CRYSTAL GROWTH & DESIGN
Flux Growth and External Morphology of KTiOPO4 Crystals Jiyang Wang,† Huaijin Zhang,† Minhua Jiang,† Mariana Nikitina,‡ Victor Maltsev,*,‡ and Nikolay Leonyuk‡ State Key Laboratory of Crystal Materials, Shandong UniVersity, Jinan 250100, China, and Moscow State UniVersity, Moscow 119991 GSP-1, Russian Federation
2009 VOL. 9, NO. 2 1190–1193
ReceiVed October 2, 2008; ReVised Manuscript ReceiVed December 3, 2008
ABSTRACT: KTiOPO4 (KTP) crystals with high quality and weight exceeding 230 g have been obtained on seeds from hightemperature solutions based on the K6P4O13 (K6) flux. The next priority was the flux crystallization of KTP material in the presence of M2SO4-type mineralizers (M ) Li, Na, K). The motivation of the work was to find the effect of flux composition on KTP morphology by an addition of alkali sulfates on the scheme: K6:M2SO4 ) 8, 6, and 4. Small spontaneous KTP crystals were also obtained from the fluxed melts doped with V, Cr, Ni, Co, Cu, Mo, Ba, Ce, Er, and W under similar conditions. Chromium admixture in solid phase was measured to be up to 0.5 wt %. Other dopants were less than 0.02 wt %. In all experiments, typical faceting of KTP crystals is {100}, {011}, and {201}. In the case of vanadium, the {111} faces are formed, additionally to the above crystal forms. In the presence of the Li2SO4 mineralizer, {111} and {031} faces were found as well. 1. Introduction Ferroelectric KTiOPO4 (KTP) crystal is an effective nonlinear optical material having relatively high damage threshold and excellent thermal stability.1 It exhibits exceptionally high frequency conversion efficiencies along with outstanding electrooptic properties allowing combining frequency doubling with Q-switching in a single device. These excellent characteristics depend greatly on the real structure of the crystals obtained, and their morphology and chemical composition, which are closely related to the specific growth conditions. KTP melts incongruently at 1160 °C, and for the first time, its single crystals are sufficient for physical experiments were obtained from hydrothermal solutions.2 In this case, the crystallization process in high-pressure autoclaves with p ) 3000 atm and t ) 500-700 °C requires several weeks.3 On the other hand, the growth procedure based on the K6P4O13 (K6) flux makes it possible to cut down on time, and to obtain KTP single crystals with comparable sizes in the temperature range of 970-900 °C.4-6 In the literature, there are data indicating ways on which the temperature range of KTP crystallization can be decreased by at least 50 °C in the case of an addition of K2SO4 to the K6 flux.1 According to ref 7, the growth rate of KTP crystals from high concentrated solutions is maximal along the Z axis, i.e., [001] crystallographic direction. In the case of diluted fluxed melts, the {201} and {011} faces become slowly growing crystal forms. At the same time, an increase in supersaturation leads to intensive growth of {201} faces in comparison with {011}, and vice versa. Nevertheless, it is unknown dependence of KTP crystal morphology on the flux composition, for example, with an addition of alkali sulfates or oxide dopants. Thus, this work is focused on the effect of K2SO4 and other mineralizers (M), namely, Li2SO4 and Na2SO4, which have not been used up to now for KTP crystallization, on the external morphology of these * Corresponding author. E-mail:
[email protected]. Tel: 7 (495) 939 2881. Fax: 7 (495) 939 2980 † Shandong University. ‡ Moscow State University.
Figure 1. Resistance-heated crystal growth furnace: seed holder, (2) quartz rod and growing crystal, (3) insulators, (4) heat tube, (5) electric alloy wire, (6) powder and fiber insulators, (7) Pt crucible lip, (8) Pt crucible, (9) alumina crucible, (10) thermocouple.
crystals. The influence of oxide dopants and other factors on the composition and habit of KTP crystals was also studied. 2. Experimental Section High-temperature solution growth of KTP crystals on the seeds was performed in a vertical electric tube furnace having CrNi alloy wire heating and equipped with a Pt-Rh/Pt thermocouple based FP21 controller (Figure 1). As for spontaneous crystallization, all experiments were done in a similar furnace with a heating element made of CrNibased alloy with a melting temperature of about 1400 °C. A PROTHERM-100 microprocessor-containing controller, also together with a Pt-Rh/Pt thermocouple, was used to control and monitor the crystal growth temperature. Growth of large KTP crystals was based on the 1KTP/2K6 flux. The starting chemicals of spectrum purity were TiO2,
10.1021/cg801100g CCC: $40.75 2009 American Chemical Society Published on Web 01/02/2009
Flux Growth and External Morphology of KTiOPO4 Crystals
Crystal Growth & Design, Vol. 9, No. 2, 2009 1191
Figure 3. Transition spectra of Cr:KTP crystal at room temperature.
Figure 2. (a) Large size KTP crystal grown on a seed (the square corresponds to 1 cm2); (b) spontaneous KTP crystals: pure KTP is in the middle of the figure, Cr:KTP and V:KTP are at the left and at the right, respectively (the large square, represented in bold lines, is also 1 cm2, but the small square, bounded by thin lines, corresponds to 1 mm2). The picture in this form has been deformed, i.e., its vertical orientation is enlarged. Table 1. KTP Crystallization Conditions fluxed melt homogenization T (°C) 1000
time (h)
temperature range of KTP crystal growth (°C)
cooling rate (°C/h)
920-890 895-830
1 2.5
48
Table 2. Goniometric Data and Theoretical Spherical Coordinates of Measured Faces of KTP Crystals spherical coordinates (deg) (h k l) (1 (0 (2 (0
0 1 0 3
0) 1) 1) 1)
φtheor
φexp
90 0 90 0
90 0 90 0
Ftheor 90 58.84 58.80 78.6
Fexp 90 59 59.6 78.6
KPO3, K4P2O7 · 3H2O. In some cases, KPO3 and K4P2O7 · 3H2O can be replaced by KH2PO4 and K2CO3. Also, the high-purity KH2PO4 material
can be easily obtained by several times recrystallization process. Spectrum purity K2CO3 is commercially available and not so expensive. The starting mixture was placed into a Pt crucible with a diameter of 100 mm and height of 100 mm. In seeded solution growth runs, [010] seeds were used. In each experiment, the saturation temperature was determined by a tentative seed method, and supersaturation was kept within certain limits by the cooling fluxed melt in the range of 0.5-5 °C/day following the earlier experimental data on KTP solubility and crystallization kinetics. At the end of crystal growth process, the crystal grown was pulled out and cooled to room temperature within about 1 week. In the case of spontaneous crystallization, the initial charge corresponded to the 1KTP/2K6 composition,3,4 to which M2SO4 mineralizers (M ) Li, Na, K) were added. KTP powder was sintered with KH2PO4 and TiO2 according to the reaction KH2PO4 + TiO2 f KTiOPO4 + H2Ov. The K6 flux was obtained following the scheme 2KH2PO4 + 2K2HPO4 f K6P4O13 + 3H2Ov. The dopants of V, Cr, Ni, Co, Cu, Mo, Ba, Ce, Er, and W were used in the form of their oxides (up to 2 wt % solvent composition). The reagents were mechanically homogenized in a ceramic mortar and heated in a Pt crucible for 20-25 h at temperatures of 350 and 400 °C for KTP and K6, respectively, in order to attain their constant mass. Crystallization runs ranged from 3 to 4 weeks. Three 15 mL Pt crucibles were used in each of three series of the crystal growth experiments. The K6:M molar ratio (n) was the same in each of the three cycles, namely n ) 8, 6, or 4 (in molar fractions). The crucible charges of the given cycle differed only in the mineralizer type: K2SO4, Li2SO4, or Na2SO4. Three crucibles were kept together in the furnace under the temperature conditions shown in Table 1, and cooled to room temperature. Then, KTP crystals were released from the crystallized melt by dissolution in distilled water. Goniometric measurements of the crystals obtained were performed by using an optical two-circle GD-1 goniometer. Chemical analysis was made with a CAMECA SX-50 electron microprobe analyzer with the sensitivity of 0.02% and the precision of ( 0.02 wt %. The room temperature absorption spectra were measured with Hitachi U-3500 spectrophotometer in the wavelength 190-3200 nm.
3. Results and Discussion KTP crystals with high quality and weight exceeded 230 g have been grown on seeds (Figure 2a). Only a few crystal forms, namely {100}, {110}, and {201}, are well developed. Because the (001) face was formed on the solution’s surface, it is not so flat and still keeps the facet orientation. The size of the crystal is normally 35 × 40 × 55 mm3, and it can be widely used for second harmonic generation devices. The maximum continuous wave 532 nm output of an intracavity diode pump Nd:YVO4/ KTP laser can reach 5W.
1192 Crystal Growth & Design, Vol. 9, No. 2, 2009
Wang et al.
Figure 4. Growth forms of KTP crystals depending on the type and concentration of M2SO4 mineralizer: (I) n ) 4, (II) n ) 6, (III) n ) 8; (a) M ) Li, (b) M ) Na, (C) M ) K.
Small spontaneous transparent KTP crystals were also obtained from the fluxed melts doped with V, Cr, Ni, Co, Cu, Mo, Ba, Ce, Er, and W under similar conditions (Figures 2b and 3). In growth experiments with K2SO4 and Na2SO4 mineralizers, KTP crystals having dimensions about 3 × 5 × 8 mm3 were obtained, but an addition of lithium sulfate made it possible to increase their sizes up to 4 × 10 × 10 mm3 under the same spontaneous crystallization conditions. The angular characteristics obtained from goniometric measurements show that crystal forms {100}, {011}, and {201} are predominant in the faceting of KTP crystals, because of the similarity of the theoretical and measured coordinates (Table 2). In the case of n ) K6:Li2SO4 ) 4 solvent, the comparison of measured crystal forms with theoretical calculations resulted in a new (031) face, which is observed in these crystals for the first time. This finding is confirmed by the calculation of hkl indices from the spherical coordinates as well as by the analysis of the network reticular density for the KTP space group, i.e., Pna21. Analysis of the face significances, following the Bravais
rule and with allowance for the symmetry elements, made it possible to identify definitively the index of a face with k + l ) 2n, because this condition must be satisfied for faces with the 0kl index. In the first series of experiments (n ) 4), the {100}, {201}, and {011} faces are well-developed. The lithium mineralizer promotes most representative faceting. Besides the abovementioned crystal forms, there is a (110) face, as an attribute of these crystals (Figure 4, Ia). Na2SO4 restricts the normal growth rates in the [100] direction, in contrast to the [101] orientation (Figure 4, Ib). The crystals grown in the presence of potassium sulfate have practically the same habit. Sometimes, however, KTP crystals flattened out parallel to the X axis can be found (Figure 4, Ic). For the n ) 6 with M ) Li2SO4, like in the first cycle of experiments, the KTP crystal shapes are rather variable, and most often characterized by the {201} and {011} crystal faces (Figure 4, IIa). In the case of Na2SO4, the {100}, {201}, and {011} faces are well-represented. The priority of the first two crystal forms is equiprobable. The {110} faces can be rarely
Flux Growth and External Morphology of KTiOPO4 Crystals
Crystal Growth & Design, Vol. 9, No. 2, 2009 1193
found, especially when the {100} faces are strongly developed. Sometimes, these crystals are elongated in the [101] crystallographic direction (Figure 4, IIb). When K2SO4 is used, they have particularly pronounced {110} simple form, in comparison to {100} faces (Figure 4, IIc). With a decrease in the mineralizer concentration, i.e., n ) 8, KTP crystals obtained in the presence of Li2SO4 have a {111} rhombic pyramid, additionally to the usual {100}, {011}, and {201} growth forms (Figure 4, IIIa). This is their genetic characteristic, which is most likely to be explained by a specific role of Li+ ions in the crystallization medium. As for M ) Na2SO4, the crystals exhibit well developed, {100}, and {011} faces, but the {201} crystal form is preferred (Figure 4, IIIb). Upon replacement of Na2SO4 with K2SO4, the {110} faces become well-pronounced, i.e., like in experiments with n ) K6: Li2SO4 ) 4 (Figure 4, IIIc). In experiments based on the flux with n ) K6:Na2SO4 ) 8 as well as in the case of Li2SO4, as was mentioned above, the crystals obtained are substantially larger than those in other runs. This may be a special effect of Li+ and Na+ ions on the flux structure and its properties. Using K6 flux doped with ion of transition or other elements, chromium up to 0.5 wt% was found in KTP crystals, whereas other dopant concentration was determined to be not exceed 0.2 wt%. They have no significant effect on crystal morphology. In Figure 4, Ia (K6:Li2SO4 ) 4) and IIIb (K6:Na2SO4 ) 8), the most characteristic faceting is represented for experiments with Cr, Ni, Ce, Er and with Co, Cu, Mo, Ba, W, respectively. KTP crystals doped with vanadium, along with the most widespread {100}, {201}, and {011} faces, also exhibit the {111} crystal form. On the basis of the results obtained, it can be noted that Na2SO4 slows down development of the {201} crystal form, whereas K2SO4 and Li2SO4 enhance the role of molecular PO43ions in the fluxed melt. As a result, the {011} faces become less developed. The mineralizers and admixtures used in this study do not affect the growth rate in the [100] direction.
temperature solutions. Spontaneous KTP crystals were also obtained from fluxed melts in presence of lithium, sodium and potassium sulfates as well as V, Cr, Ni, Co, Cu, Mo, Ba, Ce, Er, and W oxides. Chromium admixture in solid phase was measured to be up to 0.5 wt %, but other dopants were less than 0.02 wt %.
4. Conclusions
(7) Pavlova, N. I.; Garmash, V. M.; Silnickaya, G. B.; Stekolshikova, N. P.; Gerken, V. A. SoV. Phys. Crystallogr. 1986, 31, 87–90.
KTP crystals with high quality and size of 35 × 40 × 55 mm3 have been grown on seeds from the K6 based high-
In all experiments, typical faceting of KTP crystals is {100}, {011}, and {201}. In the case of vanadium, the {111} faces are formed, additionally to the above crystal forms. In the presence of the Li2SO4 mineralizer, {111}, and {031} faces were found as well, i.e., the lithium sulfate promotes more representative faceting. Also, this mineralizer makes it possible to increase sizes of KTP crystals grown under the same crystallization conditions. Acknowledgment. The authors thank I. A. Bryzgalov for electron microprobe analysis of KTP crystals and G. I. Dorokhova for her assistance in goniometric measurements. This work was supported by the RFBR Grants 07-05-00680-a`, 0805-12038_ofi, 08-05-90010_bel, and 08-05-92200_nsfc-a. The joint research is also supported by the NSFC Grants 50611120336 and 50590401.
References (1) Bolt, R. J.; Van der Mooren, M. H.; De Haas, H. J. Cryst. Growth 1991, 114, 141–152. (2) Beirlien, J. D.; Gier, T. E. U.S. Patent, 3.949.3231974. (3) Satyanarayan, M. N.; Deepthy, A.; Bhat, H. L. Crit. ReV. Solid State Mater. Sci. 1990, 24, 103–191. (4) Bordui, P. F.; Jacco, J. C.; Loiacono, G. M.; Stolzenberger, R. A.; Zola, J. J. J. Cryst. Growth 1987, 84, 403–408. (5) Tsvetkov, E. G.; Semenenko, V. N.; Chranenko, G. G.; Tjurikov, V. I. Surf., X-ray, Synchrotron Neutron Effort 2002, 5, 65–70. (6) Satyanarayan, M. N.; Bhat, H. L. J. Cryst. Growth 1997, 181, 281– 289.
CG801100G
