The Structure And Properties Of Doped Lithium Niobate Crystal

In the formwork of chemical bonds, a simple radial force constant model is used to study the lattice relaxation produced upon doping metal ions to lithium niobate (LiNbO3) crystals. And then a lattice energy model is proposed to study the occupancy of dopants in the LiNbO3 crystal on the basis of establishing the impurity bond length relaxation. Finally, we investigate the effect of various dopants on the optical absorption and photorefractive properties of LiNbO3 crystal.The local distortions of LiNbO3 lattice is occur when the doping ions enter the LiNbO3 matrix due to the difference of the ionic radius between doping ion and Li+(or Nb5+), which will have an impact notably on the macroscopic properties of material. Considering the chemical bond as a spring without mass, we calculate its force constant according to the properties of bonding atoms, to analyze the elastic deformation around the dopant. And thus the displacement of O2- joined by doping ion in the LiNbO3 crystal can be evaluated by the principle of minimum potential energy in the theory of elasticity. It shows that the bond length of impurity in LiNbO3 matrix only slightly deviates from its nature one, but has large difference from that of Li-O or Nb-O. Based on above research, the rule of doping ions in the change of optical absorption of LiNbO3 crystal is discussed.The lattice energy which is related to the cohesion of the crystal can typify the structural stability of doped LiNbO3 crystal. It can be described as the function of bond valence considering the Born-Haber cycle for the formation of an ionic oxide MmOn. The dopant occupancy in the LiNbO3 matrix can be determined by comparing the deviation of its lattice energy in different locations at both Li+ and Nb5+ sites. It shows that the optical damage resistant dopants and rare earth ions preferentially occupy the Li sites, and there is no regular rule for the occupancy of photorefractive ions, which well agree with the experiment results. Considering the occupancy and properties of dopants, we analyze the influence of doping ions on the photorefractive properties of LiNbO3 crystal.