Metal-insulator transition in the compensated sodium bronze, NaxTayW1-yO3.

We have measured the dc electrical conductivity \ensuremath{\sigma} of cubic ${\mathrm{Na}}_{\mathrm{x}}$${\mathrm{Ta}}_{\mathrm{y}}$${\mathrm{W}}_{1\mathrm{\ensuremath{-}}\mathrm{y}}$${\mathrm{O}}_{3}$ from 1.6 to 295 K for various values of x and y. A sample with x-y=0.18 appears to fall directly at the metal-insulator transition and shows an unusual temperature dependence, \ensuremath{\sigma}(T)\ensuremath{\propto}T, from 1.6 K to room temperature. A model for \ensuremath{\sigma}(T) based on the scaling theory of localization, when used to interpret the data on this and other samples, gives a conductivity exponent \ensuremath{\nu} of 1.0. This model assumes that \ensuremath{\sigma}(T) is controlled by a thermal smearing of the occupancy of one-electron energy states near the Fermi level rather than by inelastic scattering. Comparison of the conductivity transition in ${\mathrm{Na}}_{\mathrm{x}}$${\mathrm{Ta}}_{\mathrm{y}}$${\mathrm{W}}_{1\mathrm{\ensuremath{-}}\mathrm{y}}$${\mathrm{O}}_{3}$ with earlier data for the uncompensated material ${\mathrm{Na}}_{\mathrm{x}}$${\mathrm{WO}}_{3}$ indicates that the additional disorder introduced by Ta doping does not shift the critical value of electron concentration.