Indian Journal of Pure & Applied Physics Vol. 47, May 2009, pp. 356-361 Growth of KTiOPO 4 crystals by flux technique and their characterization E Gharibshahian a , M J Tafreshi a* & M Fazli b a Physics Department, b Chemistry Department, Faculty of Science, Semnan University, Semnan, Iran Received 18 September 2008 ; revised 9 January 2009 ; accepted 12 March 2009 Single crystals of KTiOPO 4 (KTP) have been grown by flux technique using K 6 P 4 O 13 flux. Crystals up to 3×2×1 mm 3 in size were grown by slow cooling and spontaneous nucleation when high temperature solution was cooled down to room temperature with the rate of 7°C/h. The structure and quality of the grown crystals were characterized by X-ray diffraction (XRD), Fourier transform infrared (FTIR) and Optical Transmission analysis. The surface morphology of the grown crystals was studied by scanning electron microscopy (SEM). Keywords: Crystal growth, Non-linear optical materials, Flux growth, KTiOPO 4 , Characterization 1 Introduction Potassium titanyle phosphate (KTP) is an excellent non-linear optical crystal and the best frequency doubling material for 1064 nm Nd 3+ emission of Nd:YAG lasers, which is widely used in non-linear optical devices 1 . Due to its large non-linear optical coefficient (10 times KDP), high damage threshold, high thermal stability and low walk off angle, KTP finds many applications in frequency doubling and mixing. It also finds wide applications in E-O modulators and Q-switches because of its high electro-optic coefficients and low dielectric constants 2 . As KTP undergoes partial decomposition on melting at 1323 K, the melt growth technique cannot be employed for its crystal growth. Therefore, in order to obtain useful large single crystals, two main growth methods, namely, the hydrothermal method and the flux method are used. Since the hydrothermal growth technique requires rather special high pressure steel vessels with noble metal liners, so the flux-growth technique is currently the most popular method for obtaining large KTP single crystals. The significant advantages of using the flux process are: (i) It operates at atmospheric pressure and hence, does not require sophisticated pressure equipment so the crystals can be grown at atmospheric pressure, (ii) it is easy to design scaled- up furnaces and (iii) crystals can be grown without incorporation of water molecules which gives rise to considerable absorption in infrared region 3 . Different types of fluxes have been used for the growth of KTP crystals by flux method. But within these fluxes, polyphosphate (K 6 P 4 O 13 ) has identified as a viable high temperature solvent because of absence of foreign-ion and better morphology of the grown crystals 4 . Owing to the high viscosity of the K 6 P 4 O 13 flux, the overall kinetic resistance for growing is large. Hence, a significant level of bulk supersaturation is required in the neighbourhood of the growing crystal. This bulk supersaturation is achieved by slow cooling of the solution 5 . The amount of cooling rate during growth of KTP crystals by flux method depends on the type of the flux and technique adopted for the growth process. When top seeded solution growth (TSSG) technique is used, cooling rates as low as 0.5-2°C/day are sufficient to proceed, the growth process in the presence of seed crystal 2 . When seed crystal is not introduced to the solution, nucleation occurs either on a cold finger or spontaneously inside the solution 6 . This paper reports the growth of KTP crystals by spontaneous nucleation in solution using K 6 P 4 O 13 flux. The solution containing synthesized material was cooled down with different cooling rates to find out the effect of cooling rate on the growth of KTP crystals and the rate at which nucleation takes place spontaneously in solution. The grown crystals were subjected to different analysis and a comparison has been made between the properties of these crystals and the KTP crystals grown by other methods. 2 Experimental Details 2.1 Charge preparation Starting materials including high purity KH 2 PO 4 , TiO 2 and K 2 HPO 4 (Merck Company), with molar