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Journal of Crystal Growth 311 (2009) 4235–4240

Contents lists available at ScienceDirect

Journal of Crystal Growth

0022-02

doi:10.1

�Corr

E-m

journal homepage: www.elsevier.com/locate/jcrysgro

Growth and properties of KTP crystals from a new flux

Jing Hu a,b, Zhanggui Hu a,�

a Key Lab of Functional Crystals and Laser Technology of Chinese Academy of Sciences, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences,

Beijing 100190, PR Chinab Graduate University of Chinese Academy of Sciences, Beijing 100049, PR China

a r t i c l e i n f o

Article history:

Received 13 February 2009

Received in revised form

24 April 2009

Accepted 24 April 2009

Communicated by R.S. Feigelsoncrystals were also measured.

Available online 6 May 2009

PACS:

42.65.Ky

42.79.Nv

81.10.�h

Keywords:

A1. Characterization

A2. Top seeded solution growth (TSSG)

B1. KTP

B1. Phosphates

B2. Nonlinear optic materials

48/$ - see front matter & 2009 Published by

016/j.jcrysgro.2009.04.035

esponding author. Tel.: +86 10 82543721; fax:

ail address: hu@mail.ipc.ac.cn (Z. Hu).

a b s t r a c t

High-quality KTiOPO4 (KTP) crystals were grown by a top seeded solution growth (TSSG) method using

K8P6O19–BaF2 as a flux. The volatility of different solvents, such as K8P6O19 (K8), K8–NaF, K8–KF, and

K8–BaF2, was measured. These fluoride additives in K8 fluxes and their compositional effects on the

growth of KTP crystals were studied and discussed. The transmissivity and optical homogeneity of KTP

& 2009 Published by Elsevier B.V.

1. Introduction

Potassium titanyl phosphate (KTiOPO4; KTP) is an excellentcrystal for second-harmonic conversion of Nd:YAG laser radiation.Adopted as a standard for many industrial and medical applica-tions, KTP possesses a vast array of properties such as highthermal stability, good mechanical characteristics, transparencyover a large wavelength range, large nonlinear-optical coefficients,high damage threshold, and broad angular acceptance [1–4].

KTP melts incongruently at 1172 1C [5]. In order to obtain moreapplicable large single crystals, two main growth methods arecommonly used, namely, the hydrothermal method [6–10] andthe flux method [11–15]. The latter method is currently the mostpopular growth process for effecting large KTP single crystals withhigh optical quality. Polyphosphate has been identified as one ofthe most available high-temperature solvents for such crystalgrowth processes.

To date, other groups have successfully grown KTP crystalswith K6P4O13 as a conventional flux [4,15]. However, a majordisadvantage of the polyphosphate flux is its high viscosity, whichis dependent on the solution temperature. Moreover, its poorer

Elsevier B.V.

+86 10 82543709.

visibility and marked slow dissolution kinetics are also detri-mental to the performance of the polyphosphate flux.. The highviscosity usually results in super-saturation gradients in solution.Here in we intended to explore new desired fluxes to improve KTPcrystal quality, as low viscosity helps crystal growth. Introductionof alkali and alkaline earth fluoride ions is also well-known to beable to reduce solution viscosity. Thus, the addition of appropriateamounts of NaF, KF, and/or BaF2 was herein employed to improvethe mass transport and reduce the solution viscosity, and thevolatilities of these solvents were also measured. These resultsshowed that, among several dopants, BaF2 was the most suitableone. In this paper, we also reported a solvent based on theKTP–K8P6O19–BaF2 system, which overcame many problemsencountered during the growth of KTP crystals by the K6P4O13

flux. The new solvent system has enabled the preparation of largeinclusion-free KTP crystals with large dimensions.

2. Crystal growth experimental

2.1. Preparation of starting materials

Starting materials KH2PO4, K2HPO4, and TiO2 were weighed,mixed, and triturated vigorously in an agate mortar, sintered at1050 1C for 1–2 h, and melted in a Pt crucible. Several samples

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J. Hu, Z. Hu / Journal of Crystal Growth 311 (2009) 4235–42404236

were prepared with different amounts of starting materials.Phosphates were allowed to dehydrate, where extreme cautionwas exercised during the dehydration process. Subsequently,metal fluoride (KF, NaF, or BaF2) was added into the solution.Fluoride was added after phosphate dehydration in order to avoidexcessive frothing.

2.2. Volatility measurements

The volatility of a flux is very important during crystal growth,whose direct impacts have been found on the quality of KTP crystals.The volatility of the top seeded solution growth (TSSG) processcauses additional problems such as spontaneous nucleation on thesolution surface, the decomposition of seed crystals, and unstablegrowth.

In this paper, KH2PO4, K2HPO4, and TiO2 were used as startingmaterials (molar ratio; KH2PO4:K2HPO4:TiO2=4:0.7:1), while NaF, KF,and BaF2 were used as additives. The molten solution was heated toand kept at a temperature 50 1C above the saturation temperaturefor at least 24 h to allow all chemical reactions to complete and thesolution to reach homogenization. After the solution was homo-genized, the molten solution was cooled to a temperature slightlyabove the supposed saturation temperature. The trial seedingmethod was then used to precisely control the saturation tempera-ture of the solution. Once the saturation temperature was underprecise control, a working seed was mounted on a platinum seed-rod. Whenever NaF or KF was used as an additive, at the end of thecrystal growth, a large amount of volatiles was observed coagulatingon the seed-rod. The weight loss during this period was defined asvolatility. In order to investigate the volatilization of the solution, thecharged crucible was weighed both before and after each growth.These results are shown in Fig. 1.

2.3. Viscosity and solubility measurements

Among the systems studied previously, the KTP–K8P6O19–BaF2

system has shown the most desired results. So, solutions with

Fig. 1. Comparison of the volatility of KTP in four fluxe

solute (KTP)/solvent (K8 or K8–BaF2) weight ratios ranging from0.4 to 0.5 were prepared and their dynamic viscosities weremeasured. Crystals were grown in a temperature region between810 and 925 1C, thus lowering the growth range by about 50 1C.A crucible of 70 mm diameter was rotated at a constant rate ofabout 30 rpm. The rotation imparts a torque to a platinum bob(spindle, 12 mm diameter) immersed in the liquid and a restoringtorque is applied electromagnetically by adjusting the currentthrough a pair of coils to produce a balance. A high-temperatureviscometer that measures the viscous damping of 50 and 106 cp ofthe solution volume was used. The absolute value was calibratedusing silica–glycerol solutions at room temperature. Fig. 2 showsthe plot of viscosity versus temperature for the K8 self-flux andthe K8–BaF2 flux.

The solubility of KTP was studied in the BaF2-containing K8

solvent systems. Solutions with solute (KTP crystal)/solute+solvent (K8 or K8–BaF2) weight ratios ranging from 0.27 to 0.42were prepared, i.e. 27–42 wt% of KTP and 73–58 wt% of K8–BaF2.The crucible was filled almost to the top. The solution was thenheated and kept at 50 1C above the supposed saturationtemperature for 24–48 h to allow the chemical reaction andhomogenization of the melt to reach completion. After homo-genization, the melt was quickly cooled to a temperature abovethe saturation temperature by 10–15 1C, and then gradually cooledby 1–2 1C decrements to reach the saturation temperatureprecisely. The position of the coldest spot on the melt surfacewas then determined and measured. The measurements werecarried out in the temperature range of 800–1000 1C as shown inFig. 3.

2.4. Crystal growth of KTP

The experiments were performed in a resistance-heatedfurnace. An A1-708P controller was used to control furnacetemperature and cooling rate. The compositions of samplesolutions in specific chemical proportions were weighed preciselyand triturated carefully in an agate mortar. The temperature was

s: (A) K8–KF, (B) K8–BaF2, (C) K8–NaF, and (D) K8.

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Fig. 3. Comparison of the solubility of KTP in K8–BaF2 and K8 fluxes.

Fig. 2. Comparison of the viscosity of KTP in K8–BaF2 and K8 fluxes.

J. Hu, Z. Hu / Journal of Crystal Growth 311 (2009) 4235–4240 4237

raised at a rate of about 100–150 1C/h to 1100 1C and held for24–48 h to ensure the solution melts completely and mixeshomogeneously. After the solution was first cooled at a rate of40 1C/h to about 900 1C, a platinum wire mounted with a [0 0 1]seed orientation crystal was slowly introduced into the furnaceand dipped into the solution. In order to form large crystals, theTSSG method was adopted. The saturation temperature wasmeasured at 900 1C by the testing seed crystal method. The seedwas dipped into the solution at 10 1C above the saturationtemperature, and the solution was kept at a constant temperaturefor 20 min to dissolve the outer surface of the seed. Subsequently,the temperature was decreased at a rate of 0.2–1 1C per day untilthe end of growth. The rotation speed employed was 60–120 rpm,and in many growths, the rotation direction was reversed every120 s with an interval pause of 5 s. After the growth wascompleted the crystal was drawn out of the solution surface andcooled down together with the furnace to room temperature at arate of 20 1C/h.

Finally, a KTP crystal with a size of 19�44�21 mm3 (Fig. 4)was obtained by the TSSG method using the K8–BaF2 flux. The

crystal is colorless, transparent, and exhibits fairly distinguishablefacets.

2.5. Results and discussion

2.5.1. Volatility of KTP

As shown in Fig. 1, the growth of KTP single crystals frequentlyencounters a serious volatility problem from solution additives ofNaF and KF, but not the BaF2 additive and K8 flux with no fluorideaddition. Additionally, in the molten solution with additives NaFand KF during crystal growth, a thin layer of white substance wasformed around the seeding-rod near the cover of the furnace.These are the components volatilized by the solution. It is ratherobvious that large crystals of high quality could not be grownusing KF or NaF as a flux. The KTP–K8–BaF2 and KTP–K8 systemsare, on the other hand, suitable for the growth of high-quality KTPcrystals.

2.5.2. Viscosity and solubility of KTP

Fig. 2 shows viscosity data of solutions used for crystallizationof BaF2 and K8, respectively. These data suggest that the BaF2

additive solute reduces the viscosity considerably, ranging from56 to 188 cp in the case of 40 wt% KTP in K8–BaF2. Solutionviscosity was found to be dependent more strongly on BaF2 thanon solution-temperature or composition. These data also suggestthat the fluoride ions can reduce the viscosity and help crystalgrowth when KTP is dissolved in the solution.

The solubility data for KTP in K8, and K8–BaF2 fluxes are shownin Fig. 3. The solubility was found increase as BaF2 was added tothe K8 self-flux. The growth experiments also show that thefavorable weight concentration of KTP in K8–BaF2 fluxes is in therange of 30–41%.

2.5.3. Crystal growth of KTP

Over 15 growth runs of the top-seed solution growth of KTPwere carried out with different fluoride additives in the K8 fluxes.They lasted for an average of 2 months, including melt stirring,temperature gradient and saturation temperature determination,growth, and cooling. Fig. 4 shows an example of crystal grownfrom the K8–BaF2 flux. Growth problems encountered during theprocesses will be examined in the following.

2.5.3.1. Effects of seed orientation and sizes. Although [10 0] and[0 10] seeds were occasionally used for our crystal growth, most ofthe experiments here were performed with [0 0 1] seeds, whichgave the highest quality crystals. Generally for the seed, thesmaller the better; however, the growth rate of KTP crystals alongdimension a is faster than along dimensions b and c. Therefore,the seed cannot be too small and the length of dimension a islonger than those of dimensions b and c. In our experiment, thesize of the seed crystal was a� b� c=6�3�3 mm3.

2.5.3.2. Effects of rotation speed. Faster rotation increases materialmixing, eliminates inclusions, and limits the occurrence of un-stable growth. During the initial stages of crystal growth, rotationspeed was kept quite high (120 rpm) for increased stirring effects,as contact areas were small. Crystal growth with high rotationrates showed no observable boundary between the seed and thegrown crystal.

2.5.3.2. Effects of temperature gradient in solution. Large tempera-ture gradient in the growth region was beneficial to the crystalgrowth of KTP. It was thought that enhancing the natural con-vection would help mass transfer, in the same way as enhancing

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Fig. 4. KTP crystals obtained by the TSSG method using the K8–BaF2 flux.

Fig. 5. X-ray diffraction of the KTP crystal in K8–NaF (a), K8–KF (b), K8–BaF2 (c), K8

(d) fluxes and standard (e).

J. Hu, Z. Hu / Journal of Crystal Growth 311 (2009) 4235–42404238

the forced convection did. Thus, the temperature gradient in so-lution was increased to 1.5 1C/cm.

Fig. 6. Transparency transmission curve of the KTP crystal grown by the K8–BaF2

flux method.

3. Analysis and characterization of the grown crystals

3.1. Element content determination and X-ray diffraction analysis

Since KTP crystals were grown from fluxes, the compositionand impurity of KTP crystals were measured by an inductive-coupled plasma (ICP) atomic-emission spectrometer. Severalknown standards were used for elemental calibration. The resultsare KTiOPO4 (mol): K:Ti:P=1.0:0:959:0.959; Impurity (mol):KTiOPO4:Ba2+=1:0.0148. These results show that 1.48 mol% ofBa2+ is doped into the KTP crystal.

X-ray powder diffraction was performed on a Bruker D8ADVANCE X-ray diffractometer with CuKa radiation (graphitemonochromator) at room temperature. The scanning step widthof 0.021 and the scanning rate of 0.011/s were applied to record thepatterns in the 2y range of 10–701. X-ray results in Fig. 5 show thatthe X-ray diffraction peak positions of these crystals are almostidentical to the standard spectrum.

3.2. Transmission spectrum

Samples were mounted on a Lambda 900UV/VIS/NIR (Perkins–Elmer) spectrophotometer for absorption spectra at roomtemperature. A KTP crystal sample sized 3�3�7 mm3 grownfrom the K8–BaF2 flux was cut according to type II second-harmonic generation (SHG) phase-matching angle of KTP in theX–Y plane for 1064–532 nm.

The measurement range extended from 200 to 2700 nm. Thetransmission spectrum of the KTP crystal is shown in Fig. 6. Theoptical transparency curve is almost flat with transmittance about80% at the range of 300–2500 nm. The result is comparable withKTP crystals grown from the K6 flux.

3.3. Optical homogeneity

Two KTP crystals with dimensions of 4.5�3�9.36 mm3 grownfrom the K8–BaF2 flux and 4� 4�5.58 mm3 grown from the K6

flux were studied in a WYKO RTI 4100 Laser Interferometer. Theoptical source in the instrument was a He–Ne laser with a

wavelength of 633 nm and the incident beam laser was parallelto the crystal optical axis. Fig. 7 shows the homogeneity plot of theKTP crystal grown from the K8–BaF2 flux. Fig. 8 showsthe homogeneity plot of the KTP crystal grown from the K6 flux.The refractive index of the KTP crystals was about 1.77 andthe color marks in the plot indicated the values of the KTPcrystal’s refractive index in various areas. The extreme values inthe areas of bright red and deep blue were the highest andthe lowest point in the data set, respectively. Theoptical homogeneities characterized by the root-mean-square ofthe gradient of refractive index were about 1.816�10�6

and 9.581�10�6 cm�1. Above results indicate that theKTP crystal grown from the K8–BaF2 flux has excellent opticalquality.

Second harmonic generation is usually an excellent test of crystalquality. We have investigated the KTP sample grown from theKTP–K8–BaF2 system. Fundamental 1064.2 nm light was generatedwith a Q-switched Nd:YAG laser. KTP from the KTP–K8–BaF2 systemwas found to have an SHG effect the same as the KTP from the K6

flux.

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Fig. 7. Homogeneity plot of the KTP crystal in the K8–BaF2 flux.

Fig. 8. Homogeneity plot of the KTP crystal in the K6 flux.

J. Hu, Z. Hu / Journal of Crystal Growth 311 (2009) 4235–4240 4239

4. Conclusion

High-quality KTP crystals are grown by a new solventcomposition based on a KTP–K8–BaF2 system. Compared to the

self-flux system and the other additive (NaF or KF) solutions, theK8–BaF2 flux system has several advantages: first, the viscosity ofthe melt is lower than that of the self-flux system. Second, thevolatility of the melt is lower than that of the NaF or KF additive

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J. Hu, Z. Hu / Journal of Crystal Growth 311 (2009) 4235–42404240

solution. Third, the melt is more stable for KTP crystal growth andoptical homogeneity can reach up to 10�6, slightly better thangrowth from K6 flux with no fluoride addition. Although a smallamount of Ba ion doping into the KTP crystal was detected, it willnot significantly affect the nature. Further investigations in thenonlinear optical properties of the KTP grown in K8–BaF2 flux arein progress.

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