KTP CRYSTAL - AN OVERVIEW IntroductionFrom the ancient days itself, human have been using crystals as gems, salts etc. All the crystals are not available in nature and purity level of the available crystals is not good enough. These negative points tend human to grow good quality crystals as an art. When people realize the science behind this art product, technology in our society get started growing. Such growth gives more devices for us. They are electrical diodes, LEDs, Photodiodes, Radiation detectors, ICs, Transistors, Tyristors, Electro Optic devices, Lenses, Prisms, Nonlinear optic devices, Bearings, Transducers, tape heads, filters etc.Among many crystals KTP crystal is a well known one. This article focuses on KTP crystal's properties, growth techniques and fluxes.Methods to grow KTP crystalAmong many methods, choosing an optimum unique method for growing crystals is very important. Each method has its own requirements. Only when the requirements are satisfied by the chosen materials then it is possible to grow the materials by that method. For melt growth, material should melt congruently and should have no destructive phase change [1]. The incongruent melting point causes KTP crystal unable to grow by pure melt and hence it is found that flux and hydrothermal method are suitable for its growth.Byrappa have compared flux and hydrothermally grown KTP crystal. The former one grows at high temperature and at atmospheric pressure but the latter one grows at moderate temperature with the pressure higher than atmospheric pressure. Chang-long Zhang et al have grown KTP crystal at temperature and pressure of 400-5400C and 120-150 MPa respectively [2] in hydrothermal method. But in the case of flux growth the growth temperature depends on the flux used. In both cases incorporation occurs in the grown KTP crystals. Incorporation due to solvent occurs in flux grown KTP crystal and for hydrothermally grown KTP crystal it is due to (OH-) ion [3]. A narrow valley nearly at 2750 nm wavelength in the UV spectrum of hydrothermally grown KTP crystal indicates the incorporation of (OH-) ion into the lattice [2, 4].Even though hydrothermal method is very expansive, hydrothermally grown KTP crystal has lower ionic conductivity than the flux grown one [3]. In the same way, it has the following good properties as the damage threshold value is 9.5 GW/cm2 for 1064 nm [2], the root-mean-squared of the gradient of refractive index is 4.15*10-6 cm-1, which characterize the optical homogeneity, and its anti-gray tracking ability is better than the flux grown KTP crystal [4].Flux growth is comparatively less expansive method than the hydrothermal method. In order to avoid the unpredictable imperfections, the seeded solution growth is widely used rather than the unseeded solution growth . In the case of KTP crystal growth Top Seeded Solution Growth is widely used by the crystal growers. In actual practice 700 1400 0C is the temperature involved in the growth of KTP crystal by flux method [1]. Depending upon the used flux, the applied temperature while growing KTP crystal gets changed and this is explained in the forthcoming section in detail. KTP crystals grown by this method have high growth rate ( 1 mm/ day) [3] than the KTP crystals grown by hydrothermal method. Another technique named Accelerated Crucible Rotation Technique (ACRT) is used to grow crystals uniformly and to avoid solvent inclusion [1].Experimental setup For flux growth, furnace is needed one and hence growth of crystals by this method is expansive than low temperature solution growth but better than hydrothermal method since in hydrothermal method high pressure is required in addition to high temperature. According to Peter Mullinger and Barrie Jenkins, objective of a furnace is To attain high processing temperature than can be achieved in the open air and this temperature is actually controlled by temperature controllers [5]. In general, Eurotherm programmable temperature controllers are mostly used [6] to control the temperature and hence grow crystals. Depending upon the fuel used, furnace is classified into many. Among them electrical furnace is one which takes electricity as fuel. It is also subdivided into inductive type and resistive type electrical furnace [7]. Resistive type electrical furnaces are widely used by crystal growth community. Some people use 3 zone and some others use 5 zone furnace for growth.Apart from that, a new type furnace has been used by Bordui et al known as heat pipe based furnace system for KTP crystal growth, which has a sodium filled Inconel heat pipe gives resistance to corrosion at high temperature. This type of furnace is able to attain spatial temperature uniformity with less variation of 1.90C throughout the volume of the flux used and also the spurious nucleation problem is largely avoided with the help of this furnace [8]. Next to furnace, crucible plays an important role in flux growth technique in order to grow KTP crystal. Depending upon the used flux, growth temperature of KTP crystal varies. It is supposed to be that KNaPo3F flux is used for KTP crystal growth then, the growth temperature is 11000C - 10200C [9]. So it is necessary to select a suitable crucible to withstand high temperature like this. In addition to that, it possesses the ability to withstand solvent attack.Platinum crucible is a better choice for flux growth because of its high melting point (17730C) and its ability to withstand solvent attack [6]. Rhodium is used by manufacturers to increase the strength of the platinum crucible. With this advantage, it is easy for Rhodium ion in the crucible to diffuse into the flux and then into the KTP crystal. This Rhodium ion diffusion is identified by colour change that is Rh - incorporated KTP crystals are reddish orange in colour. In addition to that they show decreased SHG performance for 1.064m laser source [10]. Consequently, it is better to use pure platinum crucibles since the impurities such as Rh, Ir contaminate easily into the flux [11] and these platinum alloy crucibles are more susceptible to solvent attack [6].In addition to the purity of the crucible, concentration has to be given to the faults present in the crucible. Even scratches present inside the crucible wall affect the metastable zone width of KTP crystal, magnitude of which is a measure of stability of flux [12]. Low volatility is one of the requirements for flux selection and this would be relaxed by crucible if it is covered or welded with lid [6]. Thus scratches in the crucible as well as the lid of the crucibles can give a significant effect over the physical properties of the flux used for KTP crystal growth.In the case of hydrothermal growth, autoclave is first and foremost thing. Depending upon the applied temperature and pressure, materials needed to fabricate autoclave are different. According to H. L. Bhatt, sealed glass is suitable choice for the temperature range of 2500C - 3000C and pressure of up to 6 atm [6]. Moreover steel autoclaves are used in practice and this causes contamination problem [1]. In order to avoid this problem, liners are used inside the autoclaves. Chang-long Zhang et al have used gold liners for avoiding impurity incorporation and got succeeded. Only less than 6 ppm concentration of impurity present in the KTP crystal grown using this gold lined autoclave [4].FluxIt is noteworthy, to understand the physical properties of flux, while growing KTP crystal through flux method. Generally, flux should have low volatility, viscosity and possesses well separated melting and boiling points. Moreover, it should not react with the crucible material and should be easily leachable in other words every flux should have its own solvent in order to separate the grown crystal from it [6]. Selection of suitable solvent is an important job since it can affect growth properties and also the nature of KTP crystal.Number of fluxes introduced for KTP crystal growth are polyphosphate [13,14], tungstate [15,16,17], molybdate [15], phosphate/ sulfate [11] and potassium sodium fluoride (KNaPo3F flux) [9] fluxes. K4P2O7 (K4), K6P4O13 (K6), K8P6O19 (K8) and K15P13O40 (K15) are examples of polyphosphate self flux and 3K2WO4.3KPO3.Li2WO4, K2WO4.P2O5, K2O-P2O5-TiO2-WO3 for tungstate flux and 3K2MoO4.3KPO3.Li2MoO4 for molybdate flux.Growth process equation for various fluxes are given below,For K6 flux,

[18]For 3K2WO4.3KPO3.Li2WO4 flux,

[15]For K2WO4.P2O5 flux,

[16]

For K2O-P2O5-TiO2-WO3 flux,

[17]For 3K2MoO4.3KPO3.Li2MoO4 flux,

[15]For KNaPo3F flux,

[9]It is straightforward that the polyphosphate fluxes contain only potassium, phosphorus and oxygen atom itself and hence the presence of foreign ion in KTP crystal is minimized with the usage of polyphosphate fluxes. Comparison of solubility of polyphosphate fluxes have given by G.M.Loicono et al and the derived entalpy of dissolution for K4, K6, K8, K15 fluxes are 4.8, 4.4, 4.5 and 3.3 kcal/mol respectively [13].High viscosity of self fluxes affects the growth of KTP crystal. Having this problem in mind Jing Hu and Zhanggui Hu introduced NaF, KF and BaF2 as additive for K8 flux. On comparing with other introduced fluxes, K8-BaF2 volatility curve is parallel to that of the K8 flux whereas for other fluxes this curve is very odd. Moreover, K8-BaF2 flux shows reduced viscosity than K8 flux for the temperature range of approximately 8200C-10000C. Even though 1.48 mol % Ba2+ is doped into KTP crystal, there is no change in colour was observed for grown crystal of dimension 19*44*21 mm3 and its optical property is excellent [14].For different molar fractions of K2SO4, K6 flux (xK6:yK2SO4), is modified as 1:3, 1:1, 2:1, 4:1 and 1:0 by R. J. Bolt et al. Among the all fractions, 4:1 gives better results in the point of view of least sulfur ion incorporation. Colourless crystals of size up to 30*30*15 mm3 have been harvested with repeatability for 4K6:K2SO4 flux (4:1) [11]. In this case the growth temperature range is 9250C -7900C [11] which is more or less equal to the temperature range of K8-BaF2 flux, for which the growth temperature is 9250C -8100C [14].In the group of tungstate flux 3K2WO4.3KPO3.Li2WO4 fluxs volatility and viscosity is low in nature [15]. Melt produced by K2WO4.P2O5 flux is very fluid and didnt have any glass formation [16]. In the case of 3K2WO4.3KPO3.Li2WO4 flux, addition of Lithium enhances the fluidity [15]. On comparing K2O-P2O5-TiO2-WO3 flux to the above two fluxes (3K2WO4.3KPO3.Li2WO4 and K2WO4.P2O5) the temperature range for getting single stable phase of KTiOPO4 is large in 3K2WO4.3KPO3.Li2WO4 flux and small in K2O-P2O5-TiO2-WO3 flux. These temperature range for 3K2WO4.3KPO3.Li2WO4, K2WO4.P2O5, K2O-P2O5-TiO2-WO3 fluxes are (10200C-6500C) [15], (1000 0C-7000C) [16] and (930 0C-7000C) [17] respectively. Growth rate of KTP crystal for 3K2WO4.3KPO3.Li2WO4 and K2O-P2O5-TiO2-WO3 fluxes are (0.5-1mm/side-day [15] and 0.9 mm/day [17] respectively.3K2MoO4.3KPO3.Li2MoO4 flux is produced by just changing starting material that is MoO3 instead of WO3 in the reaction mechanism for KTP in 3K2WO4.3KPO3.Li2WO4 solvent. According to L. K. Cheng et al physicochemical properties such as melting point, volatility and viscosity are similar for this two fluxes.F. Joseph Kumar et al have derived enthalpy of solution (Hs) for different fluxes with the help of the data of Bordui et al, Loiacono et al and Ballman et al and got 4778, 4391, 4535, 3336, 105000 cal/mol-K for K4, K6, K8, K15 and 3K2WO4.P2O5 fluxes respectively [12]. Convert this into SI unit, we get 19.99, 18.37, 18.974, 13.957, 43.932 kJ/mol-K. For 3K2WO4.3KPO3.Li2WO4 flux, it is found from the literature Hs value is 43.9 kJ/mol-K [15]. Obviously, for the above mentioned self fluxes, Hs value lies between 10 and 20 kJ/mol-K and for the above two tungstate fluxes, Hs value is more or less the same.In addition to the Hs value, F. Joseph Kumar et al theoretically found surface energy and metastable zone width at 1223 K for the above mentioned fluxes. The derived surface energy values are 5.572, 5.450, 5.039, 4.277, 5.781 mJ/m2 for K4, K6, K8, K15 and 3K2WO4.P2O5 fluxes respectively and their metastable zone width are 6.33, 6.67, 5.73, 6.09, 3.050C respectively and it is concluded that K6 flux has large metastable zone width and hence it is highly stable comparable to other given fluxes [12].J.C.Jacco et al found out PbF2 flux modifier is better than the B2O3, CaO, V2O5, KF and PbO flux modifiers. Even it is better, it is also produces significant incorporation into the grown crystal which is the drawback of this modifier [18]. In the case of Phosphate/sulfate flux (4K6:K2SO4) it is believed that sulfate ion (SO42-) itself act as a flux modifier by breaking the titanate chain in the flux and hence reducing the viscosity of flux [11].Depending upon the used flux, generally water [9, 18] and hot water [16] are used as a solvent for KTP crystal in order to separate it out from the flux. It is very important since these solvents are the tool to get the grown crystal out from the flux unless there is no use of this method of crystal growth.Structure of KTP crystal

The KTP crystal belongs to orthorhombic system, with lattice constants a = 12.823 , b = 6.414 , c = 10. 589 . C2v mm2, -pna21 is the point group and the space group of this crystal.When considering each unit cell of the KTP, it possesses two non-equivalent Ti lattice sites (Ti(1) and Ti(2)), K lattice sites (K(1) and K(2)) and P lattice sites (P(1) and P(2)) and also ten non-equivalent O lattice sites (O(1), O(2)O(10)). Ti and O form distorted TiO6 octahedron and P and O form the PO4 tetrahedron in its crystal structure [19].Effect of seed orientationSeed orientation is a tool in crystal growth as it solves many problems when the grower chooses the orientation properly. In the case of KTP crystals, oriented seed grown crystals have lower dislocation density and smaller capping region than the crystals grown using seed orientation under the same cooling rate (0.10C/h) rotation rate (70 RPM ) and saturation temperature (9600C) for K6 flux [20]. In addition to this, the same seed orientation used to minimize the formation of inclusion under the following growth conditions as cooling rate = 0.5-1.50C/day, rotation rate = 120 RPM and saturation temperature = 9720C and this is done for K6 flux [21]. For tungstate flux (3K2WO4.3KPO3.Li2WO4) L. K. Cheng et al found {201} seeding gave the best results than the {100} and {011} oriention [15].Physical propertiesSingle crystal XRD has gave information about the lattice constant values and space group and point group of KTP (which are mentioned earlier). While taking UV studies for KTP crystal the incident beams illumination direction slightly changes the UV-cut off wavelength. For K6 flux grown KTP, if the incident beam was parallel to [010], [100] direction of KTP crystal, then it is found that the UV- cut off wavelengths are 340nm [11] and 350 nm [18] respectively. Independent of the flux, in some cases such as 4 K6:K2SO4, K6, 3 K2WO4.3KPO3.Li2WO4, 3K2WO4.P2O5 flux grown KTP crystal shows the UV- cut off wavelength is 350nm [11,18,15,16]. 80% [14], 85% [22] of transmission was observed in K8-BaF2 flux grown and K6 flux grown KTP crystal respectively. C.V.Kannan et al found out absorption coefficient () at 532nm wavelength and the band gap of K6 flux grown KTP crystal as 6.4*10-3 cm-1 and 3.43 eV respectively [22].Relation between the absorption coefficient and the incident photon energy is also derived by C.V.Kannan et al.Optical transmission spectrum of KTP crystal grown by different methods such as ACR (Accelerated Crucible Rotation method), HTM (Hydrothermal method), IFM (Improved Flux Method) are compared by Shen Dezhong and Huang Chaoen and all the three spectrum are more or less the same except the absorption peak at 2.7m for hydrothermally grown KTP crystal [23]. The same effect was observed by Chang-long Zhang et al at 2.75m [2].From the UV analysis, the ambient suitable for the growth of KTP crystal is found out as nitrogen ambient than the oxygen ambient (80% O2) and the air (21% O2) ambient. Since in the oxygen ambient, it is easy to oxidize the platinum, which is the crucible material and hence the formed PtO2 is incorporated into the growing crystals and finally increases the absorption coefficient [24]. Laser interferometer is an instrument used to scale optical homogeneity indirectly by measuring root-mean-square of the gradient of refractive index of the crystal. Mostly for KTP crystal this value is of the order of 10-6 cm-1 [4,14] and hence its optical homogeneity is conformed. The following table gives the information about this RMS of the gradient of refractive index of KTP crystals grown under different conditions.Used instrument: WYKO RTI 4100 Laser interferometer;Source: He-Ne laser (633 nm)S.NOMethod of growthFlux / Nutrient usedroot-mean-square of the gradient of refractive indexRefractive indexReference

1Flux growth (TSSG technique)K8-BaF2 flux1.94*10-6 cm-11.7714

2Hydrothermal methodCrushed flux grown KTP crystals (nutrient)4.152*10-6 cm-11.774

3Flux growth (TSSG technique)K6 flux12.211*10-6 cm-11.7714

On seeing this table, the RMS of the gradient of refractive index of flux grown KTP crystal is approximately three times greater than that of hydrothermally grown KTP crystal. In the above all cases refractive index of KTP crystal is 1.77 [4,14].Sellmeier coefficients of KTP crystal at 200C and 300C are given by Gorachand Ghosh and Chang-Long-Zhang et al respectively.For type-II frequency doubling at 1.06 m, 1.325 m wavelengths, the phase matching angle is found to be = 2100.50 (X-Y plane), = 900 (Z axis) and = 900 (X-Y plane), = 460 (Z axis respectively) respectively and this is for the tungstate flux grown KTP crystal at room temperature [16]. The phase matching angle is found to be 23010 off the X-axis for the K6 flux grown KTP crystal and also for this crystal the maximum conversion efficiency is found out as 58% (without any antireflection coating) at an approximate intensity of about 140 mW/cm2 [22].On comparing polycrystalline KTP (produced through solid state reaction) and flux grown KTP crystal to the polycrystalline KTP grown under oxygen atmosphere, polycrystalline KTP grown under oxygen atmosphere gives relatively large SHG efficiency (4.8 times of urea) for Q switched Nd:YAG laser [25]. In addition to the above, the SHG efficiency of K8-BaF2 flux grown KTP crystals have same SHG efficiency like K6 flux grown KTP crystals for 1064.2 nm wavelength [14] IR spectra of KTP crystal was given by J.C. Jacco. According to him, absorption peaks between 1250 and 850 cm-1 correspond to vibrations of PO4 groups and the peaks at 820, 785, 712 cm-1 correspond to Ti-O vibrations of the distorted TiO6 octahedra and also the peaks in the region between 660 and 350 cm-1 correspond to the splitting of the degenerate PO4 modes [26].Due to the any one of the following modes, PO4 tetrahedra internal modes, the TiO6 octahedra internal modes, combination and overtone bands of PO4 groups, OH- ion modes and lattice modes, vibrations have been occurred in KTP crystals [27] and this was given by K. Vivekanandan et al for both flux grown and tungstate grown KTP crystal. Influence of flux in the Raman spectra is observed as shifts in the internal mode frequencies of TiO6 and PO4 towards lower wavenumber region for tungstate flux than the phosphate flux grown KTP crystals [27]. Polarization of input and stokes light [28], Pressure [29], ac and dc electric field [30] are the factors affecting the Raman spectra of KTP crystal and this was proved by G. A. Massey et al, G. A. Kourouklis et al and M. J. Bushiri et al respectively. Phase change in KTP crystal is induced by pressure and this change of phase is observed by taking Raman study as a medium.It is straightforward from high frequency Raman spectrum, at nearly 5.5GPa and 10GPa, discontinuous behavior is observed which is believed to be due to phase change of KTP crystal from ferroelectric to antiferroelectric and from antiferroelectric to paraelectric phase respectively [29].Both dc and ac fields also play their own role on the Raman spectra of KTP crystal. Mainly intensity variation is observed due to these field applications. After 30 minutes application of the dc fields of 26 V/Cm, 38 V/Cm along polar axis shows reduction and large reduction in intensity respectively at the Raman bands of KTP crystal. In contrast to that, an ac field of strength 20 V/Cm and frequency of 1 kHz enhances the intensity slightly in the Raman spectrum of KTP crystal which was taken 2 hours later the application of this field [30].In order to reveal the mechanical properties of KTP crystal, it is analyzed both by micro- and nanoindentation. Mohs hardness value of KTP crystal is 5, which is half the value of diamond (Mohs hardness values for diamond is 10). Anisotropy in hardness value of KTP for various planes is due to difference in recticular face density and nonidentity of the bonding strength in the planes and it is maximum for (100) and (011) faces. By nanoindentation method hardness value and the elastic modulus value is found out as HN = 9.48 0.36 GPa, EN =142 1 GPa respectively for KTP [31].With the help of computerized adiabatic calorimeter, heat capacity of KTP was analyzed in the range of 60-360 K. From this study is is known that KTP crystal is stable within 60-360 K thermodynamically [32].Physical properties such as mechanical strength, electrical conductivity and optical absorption are always influencing the performance of KTP crystal in devices and hence it is necessary to find defects and their distribution in the crystal [33]. Etching provides an easy way to check the quality of the grown KTP crystal by analyzing the etch pits in the crystal.As KTP crystal is insoluble in water, on seeing literature [17,23,34] HCl is found out as etchant for KTP crystal . Different concentrations (37% HCl [34], 20% HCl [17]) of HCl is used for this etching purpose. Generally etching time and temperature are the parameters which are changeable as per the experimentalist wish.Orientation of etch pits along a particular direction was observed by Cheng Gan Chao et al [17] and Shen Dezhong et al [23] on (020) face and [110] plane respectively. From the above result, it is obvious that the grown KTP crystal possesses no domains on the above mentioned face and plane. Moreover, dislocation wall is observed on the (020) face.Both spike edge pits and non-spike edge pits are presents in the (100) face of KTP crystal. Middle of the (100) face have etch pits without any spike, but near the edge of this face dislocation tend to turned into the crystal at an angle (range 300 500) and hence the spike-like etch pits were formed. The authors also confirms the presence of spike-like etch pits with the help of SEM. Big hole in the SEM image is an indication of presence of spike-like etch pits [34].According to Shen Dezhong et al, dislocation density of KTP for [110] plane is 3*103/cm2 [23]. At the same time variation in dislocation density is observed by R.J. Bolt et al in KTP for different faces and it can vary from 0 - 20,000/mm2 [34] depending upon the face.For flux grown KTP crystal, at room temperature the piezoelectricity, elasticity and dielectric permittivity tensors, have been completely determined by David K. T. Chu et al. In addition to this P-E-D matrix, piezoelectric and electromechanical coupling coefficients and also the temperature dependence of elastic constant and the thermal expansion coefficients are also determined. The determined piezoelectric coupling coefficients are kt= 34% (along z axis), k24=16% (along y axis) and k15=14% (along x axis) and this is for longitudinal mode and also it is found comparatively larger than RTP and RTS crystals. The temperature dependence of elastic constants and the thermal expansion coefficients of KTP crystal are given below. After analyzing, it is obvious that KTP crystal possesses less temperature dependence than its isomorphs such as RTA and RTP crystals so may be suitable for frequency control and signal processing applications. Moreover, the electromechanical coupling coefficient kt is 35% for the flux grown KTP crystal [35].Temperature dependence of elastic constants -->

-----220-134-143-100

Thermal expansion coefficients -->112233 ----

----6.89.6-1.3 ----

List of determined temperature dependence of elastic constants and thermal expansion coefficients.Gray tracking formation is a major problem in KTP crystal investigated by many researchers and the factors responsible for this is also found out. Humidity and temperature of used gases [36] and concentration of hydroxyl [37] in KTP crystal while annealing affect the gray tracking. Only the increased humidity of gases decrease the gray tracking but not the types of gases [36] used at annealing. Increased hydroxyl concentration in KTP crystal decreases its gray track susceptibility [37]. It is experimentally observed and tabled as,KTP CRYSTAL (as named by author)Susceptibility @ 150M W/cm2[% /hour]Content of hydroxyl[mg/g]

#F0.480.010

#B0.130.011

#D0.09(min. value in the given four)0.078(max. value in the given four)

#G5.3(max. value in the given four)0.008 (min. value in the given four)

It is also obvious that, while annealing the crystal in air atmosphere, the hydroxyl comes into the crystal and hence the gray track susceptibility gets decreased. There is another way to get the hydroxyl ion into the crystal lattice is hydrothermal growth. It may be the reason that the hydrothermally grown KTP crystal possesses high anti-gray track ability [4] than the flux grown KTP crystal.Moreover, gray tracking also depends on the direction of polarization whether it is parallel or perpendicular to the z axis of the KTP crystal. Experimentally it is proved that gray track susceptibility is one order magnitude less for z polarization irradiation than the z polarization irradiation for KTP crystals [38].ConclusionIn this article, some of the properties of KTP crystal is discussed. This may be helpful to know why KTP crystal is used in more applications.References:1)Brice, J.C, Crystal Growth Processes, Halsted Press. ISBN 0-216-91793-X.2)Chang-Long Zhang, Ling-Xiong Huang, Wei-Ning Zhou, Ge Zhang, et al., J. Crystal growth 292 (2006) 364.3) Byrappa, K, Masahiro yoshimura, Handbook of Hydrothermal Technology-A Technology for Crystal Growth and Materials Processing. ISBN 0-8155-1445-X.4)Chang-Long-Zhang, Zhang-Gui Hu, Ling-Xiong Huang, Wei-Ning Zhou, et al., J. Crystal Growth 310 (2008) 2010.5)Peter Mullinger, Barrie Jenkins, Industrial and Process Furnaces-Principles, Design and operation. ISBN 978-0-7506-8692-16)Bhatt, H.L, Introduction to Crystal Growth Principles and practice, CRC Press. ISBN-13: 978-1-1-4398-8333-4.7)Trinks, W, Mawhinney, M.H, Shannon, R.A, Reed, R.J, Garvey, J.R, Industrial furnaces, Sixth Edition, John Wiley & Sons, INC. ISBN 0-471-38706-1.8)Bordui, P.F, Jacco, J.C, Loiacona, G.M, Stolzenberger, R.A, zola, J.J, J. Crystal Growth 84 (1987) 403.9)Suma, S, Santha, N, Sebastian, M.T, Materials Lett. 34 (1998) 322.10)Bhaumik, I, Ganesamoorthy, S, Bhatt, R, Karnal, A.K, et al., Crystal. Res. Technol. 41(2006) 1180.11)Bolt, R.J, Van der Mooren, M.H, de Haas, H, J. Crystal Growth 114 (1991) 141.12)Joseph Kumar, F, Jeyaraman, D, Subramanian, C, Ramasamy, P, J. Crystal Growth 137 (1994) 535.13)Loiacona, G.M, Mcgee, T.F, Kostecky, G, J. Crystal Growth 104 (1990) 389.14)Jing Hu, Zhanggui Hu, J. Crystal Growth 311 (2009) 4235.15)Cheng, L.K, Bierlein, J.D, J. Crystal Growth 110 (1991) 697 .16)Ballman, A.A, Brown, H, Olson, D.H, J. Crystal Growth 75 (1986) 390.17)Cheng Gan Chao, Qian Zhi Qiang, Tang Guang Kui, Song Wen Bao, Tang Hong Gao, J. Crystal Growth 112 (1991) 294.18)Jacco, J.C, Loiacono, G.M, Jaso, M, Mizell, G, Greenberg, B, J. Crystal Growth 70 (1984) 484.19)Zhang Kecong, Zhang Hong, Chinese Sc. Bull. 43 (1998) 529.20)Kim, J.H, Kang, J.K, Chung, S.J, J. Crystal Growth 147 (1995) 484.21)Indranil Bhaumik, Ganesamoorthy, S, Rajeev Bhatt, Sundar, R, et al., J. Crystal Growth 243 (2002) 522.22)Kannan, C.V, Ganesamoothy, S, Kumaragurubaran, S, Subramanian, C, Cryst. Res. Technol. 37 (2002) 1049.23)Shen Dezhong, Huang Chaoen, Prog. Crystal Growth and Charat. 11 (1985) 269.24)Akio Miyamoto, Yusuke Mori, Takatomo Sasaki, Sadao Nakai, App. Phys. Lett. 69 (1996) 1032.25)Dhanaraj, G, Bhatt, H.L, Sol. State Commun. 91 (1994) 757.26)Jacco, J.C, Mat. Res. Bull. 21 (1986) 1189.27)Vivekanandan, K, Selvasekarapandian, S, Kolandaivel, P, Sebastian, M.T, Suma, S, Materials Chemistry and physics 49 (1997) 204.28) Marsey, G.A, Loehr, T.M, Willis, L.J, Johnson, J.C, Appl. Opt. 19 (1980) 4136.29)Kourouklis, G. A, Jeyaraman, A, Ballman, A.A, Sol. State Commun. 62 (1987) 379.30)Bushiri, M.J, Mahadevan Pillai, V. P, Ratheesh.R, Nayar, V.U, Journal of physics and chemistry of solids 60 (1999) 1983.31)Stus, N.V, Dub, S.N, Stratiychuk, D.A, Lisnyak, V.V, Journal of alloys and Compounds 366 (2004) L13.32)Zhi-Cheng Tan, Guang-Yu Sun, Yong-Ji-Song, Lan Wang, et al., Thermochimica Acta 352 (2000) 247.33)Springer Handbook of Crystal Growth. ISBN 978-3-540-74182-4.34)Bolt, R.J, Van der Mooren, M, J. Crystal Growth 112 (1991) 773.35)David K. T. Chu, John D. Bierlein, Robert G. Hunsperger, IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency control 39 (1992) 683.36)Shinji Motokoshi, Takahisa Jitsuno, Yasukazu Izawa, Masahiro Nakatsuka, CLEO (2001) 167.37)Shinji Motokoshi, Takahisa Jitsuno, Masahiro Nakatsuka, Yasukasu Izawa, CLEO (2000) 111.38)Hu, X.B, Wang, J.Y, Zhang, H.J, Jiang, H.D, et al., J. Crystal Growth 247 (2003) 137.