Structure, Conductivity, and Thermal Expansion Studies of Redox Stable Rutile Niobium Chromium Titanates in Oxidizing and Reducing Conditions

Materials from the solid solution with general formula Ti 1−2 x Cr x Nb x O 2 (0 20 S cm −1 in 5% H 2 /Ar. The lowest conductivity was observed for samples low in titanium content, indicating that the reduction of Ti 4+ to Ti 3+ is primarily responsible for the high electronic conductivity in these materials. All samples showed very slow reduction kinetics, and true equilibrium was not achieved even after >50 h in flowing 5% H 2 /Ar at 900 °C, suggesting that the equilibrium conductivity of these materials at 10 −20 atm may be significantly higher. Conductivity studies as a function of p (O 2 ) showed complex redox behavior with indications of a number of step transitions and very slow redox kinetics. The thermal expansion coefficient was in the range of 5.6 × 10 −6 − 8.6 × 10 −6 K −1, which changed little between reduced and fully oxidized samples.