First-principles study of microscopic properties of the Nb antisite in LiNbO3: Comparison to phenomenological polaron theory

Through the first-principles local-density-approximation+Hubbard-U (LDA+U) electronic-structure calculation method, the microscopic properties of the Nb antisite (Nb-Li) and the electron-lattice interaction are investigated. The atomic structure is found to depend on the capture of electrons at the defect level, and especially when the defect level is occupied by two electrons, the NbLi undergoes a large-lattice-relaxation (LLR), accompanied with the formation of the deep level. The main driving force toward the LLR is suggested to be the orbital hybridization of the defect level state and the conduction-band state. As a result, the NbLi defect exhibits a negative-U property. Based on the computational results, several well-known light-induced phenomena in LiNbO3 and the polaron model are discussed.