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Journal of Crystal Growth 259 (2003) 296–301

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0022-0248/

doi:10.101

Growth and characterization of KNbO3 byvertical Bridgman method

T. Takagi*, T. Fujii, Y. Sakabe

Murata Manufacturing Co., Ltd., 2-26-10 Tenjin, Nagaokakyo, Kyoto 617-8555, Japan

Received 23 May 2003; accepted 11 July 2003

Communicated by T. Hibiya

Abstract

Potassium niobate (KNbO3; KN) was grown from a K2O-Nb2O5 flux using the vertical Bridgman (VB) method. By

optimizing a flux composition, soaking temperature, the pulling down rate of crucible, and temperature gradient, we

were able to successfully grow a pale white and partially colorless KN crystal, 10mm in diameter and 50mm in length,

without a seed crystal. Blue coloration occurred when the soaking temperature was too high and low soaking

temperature resulted in red coloration. The crystal had almost no cracking under optimal temperature conditions and

optimal pulling down rate of the crucible. The X-ray pole figures of (0 4 0) and (0 0 4) reflections revealed that the white

crystal had a so-called multi-domain structure. It is possible to obtain the high quality KN crystal using VB method.

r 2003 Elsevier B.V. All rights reserved.

PACS: 81.10.fq; 81.10.�h

Keywords: A2. Bridgman technique; B1. Niobates; B2. Ferroelectric materials

1. Introduction

Potassium niobate (KNbO3; KN) is a ferro-electric material with a perovskite structure(a ¼ 0:5721 nm, b ¼ 0:5695 nm, c ¼ 0:3973 nm atroom temperature), and one of the most promisingcrystals for nonlinear-optical and electro-opticaldevices because of its large nonlinear susceptibilityand high photorefractive coefficient [1]. Also KNhas large piezoelectric constants [1], therefore it

onding author. Murata Manufacturing Co., Ltd.,

ohara, Yasu, Shiga 520-2393, Japan. Tel.: +81-77-

fax: +81-77-587-1923.

address: takagisc@murata.co.jp (T. Takagi).

$ - see front matter r 2003 Elsevier B.V. All rights reserve

6/j.jcrysgro.2003.07.022

has large electromechanical coupling constants inbulk acoustic waves [2,3] and in surface acousticwave [4,5]. This is why a great deal of attention hasbeen directed toward this material for many years.In general, top-seeded solution growth (TSSG)

is used to grow KN bulk crystal [6–8] and a50� 50� 15mm high quality crystal has beenobtained [8]. However, it needs a crystal diametercontrol system with weight sensor to use the TSSGtechnique industrially, and this results in expensiveapparatus. We chose vertical Bridgman (VB)method to grow KN crystal. The reasons are asfollows: (1) it is possible to grow KN crystal at lowcost because it does not have to control the crystaldiameter and (2) it is known as one of the low

d.

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T. Takagi et al. / Journal of Crystal Growth 259 (2003) 296–301 297

temperature gradient process. Moreover, It is saidthat growing below the melt result in good qualityKN crystal [6]. This paper is the first article of ourstudy and reports on the influence of fluxcomposition, the soaking temperature, the pullingdown rate of the crucible, and the temperaturegradient on VB growth of KN crystal.

2. Experiments

KN is an incongruent melting material [9] andhas to be grown from K2O rich, non-stoichio-metric solution. As the starting material 99%K2CO3 and 99.99% Nb2O5 raw powders (Kojun-do Chemical Laboratory Co., Ltd.) were used, andmixed. The mixture was then calcined at 900�C or850�C for 2 h in a platinum crucible. The calcinedmixture was milled and put into a cylindricalplatinum crucible, 10mm in diameter, 100mm inheight, and 0.2mm in thick. The crucible wassurrounded by an alumina tube maintain the shapeof the crucible. Fig. 1 has a cross-section of the VBapparatus. The cylindrical furnace was separatedinto two heat zones, upper and lower zones, sothat temperature bias could be controlled. A set of

Fig. 1. Cross-section of VB apparatus.

platinum plate reflectors was positioned above thecrucible to homogenize the thermal conditions atthe top. The melt temperature was monitored byan R-type thermo-couple placed under the cruci-ble. The temperature distributions are in Fig. 2.We examined three types of temperature distribu-tions (Types 1–3). The maximum temperaturegradients of these three distributions were 10�C/cm, 16�C/cm, and 24�C/cm. The crucible wasplaced into the VB apparatus at a position wherethe temperature gradient was maximum, thenheated at a rate of 300�C/h to the soakingtemperature, and melted for 12 h. Examinedconditions, such as the composition of the mixture,calcined and soaking temperatures, are in Table 1.

Fig. 2. Temperature distributions.

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T. Takagi et al. / Journal of Crystal Growth 259 (2003) 296–301298

After soaking, the melt was cooled down to1045�C at a rate of 30�C/h. Crystal growth beganwhen the crucible was pulled down at a rate of0.3–1.0mm/h after the melt was maintained for 2 hat 1040�C. When the crucible bottom temperaturefell below 800�C, pulling down was stopped. Itwas then cooled down to room temperature at arate of 15�C/h. The platinum crucible was brokenand stripped off from the solidified contents usingnippers. The contents were rinsed in water at roomtemperature for several hours to wash away theflux. In these experiments, the crystal was grown inair without a seed crystal.The X-ray powder diffraction pattern was

measured to check the crystal structure at 25�C,

Table 1

Typical melt composition

Melt composition

K2O/Nb2O5 molar

ratio

Calcined

temperature (�C)

Soaking

temperature (�C)

52/48 900 1070, 1150, 1250

54/46 850 1070, 1150

56/44 850 1070

Fig. 3. KN crystals grown from various melt

300�C, and 500�C. The crystal orientation wasdetermined by measuring the X-ray pole figures of(0 4 0) and (0 0 4) reflections. Inductively coupledplasma atomic-emission spectrometry (ICP-AES)was used to determine the chemical composition ofthe crystals.

3. Results and discussion

Fig. 3 shows crystals grown from various meltcompositions and soaking temperatures, when themoving rate of the crucible was 1.0mm/h and thetemperature distribution was Type 2. For the casethat the K2O/Nb2O5 molar ratio of the melt was52/48, the whole of crystal had been blue andcracked when the melt was soaked at 1250�C. Andred coloration occurred at the beginning ofcrystallization when the melt was soaked at1070�C. A pale white and slightly yellow crystalwas obtained when the soaking temperature was1150�C. At other melt compositions, red colora-tion did not occur, but blue coloration did at alower soaking temperature with increased K2Oconcentration of the melt.

compositions and soaking temperatures.

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Fig. 4. X-ray powder diffraction patterns.

Table 2

Chemical compositions of KN crystal

Crystal color

White Red Blue

Molar ratio of

K2O/Nb2O5

1.00 0.98 1.00

Na2O (wt%) 0.093 0.109 0.096

Pt (wt%) o0.005 o0.005 o0.005

Table 3

Results of crystal growth under various conditions

Temperature

condition

Pulling down

rate (mm/h)

Color Crack

Type 1 1.0 White, pale yellow Much

Type 2 1.0 White, pale yellow Much

Type 3 1.0 White, pale yellow A few

0.5 White, colorless, and

red in bottom part

Almost

no crack

0.3 White, colorless, and

red in bottom part

Much

T. Takagi et al. / Journal of Crystal Growth 259 (2003) 296–301 299

X-ray powder diffraction patterns (Fig. 4)confirmed that these crystals were consisted onlyKN crystals and the crystal structures wereorthorhombic at 25�C, tetragonal at 300�C, andcubic at 500�C. There were no differences indiffraction patterns at different colored crystals.The molar ratios of K2O/Nb2O5 and impurity

concentrations measured by ICP-AES are inTable 2. The red colored crystal was slightlyNb2O5 rich and Na2O was detected for all crystals.Other impurities such as Pt or Al2O3 were notdetected. It is thought that local melt compositionbecame K2O poor due to low soaking temperature,and a Nb2O5 rich phase such as K4Nb6O7, thoughundetected by X-ray powder diffraction, causedred coloration. The reasons for blue coloration arenot yet completely clear. However, consideringone report that blue color is due to free carriersinduced by oxygen vacancies [6], K2O evaporationor deformation induced by overheating may berelated to this blue coloration.Crystal grown under various conditions are

listed in Table 3, when the K2O/Nb2O5 molarratio in the melt was 52/48. Type 3 was the besttemperature distribution to reduce the number ofcracks. At a pulling down rate of 1.0mm/h, the

crystal was slightly pale yellow, while the otherswere a pale white and partially colorless. Con-sidering KN is yellow at high temperature, thisappears to be because a high temperature phasewas pinned by pores or inclusions in the paleyellow crystal. The crystal grown at a rate of0.5mm/h has almost no cracks, except for the redcoloration part at the bottom (Fig. 5). Thecolorless part, 5mm thick, cut off this crystal isalso in Fig. 5. It is surprising that such a largecolorless crystal was able to be obtained without aseed crystal using the VB method. We expectedthat a slower pulling down rate would result inlarger colorless crystals; however, many cracksoccurred in crystals grown at a rate of 0.3mm/h.The reasons for these cracks are currently beingstudied.Fig. 6 shows X-ray pole figures of (0 4 0) and

(0 0 4) reflections for a white sample cut vertical tothe direction of growth, where (0 4 0) includes(4 0 0). There are six spots in the pole figure of the(0 4 0) reflection, which we can divide into twogroups (A and B) with the three spots in eachgroup being at an angle of 60� to each other.

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Fig. 5. Crystals grown at a rate of 0.5mm/h, when K2O/Nb2O5 of melt was 52/48, temperature condition was Type 3.

Fig. 6. X-ray pole figures of (0 4 0) and (0 0 4) reflections for a white crystal.

Fig. 7. Scheme of white crystal structure.

T. Takagi et al. / Journal of Crystal Growth 259 (2003) 296–301300

Further, there are three pairs, one spot in group Aand another in group B, are at an angle of 90� toeach other. However, for (0 0 4) reflection, threespots are at an angle of 90� to one another. Theseresults indicate that the white crystal has three

different directed crystals, (a1; b1; c1), (a2; b2; c2),(a3; b3; c3), make an angle of 90� for three c-axis,and 60� for three a- and b-axis. Fig. 7 shows ascheme for this structure. Look at the a1 and b1;these axes are in a plane including the c2- and c3-axis and at 45� to c2 and c3: Considering the/1 1 0S direction in the cubic phase is a /1 0 0S or/0 1 0S axis in the tetragonal phase and these axesare the a- or b-axis in the orthorhombic phase [10],we assume that this structure was formed throughtwo phase transitions, i.e. cubic to tetragonal at435�C and tetragonal to orthorhombic at 225�Cduring the cooling process. We think this structurehas a so-called multi-domain structure and trans-forms a single-domain, colorless crystal by anelectrical or mechanical poling process.

4. Conclusion

We successfully grew a pale white and partiallycolorless KN crystal from a K2O-Nb2O5 flux usingthe vertical Bridgman method, by optimizing fluxcomposition, soaking temperature, the pulling

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T. Takagi et al. / Journal of Crystal Growth 259 (2003) 296–301 301

down rate of the crucible, and the temperaturegradient. Soaking temperature was important toavoid coloration, i.e. blue coloration occurredwhen the soaking temperature was too high and alow soaking temperature resulted in red colora-tion. A crystal grown under a temperaturegradient of 24�C/cm had almost no cracks. At apulling down rate of 1.0mm/h, the crystal wasslightly pale yellow, while a crystal grown at a rateof 0.5mm/h was pale white and partially colorless.A slower pulling down rate (0.3mm/h) causedmany cracks. The X-ray pole figures of (0 4 0) and(0 0 4) reflections revealed that white crystal hadso-called multi-domain structure. It is supposedthat white crystal transforms colorless by electricalor mechanical poling process. We believe it ispossible to obtain high quality KN crystal usingthe VB method.

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