Oxygen diffusion in single crystal barium titanate

Abstract

Oxygen diffusion in cubic, nominally undoped, (100) oriented BaTiO₃ single crystals has been studied by means of ¹⁸O₂/¹⁶O₂ isotope exchange annealing and subsequent determination of the isotope profiles in the solid by time-of-flight secondary ion mass spectrometry (ToF-SIMS). Experiments were carried out as a function of temperature 973 < T/K < 1173, at an oxygen activity of aO₂ = 0.200, and as a function of oxygen activity 0.009 < aO₂ < 0.900 at T = 1073 K. The oxygen isotope profiles comprise two parts: slow diffusion through a space-charge zone at the surface depleted of oxygen vacancies followed by faster diffusion in a homogeneous bulk phase. The entire isotope profile can be described by a single solution to the diffusion equation involving only three fitting parameters: the surface exchange coefficient k_s*, the space-charge potential Φ₀ and the bulk diffusion coefficient D*(∞). Analysis of the temperature and oxygen activity dependencies of D*(∞) and Φ₀ yields a consistent picture of both the bulk and the interfacial defect chemistry of BaTiO₃. Values of the oxygen vacancy diffusion coefficient D_V extracted from measured D*(∞) data are compared with literature data; consequently a global expression for the vacancy diffusivity in BaTiO₃ for the temperature range 466 < T/K < 1273 is obtained, with an activation enthalpy of vacancy migration, ΔH_mig,V = (0.70 ± 0.04) eV.