Abnormal lattice occupation of Mo and related polarons in LiNbO3: Hybrid density functional theory investigation

Studies have reported that Mo-doped LiNbO3 exhibits excellent photorefractive performance, which may be attributed to the Mo abnormally occupation in Nb sites (MoNb) and the MoNb photorefractive centres. Therefore, we investigate the basic characteristics of Mo-doped LiNbO3, including the preferable substitutional sites, doping stability, lattice relaxation, and electronic structures using the spin-polarised density functional theory within semi-local and hybrid functionals. Particularly, the type and properties of the polarons in Mo-doped LiNbO3 are studied intensively, and the advantages of MoNb polarons with respect to the MoLi polarons are highlighted. The calculation results of both transfer energies and defect formation energies show that MoNb are preferable to be formed in both stoichiometric and congruent LiNbO3, while Mo substitutional Li (MoLi) is consumed in small amounts. MoNb could act as small bound polarons that exhibit fast response to light with respect to the MoLi bound bipolarons. The introduced double-centre by Mo doping is partly responsible for the excellent photorefractive properties of Mo-doped LiNbO3.