KNbO3 Growth Bridgeman Process

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Journal of Crystal Growth 259 (2003) 296–301

Growth and characterization of KNbO 3 byvertical Bridgman method

T. Takagi*, T. Fujii, Y. SakabeMurata 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 (KNbO 3 ; KN) was grown from a K 2 O-Nb 2 O 5 ux using the vertical Bridgman (VB) method. Byoptimizing a ux composition, soaking temperature, the pulling down rate of crucible, and temperature gradient, wewere able to successfully grow a pale white and partially colorless KN crystal, 10 mm in diameter and 50 mm in length,without a seed crystal. Blue coloration occurred when the soaking temperature was too high and low soakingtemperature resulted in red coloration. The crystal had almost no cracking under optimal temperature conditions andoptimal pulling down rate of the crucible. The X-ray pole gures of (0 4 0) and (0 0 4) reections revealed that the whitecrystal 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. À h

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

1. Introduction

Potassium niobate (KNbO 3 ; KN) is a ferro-electric material with a perovskite structure

(a¼

0 : 5721 nm, b¼

0 : 5695 nm, c¼

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 coefcient [1]. Also KNhas large piezoelectric constants [1], therefore it

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 Â 50 Â 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

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*Corresponding author. Murata Manufacturing Co., Ltd.,2288 Oshinohara, Yasu, Shiga 520-2393, Japan. Tel.: +81-77-586-8209; fax: +81-77-587-1923.

E-mail address: [email protected] (T. Takagi).

0022-0248/$- see front matter r 2003 Elsevier B.V. All rights reserved.doi:10.1016/j.jcrysgro.2003.07.022



temperature gradient process. Moreover, It is saidthat growing below the melt result in good qualityKN crystal [6]. This paper is the rst article of our

study and reports on the inuence of uxcomposition, 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 K 2 O rich, non-stoichio-metric solution. As the starting material 99%K 2 CO 3 and 99.99% Nb 2 O 5 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, 10 mm in diameter, 100 mm inheight, and 0.2 mm 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

platinum plate reectors was positioned above thecrucible to homogenize the thermal conditions atthe top. The melt temperature was monitored by

an 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 12h. Examinedconditions, such as the composition of the mixture,calcined and soaking temperatures, are in Table 1 .

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Fig. 1. Cross-section of VB apparatus. Fig. 2. Temperature distributions.

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



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 of

0.3–1.0 mm/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 solidied contents usingnippers. The contents were rinsed in water at roomtemperature for several hours to wash away theux. In these experiments, the crystal was grown inair without a seed crystal.

The X-ray powder diffraction pattern wasmeasured to check the crystal structure at 25 C,

300 C, and 500 C. The crystal orientation wasdetermined by measuring the X-ray pole gures of (0 4 0) and (0 0 4) reections. Inductively coupled

plasma atomic-emission spectrometry (ICP-AES)was used to determine the chemical composition of the crystals.

3. Results and discussion

Fig. 3 shows crystals grown from various meltcompositions and soaking temperatures, when themoving rate of the crucible was 1.0 mm/h and thetemperature distribution was Type 2. For the casethat the K 2 O/Nb 2 O 5 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 of crystallization 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 K 2 Oconcentration of the melt.

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Table 1Typical melt composition

Melt compositionK 2 O/Nb 2 O 5 molarratio

Calcinedtemperature ( C)

Soakingtemperature ( C)

52/48 900 1070, 1150, 125054/46 850 1070, 115056/44 850 1070

Fig. 3. KN crystals grown from various melt compositions and soaking temperatures.

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



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) reection, 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 0 S 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 structure

has 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 K 2 O-Nb 2 O 5 ux usingthe vertical Bridgman method, by optimizing uxcomposition, soaking temperature, the pulling

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Fig. 5. Crystals grown at a rate of 0.5 mm/h, when K 2 O/Nb 2 O 5 of melt was 52/48, temperature condition was Type 3.

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

Fig. 7. Scheme of white crystal structure.

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



down rate of the crucible, and the temperaturegradient. Soaking temperature was important toavoid coloration, i.e. blue coloration occurred

when 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 1.0 mm/h, the crystal wasslightly pale yellow, while a crystal grown at a rateof 0.5 mm/h was pale white and partially colorless.A slower pulling down rate (0.3 mm/h) causedmany cracks. The X-ray pole gures of (0 4 0) and(0 0 4) reections 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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