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Growth and properties of ferroelectric potassium lithium niobate (KLN) crystal grown by the Czochralski method
Journal of Crystal Growth 223 (2001) 376–382
Growth and properties of ferroelectric potassium lithium niobate (KLN) crystal grown by the Czochralski method Jin Soo Kima,*, Ho-Sueb Leeb b
a Department of Physics, Pusan National University, Pusan 609-735, South Korea Department of Physics, Changwon National University, Changwon 641-773, South Korea
Received 12 July 2000; accepted 18 December 2000 Communicated by T. Nishinaga
Abstract The ferroelectric potassium lithium niobate (KLN) crystals with tungsten–bronze-type structure were grown by the Czochralski method. Usually, the growth of ferroelectric KLN crystal was very difficult and as-grown KLN crystals have cracks. These problems were investigated in relation with growth axis and different solid solution. For the growth of high quality and crack-free KLN crystals, the growth conditions and properties of ferroelectric KLN crystal were investigated. The grown KLN crystals are single-crystalline and belong to the tetragonal system with the lattice constants a ¼ b ¼ 1.2500–1.2551 nm and c ¼ 0:3996–0.4009 nm. The diffuse dielectric anomaly was observed for these KLN crystals. The width of phase-transition region and phase-transition temperature depends both on the compositional fluctuations and the compositional variations. # 2001 Published by Elsevier Science B.V. PACS: 81.10.Fq; 77.84.Dy; 42.70.Mp Keywords: A2. Bulk crystal growth; A2. Czochralski method; A2. Growth from melt; B1. Materials by type; B1. Oxides; B1. Potassium compounds; B1. Tungsten bronzes; B2. Ferroelectric materials; B2. Materials by property class
1. Introduction The potassium lithium niobate (K3 Li2 Nb5 O15 ; KLN) with tungsten–bronze (TB)-type structure is a useful material for applications in nonlinear optics, electrooptic and piezoelectric devices [1–3]. Recently, KLN crystals have been considered to be superior materials for blue laser radiation. The blue laser source is obtained by second-harmonic generation (SHG) i.e. double the frequency of a diode laser in a nonlinear optical crystal [4].
*Corresponding author. E-mail address: [email protected] (J. Soo Kim).
For the application, it is necessary to develop a growth technique of high-quality KLN bulk crystals. We have carried out KLN crystal growing by Czochralski method and successfully obtained transparent large-size KLN crystals. In this paper, we report the crystal growth by Czochralski method and the dielectric and optical properties of KLN crystals.
2. Experiment Several methods have been used to grow KLN crystals, such as Kyropoulos [5], Czochralski (CZ) [6], top-seeded solution growth (TSSG) [4,7],
0022-0248/01/$ - see front matter # 2001 Published by Elsevier Science B.V. PII: S 0 0 2 2 - 0 2 4 8 ( 0 1 ) 0 0 6 0 9 - 1
J. Soo Kim, H.-S. Lee / Journal of Crystal Growth 223 (2001) 376–382
micro pulling-down (m-PD) [8] and laser-heated pedestal growth (LHPG) methods [9]. Usually, KLN crystal is grown by CZ and Kyropoulos methods. However, the growth of KLN crystal with high optical quality was difficult due to cracks induced by a change of composition and structural characteristics [7–11]. In this experiment, the melt composition of xK2 CO3 þ ð1 xÞ Li2 CO3 +Nb2 O5 (x ¼ 0:4–0.7) and non-stoichiometric composition with 45 mol% Nb2 O5 were used. The crystal growth and several physical properties of grown KLN crystal were studied. For the measurements, the specimens were prepared by cutting the crystals perpendicular to the c-axis. Both faces of the specimens were pasted with silver electrodes and fired at 5008C for several hours. The thickness and the area of the specimens were about 1 mm and 20 mm2 , respectively. The dielectric properties of the KLN crystals were investigated over the frequency range of 100 Hz– 1 MHz in the temperature range of 30–7008C. The dielectric constant was measured by an impedance analyzer (HP4194A, Gain/Impedance analyzer) with a heating rate of 0.58C/min. The real composition was measured by a micro-chemical analyzer (EPMA, Shimazu) and a chemical analyzer (ICP-AES, Jovin Yvon 138 Ultrace). For the observation of microstructure, the specimens of KLN crystal were polished and etched in an etchant (HF : HNO3 =1 : 2) at the room temperature.
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annealed at 9508C for 12 h. X-ray diffraction studies have been carried out to check a formation of single phase and a structure of the sample. The mixture of the raw materials was charged in a Pt crucible (50 mm diameter by 35 mm height, 60 ml) and was melted two times by a RF-heating. Four or five KLN crystals, weighing 12–15 g, were continuously grown in the same melted raw materials in the crucible. To reduce the thermal gradient above the melt surface, a Pt after-heater was attached. KLN crystal was not grown at the composition x=0.4 and 0.7, but KLN crystal with ferroelectric properties was grown at the composition x=0.5 and 0.6. KLN crystals, grown along the [0 0 1]-axis in each composition of xK2 CO3 +(1x)Li2 CO3 +Nb2 O5 , have cracks in the early stage of a crystal growth. It implies that the growth of high-quality KLN crystals in the [0 0 1]axis was generally difficult because of the complex nature of the compounds. However, crack-free KLN crystal was successfully obtained by choosing the [1 0 0] growth direction and using Pt afterheater. As shown in Fig. 1, the shapes of the wellgrown crystals in the [1 0 0] and [0 0 1] direction were plate and cylindrical, respectively. The color of as-grown KLN crystal was pale yellow. The pulling rate was 0.5–1.0 mm/h, and the seedrotation speed was 20–36 rpm. Some results for growth conditions are summarized in Table 1. 3.2. Dielectric properties of KLN crystals
3. Results and discussion 3.1. Crystal growth Scott et al. [12] and Ikeda et al. [13] studied the phase equilibrium of K2 O–Li2 O–Nb2 O5 ternary system. KLN crystal grown in this composition range was incongruently melted and was grown only from high-temperature solutions. In this study, for a KLN crystal growth, the raw materials of xK2 CO3 +(1x)Li2 CO3 +Nb2 O5 (x ¼ 0:4–0.7) and nonstoichimetric composition with 45 mol% Nb2 O5 were prepared from the chemical reagents Li2 CO3 , Nb2 O5 and K2 CO3 , which were sufficiently mixed and calcined at 6508C for 12 h, and
The grown KLN crystals were divided into three groups depending on their dielectric properties. KLN crystals that belong to group I have a phase transition at the temperatures 400–5408C and have a dielectric constant of about 100–140 at room temperature. KLN crystals that belong to group II have a very broad dielectric anomaly at the temperature below 3008C and have a dielectric constant of 400 at room temperature. The crystal that belong to group III has a similar shape to other KLN crystals, but the dielectric properties were quite different from typical KLN crystals. In the case of easily grown KLN crystals at composition x=0.6, it was identified that KLN crystals belong to group III.
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Fig. 1. The KLN crystals grown by the Czochralski technique. Crystal growth is performed in two orientations along [1 0 0] (upper) and [0 0 1] (lower).
Fig. 2 shows that the temperature-dependent dielectric constant of KLN crystals belong to groups I and II at the temperature range of 100– 7008C. To demonstrate a variety of physical properties of KLN crystals, four kinds of KLN crystals were chosen according to the dielectric anomaly of phase transition. The phase-transition temperature of the three KLN crystals, abbreviated as KLN(2), KLN(3) and KLN(4), were 4208C, 4838C and 5348C, respectively. While KLN(1) crystal has quite different dielectric properties than others. The phase-transition temperatures of KLN(3) and KLN(4) crystals are higher than that of the well-known 4308C [14]. Different from KLN(2), both KLN(3) and KLN(4) crystals exhibit a considerably sharp dielectric anomaly of the phase transition. There are differences in phase transition temperature and broadness of dielectric constant between crystals. Since the peak of dielectric constant of KLN(2) crystal is notably broadened, it is difficult to determine the phase transition temperature accurately. Namely, this dielectric behavior of KLN(2) crystal shows a broad anomaly at the transition temperature of about 4208C, and this indicates that the transition is diffuse phase transition (DPT), which is caused by the compositional fluctuations and structural disorder [15–18].
Table 1 The growth conditions of KLN crystals. Nos. 8 and 9 indicate crystal grown in the melt composition of 33 mol% K2 CO3 , 22 mol% Li2 CO3 and 45 mol% Nb2 O5
1 2 3 4 5 6 7 8 9
x
Growth
Rotation (rpm)
Pulling (mm/h)
Group
Crystals
0.5 0.5 0.6 0.6 0.6 0.7 0.7 } }
[1 0 0] [0 0 1] [1 0 0] [1 0 0] [0 0 1] [0 0 1] [1 0 0] [1 0 0] [1 0 0]
22 22 21 36 12 16 16 34 36
0.7 0.7 0.9 1.0 0.7 0.7 0.7 1.0 1.0
I II I I III III III I I
KLN(1) } KLN(2) } } } } KLN(3) KLN(4)
Fig. 2. The temperature dependence of dielectric constant of KLN(1), KLN(2), KLN(3) and KLN(4) along c-axis at 10 kHz.
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3.3. Structural characteristics related to composition, lattice constants, microstructure and optical properties To investigate the composition-dependent phase transition, the composition of KLN crystal was measured by the inductively coupled plasmaatomic emission (ICP) spectroscopy and electro probe micro-analysis (EPMA) method, and the results were shown in Table 2. The content of K, Nb and O in the crystals was determined directly by the EPMA and Li concentration determined by the ICP. Table 2 shows that there are different compositions in KLN(1), KLN(2), KLN(3) and KLN(4) crystals. The completely filled TB-type structure K3 Li2 Nb5 O15 is not stable, it is only stable in the presence of Nb excess [19]. As shown in Table 2, this result of composition analysis agrees with that of Nb excess. In particular, the content of Nb in KLN(1) crystal is larger than those of others. From the DPT characteristics of dielectric properties and composition analysis, it can be concluded that the width of phase-transition region and the sharpness at the phasetransition temperature depends both on the compositional fluctuations and the compositional variations.
Table 2 The real composition of KLN(1), KLN(2), KLN(3) and KLN(4) crystals Crystals
K (wt%)
Li (wt%)
Nb (wt%)
Molecular formula
KLN(1) KLN(2) KLN(3) KLN(4)
11.81 12.77 12.61 13.59
1.42 1.45 1.72 1.65
56.81 56.77 55.61 56.23
K2:67 Li1:67 Nb5:28 O15:0 K2:89 Li1:77 Nb5:22 O15:0 K2:83 Li2:10 Nb5:02 O15:0 K2:99 Li2:01 Nb5:17 O15:0
Lattice constants, densities and axial ratios of KLN crystals were also determined and the results were summarized in Tables 3 and 4. We have subjected these crystal to X-ray powder diffraction using Cu Ka lines studies (a1 =1.54056 A˚) and found that they are single crystalline. Also, the X-ray diffraction lines could be indexed in a tetragonal system and the lattice constants with a ¼ b ¼12.500–12.551 A˚ and c=3.996–4.009 A˚ determined by least squares fitting, which agree with the previously reported value [10,19]. The pffiffiffiffiffi axial ratios 10c=a of KLN(2), KLN(3) and KLN(4) are about 1.01. The densities of KLN(1), KLN(2), KLN(3) and KLN(4) were determined by the Archimedes method to be 4.36, 4.36, 4.34 and 4:33 g=cm3 , respectively. While the density of group III was larger than 4:4 g=cm3 . It is well known that the theoretical density of KLN crystal was 4:30 g=cm3 . The density with 4.33–4.36 g/cm3 of the crystals belong to group I were found to be slightly higher than that of theoretical density. It is explained that the completely filled TB-type structured KLN crystal is not stable and it is only stable in the presence of Nb excess. By the measurement of dielectric constant as a function of temperature, KLN crystal belong to groups I–III in characters easily identified. Briefly, this was achieved by measurement of density. At least, crystals with density of 4.30–4.36 g/cm3 may be KLN crystal with ferroelectric properties. KLN crystals were etched in a solution of 1HF : 2HNO3 for 8 h at 258C and the etch-pit patterns of KLN crystals by optical microscopy are shown in Fig. 3. The etch-pit grains of KLN crystal belong to group I show a rectangular shape toward c-axis in [1 0 0] faces and square shape in [0 0 1] faces. The etch-pit grain sizes of KLN crystal grown by Czochralski method in this work
Table 3 The phase-transition temperature, lattice constants and densities of KLN(1), KLN(2), KLN(3) and KLN(4) crystals pffiffiffiffiffi a (A˚) c (A˚) 10c=a Density (g/cm3 ) Crystals Tc (8C) KLN(1) KLN(2) KLN(3) KLN(4)
} 420.0 483.0 534.0
12.568 12.551 12.500 12.525
3.991 4.009 3.996 3.997
1.004 1.010 1.011 1.009
4.36 4.36 4.34 4.33
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Table 4 Miller index of K3 Li2 Nb5 O15 single crystal 2Y
hkl
I=I0
d (A˚)
17.10 22.48 23.96 24.19 25.66 27.63 28.62 29.42 30.19 31.04 31.76 32.49 34.22 36.82 37.17 42.05 45.75 48.64 50.02 51.48 52.70 54.10 54.85
210 310 001 111 320 211 400 410 330 301 311 420 321 401 411 530 620 630 222 710 322 631 412
21 58 30 46 64 54 22 65 22 24 100 30 70 27 19 19 19 30 18 43 27 17 26
5.181 3.951 3.710 3.676 3.468 3.225 3.116 3.033 2.957 2.878 2.815 2.753 2.618 2.439 2.416 2.146 1.981 1.870 1.821 1.773 1.735 1.693 1.672
were found to be approximately 20–30 mm. The etch-pit grains of KLN crystal in the (0 0 1) faces have the four-fold symmetry and were arranged on a regular lattice. Choosing the KLN(3) crystals belong to group I, optical transmittance was measured and the results was shown in Fig. 4. The transmittance of KLN(3) crystal increase rapidly at the wavelength of about 400 nm and is about 75% and absorption edge is 370 nm. These results are similar to those of the previous works [7]. The SHG experiment was carried out at room temperature. A Nd : YAG laser operating at 1064 nm wavelength with pulse output power was employed as a fundmental wave to generate a 532 nm green beam. It was identified that SHG of KLN crystal belong to group I was only possible. Also, the SHG characteristics of KLN crystal belonging to group I were influenced by the crystal composition. It is known that noncritical phase matched wavelength for SHG can be tuned from 1050 to 820 nm by decreasing Nb concentration in a previous article [12]. Thus, it indicated that nonlinear properties for SHG was
Fig. 3. (a) The etch-pit grains in (1 0 0) faces of KLN crystal show a rectangular shape toward c-axis. (b) The etch-pit grains in (0 0 1) faces of KLN crystal show a square shape (a photograph 400 mm in width).
Fig. 4. The transmission spectrum of the KLN(3) crystal.
J. Soo Kim, H.-S. Lee / Journal of Crystal Growth 223 (2001) 376–382
enhanced by the variation of composition. More detailed measurements of SHG depending on compositions are currently in progress. 3.4. Characteristics of KLN crystal growth of stoichiometric and nonstoichiometric composition It is well known that the single-phase region can be divided into parts I–III [20,21]. The KLN crystal belonging to phase I is ferroelectric and Curie temperature shifts from 5408C to 3268C as the Nb2 O5 concentration changes from 0.51 to 0.55. Crystal in phase II, where the Nb2 O5 concentration changes from 0.55 to 0.68, has pseudo-tetragonal structure and is not a ferroelectric. Phase III is located in the area where the concentration of K2 O is 0.17–0.28 and that of Nb2 O5 is 0.63–0.73. From the results of the dielectric behavior and composition analysis, the characteristics of KLN(2), KLN(3) and KLN(4) crystals belong to that group I are similar to that of phase I, while the characteristics of KLN(1) crystals that belong to the group II are similar to that of phase II. Therefore, KLN(2), KLN(3) and KLN(4) crystals are ferroelectrics, while the KLN(1) crystal may not be a ferroelectric. The KLN crystal was grown from a solution having stoichiometric composition, xK2 CO3 + (1x)Li2 CO3 +Nb2 O5 (x=0.6) which crystallizes in orthorhombic structure and is therefore unlikely to be ferroelectric. These results agree with the previous reports that the growth of ferroelectric KLN crystal is difficult in stoichiometric composition. Moreover, these KLN crystals should be pulled from melts with the content of Nb2 O5 much lower than 50 mol%. In about third growth attempt, ferroelectric KLN crystals were grown in the melts. It was explained that once Nb-rich KLN crystals with orthorhombic structure (KLN crystal belongs to group II or III) were grown in the growth of first or second attempt, it seems more promising in terms of growing larger and better-quality crystals. Namely the melt composition changed the proper melts for the growth of ferroelectric KLN crystal, such as Adachi melt composition of 35 mol% K2 CO3 , 17.3 mol% Li2 CO3 and 47.7 mol% Nb2 O5 . The Nb2 O5 reducive has been shown to play an important
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role in the stable growth of pure TB-type KLN crystals by providing an optimum degree of complex formation. To identify the variation of composition and optimum conditions, we prepared the non-stoichiometric composition of 33 mol% K2 CO3 , 22 mol% Li2 CO3 and 45 mol% Nb2 O5 instead of stoichiometric composition of 30 mol% K2 CO3 , 20 mol% Li2 CO3 and 50 mol% Nb2 O5 (x=0.6). In these melts, as-grown KLN crystals have many cracks in [0 0 1] growth direction, but KLN crystals with high quality have grown in [1 0 0] growth direction. The phase-transition temperature of KLN crystal (KLN(3) and KLN(4)) grown in this melt is higher than that of stoichiometric composition melt (KLN(2)). By the previous report, KLN crystal characterized by strongly diffused dielectric peaks (DPT) [7]. However, KLN crystals grown in the non-stoichiometric composition exhibit some characteristics of relatively sharp dielectric peaks.
4. Conclusion The growth of KLN crystals is considerably difficult due to the cracks induced by the change of composition and structural characteristics. Nevertheless, CZ method is a method for the growth of large KLN crystal. We have successfully grown the tetragonal TB-type KLN crystals, which do not crack when cooling through the paraelectric/ferroelectric phase transition. Dielectric constant measurement, XRD, optical transmittance, etching pattern and composition analysis have been used to characterize the physical properties of KLN crystals. KLN crystals grown along the [0 0 1] direction have many cracks, therefore choose other growth direction such as [1 0 0] direction. Above all, it is important to choose the melt composition as well as growth environments. The phase transition of KLN(2), KLN(3) and KLN(4) crystal were 4208C, 4838C and 5348C, respectively. Different from the case of KLN(2) crystal, both KLN(3) and KLN(4) crystals exhibit a considerably sharp dielectric anomaly of the phase transition. The grown KLN crystals at different melts have different composition. The composition and
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structural disorder of ferroelectric TB-type KLN crystals have a strong influence on the ferroelectric properties as well as some other properties.
References [1] M. Adachi, A. Kawabata, F. Takeda, Jpn. J. Appl. Phys. 30 (1991) 2208. [2] T. Karaki, K. Miyashita, M. Nakatsuji, M. Adachi, Jpn. J. Appl. Phys. 37 (1998) 5277. [3] Wen-Dan Cheng, Jiu-Tong Chen, Jin-Shun Huang, QianEr Zhang, Chem. Phys. Lett. 261 (1996) 66. [4] X.W. Xu, T.C. Chong, L. Li, H. Kumagai, M. Hirano, J. Crystal Growth 198/199 (1999) 536. [5] T. Fukuda, Jpn. J. Appl. Phys. 8 (1969) 122. [6] L.G. Van Uitert, S. Singh, H.J. Levinstein, J.E. Geusic, W.A. Bonner, Appl. Phys. Lett. 11 (1967) 161. [7] G.Y. Kang, J.K. Yoon, J. Crystal Growth 193 (1998) 615. [8] K. Imai, M. Imaeda, S. Uda, T. Taniuchi, T. Fukuda, J. Crystal Growth 177 (1997) 79.
[9] M. Matsukura, Z. Chen, M. Adachi, A. Kawabata, Jpn. J. Appl. Phys. 36 (1997) 5947. [10] H.R. Xia, L.J. Hu, J.Q. Wei, J.Y. Wang, Y.G. Liu, Crystallogr. Res. Technol 32 (1997) 311. [11] J.S. Kim, J.N. Kim, Jpn. J. Appl. Phys. 39 (6A) (2000) 3502. [12] B.A. Scott, E.A. Giess, B.L. Olson, G. Burns, A.W. Smith, D.F. O’Kane, Mater. Res. Bull. 5 (1970) 47. [13] T. Ikeda, K. Kiyohashi, Jpn. J. Appl. Phys. 9 (1970) 1541. [14] L.G. Van Uitert, S. Singh, H.J. Levinstein, J.E. Geusic, W.A. Bonner, Appl. Phys. Lett. 11 (1967) 161. [15] G.A. Smolenskii, Ferroelectrics and Related Materials, Gordon and Breach, Amsterdam, 1984, (Chapter 12). [16] J.S. Kim, D.J. Kim, J.N. Kim, J. Kor. Phys. Soc. 32 (1998) S316. [17] N. Setter, L.E. Cross, J. Appl. Phys. 51 (1980) 4356. [18] J.S. Kim, J.N. Kim, J. Phys. Soc. Jpn. 69 (5) (2000) 1880. [19] S.C. Abrahams, P.B. Jamieson, J.L. Bernstein, J. Chem. Phys. 15 (1971) 2355. [20] T. Ikeda, K. Kiyohashi, Jpn. J. Appl. Phys. 9 (1970) 1541. [21] T. Nagai, T. Ikeda, Jpn. J. Appl. Phys. 12 (1973) 199.
Growth and properties of ferroelectric potassium lithium niobate (KLN) crystal grown by the Czochralski method Jin Soo Kima,*, Ho-Sueb Leeb b
a Department of Physics, Pusan National University, Pusan 609-735, South Korea Department of Physics, Changwon National University, Changwon 641-773, South Korea
Received 12 July 2000; accepted 18 December 2000 Communicated by T. Nishinaga
Abstract The ferroelectric potassium lithium niobate (KLN) crystals with tungsten–bronze-type structure were grown by the Czochralski method. Usually, the growth of ferroelectric KLN crystal was very difficult and as-grown KLN crystals have cracks. These problems were investigated in relation with growth axis and different solid solution. For the growth of high quality and crack-free KLN crystals, the growth conditions and properties of ferroelectric KLN crystal were investigated. The grown KLN crystals are single-crystalline and belong to the tetragonal system with the lattice constants a ¼ b ¼ 1.2500–1.2551 nm and c ¼ 0:3996–0.4009 nm. The diffuse dielectric anomaly was observed for these KLN crystals. The width of phase-transition region and phase-transition temperature depends both on the compositional fluctuations and the compositional variations. # 2001 Published by Elsevier Science B.V. PACS: 81.10.Fq; 77.84.Dy; 42.70.Mp Keywords: A2. Bulk crystal growth; A2. Czochralski method; A2. Growth from melt; B1. Materials by type; B1. Oxides; B1. Potassium compounds; B1. Tungsten bronzes; B2. Ferroelectric materials; B2. Materials by property class
1. Introduction The potassium lithium niobate (K3 Li2 Nb5 O15 ; KLN) with tungsten–bronze (TB)-type structure is a useful material for applications in nonlinear optics, electrooptic and piezoelectric devices [1–3]. Recently, KLN crystals have been considered to be superior materials for blue laser radiation. The blue laser source is obtained by second-harmonic generation (SHG) i.e. double the frequency of a diode laser in a nonlinear optical crystal [4].
*Corresponding author. E-mail address: [email protected] (J. Soo Kim).
For the application, it is necessary to develop a growth technique of high-quality KLN bulk crystals. We have carried out KLN crystal growing by Czochralski method and successfully obtained transparent large-size KLN crystals. In this paper, we report the crystal growth by Czochralski method and the dielectric and optical properties of KLN crystals.
2. Experiment Several methods have been used to grow KLN crystals, such as Kyropoulos [5], Czochralski (CZ) [6], top-seeded solution growth (TSSG) [4,7],
0022-0248/01/$ - see front matter # 2001 Published by Elsevier Science B.V. PII: S 0 0 2 2 - 0 2 4 8 ( 0 1 ) 0 0 6 0 9 - 1
J. Soo Kim, H.-S. Lee / Journal of Crystal Growth 223 (2001) 376–382
micro pulling-down (m-PD) [8] and laser-heated pedestal growth (LHPG) methods [9]. Usually, KLN crystal is grown by CZ and Kyropoulos methods. However, the growth of KLN crystal with high optical quality was difficult due to cracks induced by a change of composition and structural characteristics [7–11]. In this experiment, the melt composition of xK2 CO3 þ ð1 xÞ Li2 CO3 +Nb2 O5 (x ¼ 0:4–0.7) and non-stoichiometric composition with 45 mol% Nb2 O5 were used. The crystal growth and several physical properties of grown KLN crystal were studied. For the measurements, the specimens were prepared by cutting the crystals perpendicular to the c-axis. Both faces of the specimens were pasted with silver electrodes and fired at 5008C for several hours. The thickness and the area of the specimens were about 1 mm and 20 mm2 , respectively. The dielectric properties of the KLN crystals were investigated over the frequency range of 100 Hz– 1 MHz in the temperature range of 30–7008C. The dielectric constant was measured by an impedance analyzer (HP4194A, Gain/Impedance analyzer) with a heating rate of 0.58C/min. The real composition was measured by a micro-chemical analyzer (EPMA, Shimazu) and a chemical analyzer (ICP-AES, Jovin Yvon 138 Ultrace). For the observation of microstructure, the specimens of KLN crystal were polished and etched in an etchant (HF : HNO3 =1 : 2) at the room temperature.
377
annealed at 9508C for 12 h. X-ray diffraction studies have been carried out to check a formation of single phase and a structure of the sample. The mixture of the raw materials was charged in a Pt crucible (50 mm diameter by 35 mm height, 60 ml) and was melted two times by a RF-heating. Four or five KLN crystals, weighing 12–15 g, were continuously grown in the same melted raw materials in the crucible. To reduce the thermal gradient above the melt surface, a Pt after-heater was attached. KLN crystal was not grown at the composition x=0.4 and 0.7, but KLN crystal with ferroelectric properties was grown at the composition x=0.5 and 0.6. KLN crystals, grown along the [0 0 1]-axis in each composition of xK2 CO3 +(1x)Li2 CO3 +Nb2 O5 , have cracks in the early stage of a crystal growth. It implies that the growth of high-quality KLN crystals in the [0 0 1]axis was generally difficult because of the complex nature of the compounds. However, crack-free KLN crystal was successfully obtained by choosing the [1 0 0] growth direction and using Pt afterheater. As shown in Fig. 1, the shapes of the wellgrown crystals in the [1 0 0] and [0 0 1] direction were plate and cylindrical, respectively. The color of as-grown KLN crystal was pale yellow. The pulling rate was 0.5–1.0 mm/h, and the seedrotation speed was 20–36 rpm. Some results for growth conditions are summarized in Table 1. 3.2. Dielectric properties of KLN crystals
3. Results and discussion 3.1. Crystal growth Scott et al. [12] and Ikeda et al. [13] studied the phase equilibrium of K2 O–Li2 O–Nb2 O5 ternary system. KLN crystal grown in this composition range was incongruently melted and was grown only from high-temperature solutions. In this study, for a KLN crystal growth, the raw materials of xK2 CO3 +(1x)Li2 CO3 +Nb2 O5 (x ¼ 0:4–0.7) and nonstoichimetric composition with 45 mol% Nb2 O5 were prepared from the chemical reagents Li2 CO3 , Nb2 O5 and K2 CO3 , which were sufficiently mixed and calcined at 6508C for 12 h, and
The grown KLN crystals were divided into three groups depending on their dielectric properties. KLN crystals that belong to group I have a phase transition at the temperatures 400–5408C and have a dielectric constant of about 100–140 at room temperature. KLN crystals that belong to group II have a very broad dielectric anomaly at the temperature below 3008C and have a dielectric constant of 400 at room temperature. The crystal that belong to group III has a similar shape to other KLN crystals, but the dielectric properties were quite different from typical KLN crystals. In the case of easily grown KLN crystals at composition x=0.6, it was identified that KLN crystals belong to group III.
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Fig. 1. The KLN crystals grown by the Czochralski technique. Crystal growth is performed in two orientations along [1 0 0] (upper) and [0 0 1] (lower).
Fig. 2 shows that the temperature-dependent dielectric constant of KLN crystals belong to groups I and II at the temperature range of 100– 7008C. To demonstrate a variety of physical properties of KLN crystals, four kinds of KLN crystals were chosen according to the dielectric anomaly of phase transition. The phase-transition temperature of the three KLN crystals, abbreviated as KLN(2), KLN(3) and KLN(4), were 4208C, 4838C and 5348C, respectively. While KLN(1) crystal has quite different dielectric properties than others. The phase-transition temperatures of KLN(3) and KLN(4) crystals are higher than that of the well-known 4308C [14]. Different from KLN(2), both KLN(3) and KLN(4) crystals exhibit a considerably sharp dielectric anomaly of the phase transition. There are differences in phase transition temperature and broadness of dielectric constant between crystals. Since the peak of dielectric constant of KLN(2) crystal is notably broadened, it is difficult to determine the phase transition temperature accurately. Namely, this dielectric behavior of KLN(2) crystal shows a broad anomaly at the transition temperature of about 4208C, and this indicates that the transition is diffuse phase transition (DPT), which is caused by the compositional fluctuations and structural disorder [15–18].
Table 1 The growth conditions of KLN crystals. Nos. 8 and 9 indicate crystal grown in the melt composition of 33 mol% K2 CO3 , 22 mol% Li2 CO3 and 45 mol% Nb2 O5
1 2 3 4 5 6 7 8 9
x
Growth
Rotation (rpm)
Pulling (mm/h)
Group
Crystals
0.5 0.5 0.6 0.6 0.6 0.7 0.7 } }
[1 0 0] [0 0 1] [1 0 0] [1 0 0] [0 0 1] [0 0 1] [1 0 0] [1 0 0] [1 0 0]
22 22 21 36 12 16 16 34 36
0.7 0.7 0.9 1.0 0.7 0.7 0.7 1.0 1.0
I II I I III III III I I
KLN(1) } KLN(2) } } } } KLN(3) KLN(4)
Fig. 2. The temperature dependence of dielectric constant of KLN(1), KLN(2), KLN(3) and KLN(4) along c-axis at 10 kHz.
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3.3. Structural characteristics related to composition, lattice constants, microstructure and optical properties To investigate the composition-dependent phase transition, the composition of KLN crystal was measured by the inductively coupled plasmaatomic emission (ICP) spectroscopy and electro probe micro-analysis (EPMA) method, and the results were shown in Table 2. The content of K, Nb and O in the crystals was determined directly by the EPMA and Li concentration determined by the ICP. Table 2 shows that there are different compositions in KLN(1), KLN(2), KLN(3) and KLN(4) crystals. The completely filled TB-type structure K3 Li2 Nb5 O15 is not stable, it is only stable in the presence of Nb excess [19]. As shown in Table 2, this result of composition analysis agrees with that of Nb excess. In particular, the content of Nb in KLN(1) crystal is larger than those of others. From the DPT characteristics of dielectric properties and composition analysis, it can be concluded that the width of phase-transition region and the sharpness at the phasetransition temperature depends both on the compositional fluctuations and the compositional variations.
Table 2 The real composition of KLN(1), KLN(2), KLN(3) and KLN(4) crystals Crystals
K (wt%)
Li (wt%)
Nb (wt%)
Molecular formula
KLN(1) KLN(2) KLN(3) KLN(4)
11.81 12.77 12.61 13.59
1.42 1.45 1.72 1.65
56.81 56.77 55.61 56.23
K2:67 Li1:67 Nb5:28 O15:0 K2:89 Li1:77 Nb5:22 O15:0 K2:83 Li2:10 Nb5:02 O15:0 K2:99 Li2:01 Nb5:17 O15:0
Lattice constants, densities and axial ratios of KLN crystals were also determined and the results were summarized in Tables 3 and 4. We have subjected these crystal to X-ray powder diffraction using Cu Ka lines studies (a1 =1.54056 A˚) and found that they are single crystalline. Also, the X-ray diffraction lines could be indexed in a tetragonal system and the lattice constants with a ¼ b ¼12.500–12.551 A˚ and c=3.996–4.009 A˚ determined by least squares fitting, which agree with the previously reported value [10,19]. The pffiffiffiffiffi axial ratios 10c=a of KLN(2), KLN(3) and KLN(4) are about 1.01. The densities of KLN(1), KLN(2), KLN(3) and KLN(4) were determined by the Archimedes method to be 4.36, 4.36, 4.34 and 4:33 g=cm3 , respectively. While the density of group III was larger than 4:4 g=cm3 . It is well known that the theoretical density of KLN crystal was 4:30 g=cm3 . The density with 4.33–4.36 g/cm3 of the crystals belong to group I were found to be slightly higher than that of theoretical density. It is explained that the completely filled TB-type structured KLN crystal is not stable and it is only stable in the presence of Nb excess. By the measurement of dielectric constant as a function of temperature, KLN crystal belong to groups I–III in characters easily identified. Briefly, this was achieved by measurement of density. At least, crystals with density of 4.30–4.36 g/cm3 may be KLN crystal with ferroelectric properties. KLN crystals were etched in a solution of 1HF : 2HNO3 for 8 h at 258C and the etch-pit patterns of KLN crystals by optical microscopy are shown in Fig. 3. The etch-pit grains of KLN crystal belong to group I show a rectangular shape toward c-axis in [1 0 0] faces and square shape in [0 0 1] faces. The etch-pit grain sizes of KLN crystal grown by Czochralski method in this work
Table 3 The phase-transition temperature, lattice constants and densities of KLN(1), KLN(2), KLN(3) and KLN(4) crystals pffiffiffiffiffi a (A˚) c (A˚) 10c=a Density (g/cm3 ) Crystals Tc (8C) KLN(1) KLN(2) KLN(3) KLN(4)
} 420.0 483.0 534.0
12.568 12.551 12.500 12.525
3.991 4.009 3.996 3.997
1.004 1.010 1.011 1.009
4.36 4.36 4.34 4.33
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Table 4 Miller index of K3 Li2 Nb5 O15 single crystal 2Y
hkl
I=I0
d (A˚)
17.10 22.48 23.96 24.19 25.66 27.63 28.62 29.42 30.19 31.04 31.76 32.49 34.22 36.82 37.17 42.05 45.75 48.64 50.02 51.48 52.70 54.10 54.85
210 310 001 111 320 211 400 410 330 301 311 420 321 401 411 530 620 630 222 710 322 631 412
21 58 30 46 64 54 22 65 22 24 100 30 70 27 19 19 19 30 18 43 27 17 26
5.181 3.951 3.710 3.676 3.468 3.225 3.116 3.033 2.957 2.878 2.815 2.753 2.618 2.439 2.416 2.146 1.981 1.870 1.821 1.773 1.735 1.693 1.672
were found to be approximately 20–30 mm. The etch-pit grains of KLN crystal in the (0 0 1) faces have the four-fold symmetry and were arranged on a regular lattice. Choosing the KLN(3) crystals belong to group I, optical transmittance was measured and the results was shown in Fig. 4. The transmittance of KLN(3) crystal increase rapidly at the wavelength of about 400 nm and is about 75% and absorption edge is 370 nm. These results are similar to those of the previous works [7]. The SHG experiment was carried out at room temperature. A Nd : YAG laser operating at 1064 nm wavelength with pulse output power was employed as a fundmental wave to generate a 532 nm green beam. It was identified that SHG of KLN crystal belong to group I was only possible. Also, the SHG characteristics of KLN crystal belonging to group I were influenced by the crystal composition. It is known that noncritical phase matched wavelength for SHG can be tuned from 1050 to 820 nm by decreasing Nb concentration in a previous article [12]. Thus, it indicated that nonlinear properties for SHG was
Fig. 3. (a) The etch-pit grains in (1 0 0) faces of KLN crystal show a rectangular shape toward c-axis. (b) The etch-pit grains in (0 0 1) faces of KLN crystal show a square shape (a photograph 400 mm in width).
Fig. 4. The transmission spectrum of the KLN(3) crystal.
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enhanced by the variation of composition. More detailed measurements of SHG depending on compositions are currently in progress. 3.4. Characteristics of KLN crystal growth of stoichiometric and nonstoichiometric composition It is well known that the single-phase region can be divided into parts I–III [20,21]. The KLN crystal belonging to phase I is ferroelectric and Curie temperature shifts from 5408C to 3268C as the Nb2 O5 concentration changes from 0.51 to 0.55. Crystal in phase II, where the Nb2 O5 concentration changes from 0.55 to 0.68, has pseudo-tetragonal structure and is not a ferroelectric. Phase III is located in the area where the concentration of K2 O is 0.17–0.28 and that of Nb2 O5 is 0.63–0.73. From the results of the dielectric behavior and composition analysis, the characteristics of KLN(2), KLN(3) and KLN(4) crystals belong to that group I are similar to that of phase I, while the characteristics of KLN(1) crystals that belong to the group II are similar to that of phase II. Therefore, KLN(2), KLN(3) and KLN(4) crystals are ferroelectrics, while the KLN(1) crystal may not be a ferroelectric. The KLN crystal was grown from a solution having stoichiometric composition, xK2 CO3 + (1x)Li2 CO3 +Nb2 O5 (x=0.6) which crystallizes in orthorhombic structure and is therefore unlikely to be ferroelectric. These results agree with the previous reports that the growth of ferroelectric KLN crystal is difficult in stoichiometric composition. Moreover, these KLN crystals should be pulled from melts with the content of Nb2 O5 much lower than 50 mol%. In about third growth attempt, ferroelectric KLN crystals were grown in the melts. It was explained that once Nb-rich KLN crystals with orthorhombic structure (KLN crystal belongs to group II or III) were grown in the growth of first or second attempt, it seems more promising in terms of growing larger and better-quality crystals. Namely the melt composition changed the proper melts for the growth of ferroelectric KLN crystal, such as Adachi melt composition of 35 mol% K2 CO3 , 17.3 mol% Li2 CO3 and 47.7 mol% Nb2 O5 . The Nb2 O5 reducive has been shown to play an important
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role in the stable growth of pure TB-type KLN crystals by providing an optimum degree of complex formation. To identify the variation of composition and optimum conditions, we prepared the non-stoichiometric composition of 33 mol% K2 CO3 , 22 mol% Li2 CO3 and 45 mol% Nb2 O5 instead of stoichiometric composition of 30 mol% K2 CO3 , 20 mol% Li2 CO3 and 50 mol% Nb2 O5 (x=0.6). In these melts, as-grown KLN crystals have many cracks in [0 0 1] growth direction, but KLN crystals with high quality have grown in [1 0 0] growth direction. The phase-transition temperature of KLN crystal (KLN(3) and KLN(4)) grown in this melt is higher than that of stoichiometric composition melt (KLN(2)). By the previous report, KLN crystal characterized by strongly diffused dielectric peaks (DPT) [7]. However, KLN crystals grown in the non-stoichiometric composition exhibit some characteristics of relatively sharp dielectric peaks.
4. Conclusion The growth of KLN crystals is considerably difficult due to the cracks induced by the change of composition and structural characteristics. Nevertheless, CZ method is a method for the growth of large KLN crystal. We have successfully grown the tetragonal TB-type KLN crystals, which do not crack when cooling through the paraelectric/ferroelectric phase transition. Dielectric constant measurement, XRD, optical transmittance, etching pattern and composition analysis have been used to characterize the physical properties of KLN crystals. KLN crystals grown along the [0 0 1] direction have many cracks, therefore choose other growth direction such as [1 0 0] direction. Above all, it is important to choose the melt composition as well as growth environments. The phase transition of KLN(2), KLN(3) and KLN(4) crystal were 4208C, 4838C and 5348C, respectively. Different from the case of KLN(2) crystal, both KLN(3) and KLN(4) crystals exhibit a considerably sharp dielectric anomaly of the phase transition. The grown KLN crystals at different melts have different composition. The composition and
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structural disorder of ferroelectric TB-type KLN crystals have a strong influence on the ferroelectric properties as well as some other properties.
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