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(1-2x)Cr(x)Nb(x)O(2) (0<= 0.5) were synthesised and their structure, conductivity, redox behavior, and thermal expansion Studied in air and under reducing conditions of up to 10(-20) atm p(O(2)). Their rutile structure (P4(2/)mnm) is retained after reduction, though TEM analysis showed that crystallographic shear planes were formed on reduction to accommodate oxygen vacancies. The conductivity of these materials at 900 degrees C in air was 1.5 - 1.7 X 10(-2) S cm(-1), increasing to > 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 degrees 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 x 10(-6) - 8.6 x 10(-6) K(-1), which changed little between reduced and fully oxidized samples.