Alkali diffusion in niobium doped SrTiO₃ - transport pathways, diffusion coefficients and activation energies

The diffusion characteristics of potassium and rubidium in niobium-doped strontium titanate single crystals were systematically investigated over a range of temperatures. Charge attachment induced transport (CAIT) experiments were employed to induce diffusion by continuous deposition of alkali ions onto the sample surface. Subsequent analysis of the samples using time-of-flight secondary ion mass spectrometry (ToF-SIMS) depth profiling revealed bimodal concentration profiles for all experiments, for both potassium and rubidium. These profiles were modeled assuming a superposition of two mechanisms, identifying two distinct transport pathways, each characterized by a specific diffusion coefficient. The temperature dependence of the diffusion coefficients was evaluated using an Arrhenius approach, enabling the determination of two activation energies for each alkali ion. The process with a lower diffusion coefficient and lower activation energy is attributed to interstitial transport, while the process with a higher diffusion coefficient and higher activation energy correlates with diffusion mediated by defects and is similar between the alkali ions. This study provides insight into the complex interplay between defect chemistry and ion transport mechanisms in niobium-doped strontium titanate, following a previous study on potassium diffusion in undoped strontium titanate.