The photorefractive performance and growth quality of Tungsten bronze (TB) niobate can be effectively improved by doping because of its special crystal structure. For example, potassium and sodium strontium barium niobate crystals doped with copper (Cu: KNSBN) has breakthrough on obtaining large crystal size and performance improvements. Doping modification in crystalline materials research has caught more and more attention. Therefore, studies on the mechanism of doping, the relationship of doping and crystals molecular design and changes of the compositions are necessary.

 

There are five types of structure position (Al, A2, C, Bl, B2)inside the TB niobate crystal structure, which provides favorable conditions for introducing other ions to doping modification. Common dopant ions are: monovalent cations Cu ⁺, divalent cations Mg^{2 +}, Ca^{2 +} and Cu^{2 +} etc., trivalent cation Cr^{3 +}, Fe^{3 +}, etc., tetravalent cations Zr^{4 +}, Ti^{4 +}, etc., and pentavalent cation Ta^{5 +}, V^{5 +} and hexavalent cation W^{6 +}, etc. .

 

Dopant ions in crystal have the following functions. First, to make crystal growth streak reduced and crystal not easy to crack, so that large size crystalline material is easily to grow. Second, to make the crystal has good polarization performance, and the depolarization phenomenon is not easy to appear. In addition, the dopant ions into the crystal also have the following effects on crystal : (a) Changes of lattice constant. (b) Distortion of crystal symmetry. (c) effects on crystals phase transformation. At the same time, the amount of dopant ions into TB niobate crystal will lead to changes in other cations in the crystal more or less. Dopant ions get into the crystal structure of different positions due to its own characteristics and types of cations in crystal and occupying circumstances. So in the synthesis of crystal material, doping can improve the quality of the crystal growth and the performance of the crystal.