Bound electron polarons in lithium niobate

Small bound polarons and bipolarons in lithium niobate are investigated in the framework of the density functional theory. The LDA+U method is employed to account for the strong correlation of the electronic states in the Nb 4d orbitals. Charge distributions as well as structural and electronic properties are discussed depending on the charge state. The Nb_Li⁵⁺ antisite is found to introduce a localized level within the LiNbO₃ fundamental gap that is almost resonant with the conduction band edge. This level can be occupied either by a one or by two electrons. In the latter case, a major rearrangement of the atomic position occurs, and a covalent bond along the rhombohedral [111] direction between the antisite NbLi and a Nb at a regular lattice site is formed.