Zuo-Guang. Ye Advanced Dielectric Piezoelectric and Ferroelectric Materials: Synthesis, Characterisation and Applications

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29.1 Introduction

The future use of piezoelectric materials containing lead, which is extensive

at present, is likely to be restricted because of environmental regulations.

Anticipating that situation, it is necessary for lead-free piezoelectric materials

to be developed. Potassium niobate (KNbO

) is a material that has extremely

high nonlinear optical characteristics and a high electromechanical coupling

coefficient (efficiency of conversion between electrical energy and mechanical

energy). Therefore, it is considered to be most promising for use as an

alternative to current lead-based piezoelectric materials

1,2

. Nevertheless, KNbO

poses many practical disadvantages for use in piezoelectric and ferroelectric

materials.

First, polycrystal KNbO

is a poorly sinterable material

. It is difficult to

obtain sintered ceramics that are suitable for practical applications, even if

special methods such as hot press are used. Although attempts have been

made to improve the sintering using various additives, many additives degrade

the electrical properties of the KNbO

itself. Utilization of single crystals

also involves such problems as difficulty in growing the stoichiometric single

crystals KNbO

shows incongruent melting at high-temperature; moreover,

fine cracks occur as a result of the phase transition from the high-temperature

phase to the room temperature phase upon cooling, thereby engendering

deterioration of the single-crystal quality.

To overcome these disadvantages, we specifically examined the layered

perovskite K

NbO

F, with the structure as shown in Fig. 29.1, for use as a

precursor for producing nanoparticles, single crystals in plate-shaped and

thin films

. A distinctive feature of this compound is its unique layered

perovskite structure, which includes an ordered KF block and a KO block

between layers. An interlayer KF block composed of fluorine of negative

charge one has weaker bonding than a perovskite block. For that reason, the

potassium ions and fluorine ions in K

NbO

F are desorbed easily when

stirred into water or calcinated at low temperature. Using this characteristic,

Room temperature preparation of KNbO

nanoparticles and thin film from a

perovskite nanosheet

K T O D A and M S AT O, Niigata University, Japan

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Room temperature preparation of KNbO

nanoparticles 885

we obtained KNbO

nanoparticles and formed a crystalline KNbO

thin film

at room temperature. This paper describes the reaction mechanism and

formation of thin films.

29.2 Mechanisms of generation of KNbO

nanoparticles

29.2.1 Ion exchangeable layered perovskite

Since the late 1980s, we have been engaged in explorative research of ion

exchangeable layered perovskite compounds, in which interlayer ions include

alkali metals with lower valence that are readily exchangeable with other

ions. Typical ion exchange layered perovskite compounds are the Dion–

Jacobson phase and the Ruddlesden–Popper phase, as shown in Fig. 29.2

5,6

The difference between the two structures lies in the charge amount of

interlayer ions. These materials attract attention because manipulations on

the nano-scale, such as conversion to a nanosheet by cleavage of layers, can

be realized in easy-to-handle environments such as in a beaker. Moreover,

29.1

Crystal structure of layered perovskite K

NbO

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metastable compounds, which cannot be obtained by conventional solid-

state high-temperature reactions, are obtainable through ion exchange reaction.

These nanostructures are expected to form photocatalysts, phosphors, ionic

conductors and superconducting materials which will find applications in

new devices.

We have discovered the conversion of the layered perovskite K

NbO

F, a

precursor phase, to perovskite KNbO

at room temperature. Figure 29.3

shows the X-ray diffraction (XRD) patterns before and after the conversion.

It is clear from Fig. 29.3 that KNbO

obtained at room temperature is a

crystalline compound. The synthesis of K

NbO

F was first reported by Galasso

and Darby in 1962

. Its crystal structure is presumed to be a layered perovskite

structure of K

NiF

type in which 50% of the apex oxygen is replaced by

fluorine. In 1998, from the experiments using

F NMR (nuclear magnetic

resonance), Du et al. found that the averagely positioned fluorine model is

incorrect. Instead, they proposed a structural model in which the KO block

and KF block are built in a perfect order, as depicted in Fig. 29.1

. This

peculiar pyramidal coordination structure plays an important role in the low-

temperature reaction, as described in the following section.

Dion–Jacobson Ruddlesden–Popper

RbLaNb

O CsCa

SrTa

29.2

Model structure of typical Dion–Jacobson phase and

Ruddlesden–Popper phases.

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Room temperature preparation of KNbO

nanoparticles 887

29.2.2 Conversion of K

NbO

F to nanosheets in water

and re-stacking

When the layered perovskite K

NbO

F is immersed in water at room

temperature, exfoliation occurs first. Figure 29.4 shows a photograph of the

solution together with pure water. Because the light path of a laser pointer is

visible, this solution is inferred to be a colloidal solution. Observation by

transmission electron microscopy (TEM) reveals that the colloid particles

are uniform nanosheets of approximately 100nm size. Such a size coincides

with the crystallite size of the precursor phase. The conversion to nanosheets

occurs rapidly and is almost completed within minutes. The insertion of

bulky molecules to assist peeling, as is needed in the formation of nanosheet

by ordinary cleavage of layers, is found to be unnecessary. This process is

usually confused with the hydrothermal method. However, in contrast to the

hydrothermal method, in which the formation of a crystal lattice occurs in

hot water under high pressure, this method shows its characteristic in that the

perovskite block is maintained, even in the solution. Therefore, there is no

need for an external field for crystallization. Instead, voltage application or

heating might hinder the self-organizing re-stacking of nanosheets. The simplest

method, which is also effective for collecting powders, is centrifugation.

Such a selective dissolution takes place between layers because of their

binding characteristics: KF maintains higher solubility with regard to water,

even at room temperature. For that reason, the KF block is dissolved solely

Intensity/a.u.

10 20 30 40 50 60

2θ/deg.

(b) KNbO

(a) K

NbO

(002)

(101)

(103)

(110)

(006)

(200)

29.3

Powder XRD pattern of layered perovskite K

NbO

F and KNbO

prepared after immersion into water.

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from K

NbO

F in water, but the perovskite block is held, strongly bound to

pentavalent Nb. In fact, the conversion to nanosheets is not promoted because

the KF block is not dissolved in the saturated aqueous solution of KF.

29.2.3 Re-stacking of nanosheets

Formation of KNbO

from nanosheets occurs through self-organizing re-

stacking. In this process, the layered perovskite is peeled to a monolayer

perovskite nanosheet at room temperature; then a restructuring of the perovskite

lattice occurs without any artificial manipulation. This self-organization is

driven by the difference in the stability of lattice between the precursor and

the intermediate product in a transition state in aqueous solution. The instability

of the nanosheets, an intermediate product, is attributable to their peculiar

pyramidal coordination structure, as mentioned previously. The nanosheets

from which the KF block is dissolved selectively in water maintain a pyramid-

type coordination of NbO

type because they lack apex oxygen. Figure 29.5

shows the extended X-ray absorption fine structure (EXAFS) data of nanosheets

in an aqueous solution. Because the bonding state of Nb–O–Nb in the solution

is different, it is known that the coordination of the nanosheets differs from

that of KNbO

, which is the final product. Furthermore, Fig. 29.6 shows the

Raman spectra of the colloidal solution, the KNbO

product and the K

NbO

precursor for comparison. The electronic state of K

NbO

F changes because

of the ordering of KO and KF, and a peak of Nb=O appears at 822cm

–1

Pure water Colloidal solution

29.4

Colloidal solution obtained after immersion of layered

perovskite K

NbO

F into water, as compared with pure water.

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Room temperature preparation of KNbO

nanoparticles 889

double-bond character

. The peak in the colloidal solution resembles that of

KNbO

of the final product, indicating the formation of a Nb–O polyhedron,

although a slight shift of the peak position is observed

. These data imply

that the nanosheets in the aqueous solution exhibit a distorted structure such

that the NbO

pyramids are turned to each other, as shown in Fig. 29.7. Such

a peculiar five-coordinate structure is unstable. Therefore, more unstable

KNbO

is formed. A schematic representation of these structural changes is

shown in Fig. 29.8. KNbO

is obtained as nanoparticles in plate-shape, high-

density sintered ceramics with a density of 98–99%, even under pressure-

free conditions.

29.3 Fabrication of KNbO

thin film

The precursor aqueous solution is a uniform colloidal solution. Using this

solution in a simple process, we attempted to produce uniform thin films at

| FT |

Nb–O Nb–O–Nb

(a)

(b)

(c)

(d)

29.5

EXAFS data of (a) colloidal solution, (b) KNbO

powder produced

using the present process, (c) KNbO

nano-powder annealed at

1000°C and (d) KNbO

powder synthesized using a solid state reaction.

0.0 0.1 0.2 0.3 0.4 0.5 0.6

R/nm

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