Cryst. Res. Technol. 46, No. 1, 37 – 40 (2011) / DOI 10.1002/crat.201000558

© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Crystal growth and electric-property change by rubidium or

cesium doping on potassium-sodium-niobate

Hideo Kimura*, Rumi Tanahashi, Hongyang Zhao, and Koji Maiwa

National Institute for Materials Science, 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan

Received 1 November 2010, accepted 5 November 2010

Published online 19 November 2010

Key words oxides, crystal growth, ferroelectricity, piezoelectricity.

Alkali metals (Na, Rb or Cs) co-doped with fiber- and bulk-shaped KNbO3 single crystals were grown using

two original methods by means of doping together of small ionic Na and large ionic Rb or Cs into KNbO3.

Single-phase crystals could be grown with an orthorhombic system at room temperature as well as pure

KNbO3. Piezoelectric and ferroelectric property changes by the co-doping of Rb or Cs with Na were

estimated using d33 values and a polarization-electric field hysteresis curve in fiber- and bulk-shaped crystals.

© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

1 Introduction

One of the potassium niobate crystals denoted as KNbO3 (KN) has attracted attention among ferroelectric and

piezoelectric crystals [1,2]. High quality bulk single crystals of KN were difficult to grow from their melts

because of incongruent melting and twice solid-state phase transformations [1]. On the other hand, NaNbO3

(NN) thin films as anti-ferroelectric materials were also of interest because of Curie temperature as high as

365 °C [3-5]. However, a low temperature process was necessary to fabricate NN thin films, such as a sol-gel

method [6].

Recently a solid solution of KN and NN, such as (KNa)NbO3 (KNN), has also been generating interest

[7-10]. On KNN, properties will change depending on the K/Na ratio [11]. To date, KNN has been fabricated

as bulk ceramics and thin films. KNN single crystals have rarely been reported. A phase diagram on the KN-

NN (KNbO3-NaNbO3) system was shown as a complete solid solution [12,13]. Thus the composition of grown

single crystals would change during the growth by the difference of ionic radius, i.e., Na ion was too small.

Such difficulties will be improved by the doping of large Rb or Cs ions as a co-dopant of Na, such as

(KNaRb)NbO3 (KNRN) or (KNaCs)NbO3 (KNCN) [14,15].

A popular growth method for bulk-shaped KN single crystals is the Top Seeded Solution Growth (TSSG)

method. In contrast, fiber-shaped KN crystals have been grown with high quality. We have developed a new

growth method in fiber-shaped KN crystals using a pull down technique [15]. This method was useful for

growing small-size single crystals in solid solution and/or crystals having phase transformation [16]. However,

it was difficult to measure d33 piezoelectric and P-E (Polarization-Electric field) ferroelectric properties using

small fiber-shaped crystals. To achieve this, larger crystals grown by the Czochralski method are useful.

In the present work, we grew Rb or Cs co-doped (KNa)NbO3 (KNN) single crystals using our two modified

growth methods for fiber and bulk shapes, and then characterized the d33 piezoelectric and the P-E ferroelectric

properties.

2 Experimental

Two modified methods were selected to grow crystals. One method is called the Floating zone Pulling Down

(Fz-PD) method for fiber shapes using a Pt tube as a melt feeder and keen Pt wire instead of a seed crystal [17].

This system installed a conventional double ellipsoidal mirror halogen lamp furnace and Pt tube with a

diameter of 3 or 4 mm. Even using the conventional furnace, long but small diameter crystals could be grown

in the present work. ____________________

* Corresponding author: e-mail: kimura.hideo@nims.go.jp

38 Hideo Kimura et al.: Rubidium or cesium doping on potassium-sodium-niobate

© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.crt-journal.org

The other method is called the Tube seed Czochralski method (Tube-Cz) for bulk shapes [18]. Larger diameter

crystals were grown. A Pt tube was used instead of a seed crystal. After dipping the Pt tube into the melt, the

melt rose up inside the tube by capillary forces and solidified in the upper colder part. This part was used as the

seed crystal. The furnace was an RF heating system.

The ratio of alkali metals and niobium was not 50:50 but 54:46 referring to the previous KNbO3 studies

[15]. Crystals were grown in an Ar gas flow condition following the Pt wire with a pulling-down rate of

10 mm/h and no crystal rotation. Amounts of Na, Rb, or Cs reached 10-20 mol% in alkali metals of K-Na-A

(A: Rb or Cs).

The starting materials of Rb and Cs carbonates absorbed moisture easily. We dared to use low purity

carbonates such as 97% (Rb) and 95% (Cs). The moisture absorption of low purity carbonates was smaller than

that of high purity carbonates of 99% [19].

The sample size for the characterization was almost a 2 mm rectangle, 2-3 mm in length for d33

measurement and a 1-2 mm rectangle, 1 mm in thickness for P-E hysteresis curve measurement. Ag paste was

used for electrode fabrication. For the crystals grown by the Tube-Cz method, poling treatment was conducted

by applying 2-3 kV at 150-160 °C in silicone oil. The d33 measurement was conducted using a H.C. Materials

ZJ-4B d33 meter at room temperature. The ferroelectric properties in the P-E hysteresis curve were measured at

room temperature in silicone oil using an aix ACCT EASY CHECK 300 ferroelectric tester and Matsusada

high voltage amplifier.

3 Results and discussion

Single crystals could be grown with a single-phase even by the co-doping. Crystals were grown along the c-

axis (shortest axis in the orthorhombic system). The quality of grown crystals was good from an observation

under cross-nicols. In fiber crystals, a self-poling effect was expected as well as observed in LiNbO3 [20].

Unfortunately, this effect was difficult to observe in the present crystals.

Figure 1 shows typical as-grown single crystals of KNRN: (KNaRb)NbO3. Figure 1a was grown using a

3 mm in diameter Pt tube and figure 1b was grown using a 4 mm in diameter Pt tube. Single crystals were

brownish and transparent. In a double ellipsoidal mirror furnace, the homogeneous temperature region is small

in the vertical direction. In this case, the 4 mm in diameter Pt tube direction was better than the 3 mm in

diameter Pt tube as it enabled extension of the homogeneous temperature region in the horizontal direction.

Details of the crystal growth conditions have been reported [19]. Crystals had an orthorhombic system at room

temperature even at KNRN and KNCN as well as KN. The lattice volume and the chemical composition of

alkali metals changed significantly on KNN, but there was little change on KNRN [18]. The composition

change was improved by the co-doping of Rb or Cs with Na.

Fig. 1 Typical as-grown crystals by the Fz-PD method of KNRN: (KNaRb)NbO3. (a) Grown using 3 mm

in diameter and (b) using 4 mm in diameter Pt tubes. Scale is 10 mm.

Fig. 2 Typical as-grown crystals by the Tube-Cz method. (a) KN: KNbO3, (b) KNRN: (KNaRb)NbO3 and

(c) KNCN: (KNaCs)NbO3. Scale is 10 mm.

Figure 2 shows typical crystals grown by the Tube-Cz method on KN: KNbO3, KNRN: (KNaRb)NbO3 and

KNCN: (KNaCs)NbO3. The c-axis was easily estimated from the rectangular crystal habit. Since the

composition change of KNN crystals was the largest, the dielectric constant was almost the same by the co-

Cryst. Res. Technol. 46, No. 1 (2011) 39

www.crt-journal.org © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

doping of Rb or Cs with Na [21]. Table 1 shows typical d33 values of bulk-shaped crystals after the poling. The

d33 values tended to increase by the co-doping. Values of KNRN and KNCN were larger than that of KN. In

the present work, it was difficult to measure the d33 values in fiber crystals due to the small diameter. Thus, we

cannot discuss the differences between fiber and bulk crystals.

Table 1 Piezoelectric constant d33 after poling measured along the c-axis by the

Tube-Cz method. KN: KNbO3, KNRN: (KNaRb)NbO3 and KNCN: (KNaCs)NbO3.

Crystals d33, pC/N

KN 16-23

KNRN 55-63

KNCN 59-92

Fig. 3 Typical P-E hysteresis curves on (a) KN: KNbO3, (b) KNRN: (KNaRb)NbO3 and (c) KNCN: (KNaCs)NbO3 bulk

crystals grown by the Tube-Cz method. Electric voltage is applied to the inserted values.

Fig. 4 Typical P-E hysteresis curve on KNRN: (KNaRb)NbO3

fiber crystals grown by the Fz-PD method. Electric voltage is

applied to the inserted values.

Figure 3 shows a typical P-E hysteresis curve on KN: KNbO3, KNRN: (KNaRb)NbO3 and KNCN: (KNaCs)

larger voltage was applied to the samples as indicated, the sample was broken. KNCN is not good at higher

voltage. In the P-E hysteresis curve on bulk crystals, ferroelectric properties are not so improved.

On the other hand, figure 4 shows a typical P-E hysteresis curve on KNRN: (KNaRb)NbO3 fiber crystals

grown by the Fz-PD method. Electric voltage is also applied to inserted values. Compared with the bulk and

fiber crystals in figures 3 and 4, fiber crystal has better ferroelectric properties on remnant polarization

although the coercive electric field is large. This can be attributed to its twin structure. On KNRN:

(KNaRb)NbO3 fiber crystals, the twin image was observed along the growth direction. The twin structure

affects the ferroelectric domains.

4 Conclusion

Alkali metals (Na, Rb or Cs) co-doped with fiber- and bulk-shaped KNbO3 single crystals were grown using

two original methods, by means of co-doping of small ionic Na and large ionic Rb or Cs into KNbO3. Even by

the co-doping, single-phase crystals could be grown with the orthorhombic system at room temperature, as

40 Hideo Kimura et al.: Rubidium or cesium doping on potassium-sodium-niobate

© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.crt-journal.org

well as pure KNbO3. It was easy to grow long fiber crystals using the present method through a 4 mm in

diameter Pt tube and conventional double ellipsoidal mirror furnace. Piezoelectric properties of d33 on bulk

crystals were improved by the co-doping. Ferroelectric properties estimated using the P-E hysteresis curve on

bulk crystals were not so improved. But the P-E hysteresis curve on KNRN fiber crystals was better than that

on KNRN bulk crystal.

Acknowledgements Part of this work was supported by grants from the bilateral program with Australia promoted by

JSPS, JSPS Grant-in-Aid for Scientific Research (c) (21605012), and JSPS Fellowship (P09608).

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