Investigation of proton conduction in the tetragonal tungsten bronze niobates: A0.6(M,Nb)O3 (A = Sr, Ba; M = Mg, Ti)

In this paper we report investigations of high temperature proton conduction in the tetragonal tungsten bronze niobates, A0.6Ti0.2Nb0.8O3 and A0.6Mg0.067Nb0.933O3 (A = Sr, Ba). For all samples, conductivities in dry N2 were higher than those in dry O2 indicating that these materials show n-type electronic conduction. In wet atmospheres, the conductivities for the Ba containing systems and Sr0.6Ti0.2Nb0.8O3 were higher than those in dry atmospheres, indicating the presence of proton conduction. The conductivities were, however, low (σ600 °C ≈ 2 × 10− 6 and 6 × 10− 6 S cm− 1 in wet O2 and N2, respectively).In the case of Sr0.6Mg0.067Nb0.933O3 the conductivities were similar for both wet and dry atmospheres, with the exception of a small increase in conductivity at lower temperatures (below 650 °C) in wet O2, suggesting a lower proton conductivity for this system.The low proton conductivity in these samples is attributed to a low oxygen vacancy concentration. Attempts to vary the Ti/Nb or Mg/Nb ratio to raise this oxide ion vacancy concentration, and so improve the proton conduction, were unsuccessful.