Potassium Diffusion in Mono- and Bi-Crystalline SrTiO3 – Mechanisms and Activation Energies

Abstract

The diffusion of potassium in SrTiO₃ (STO) single crystals is investigated as a function of temperature. Charge attachment induced transport experiments are employed inducing diffusion profiles in STO evolving with time. Potassium concentration profiles, characterized ex-situ by ToF-SIMS depth profiling, exhibited a bimodal shape indicating two different transport pathways. Two diffusion coefficients are obtained for the two profiles. Their temperature dependence is described using an Arrhenius approach, allowing two activation energies to be derived from the data set. Utilizing DFT+U, the ionic mobility of potassium along the crystal structure is simulated and activation energies are determined for every trajectory. The transport pathway with the larger diffusion coefficient is assigned to defect transport, most likely via Sr vacancies. The transport pathway exhibiting the lower diffusion coefficient is assigned to interstitial transport. In addition, the experiment is repeated on a bicrystalline STO sample to investigate the effect of a grain boundary on the ionic conductivity of the sample. As expected, the grain boundary represents an additional diffusion path for the potassium ions. The corresponding diffusion coefficient along the grain boundary is three orders of magnitude larger than that for bulk diffusion. Atomically resolved structural information in the grain boundary region is presented.