Electrical conductivity of nonstoichiometric rutile single crystals from 1000° to 1500°C

Abstract

The electrical conductivity of single crystals of rutile was measured in the ‘ c’ direction over the temperature range 1000°–1500°C and from 1 to 10 −15 atm of oxygen. Based on the excellent fit observed between the theoretically derived relation σ 5 = ( Aσ + B) P −1 O 2 + D'σ 4 and the experimental conductivity data, the nonstoichiometric defect structure of rutile was rationalized in terms of a defect model involving quasi-free electrons and both triply and quadruply ionized titanium interstitials. The standard enthalpy of formation for the following defect reactions in rutile: (a) Ti + 20 = O 2( g) + Ti i +3 + 3 e; Δ H 0 a = 9.24(eV) (b) Ti + 20 = O 2( g) + Ti i +4 + 4 e; Δ H 0 b = 10.67(eV) (c) Ti +3 i = Ti +4 i + e; Δ H 0 c = 1.43(eV) were determined from the temperature dependence of A and B obtained from the above relation and from the experimental expression between the electron mobility and temperature. Excellent agreement was observed between the value of Δ H 0 a = 9.24(eV) and the negative value of the relative partial molar enthalpy of oxygen—Δ H̄o 2 = 9.16(eV), for nonstoichiometric rutile obtained by employing an electrochemical technique. The electrical conductivity of rutile in air below approximately 950°C appears, on the basis of this investigation to be impurity controlled due to the presence of aluminum rather than intrinsic conduction.