Study Of Defects And Properties Of Doped Lithium Niobate Single Crystals

From the viewpoint of chemical bond, a bonding energy model is proposed to study the occupancy and properties of dopants in the lithium niobate (LiNbO₃) single crystals based on the impact of the ionic radii and valence state.Dopants usually incorporate into Li or Nb sites in the LiNbO₃ crystallographic frame. Generally speaking, the properties of chemical bonds are more similar if they have closer bonding energies. Thus dopants make the crystal lattice change little. According to this opinion, the dopant occupancy in the LiNbO₃ matrix can be determined by comparing the deviation of its bonding energy in different lattice location at both Li and Nb sites. The theoretical occupancy of dopants agrees well with the experimental results.By analyzing the variation of bonding energy of dopants in different lattice sites (i.e., Li or Nb sites), we find that photorefractive (PR) ions decrease the crystal bonding energy whether they occupy Li or Nb sites, whereas those optical damage resistant (ODR) dopants have the ability to increase the crystal bonding energy when they move to occupy Nb sites. It gives us a bonding energy criterion to distinguish ODR and PR ions and predict some ODR ions to explore new optical devices for the practical applications.The threshold concentration of ODR ions in the LiNbO₃ single crystals is quantitatively obtained using bonding energy method. Take the reliable threshold value 5.0 mol% of Mg doped congruent LiNbO₃ as reference, the crystal bonding energies of pure and Mg doped LiNbO₃ are calculated and the bonding energy of Mg doped LiNbO₃ at the threshold decreases 2.14 kcal compared to that of undoped congruent LiNbO₃. The threshold concentration of ODR ions in both congruent and near-stoichiometric LiNbO₃ crystals can be calculated in the light of the same reduced bonding energy (2.14 kcal) at the same growth condition, and it is assumed due to the same property at the threshold of various ODR dopants. The calculated results agree well with the literature reports.The doping mechanism of ODR ions in the LiNbO₃ crystal is proposed according to the structure formula. The internal relation between crystal bonding energy and defect structure is investigated. We find that with the increased doping concentration, the crystal bonding energy first increase and then decrease and then increase, while the change trend of intrinsic defect Li vacancy is opposite, it first decrease and then increase and then decrease.In addition, based on the crystal structure and chemical bonding, a charge transfer model is proposed for the calculation of energy gap of defects in LiNbO₃ single crystals. It is shown that the PR centers should have appropriate energy level in the LiNbO₃ crystals, which provides us a theoretical guide to obtain promising nonvolatile holographic recording devices through incorporating impurities intentionally.