The effect of the Nb-W distribution on polaronic transport in ternary Nb-W oxides: electrical and optical properties

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Abstract
Polaronic conductivity in ternary oxides NbO2.5-x-WO3-y follows the Mott T-1/4 law. The transport properties differ from those of the end-member binary oxides in the higher degree of localisation of carriers (2rA approximately=6.4 AA). Comparison between transport in different crystal structures with various Nb to W ratios shows that the plasma frequencies omega p depend on the total carrier concentration N via, omega p2=( epsilon x/4 pi e2)(m*/N), where m*=0.65me is the effective mass. The polaronic ground state is correlated mainly with W5+ states; Franck-Condon transitions occur at 0.7 eV (W5+ to W6+) and 1.1 eV (W5+ to Nb5+). An increasing degree of chemical reduction leads to an increasing carrier concentration. The polaronic part of the carrier concentration saturates at a critical Anderson point with NA=6*1021 cm-1, with a simultaneous appearance of polarons and quasi-free carriers for higher concentrations.