Tantalum and niobium diffusion in single crystalline lithium niobate

Abstract LiNbO 3 and LiTaO 3 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 LiNbO 3 . The diffusion system consisted of a 20 nm layer of LiTaO 3 sputter deposited on top of (i) a congruent LiNbO 3 single crystal, i.e. (48.3 ± 0.1) mol% Li 2 O, and on top of (ii) a VTE processed LiNbO 3 single crystal with nearly stoichiometric composition, i.e. (49.9 ± 0.1) mol% Li 2 O. 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 LiNbO 3 . For sub-stoichiometric LiNbO 3 the joint discussion of this work and of literature data on the basis of the generally accepted defect model, 4[ Nb Li 4• ] = [ V Li ′], suggests Nb transport in the Li sublattice. For hyper-stoichiometric LiNbO 3 the defect model 5 V Nb 5 ′ = L i i • is derived from the Li 3 NbO 4 /LiNbO 3 solution reaction of the VTE process designed to obtain Li 2 O-rich LiNbO 3 in accordance with the Li 2 O–Nb 2 O 5 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 Nb Li 4• in the Li sublattice of congruent, i.e. sub-stoichiometric, LiNbO 3 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 LiNbO 3 , 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.