Journal of Crystal Growth 223 (2001) 376–382

Growth and properties of ferroelectric potassium lithiumniobate (KLN) crystal grown by the Czochralski method

Jin Soo Kima,*, Ho-Sueb Leeb

aDepartment of Physics, Pusan National University, Pusan 609-735, South KoreabDepartment 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 crystalshave cracks. These problems were investigated in relation with growth axis and different solid solution. For the growthof 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 latticeconstants a ¼ b ¼ 1.2500–1.2551 nm and c ¼ 0:3996–0.4009 nm. The diffuse dielectric anomaly was observed for theseKLN crystals. The width of phase-transition region and phase-transition temperature depends both on thecompositional 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 (K3Li2Nb5O15;KLN) with tungsten–bronze (TB)-type structure isa useful material for applications in nonlinearoptics, electrooptic and piezoelectric devices [1–3].Recently, KLN crystals have been considered to besuperior materials for blue laser radiation. Theblue laser source is obtained by second-harmonicgeneration (SHG) i.e. double the frequency of adiode laser in a nonlinear optical crystal [4].

For the application, it is necessary to develop agrowth technique of high-quality KLN bulkcrystals. We have carried out KLN crystal growingby Czochralski method and successfully obtainedtransparent large-size KLN crystals. In this paper,we report the crystal growth by Czochralskimethod and the dielectric and optical propertiesof KLN crystals.

2. Experiment

Several methods have been used to grow KLNcrystals, such as Kyropoulos [5], Czochralski (CZ)[6], top-seeded solution growth (TSSG) [4,7],

*Corresponding author.

E-mail address: kimjjin@hyowon.pusan.ac.kr (J. Soo Kim).

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

micro pulling-down (m-PD) [8] and laser-heatedpedestal growth (LHPG) methods [9]. Usually,KLN crystal is grown by CZ and Kyropoulosmethods. However, the growth of KLNcrystal with high optical quality was difficult due tocracks induced by a change of composition andstructural characteristics [7–11]. In this experiment,the melt composition of xK2CO3 þ ð1� xÞLi2CO3+Nb2O5 (x ¼ 0:4–0.7) and non-stoichio-metric composition with 45mol% Nb2O5 wereused. The crystal growth and several physicalproperties of grown KLN crystal were studied.For the measurements, the specimens were

prepared by cutting the crystals perpendicular tothe c-axis. Both faces of the specimens were pastedwith silver electrodes and fired at 5008C for severalhours. The thickness and the area of the specimenswere about 1mm and 20mm2, respectively. Thedielectric properties of the KLN crystals wereinvestigated over the frequency range of 100Hz–1MHz in the temperature range of 30–7008C. Thedielectric constant was measured by an impedanceanalyzer (HP4194A, Gain/Impedance analyzer)with a heating rate of 0.58C/min. The realcomposition was measured by a micro-chemicalanalyzer (EPMA, Shimazu) and a chemical analy-zer (ICP-AES, Jovin Yvon 138 Ultrace). For theobservation of microstructure, the specimens ofKLN crystal were polished and etched in anetchant (HF :HNO3=1 : 2) at the room tempera-ture.

3. Results and discussion

3.1. Crystal growth

Scott et al. [12] and Ikeda et al. [13] studied thephase equilibrium of K2O–Li2O–Nb2O5 ternarysystem. KLN crystal grown in this compositionrange was incongruently melted and was grownonly from high-temperature solutions. In thisstudy, for a KLN crystal growth, the raw materialsof xK2CO3+(1�x)Li2CO3+Nb2O5 (x ¼ 0:4–0.7)and nonstoichimetric composition with 45mol%Nb2O5 were prepared from the chemical reagentsLi2CO3, Nb2O5 and K2CO3, which were suffi-ciently mixed and calcined at 6508C for 12 h, and

annealed at 9508C for 12 h. X-ray diffractionstudies have been carried out to check a formationof single phase and a structure of the sample. Themixture of the raw materials was charged in a Ptcrucible (50mm diameter by 35mm height, 60ml)and was melted two times by a RF-heating. Fouror five KLN crystals, weighing 12–15 g, werecontinuously grown in the same melted rawmaterials in the crucible. To reduce the thermalgradient above the melt surface, a Pt after-heaterwas attached.KLN crystal was not grown at the composi-

tion x=0.4 and 0.7, but KLN crystal with ferro-electric properties was grown at the compositionx=0.5 and 0.6. KLN crystals, grown along the[0 0 1]-axis in each composition of xK2CO3

+(1�x)Li2CO3+Nb2O5, have cracks in the earlystage of a crystal growth. It implies that thegrowth of high-quality KLN crystals in the [0 0 1]-axis was generally difficult because of the complexnature of the compounds. However, crack-freeKLN crystal was successfully obtained by choos-ing the [1 0 0] growth direction and using Pt after-heater. As shown in Fig. 1, the shapes of the well-grown crystals in the [1 0 0] and [0 0 1] directionwere plate and cylindrical, respectively. The colorof as-grown KLN crystal was pale yellow. Thepulling rate was 0.5–1.0mm/h, and the seed-rotation speed was 20–36 rpm. Some results forgrowth conditions are summarized in Table 1.

3.2. Dielectric properties of KLN crystals

The grown KLN crystals were divided into threegroups depending on their dielectric properties.KLN crystals that belong to group I have a phasetransition at the temperatures 400–5408C and havea dielectric constant of about 100–140 at roomtemperature. KLN crystals that belong to group IIhave a very broad dielectric anomaly at thetemperature below 3008C and have a dielectricconstant of 400 at room temperature. The crystalthat belong to group III has a similar shape toother KLN crystals, but the dielectric propertieswere quite different from typical KLN crystals. Inthe case of easily grown KLN crystals at composi-tion x=0.6, it was identified that KLN crystalsbelong to group III.

J. Soo Kim, H.-S. Lee / Journal of Crystal Growth 223 (2001) 376–382 377

Fig. 2 shows that the temperature-dependentdielectric constant of KLN crystals belong togroups I and II at the temperature range of 100–7008C. To demonstrate a variety of physicalproperties of KLN crystals, four kinds of KLNcrystals were chosen according to the dielectricanomaly of phase transition. The phase-transitiontemperature of the three KLN crystals, abbre-viated as KLN(2), KLN(3) and KLN(4), were4208C, 4838C and 5348C, respectively. WhileKLN(1) crystal has quite different dielectricproperties than others. The phase-transition tem-peratures of KLN(3) and KLN(4) crystals arehigher than that of the well-known 4308C [14].Different from KLN(2), both KLN(3) andKLN(4) crystals exhibit a considerably sharpdielectric anomaly of the phase transition. Thereare differences in phase transition temperature andbroadness of dielectric constant between crystals.Since the peak of dielectric constant of KLN(2)crystal is notably broadened, it is difficult todetermine the phase transition temperature accu-rately. Namely, this dielectric behavior of KLN(2)crystal shows a broad anomaly at the transitiontemperature of about 4208C, and this indicatesthat the transition is diffuse phase transition(DPT), which is caused by the compositionalfluctuations and structural disorder [15–18].

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).

Table 1

The growth conditions of KLN crystals. Nos. 8 and 9 indicate

crystal grown in the melt composition of 33mol% K2CO3,

22mol% Li2CO3 and 45mol% Nb2O5

x Growth Rotation

(rpm)

Pulling

(mm/h)

Group Crystals

1 0.5 [1 0 0] 22 0.7 I KLN(1)

2 0.5 [0 0 1] 22 0.7 II }

3 0.6 [1 0 0] 21 0.9 I KLN(2)

4 0.6 [1 0 0] 36 1.0 I }

5 0.6 [0 0 1] 12 0.7 III }

6 0.7 [0 0 1] 16 0.7 III }

7 0.7 [1 0 0] 16 0.7 III }

8 } [1 0 0] 34 1.0 I KLN(3)

9 } [1 0 0] 36 1.0 I 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.

J. Soo Kim, H.-S. Lee / Journal of Crystal Growth 223 (2001) 376–382378

3.3. Structural characteristics related tocomposition, lattice constants, microstructureand optical properties

To investigate the composition-dependent phasetransition, the composition of KLN crystal wasmeasured by the inductively coupled plasma-atomic emission (ICP) spectroscopy and electroprobe micro-analysis (EPMA) method, and theresults were shown in Table 2. The content of K,Nb and O in the crystals was determined directlyby the EPMA and Li concentration determined bythe ICP. Table 2 shows that there are differentcompositions in KLN(1), KLN(2), KLN(3) andKLN(4) crystals. The completely filled TB-typestructure K3Li2Nb5O15 is not stable, it is onlystable in the presence of Nb excess [19]. As shownin Table 2, this result of composition analysisagrees with that of Nb excess. In particular, thecontent of Nb in KLN(1) crystal is larger thanthose of others. From the DPT characteristics ofdielectric properties and composition analysis, itcan be concluded that the width of phase-transi-tion region and the sharpness at the phase-transition temperature depends both on thecompositional fluctuations and the compositionalvariations.

Lattice constants, densities and axial ratios ofKLN crystals were also determined and the resultswere summarized in Tables 3 and 4. We havesubjected these crystal to X-ray powder diffractionusing CuKa lines studies (a1=1.54056 A) andfound that they are single crystalline. Also, theX-ray diffraction lines could be indexed in atetragonal system and the lattice constants witha ¼ b ¼12.500–12.551 A and c=3.996–4.009 Adetermined by least squares fitting, which agreewith the previously reported value [10,19]. Theaxial ratios

ffiffiffiffiffi

10p

c=a of KLN(2), KLN(3) andKLN(4) are about 1.01. The densities ofKLN(1), KLN(2), KLN(3) and KLN(4) weredetermined by the Archimedes method to be4.36, 4.36, 4.34 and 4:33 g=cm3, respectively. Whilethe density of group III was larger than 4:4 g=cm3.It is well known that the theoretical density ofKLN 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 higherthan that of theoretical density. It is explained thatthe completely filled TB-type structured KLNcrystal is not stable and it is only stable in thepresence of Nb excess. By the measurement ofdielectric constant as a function of temperature,KLN crystal belong to groups I–III in characterseasily identified. Briefly, this was achieved bymeasurement of density. At least, crystals withdensity of 4.30–4.36 g/cm3 may be KLN crystalwith ferroelectric properties.KLN crystals were etched in a solution of

1HF : 2HNO3 for 8 h at 258C and the etch-pitpatterns of KLN crystals by optical microscopyare shown in Fig. 3. The etch-pit grains of KLNcrystal belong to group I show a rectangular shapetoward c-axis in [1 0 0] faces and square shape in[0 0 1] faces. The etch-pit grain sizes of KLNcrystal grown by Czochralski method in this work

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) 11.81 1.42 56.81 K2:67Li1:67Nb5:28O15:0

KLN(2) 12.77 1.45 56.77 K2:89Li1:77Nb5:22O15:0

KLN(3) 12.61 1.72 55.61 K2:83Li2:10Nb5:02O15:0

KLN(4) 13.59 1.65 56.23 K2:99Li2:01Nb5:17O15:0

Table 3

The phase-transition temperature, lattice constants and densities of KLN(1), KLN(2), KLN(3) and KLN(4) crystals

Crystals Tc (8C) a (A) c (A)ffiffiffiffiffi

10p

c=a Density (g/cm3)

KLN(1) } 12.568 3.991 1.004 4.36

KLN(2) 420.0 12.551 4.009 1.010 4.36

KLN(3) 483.0 12.500 3.996 1.011 4.34

KLN(4) 534.0 12.525 3.997 1.009 4.33

J. Soo Kim, H.-S. Lee / Journal of Crystal Growth 223 (2001) 376–382 379

were found to be approximately 20–30 mm. Theetch-pit grains of KLN crystal in the (0 0 1) faceshave the four-fold symmetry and were arranged ona regular lattice.Choosing the KLN(3) crystals belong to group

I, optical transmittance was measured and theresults was shown in Fig. 4. The transmittance ofKLN(3) crystal increase rapidly at the wavelengthof about 400 nm and is about 75% and absorptionedge is 370 nm. These results are similar to those ofthe previous works [7]. The SHG experiment wascarried out at room temperature. A Nd :YAGlaser operating at 1064 nm wavelength with pulseoutput power was employed as a fundmental waveto generate a 532 nm green beam. It was identifiedthat SHG of KLN crystal belong to group I wasonly possible. Also, the SHG characteristics ofKLN crystal belonging to group I were influencedby the crystal composition. It is known thatnoncritical phase matched wavelength for SHGcan be tuned from 1050 to 820 nm by decreasingNb concentration in a previous article [12]. Thus,it indicated that nonlinear properties for SHG was

Table 4

Miller index of K3Li2Nb5O15 single crystal

2 Y h k l I=I0 d (A)

17.10 2 1 0 21 5.181

22.48 3 1 0 58 3.951

23.96 0 0 1 30 3.710

24.19 1 1 1 46 3.676

25.66 3 2 0 64 3.468

27.63 2 1 1 54 3.225

28.62 4 0 0 22 3.116

29.42 4 1 0 65 3.033

30.19 3 3 0 22 2.957

31.04 3 0 1 24 2.878

31.76 3 1 1 100 2.815

32.49 4 2 0 30 2.753

34.22 3 2 1 70 2.618

36.82 4 0 1 27 2.439

37.17 4 1 1 19 2.416

42.05 5 3 0 19 2.146

45.75 6 2 0 19 1.981

48.64 6 3 0 30 1.870

50.02 2 2 2 18 1.821

51.48 7 1 0 43 1.773

52.70 3 2 2 27 1.735

54.10 6 3 1 17 1.693

54.85 4 1 2 26 1.672

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 400mm in width).

Fig. 4. The transmission spectrum of the KLN(3) crystal.

J. Soo Kim, H.-S. Lee / Journal of Crystal Growth 223 (2001) 376–382380

enhanced by the variation of composition. Moredetailed measurements of SHG depending oncompositions are currently in progress.

3.4. Characteristics of KLN crystal growth ofstoichiometric and nonstoichiometric composition

It is well known that the single-phase region canbe divided into parts I–III [20,21]. The KLNcrystal belonging to phase I is ferroelectric andCurie temperature shifts from 5408C to 3268C asthe Nb2O5 concentration changes from 0.51 to0.55. Crystal in phase II, where the Nb2O5

concentration changes from 0.55 to 0.68, haspseudo-tetragonal structure and is not a ferro-electric. Phase III is located in the area where theconcentration of K2O is 0.17–0.28 and that ofNb2O5 is 0.63–0.73. From the results of thedielectric behavior and composition analysis, thecharacteristics of KLN(2), KLN(3) and KLN(4)crystals belong to that group I are similar to thatof phase I, while the characteristics of KLN(1)crystals that belong to the group II are similar tothat of phase II. Therefore, KLN(2), KLN(3) andKLN(4) crystals are ferroelectrics, while theKLN(1) crystal may not be a ferroelectric.The KLN crystal was grown from a solution

having stoichiometric composition, xK2CO3+(1�x)Li2CO3+Nb2O5 (x=0.6) which crystallizesin orthorhombic structure and is therefore unlikelyto be ferroelectric. These results agree with theprevious reports that the growth of ferroelectricKLN crystal is difficult in stoichiometric composi-tion. Moreover, these KLN crystals should bepulled from melts with the content of Nb2O5 muchlower than 50mol%. In about third growthattempt, ferroelectric KLN crystals were grownin the melts. It was explained that once Nb-richKLN crystals with orthorhombic structure (KLNcrystal belongs to group II or III) were grown inthe growth of first or second attempt, it seemsmore promising in terms of growing larger andbetter-quality crystals. Namely the melt composi-tion changed the proper melts for the growth offerroelectric KLN crystal, such as Adachi meltcomposition of 35mol% K2CO3, 17.3mol%Li2CO3 and 47.7mol% Nb2O5. The Nb2O5

reducive has been shown to play an important

role in the stable growth of pure TB-type KLNcrystals by providing an optimum degree ofcomplex formation.To identify the variation of composition and

optimum conditions, we prepared the non-stoi-chiometric composition of 33mol% K2CO3,22mol% Li2CO3 and 45mol% Nb2O5 instead ofstoichiometric composition of 30mol% K2CO3,20mol% Li2CO3 and 50 mol% Nb2O5 (x=0.6).In these melts, as-grown KLN crystals have manycracks in [0 0 1] growth direction, but KLNcrystals with high quality have grown in [1 0 0]growth direction. The phase-transition tempera-ture of KLN crystal (KLN(3) and KLN(4)) grownin this melt is higher than that of stoichiometriccomposition melt (KLN(2)). By the previousreport, KLN crystal characterized by stronglydiffused dielectric peaks (DPT) [7]. However, KLNcrystals grown in the non-stoichiometric composi-tion exhibit some characteristics of relatively sharpdielectric peaks.

4. Conclusion

The growth of KLN crystals is considerablydifficult due to the cracks induced by the change ofcomposition and structural characteristics. Never-theless, CZ method is a method for the growth oflarge KLN crystal. We have successfully grown thetetragonal TB-type KLN crystals, which do notcrack when cooling through the paraelectric/ferro-electric phase transition. Dielectric constant mea-surement, XRD, optical transmittance, etchingpattern and composition analysis have been usedto characterize the physical properties of KLNcrystals. KLN crystals grown along the [0 0 1]direction have many cracks, therefore choose othergrowth direction such as [1 0 0] direction. Aboveall, it is important to choose the melt compositionas well as growth environments. The phasetransition of KLN(2), KLN(3) and KLN(4) crystalwere 4208C, 4838C and 5348C, respectively. Differ-ent from the case of KLN(2) crystal, both KLN(3)and KLN(4) crystals exhibit a considerablysharp dielectric anomaly of the phase transition.The grown KLN crystals at different meltshave different composition. The composition and

J. Soo Kim, H.-S. Lee / Journal of Crystal Growth 223 (2001) 376–382 381

structural disorder of ferroelectric TB-type KLNcrystals have a strong influence on the ferroelectricproperties as well as some other properties.

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