| KTiOPO4 crystal is an excellent material with perfect nonlinear optical properties, whereas there are two limitations in the practical application. Firstly, the cut off wavelength for typeⅡphase matched (PM) second harmonic generation (SHG) is 994 nm, therefore bule SHG light can not produce in the KTP crystal, the KTP crystal is not fit for blue and violet device. Secondly, the optimum typeⅡPM angle for SHG of Nd:YAG 1064.2 nm laser isθ=90°,Φ=23.1°, it can not achieve 90°non-critical PM direction, so the improving of green light transfer efficiency is limited. These two limitations restrict the application of KTP crystal to a large extent. In order to solving these problems, doping methods that replaced or substituted the elements of K,Ti,P in the crystal are used universally.The properties of KTP crystal will have some improvements by the doping of niobium, the position of Ti in the unit cell is replaced, so bule SHG light can produce and 90°non-critical PM direction can achieve in the KTP crystal. But the number of potassium vacancy is increasing and the holistic quality of crystal is decreasing after niobium entered crystal lattice, it would affect the application of KTP crystal. If the content of niobium is too high, crystal quality will be influenced, the efficiency of SHG will have a descent. Therefore, it is a difficult problem to balance properties and qualities in the process of KTP crystals growth. Presently, the research of Nb:KTP and KTP crystals fasten on crystal growth and properties of SHG, there is little study on composition and structure of KTP crystal, the research on content and distribution of niobium, the characteristic of point defect have little investigation in the Nb:KTP crystal too. It is important to investigate on composition and structure of Nb:KTP and KTP crystals, the results are propitious to the process of Nb:KTP crystals growth.Six pieces KTP crystal samples and three pieces Nb:KTP crystal samples were growth by flux method successfully. The influence of niobium doping on the component and Raman spectrum character of KTP crystals was investigated by XRD, LRM, LA-ICP-MS, XPS. The connection of crystal structure and growth method in KTP crystals was explored. The segregation phenomenon of Nb was researched in the Nb:KTP crystals. The characteristic peaks of chemical bond in the KTP and Nb:KTP crystals was analyzed and compared. The characteristic of point defect and valence state of Ti were analyzed, the effect of niobium on nonstoichiometry in Nb:KTP crystals was discussed too. The research results show as follows:(1) The morphology and inclusions character of KTP and Nb:KTP crystals have obvious distinguish. The number of inclusions increase and more cloud layer appear in Nb:KTP crystal after niobium entered crystal lattice, the SHG properties have some improvement thus, the crystal growth habit is affected and the crystal quality decreases. It would affect crystal performance to some extent.(2) The unit cell volume is related to growth methods of KTP crystals. The unit cell volume of KTP crystals grown by spontaneous crystallization method is small due to their cooling rate is fast. The unit cell volume of KTP crystals grown by s top seeded solution growth method is big due to their cooling rate is slow. The KTP crystals grown by hydrothermal method are made from KTP crystals grown by spontaneous crystallization method, its unit cell volume is between the above two crystals.The unit cell volume in Nb:KTP crystals grown by flux method is bigger than those grown by hydrothermal method when the raw material mixture ratio is same, so the growth method can be distinguished by unit cell volume. The unit cell volume of Nb:KTP crystals is under the control of synergistic reaction by blockage effect and expansion effect. If the content of Nb is low, the expansion effect is obvious, the unit cell volume increases. If the content of niobium is high, the blockage effect is obvious, the unit cell volume decreases.(3) The Raman shifts of characteristic peaks in KTP crystals have certain difference on account of the growth methods. The characteristic peaks in 782cm-1,744cm-1,515cm-1, are the intrinsic character of KTP crystals grown by hydrothermal method that can be used to distinguish KTP crystals grown from flux.The interaction of PO4 tetrahedrons and TiO6 octahedrons is changing after Nb entered crystal lattice, thus, a few of characteristic peaks are drift. The relative intensity ratio of asymmetric bending of TiO6 octahedral and PO4 tetrahedral are diminishing by the content of niobium increasing. In contrast, The relative intensity ratio of TiO6 octahedral symmetric stretching is expanding by the content of niobium increasing. The characteristic peaks in 327cm-1 and 1122 cm-1 are the symbols of KTP crystals grown by flux method and can be use to distinguish KTP crystals grown by flux from those grown by hydrothermal method.(4) KTP and Nb:KTP crystals grown by hydrothermal and flux method belong to nonstoichiometry compound. The elements ratio deviates from stoichiometry in KTP and Nb:KTP crystals, the atomicity ratio of potassium is bigger than theoretical value and the atomicity ratios of phosphorus and oxygen are smaller than theoretical value. The elements ratio is changing with the growth system. Nb5+ is replacing Ti4+ and the potassium vacancy is forming to balance the charge with the entry of niobium in Nb:KTP crystals. The elements ratio of potassium have a descent direction by the content increasing of niobium. Otherwise, the formation of phosphorus and oxygen vacancy by the volatilization of phosphorus makes their atomicity ratio lower than theoretical value, and the volafHization of phosphorus decreases with increasing concentration of niobium.Ti3+ center in the samples is forming in the process of crystal growth, Furthermore, when the growth temperature and surroundings have some alteration, the transmission of heat and mass are influenced in crystal growth system, the TiO6 octahedrons of negatively charged with Ti3+ center are forming by the supply of electron from O2- to Ti center, and potassium is filling in interspaces of TiO6 octahedrons and PO4 tetrahedrons in order to balance the negative charge of TiO6 octahedrons, so the atomicity ratio of potassium is bigger than theoretical value in KTP and Nb:KTP crystals.
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