal-semiconductor transition occurring in the bronzes, the NMR results seem incompatible with simple nearest-neighbor linkage and correlated site percolation models of conduction for the bronzes. The results give no evidence of a Mott-type transition occurring as no correlation effects are observable.