Collective and tracer diffusion via a defect cluster in LSGM

It was recently shown by Schulz and co-workers that in doped lanthanum gallate: La₍₁₋x₎ SrxGa₍₁₋y₎ MgyO₍₃₋₍x₊y)/₂)(LSGM) the three cations: La, Sr and Mg have tracer diffusivities that are nearly identical. To explain these findings, a bound defect cluster mechanism containing a cation vacancy from both the A- and the B- sublattices and an anion vacancy was proposed as the principal vehicle for cation diffusion in LSGM. In this paper, implications of this mechanism are considered for the first time. Sum-rule expressions for the collective correlation factors are derived and found to be in excellent agreement with Monte Carlo calculations. Expressions are also developed for the tracer correlation factors of lanthanum and gallium for diffusion via the cluster mechanism and tested by Monte Carlo computer simulation. Good agreement was found. The ratio of the tracer diffusivities of lanthanum and gallium are shown to be consistent with the cluster mechanism.