Li Self-Diffusivities in Lithium Niobate Single Crystals as a Function of Li2O Content

Li self-diffusion is investigated in LiNbO3 single crystals at temperatures below 773 K by tracer diffusion experiments. Compared are single crystals with 49.9 mol % Li2O (near stoichiometric), 49.4 mol % Li2O, and 48.6 mol % Li2O (congruent). For the experiments, a thin ion-beam sputtered isotope-enriched (LiNbO3)-Li-6 layer is used as a tracer source, and isotope depth profiling is done by secondary ion mass spectrometry. The temperature dependence of the diffusivities of all three types of samples can be described by Arrhenius laws with the same activation enthalpy of 1.33 eV; however, the diffusivities increase with decreasing amount of Li2O in the sample over the whole temperature range investigated, which is traced back to an increase in Li vacancy concentration. This is in agreement with a model of defect disorder consisting of a niobium antisite atom that is compensated by four lithium vacancies, (Nb-Li(center dot center dot center dot center dot)+ 4 V-Li').