Oxygen-18 tracer diffusion in nominally undoped and Sr-doped single crystals of mullite-type Bi2Ga4O9

Bi2M4O9 (M = Al, Ga, Fe) mullite-type compounds are currently being investigated with respect to their potential application as oxygen ion conductors or mixed ionic-electronic conductors. In the framework of these studies oxygen transport in (nominally) undoped and Sr-doped single crystals of Bi2Ga4O9 is of prime interest. 18O tracer diffusion in combination with secondary ion mass spectrometry (SIMS) depth profiling reveals that oxygen transport occurs via oxygen vacancies introduced by impurities of cations with lower valences in the undoped crystals grown by the top-seeded solution growth (TSSG) method. The fairly small enhancement of the oxygen diffusivity due to Sr doping leads to the conclusion that the solubility of Sr in Bi2Ga4O9 is extremely low with respect to the melt during the single crystal growing process. The measured enthalpies of activation ((163 ± 15) kJ/mol for undoped and (140 ± 15) kJ/mol for Sr-doped Bi2Ga4O9, respectively) must be considered as migration enthalpies.