First-principles study on the formation energies of intrinsic defects in LiNbO3

First-principles plane-wave ultrasoft pseudopotential method within local density approach (LDA) has been used to study three possible vacancy-defect models for non-stoichiometric lithium niobate (LiNbO3): (1) the oxygen-vacancy model V-O(2+) + 2V(Li)(-), (2) the niobium-vacancy model 5Nb(Li)(4+) + 4V(Nb)(5-), and (3) the lithium-vacancy model 4V(Li)(-) + Nb-Li(4+). The corresponding formation energies are obtained via energy minimization of a supercell. In Nb-rich environment, the calculated defect formation energies, both under oxidation and reduction conditions, show little effect on the intrinsic defect structures. We find that the lithium vacancy model 4V(Li)(-) + Nb-Li(4+) has the most stable configuration in the non-stoichiometric lithium niobate crystals. Our calculations also show that the formation of any type of neutral defects and Frenkel pairs in a Nb-rich environment is difficult. (c) 2007 Elsevier Ltd. All rights reserved.