Oxygen stoichiometry, chemical expansion or contraction, and electrical properties of rutile, TiO2±δ ceramics

Abstract

Rutile, TiO2 is increasingly oxygen-deficient on heating in air above ~700°C. The weight loss is generally too small for accurate measurement, but the electrical properties of quenched samples provide a sensitive qualitative indicator of oxygen content since their conductivity can vary by many orders of magnitude. The oxygen lost at high temperature is fully recovered if samples are cooled slowly. With rapid quenching, by dropping samples into liquid N2, the oxygen stoichiometry at high temperature is preserved to ambient and the resulting materials are kinetically stable but thermodynamically metastable. The lattice parameters of quenched samples showed an unusual dependence on quench temperature and, by implication, on oxygen stoichiometry. Lattice parameters increased with a small oxygen loss, δ; chemical expansion of the lattice occurred and is attributed to reduction in average Ti oxidation state and increase in Ti–O bond lengths. At higher δ, lattice parameters started to decrease giving a chemical contraction effect attributed to partial collapse of columns of edge-sharing TiO6 octahedra in the rutile structure and elimination of oxygen vacancies by crystallographic shear plane formation. Oxygen-deficient samples quenched from above 700°C were n-type, as were samples annealed and measured at 650 and 700°C. Samples measured at 450-500°C were p-type and believed to be slightly oxygen-rich; it is suggested that holes located on oxide ions at or near the sample surface arose from redox electron transfer between underbonded surface oxide ions and adsorbed O2 molecules. Samples annealed between 550 and 600°C showed cross-over between n- and p-type behavior.