Tracer diffusion of ⁹⁶Zr and ¹³⁴Ba in polycrystalline BaZrO₃

Cation tracer diffusion in polycrystalline cubic BaZrO₃ perovskites was studied using the stable isotopes ¹³⁴Ba and ⁹⁶Zr in air at 1015-1200 and 1300-1500 °C, respectively. Thin films of ¹³⁴BaO and ⁹⁶ZrO₂ were deposited on polished BaZrO₃ pellets by drop casting of aqueous precursor solutions containing the tracers. Isotope distribution profiles were recorded using secondary ion mass spectrometry. All the depth profiles exhibited two distinct regions, which enabled the assessment of both lattice and grain boundary diffusion using Fick's second law and Whipple-Le Clair's equation. The grain boundary diffusion of both cations was several orders of magnitude higher than the lattice diffusion. The lattice diffusion of Ba²⁺ was found to be significantly faster than the lattice diffusion of Zr⁴⁺, while the activation energies were comparable, respectively 395 ± 44 and 435 ± 67 kJ mol^-1. Activation energies for the diffusion of Ba²⁺ and Zr⁴⁺ through a Ba²⁺ vacancy were calculated by density functional theory using the nudged elastic band method. The calculated and experimental activation energies were in excellent agreement. The cation diffusion data in BaZrO₃ are compared to previous data on A and B-site diffusivity in perovskites. Finally, the diffusivity of Zr⁴⁺ in compounds with perovskite and fluorite crystal structures is discussed in relation to the chemical stability of BaZrO₃-based materials.