Tantalum and niobium diffusion in single crystalline lithium niobate

• Ta diffusion coefficients in nearly stoichiometric and congruent lithium niobate
• Ta as a tracer to probe the self-diffusion of Nb in lithium niobate
• Li2O activity was kept constant during diffusion annealing.
• Nb transport in the Li sublattice is suggested for sub-stoichiometric LiNbO3.
• Nb transport via Nb vacancies is suggested for hyper-stoichiometric LiNbO3.

LiNbO3 and LiTaO3 are isomorphous and Nb and Ta have the same valence electron configuration and the same ionic radii. This suggests the use of Ta as a tracer to probe the self-diffusion of Nb in LiNbO3. The diffusion system consisted of a 20 nm layer of LiTaO3 sputter deposited on top of (i) a congruent LiNbO3 single crystal, i.e. (48.3 ± 0.1) mol% Li2O, and on top of (ii) a VTE processed LiNbO3 single crystal with nearly stoichiometric composition, i.e. (49.9 ± 0.1) mol% Li2O. The diffusion anneals (1000 °C ≤ T ≤ 1100 °C) were performed under a constant oxygen partial pressure of 200 mbar. From the resulting SIMS depth profiles of tantalum a constant diffusivity was extracted which can be assumed to reflect the niobium self-diffusivity in LiNbO3.For sub-stoichiometric LiNbO3 the joint discussion of this work and of literature data on the basis of the generally accepted defect model, 4[NbLi4
• ] = [VLi′], suggests Nb transport in the Li sublattice. For hyper-stoichiometric LiNbO3 the defect model 5VNb5′=Lii
• is derived from the Li3NbO4/LiNbO3 solution reaction of the VTE process designed to obtain Li2O-rich LiNbO3 in accordance with the Li2O–Nb2O5 phase diagram. This model is supported by theoretical calculations of the defect formation energy. Interestingly, the migration enthalpy for the Li vacancy mediated transport of the anti-site defect NbLi4
• in the Li sublattice of congruent, i.e. sub-stoichiometric, LiNbO3 is similar (within a ± 10% error) to the one derived for the Nb vacancy mediated transport of Nb in the Nb sublattice of hyper-stoichiometric LiNbO3, i.e. about 3 eV.The significant discrepancies between our results and some earlier literature data can be consistently rationalised if the experimental procedures of those studies are carefully analysed.