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Growth of Na modified potassium lithium niobate crystal by the Czochralski method
ARTICLE IN PRESS
Journal of Crystal Growth 270 (2004) 370–375 www.elsevier.com/locate/jcrysgro
Growth of Na modified potassium lithium niobate crystal by the Czochralski method Byeong-Eog Juna, Chung-Sik Kimb, Dong-Jin Kimc, Yoon-Hwae Hwanga,, Hyung-Kook Kima, Jung-Nam Kima a
Department of Physics, Research Center for Dielectrics and Advanced Matter Physics, Pusan National University, Busan 609-735, Republic of Korea b Basic Science Research Institute, Pukyong National University, Busan 608-737, Republic of Korea c Electronic Ceramics Center, Dong-eui University, Busan 614-714, Republic of Korea Received 4 February 2004; accepted 21 May 2004 Communicated by T. Hibiya Available online 28 August 2004
Abstract Transparent Na modified potassium lithium niobate (Na0.23K2.60Li1.82Nb5.35O15.70; NKLN) crystal was successively grown by the Czochralski method using RF induction heating from melt composition Na2O:K2O : Li2O:Nb2O5=2:30:25:43 mol%. NKLN crystal showed a tetragonal tungsten bronze structure with lattice constants a=12.544670.0010 A˚ and c=4.012970.0005 A˚ at room temperature. The dielectric constant along the c-axis 33 showed a sharp maximum around 480 1C. Optical transmission edge was 370 nm and optical transmission spectra showed no absorption at wavelengths ranging from 380 to 800 nm. The structural and optical properties of NKLN were similar to those of the near stoichiometric KLN crystals. We believe that the growth of NKLN by the Czochralski method has an advantage for a large size and high-quality crystal. r 2004 Elsevier B.V. All rights reserved. PACS: 81.10.A; 42.70.M Keywords: A1. X-ray diffraction; A2. Czochralski method; B1. Tungsten bronzes; B2. Nonlinear optic materials
1. Introduction
Corresponding author.
E-mail address: [email protected] (Y.-H. Hwang).
The compact blue laser based on second harmonic generation (SHG) is an attractive device due to its potential application in nonlinear optics and high-density optical storage. Ferroelectric
0022-0248/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2004.05.118
ARTICLE IN PRESS B.-E. Jun et al. / Journal of Crystal Growth 270 (2004) 370–375
potassium lithium niobate (K3Li2yNb5+yO15+2y; KLN) crystal was discovered by L.G. Van Uitert et al. in 1967 and has a tetragonal tungsten bronze (TTB) structure. It is a useful material for applications in nonlinear optics and electro-optic devices because its nonlinear optical coefficient d31 was larger than that of LiNbO3, the optically induced refractive-index inhomogeneity for a intense laser such as Ar+ ion laser was not observed, and it shows no depoling by a mechanical shock or a temperature variation [1]. Particularly, it exhibits the ability to tune the noncritical phase matching wavelengths at room temperature ranging from 790 to 920 nm by changing the Li/Nb content in the crystals [2,3]. Although this freedom to tune the wavelength range leads to attractive potential applications, the growth of a large size and high-quality KLN crystal with high Li/Nb content is a challenging problem. The phase equilibrium study of K2O-Li2ONb2O5 ternary system was carried out by Scott et al. [4]. Crystal growth of KLN single crystals and crystal fibers were attempted by various methods [5-8]. However, the growth of desirable size and high-quality KLN crystals with high Li/Nb content is generally difficult because of the occurrence of an eutectic between KLN, KNbO3, and Li3NbO4 at the melt composition K2O:Li2O:Nb2O5=32.0:26.3:41.7 mol% [4]. In this report, in order to avoid crack problem in the growth of high-quality KLN crystal during cooling through the Curie temperature, we have grown Na modified KLN (NKLN) crystals by the Czochralski method using RF induction heating from the potassium and lithium rich melt composition (Na2O:K2O:Li2O:Nb2O5=2:30:25:43 mol%) by modifying site filling ratio of ions. The structural and optical properties of Na modified KLN crystals were investigated by X-ray diffraction, dielectric constant at low frequencies and optical transmittance in the UV-VIS range.
2. Experimental procedure For the growth of NKLN crystal, the regent grade (99.99% purity) raw materials were mixed as Na2CO3:K2CO3:Li2CO3:Nb2O5=2.0:30.0:25.0:
371
43.0 mol%. Two-step calcination processes were performed at 700 1C for 10 h and at 900 1C for 10 h in Pt crucible to get crystalline powder. The material was ground into a fine powder before each step. The prepared powder was put into Pt crucible and was melted at 1000 1C in air using RF heating system (5 kW max.). The seed was prepared from KLN o0014 bar which was previously grown from the stoichiometric melt K2O:Li2O:Nb2O5=30:20:50 mol%. The rotation and pulling rates were 8–10 rpm and 0.1–0.5 mm/h, respectively. The seeding temperature was 940–960 1C at the bottom of Pt crucible. The growth temperature was increased about 2 1C/day during the growth. The grown crystal boule was separated by slowly heating the melt to 1000 1C to form a cone shape to avoid a bubble inclusion. The size of grown crystal was 8–15 mm in width and 15–20 mm in length. However, in spite of the slow cooling some cracks were produced when the temperature passed through the Curie temperature. KLN crystal was grown with the same procedures for NKLN crystal except the melt composition (K2CO3:Li2CO3:Nb2O5=30:25:45 mol%) and growth temperatures ranging from 980 to 1000 1C. The metallic compositions of KLN and NKLN crystals were analyzed by the inductively coupled plasma-atomic emission spectrometry (Thermo Jarrell Ash, ICP-IRIS) [9]. Step scan (2y=5–901; step:0.051) powder X-ray diffraction patterns were measured by X-ray diffractometer (Rigaku, GDX1193A) using Cu Ka radiation. Kb radiation was filtered with Ni foil. The tetragonal lattice constants were calculated by using WinTREOR90 program with WinPLOTR packages [10]. For dielectric studies, KLN and NKLN crystals were cut into (0 0 1) plate perpendicular to the polar c-axis. Both faces of the specimens were coated with gold by thermal evaporation in a high vacuum. The dielectric constants were measured by an impedance analyzer (Hewlett Packerd, 4194A) in the temperature range of 30–700 1C with a heating rate of 1 1C/min. Optical transmittance of (1 1 0) plate of KLN and NKLN crystal was measured by UV-VIS spectrometer (Varian, Cary 5E) in the UV-VIS range (200–800 nm). Baseline was corrected by
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measuring an empty cell in air. Scan rate and sampling interval were 600 nm/min and 1.0 nm, respectively.
Table 1 The chemical compositions of KLN and NKLN crystals. The oxygen content was estimated from molar ratios of Na2O, K2O, Li2O and Nb2O5 Crystal Na/Nb K/Nb Li/Nb Chemical formula
3. Results and discussion Fig. 1 shows the NKLN crystals grown along the (0 0 1) direction by the Czochralski method using RF induction heating. From the left side to the right side of the figure, a cleaved section of asgrown crystal, a cubic and a (1 1 0) plate were shown, respectively. The as-grown crystals were transparent and had a rectangular cross-section with some cracks, which were formed when the temperature passed through the Curie temperature during cooling. We could obtain a cubic with the size of a b c ¼ 2:81 2:71 2:20 mm3. We observed that the cracks mainly originated from the (0 0 1) cleavage by the optical conoscope. We also observed that the edges were mostly (1 1 0) oriented by X-ray diffraction method. The crystal compositions of KLN and NKLN by the ICP-AES method are listed in Table 1. The crystal composition of NKLN was Na0.23K2.60Li1.82Nb5.35O15.70. The oxygen content was estimated from molar ratios of Na2O, K2O, Li2O and Nb2O5. The Li contents in NKLN and KLN crystals were almost the same but the Li/Nb ratio in NKLN was larger. The sum of Na and K
Fig. 1. Photograph of Na modified KLN crystals. Right side is the (1 1 0) plate, left side is the cleaved section of as-grown crystal and center is the top view along the o1104 of (a) cubic.
KLN — NKLN 0.043
0.520 0.486
0.336 0.340
K2.80Li1.81Nb5.39O15.78 Na0.23K2.60Li1.82Nb5.35O15.70
content in NKLN crystal was almost equal to K content in KLN crystal. The conventional Czochralski method is very useful technique for commercial crystal growth, but it showed many shortcomings to grow KLN crystal with high quality and large size. A few examples are a large temperature gradient between the top and the bottom of the melt, a variation of the growth temperature due to the change of melt composition and etc. The main problem to obtain KLN crystal with a large size and high quality was the occurrence of an eutectic between KLN, KNbO3, and Li3NbO4 in the Li and K rich melt [4]. Xu et al. have reported that the KLN crystals grown from the melt composition of K2O : Li2O : Nb2O5=32:25:43 mol% had a composition nearest to the stoichiometry [8]. The KLN crystal grown by the Czochralski method from this melt composition showed severe cracks during cooling through the Curie temperature. But we obtained a transparent and high quality NKLN crystal with a comparable size and high Li/Nb content by a small substitution of Na for K. It is expected that the growth of NKLN by the Czochralski method has a potential for a larger size and high-quality crystal. Fig. 2 shows powder X-ray diffraction patterns of NKLN and KLN crystals. The patterns matched with the JCPDS data (34-0122) of K3Li2Nb5O15. Additional lines of Li3NbO4 phase (16-0459) and/or KNbO3 phase (32-0822) due to the congruent growth characteristics of KLN crystals at lower Nb2O5 content in melt were not observed [11]. All peaks were indexed as a tetragonal structure with lattice constants a=12.544670.0010 A˚, c=4.012970.0005 A˚, and the axial ratio O10 c/a=1.0116. Similarly, lattice constants of KLN were a=12.558070.0006 A˚,
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373
3000
NKLN
Dielectric constant ε33
(302)/(550) (640) (402) (412) (422)
(212)/(630)
(002) (620) (202)
(321)
(530)
(410) (221)/(330) (311)/(420)
(320)
(400)
(001)/(310)
(211)
(220)
(210)
KLN
(200)
Intensity (arb. units)
Measured at 10 kHz KLN NKLN
2000
1000
KLN (32-0122) L3N (16-0459)
0 KN (32-0822)
10
20
30
40
50
0
60
2θ (degrees) Fig. 2. X-ray powder diffraction pattern of Na modified KLN. In the figure KLN, KN, and L3N stand for K3Li2Nb5O15, KNbO3 and Li3NbO4, respectively.
c=4.021670.0004 A˚, and the axial ratio O10 c/ a=1.0127. Those results indicates that NKLN crystal also had a tetragonal tungsten bronze structure but the lattice constants a and c decreased by substituting of Na for K. Fig. 3 shows the dielectric constants along the caxis, 33 , of KLN and NKLN at the frequency of 10 kHz. The dielectric maximum temperatures of KLN and NKLN were around 430 1C and 480 1C, respectively. The dielectric behavior of NKLN at the transition region showed a sharper and higher maximum. Many authors reported that the lattice constant along the c-axis and the Curie temperature increase as Li/Nb content in KLN crystal increases [4,8]. Kim et al. [12] reported that the dielectric constants along the polar c-axis showed a dielectric anomaly at 410–560 1C and the broad dielectric maximum depending on Li/Nb content in KLN crystals. When Li/Nb content in crystals decreased, the Curie temperature decreased and broader dielectric maximum was observed [12]. It is still unclear why the dielectric behavior of NKLN crystal is similar to that of the near stoichiometric KLN crystal in spite of Na+ and
100
200
300
400
500
600
700
Temperature (°C) Fig. 3. Dielectric constant 33 vs. temperature of KLN and Na modified KLN crystals at the frequency of 10 kHz.
K+ distribution [13]. We consider that the vacancy or defect concentration in NKLN crystal was much reduced by substitution of Na+ for K+. The Nb content in KLN and NKLN measured by ICP-AES method were 53.9 and 53.5 mol%, respectively. It was comparable to the value of the Nb content in KLN crystal, which was 52.870.8 mol%, by Scott et al. [4]. Therefore, it was considered that the Li/Nb ratio in NKLN crystal was larger than that in KLN crystal. Fig. 4 shows the optical transmission spectra in the wavelength range of 300–800 nm. The transmission spectra were not calibrated for reflection loss. When transparent KLN and NKLN crystals were heated up to 200–400 1C in air, their color turned into opaque yellow. The transmission spectra was almost constant at 380–800 nm and we did not observed a slight absorption in 380–500 nm which attributed to the pale yellow color of KLN crystal at RT. Transmission edges of KLN and NKLN were 373 and 370 nm, respectively. The Curie temperature and the lattice constants of KLN were comparable to the well-known values [4]. The lattice constants along the a and c axes of NKLN were smaller than those of KLN
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374
100
Transmission ( %)
80
60
40
NKLN KLN
20
0 300
400
500
600
700
800
Wavelength (nm) Fig. 4. Optical transmission spectra of KLN and Na modified KLN crystals.
but the Curie temperature was higher. This originated from substitution of Na+ for K+. The structure of KLN projected along the polar axis was shown in Fig. 5. The site occupancy formula of TTB structure can be written as (A1)2(A2)4C4(B1)2(B2)8O30. In the case of niobates, the B1 and B2 sites can be occupied by Nb5+, while the A1, A2 and C cites can be occupied by alkaline earth ions, alkali ions, or both. A completely filled TTB structure occurs when all A1, A2 and C sites are fully occupied [14]. In the case of KLN, it was known that A1 site is occupied by 87% K and 13% Li, A2 site is occupied by 99% K and 1% Li, C site is occupied by 94% Li and 6% Nb, while B1 and B2 sites are fully occupied by Nb5+ ions [15]. Because the size of A1 site is smaller than that of A2 site as shown in Fig. 5, the site occupancy of K+ for A1 site is less than for A2 site. When Na+ is modified into KLN structure, Na+ may occupy A1 or A2 site. A1 site was preferably occupied by Na+ ion because Na+ has a smaller ionic radius of than K+. The decreases of the lattice constants a and c originated from the reduction in A1 site occupancy of K+ by Na modification.
Fig. 5. Schematic diagram of tetragonal tungsten bronze structure viewed along the [0 0 1] axis.
4. Conclusion Na modified KLN (Na0.23K2.60Li1.82Nb5.35O15.70; NKLN) crystal was successfully grown by the Czochralski method using RF induction heating from the melt composition Na2O:K2O:Li2O: Nb2O5=2:30:25:43 mol%. As-grown crystal was transparent and colorless and had a larger size and high-quality crystal respect to KLN crystal. X-ray diffraction pattern showed a tetragonal tungsten bronze structure with lattice constants similar to the KLN crystal at room temperature. Dielectric constant 33 along the polar c-axis showed a phase transition at 480 1C and the temperature range of phase transition was narrower than that of pure KLN. The transmission edge was 370 nm and no absorption band was observed in the optical transmittance spectra in the wavelength range of 380–800 nm. In spite of the cation configuration at A1 or A2 site, the dielectric behavior of NKLN was similar to that of the near stoichiometric KLN. The substitution of Na+ for K+ at A1 or A2 cite in
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tetragonal tungsten bronze structure may reduce the vacancy or defect concentration in KLN crystal.
Acknowledgements This work was supported by Korean Research Foundation Grant (KRF-2002-005-C20001). References [1] L.G. Van Uitert, S. Singh, H.J. Levinstein, J.E. Geusic, W.A. Bonner, Appl. Phys. Lett. 11 (1967) 161. [2] J.J.E. Reid, M. Ouwerkerk, L.J.A.M. Beckers, Philips J. Res. 46 (1992) 199. [3] J.J.E. Reid, Appl. Phys. Lett. 62 (1993) 19. [4] B.A. Scott, E.A. Giess, B.L. Olson, G. Burns, A.W. Smith, D.F. O’Kane, Mater. Res. Bull. 5 (1970) 47. [5] S. Youting, Z. daofan, L. Hongbin, Z. Tang, F. Feidi, Y. Changxi, Z. Yong, W. Xing, J. Crystal Growth 194 (1998) 379.
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[6] T. Karaki, K. Miyashita, M. Nakatsuji, M. Adachi, Jpn. J. Appl. Phys. 37 (1998) 5277. [7] M. Matsukura, T. Karaki, T. Takeyama, T. Fujii, M. Adachi, Jpn. J. Appl. Phys. 38 (Part 1) (1999) 5638. [8] X.W. Xu, T.C. Chong, G.Y. Zhang, H. Kumagai, J. Crystal Growth 225 (2001) 458. [9] Korea Basic Science Institute, Busan Branch, Pusan National University, Busan 609–735, Korea, http:// www.kbsi.re.kr. [10] T. Roisnel, J. Rodriguez-Carvajal, Laboratoire Leon Brillouin (CEA/CNRS) CEA-Saclay 91191 Gif-sur-Yvette Cedex, France, http://www-llb.cea.fr/fullweb/winplotr/ winplotr.htm. [11] M. Ferriol, G. Boulon, Mater. Res. Bull. 34 (1999) 533. [12] J.S. Kim, J.N. Kim, J. Phys. Soc. Japan 69 (2000) 1880. [13] B.E. Jun, S.C. Song, D.J. Kim, C.S. Kim, H.K. Kim, G.S. J., J.N. Kim, Y.H. Hwang, J. Korean Phys. Soc. 42 (Suppl. 4) (2003) S1252. [14] M.E. Lines, A.M. Glass, Principles and Applications of Ferroelectrics and Related Materials, Clarendon Press, Oxford, 1977. [15] S.C. Abrahams, P.B. Jamieson, J.L. Bernstein, J. Chem. Phys. 54 (1971) 2355.
Journal of Crystal Growth 270 (2004) 370–375 www.elsevier.com/locate/jcrysgro
Growth of Na modified potassium lithium niobate crystal by the Czochralski method Byeong-Eog Juna, Chung-Sik Kimb, Dong-Jin Kimc, Yoon-Hwae Hwanga,, Hyung-Kook Kima, Jung-Nam Kima a
Department of Physics, Research Center for Dielectrics and Advanced Matter Physics, Pusan National University, Busan 609-735, Republic of Korea b Basic Science Research Institute, Pukyong National University, Busan 608-737, Republic of Korea c Electronic Ceramics Center, Dong-eui University, Busan 614-714, Republic of Korea Received 4 February 2004; accepted 21 May 2004 Communicated by T. Hibiya Available online 28 August 2004
Abstract Transparent Na modified potassium lithium niobate (Na0.23K2.60Li1.82Nb5.35O15.70; NKLN) crystal was successively grown by the Czochralski method using RF induction heating from melt composition Na2O:K2O : Li2O:Nb2O5=2:30:25:43 mol%. NKLN crystal showed a tetragonal tungsten bronze structure with lattice constants a=12.544670.0010 A˚ and c=4.012970.0005 A˚ at room temperature. The dielectric constant along the c-axis 33 showed a sharp maximum around 480 1C. Optical transmission edge was 370 nm and optical transmission spectra showed no absorption at wavelengths ranging from 380 to 800 nm. The structural and optical properties of NKLN were similar to those of the near stoichiometric KLN crystals. We believe that the growth of NKLN by the Czochralski method has an advantage for a large size and high-quality crystal. r 2004 Elsevier B.V. All rights reserved. PACS: 81.10.A; 42.70.M Keywords: A1. X-ray diffraction; A2. Czochralski method; B1. Tungsten bronzes; B2. Nonlinear optic materials
1. Introduction
Corresponding author.
E-mail address: [email protected] (Y.-H. Hwang).
The compact blue laser based on second harmonic generation (SHG) is an attractive device due to its potential application in nonlinear optics and high-density optical storage. Ferroelectric
0022-0248/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2004.05.118
ARTICLE IN PRESS B.-E. Jun et al. / Journal of Crystal Growth 270 (2004) 370–375
potassium lithium niobate (K3Li2yNb5+yO15+2y; KLN) crystal was discovered by L.G. Van Uitert et al. in 1967 and has a tetragonal tungsten bronze (TTB) structure. It is a useful material for applications in nonlinear optics and electro-optic devices because its nonlinear optical coefficient d31 was larger than that of LiNbO3, the optically induced refractive-index inhomogeneity for a intense laser such as Ar+ ion laser was not observed, and it shows no depoling by a mechanical shock or a temperature variation [1]. Particularly, it exhibits the ability to tune the noncritical phase matching wavelengths at room temperature ranging from 790 to 920 nm by changing the Li/Nb content in the crystals [2,3]. Although this freedom to tune the wavelength range leads to attractive potential applications, the growth of a large size and high-quality KLN crystal with high Li/Nb content is a challenging problem. The phase equilibrium study of K2O-Li2ONb2O5 ternary system was carried out by Scott et al. [4]. Crystal growth of KLN single crystals and crystal fibers were attempted by various methods [5-8]. However, the growth of desirable size and high-quality KLN crystals with high Li/Nb content is generally difficult because of the occurrence of an eutectic between KLN, KNbO3, and Li3NbO4 at the melt composition K2O:Li2O:Nb2O5=32.0:26.3:41.7 mol% [4]. In this report, in order to avoid crack problem in the growth of high-quality KLN crystal during cooling through the Curie temperature, we have grown Na modified KLN (NKLN) crystals by the Czochralski method using RF induction heating from the potassium and lithium rich melt composition (Na2O:K2O:Li2O:Nb2O5=2:30:25:43 mol%) by modifying site filling ratio of ions. The structural and optical properties of Na modified KLN crystals were investigated by X-ray diffraction, dielectric constant at low frequencies and optical transmittance in the UV-VIS range.
2. Experimental procedure For the growth of NKLN crystal, the regent grade (99.99% purity) raw materials were mixed as Na2CO3:K2CO3:Li2CO3:Nb2O5=2.0:30.0:25.0:
371
43.0 mol%. Two-step calcination processes were performed at 700 1C for 10 h and at 900 1C for 10 h in Pt crucible to get crystalline powder. The material was ground into a fine powder before each step. The prepared powder was put into Pt crucible and was melted at 1000 1C in air using RF heating system (5 kW max.). The seed was prepared from KLN o0014 bar which was previously grown from the stoichiometric melt K2O:Li2O:Nb2O5=30:20:50 mol%. The rotation and pulling rates were 8–10 rpm and 0.1–0.5 mm/h, respectively. The seeding temperature was 940–960 1C at the bottom of Pt crucible. The growth temperature was increased about 2 1C/day during the growth. The grown crystal boule was separated by slowly heating the melt to 1000 1C to form a cone shape to avoid a bubble inclusion. The size of grown crystal was 8–15 mm in width and 15–20 mm in length. However, in spite of the slow cooling some cracks were produced when the temperature passed through the Curie temperature. KLN crystal was grown with the same procedures for NKLN crystal except the melt composition (K2CO3:Li2CO3:Nb2O5=30:25:45 mol%) and growth temperatures ranging from 980 to 1000 1C. The metallic compositions of KLN and NKLN crystals were analyzed by the inductively coupled plasma-atomic emission spectrometry (Thermo Jarrell Ash, ICP-IRIS) [9]. Step scan (2y=5–901; step:0.051) powder X-ray diffraction patterns were measured by X-ray diffractometer (Rigaku, GDX1193A) using Cu Ka radiation. Kb radiation was filtered with Ni foil. The tetragonal lattice constants were calculated by using WinTREOR90 program with WinPLOTR packages [10]. For dielectric studies, KLN and NKLN crystals were cut into (0 0 1) plate perpendicular to the polar c-axis. Both faces of the specimens were coated with gold by thermal evaporation in a high vacuum. The dielectric constants were measured by an impedance analyzer (Hewlett Packerd, 4194A) in the temperature range of 30–700 1C with a heating rate of 1 1C/min. Optical transmittance of (1 1 0) plate of KLN and NKLN crystal was measured by UV-VIS spectrometer (Varian, Cary 5E) in the UV-VIS range (200–800 nm). Baseline was corrected by
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measuring an empty cell in air. Scan rate and sampling interval were 600 nm/min and 1.0 nm, respectively.
Table 1 The chemical compositions of KLN and NKLN crystals. The oxygen content was estimated from molar ratios of Na2O, K2O, Li2O and Nb2O5 Crystal Na/Nb K/Nb Li/Nb Chemical formula
3. Results and discussion Fig. 1 shows the NKLN crystals grown along the (0 0 1) direction by the Czochralski method using RF induction heating. From the left side to the right side of the figure, a cleaved section of asgrown crystal, a cubic and a (1 1 0) plate were shown, respectively. The as-grown crystals were transparent and had a rectangular cross-section with some cracks, which were formed when the temperature passed through the Curie temperature during cooling. We could obtain a cubic with the size of a b c ¼ 2:81 2:71 2:20 mm3. We observed that the cracks mainly originated from the (0 0 1) cleavage by the optical conoscope. We also observed that the edges were mostly (1 1 0) oriented by X-ray diffraction method. The crystal compositions of KLN and NKLN by the ICP-AES method are listed in Table 1. The crystal composition of NKLN was Na0.23K2.60Li1.82Nb5.35O15.70. The oxygen content was estimated from molar ratios of Na2O, K2O, Li2O and Nb2O5. The Li contents in NKLN and KLN crystals were almost the same but the Li/Nb ratio in NKLN was larger. The sum of Na and K
Fig. 1. Photograph of Na modified KLN crystals. Right side is the (1 1 0) plate, left side is the cleaved section of as-grown crystal and center is the top view along the o1104 of (a) cubic.
KLN — NKLN 0.043
0.520 0.486
0.336 0.340
K2.80Li1.81Nb5.39O15.78 Na0.23K2.60Li1.82Nb5.35O15.70
content in NKLN crystal was almost equal to K content in KLN crystal. The conventional Czochralski method is very useful technique for commercial crystal growth, but it showed many shortcomings to grow KLN crystal with high quality and large size. A few examples are a large temperature gradient between the top and the bottom of the melt, a variation of the growth temperature due to the change of melt composition and etc. The main problem to obtain KLN crystal with a large size and high quality was the occurrence of an eutectic between KLN, KNbO3, and Li3NbO4 in the Li and K rich melt [4]. Xu et al. have reported that the KLN crystals grown from the melt composition of K2O : Li2O : Nb2O5=32:25:43 mol% had a composition nearest to the stoichiometry [8]. The KLN crystal grown by the Czochralski method from this melt composition showed severe cracks during cooling through the Curie temperature. But we obtained a transparent and high quality NKLN crystal with a comparable size and high Li/Nb content by a small substitution of Na for K. It is expected that the growth of NKLN by the Czochralski method has a potential for a larger size and high-quality crystal. Fig. 2 shows powder X-ray diffraction patterns of NKLN and KLN crystals. The patterns matched with the JCPDS data (34-0122) of K3Li2Nb5O15. Additional lines of Li3NbO4 phase (16-0459) and/or KNbO3 phase (32-0822) due to the congruent growth characteristics of KLN crystals at lower Nb2O5 content in melt were not observed [11]. All peaks were indexed as a tetragonal structure with lattice constants a=12.544670.0010 A˚, c=4.012970.0005 A˚, and the axial ratio O10 c/a=1.0116. Similarly, lattice constants of KLN were a=12.558070.0006 A˚,
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373
3000
NKLN
Dielectric constant ε33
(302)/(550) (640) (402) (412) (422)
(212)/(630)
(002) (620) (202)
(321)
(530)
(410) (221)/(330) (311)/(420)
(320)
(400)
(001)/(310)
(211)
(220)
(210)
KLN
(200)
Intensity (arb. units)
Measured at 10 kHz KLN NKLN
2000
1000
KLN (32-0122) L3N (16-0459)
0 KN (32-0822)
10
20
30
40
50
0
60
2θ (degrees) Fig. 2. X-ray powder diffraction pattern of Na modified KLN. In the figure KLN, KN, and L3N stand for K3Li2Nb5O15, KNbO3 and Li3NbO4, respectively.
c=4.021670.0004 A˚, and the axial ratio O10 c/ a=1.0127. Those results indicates that NKLN crystal also had a tetragonal tungsten bronze structure but the lattice constants a and c decreased by substituting of Na for K. Fig. 3 shows the dielectric constants along the caxis, 33 , of KLN and NKLN at the frequency of 10 kHz. The dielectric maximum temperatures of KLN and NKLN were around 430 1C and 480 1C, respectively. The dielectric behavior of NKLN at the transition region showed a sharper and higher maximum. Many authors reported that the lattice constant along the c-axis and the Curie temperature increase as Li/Nb content in KLN crystal increases [4,8]. Kim et al. [12] reported that the dielectric constants along the polar c-axis showed a dielectric anomaly at 410–560 1C and the broad dielectric maximum depending on Li/Nb content in KLN crystals. When Li/Nb content in crystals decreased, the Curie temperature decreased and broader dielectric maximum was observed [12]. It is still unclear why the dielectric behavior of NKLN crystal is similar to that of the near stoichiometric KLN crystal in spite of Na+ and
100
200
300
400
500
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Temperature (°C) Fig. 3. Dielectric constant 33 vs. temperature of KLN and Na modified KLN crystals at the frequency of 10 kHz.
K+ distribution [13]. We consider that the vacancy or defect concentration in NKLN crystal was much reduced by substitution of Na+ for K+. The Nb content in KLN and NKLN measured by ICP-AES method were 53.9 and 53.5 mol%, respectively. It was comparable to the value of the Nb content in KLN crystal, which was 52.870.8 mol%, by Scott et al. [4]. Therefore, it was considered that the Li/Nb ratio in NKLN crystal was larger than that in KLN crystal. Fig. 4 shows the optical transmission spectra in the wavelength range of 300–800 nm. The transmission spectra were not calibrated for reflection loss. When transparent KLN and NKLN crystals were heated up to 200–400 1C in air, their color turned into opaque yellow. The transmission spectra was almost constant at 380–800 nm and we did not observed a slight absorption in 380–500 nm which attributed to the pale yellow color of KLN crystal at RT. Transmission edges of KLN and NKLN were 373 and 370 nm, respectively. The Curie temperature and the lattice constants of KLN were comparable to the well-known values [4]. The lattice constants along the a and c axes of NKLN were smaller than those of KLN
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100
Transmission ( %)
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NKLN KLN
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0 300
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Wavelength (nm) Fig. 4. Optical transmission spectra of KLN and Na modified KLN crystals.
but the Curie temperature was higher. This originated from substitution of Na+ for K+. The structure of KLN projected along the polar axis was shown in Fig. 5. The site occupancy formula of TTB structure can be written as (A1)2(A2)4C4(B1)2(B2)8O30. In the case of niobates, the B1 and B2 sites can be occupied by Nb5+, while the A1, A2 and C cites can be occupied by alkaline earth ions, alkali ions, or both. A completely filled TTB structure occurs when all A1, A2 and C sites are fully occupied [14]. In the case of KLN, it was known that A1 site is occupied by 87% K and 13% Li, A2 site is occupied by 99% K and 1% Li, C site is occupied by 94% Li and 6% Nb, while B1 and B2 sites are fully occupied by Nb5+ ions [15]. Because the size of A1 site is smaller than that of A2 site as shown in Fig. 5, the site occupancy of K+ for A1 site is less than for A2 site. When Na+ is modified into KLN structure, Na+ may occupy A1 or A2 site. A1 site was preferably occupied by Na+ ion because Na+ has a smaller ionic radius of than K+. The decreases of the lattice constants a and c originated from the reduction in A1 site occupancy of K+ by Na modification.
Fig. 5. Schematic diagram of tetragonal tungsten bronze structure viewed along the [0 0 1] axis.
4. Conclusion Na modified KLN (Na0.23K2.60Li1.82Nb5.35O15.70; NKLN) crystal was successfully grown by the Czochralski method using RF induction heating from the melt composition Na2O:K2O:Li2O: Nb2O5=2:30:25:43 mol%. As-grown crystal was transparent and colorless and had a larger size and high-quality crystal respect to KLN crystal. X-ray diffraction pattern showed a tetragonal tungsten bronze structure with lattice constants similar to the KLN crystal at room temperature. Dielectric constant 33 along the polar c-axis showed a phase transition at 480 1C and the temperature range of phase transition was narrower than that of pure KLN. The transmission edge was 370 nm and no absorption band was observed in the optical transmittance spectra in the wavelength range of 380–800 nm. In spite of the cation configuration at A1 or A2 site, the dielectric behavior of NKLN was similar to that of the near stoichiometric KLN. The substitution of Na+ for K+ at A1 or A2 cite in
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tetragonal tungsten bronze structure may reduce the vacancy or defect concentration in KLN crystal.
Acknowledgements This work was supported by Korean Research Foundation Grant (KRF-2002-005-C20001). References [1] L.G. Van Uitert, S. Singh, H.J. Levinstein, J.E. Geusic, W.A. Bonner, Appl. Phys. Lett. 11 (1967) 161. [2] J.J.E. Reid, M. Ouwerkerk, L.J.A.M. Beckers, Philips J. Res. 46 (1992) 199. [3] J.J.E. Reid, Appl. Phys. Lett. 62 (1993) 19. [4] B.A. Scott, E.A. Giess, B.L. Olson, G. Burns, A.W. Smith, D.F. O’Kane, Mater. Res. Bull. 5 (1970) 47. [5] S. Youting, Z. daofan, L. Hongbin, Z. Tang, F. Feidi, Y. Changxi, Z. Yong, W. Xing, J. Crystal Growth 194 (1998) 379.
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