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

Abstract

Materials from the solid solution with general formula Ti1−2xCrxNbxO2 (0<x≤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(O2). Their rutile structure (P42/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 °C in air was 1.5 − 1.7 × 10−2 S cm−1, increasing to >20 S cm−1 in 5% H2/Ar. The lowest conductivity was observed for samples low in titanium content, indicating that the reduction of Ti4+ to Ti3+ 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% H2/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(O2) 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.