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

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Handbook of dielectric, piezoelectric and ferroelectric materials40

PMN–PT single crystals of 50–76 mm or larger in diameter and more than

100mm in boule length have been successfully grown by several groups

(Lee et al., 1999; Zhu and Han, 1999; Luo et al., 2000a,b; Xu et al., 2000,

2001; Santailler et al., 2002; Wan et al., 2004; Shin et al., 2004; Nicoara

et al., 2005).

Despite the extensive efforts made in the flux growth of PZN–PT and

PMN–PT and the melt-growth of PMN–PT single crystals, currently grown

PZN–PT and PMN–PT single crystal are plagued with a host of problems.

One of the major issues has been compositional segregation in the grown

crystals. This has led to large scatter in measured dielectric and

electromechanical properties reported by various research groups as well as

within individual groups (Luo et al., 2000a,b; Xu et al., 2000, 2001a,b;

Santailler et al., 2002; Karaki et al., 2002b; Guo et al., 2003; Zawilski et al.,

2003; Shin et al., 2004; Wan et al., 2004; Nicoara et al., 2005). Developing

ways of growing large high-homogeneity PZN–PT and PMN–PT single crystals

is thus one of the major challenges faced by scientists and researchers before

these crystals find wide spread applications.

This chapter describes the results of flux growth of PZN–PT and PMN–

PT single crystals performed in the author’s laboratory over the past seven

years (Kumar and Lim, 2000; Lim, 2004; Lim and Rajan, 2004; Rajan et al.,

2005; Lim et al., 2005). The flux growth mechanisms of PZN–PT and PMN–

PT single crystals and the effects of flux and crystal compositions and growth

conditions on the quality of grown crystals are described and discussed.

Large single crystals (≥35mm edge length for PZN–PT and ≥ 25 mm PMN–

PT) of improved homogeneity have been successfully grown by an improved

flux growth technique which promotes (001) layer growth of the crystals.

The properties of the crystals obtained were evaluated and the results are

presented and discussed.

2.2 Flux growth of PZN–PT and PMN–PT single

crystals

Figure 2.1 shows the typical set-up (a) and growth result (b) of high-temperature

flux growth of PZN–PT and PMN–PT single crystals. The most commonly

used fluxes for the growth of PZN–PT and PMN–PT single crystals are PbO

(or Pb

) and complex fluxes consisting of a mixture of PbO and B

respectively, with the solute-to-flux mole ratios varying from 0.55:0.45 to

0.30:0.70.

The component oxide powders, of 99.95% or higher purity, are weighed

to desired proportions and then mixed thoroughly. The charge mixture may

be loaded directly into the Pt crucible, although pre-calcined charge has also

been commonly used. Then, the Pt crucible is covered with the lid and

further placed in a sealed alumina crucible assembly, as shown in Fig. 2.1(a),

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Flux-growth and characterization of PZN–PT and PMN–PT 41

to contain potential PbO loss during the crystal growth run. The assembly is

placed in the crystal growth furnace, which is often equipped with a local

cooling arrangement to eliminate unwanted nucleation. Techniques used for

local point cooling include the use of a thin metal rod (Shimanuki et al.,

1998; Harada et al., 1998, 2001; Matsushita et al., 2002; Babu et al., 2006),

metal wire (Chen and Ye, 2001; Karaki et al., 2002a) and controlled stream

of gas flow (Kobayashi et al., 1997, 1998; Saitoh et al., 1999; Kumar and

Lim, 2000; Benayad et al., 2004; Dabkowski et al., 2004).

The crucible assembly with the charge is then heated to high temperature,

typically between 1150 and 1250°C, and held for a sufficiently long period

of up to 10h to melt and homogenize the solution. Then, the assembly is

cooled at a controlled rate, typically in the range of 0.5–1.5°C/h, to start the

crystal growth process.

When perfect single-crystal nucleation is initiated in the middle of the

Heating

element

Sealed

alumina

crucible

Pt crucible

Crystal

Oxygen

Control

T.C.

Growth

solution

(a)

Parasitic

crystals

Satellite

crystals

Side-wall

nucleation

Main

crystal

Pyrochlore

crystal

(b)

2.1

Schematics showing (a) set-up for flux growth of lead-based

relaxor single crystals from high temperature via spontaneous

nucleation and (b) typical growth results.

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Handbook of dielectric, piezoelectric and ferroelectric materials42

base of the Pt crucible, the crystal will grow in a stress-free manner with

minimum constraints and is totally protected by the PbO-rich solution. The

latter is important because most relaxor–PT systems, in particular PZN–PT,

are unstable at high temperatures even in PbO-rich vapour environments

(Jang et al., 1992; Wakiya et al., 1995; Lim et al., 2002). The entire crystal

growth run may last for one to several weeks. Depending on the crystal type,

slow cooling is stopped after the growth temperature reaches about 800–

1000 °C, whereupon the assembly is cooled at a fast rate to room conditions.

It has also been common practice to use gradually increasing cooling rate in

the slow cooling stage (Mulvihill et al., 1996; Saitoh et al., 1999; Zhang et

al., 2000; Fan et al., 2003; Babu et al., 2006), in order to achieve a more

constant crystal growth rate. This, however, requires the knowledge of the

ternary phase diagram of the material systems concerned. Although relevant

ternary phase diagrams remain unavailable to date, binary phase diagrams

for the PMN–PbO system and for the (PZN–9%PT)–PbO system have been

reported by Ye et al. (1990) and Dong and Ye (2001), respectively.

At the conclusion of the crystal growth run, the crystal is retrieved from

the remaining solidified flux by leaching in boiling concentrated nitric acid.

It has also been common practice to decant the remaining flux solution by

flipping over the alumina crucible assembly at the end stage of the cooling

process. This, however, must be executed with care because the crystal may

crack due to thermal shock, especially for large crystals.

2.3 Effect of flux composition

PbO melts at 886°C and the PbO–B

mixture has a relatively low eutectic

point of 493°C. They are commonly used fluxes in the growth of PZN–PT

and PMN–PT single crystals. Being an end element of the final compound,

the use of PbO in the growth of PZN–PT and PMN–PT is advantageous in

that introduction of foreign ions can be avoided.

PbO melt has a reasonably low viscosity and high solubility for complex

oxides. However, its major drawbacks include its toxicity, volatility above

1100 °C and a tendency to corrode platinum above 1300°C. Lead oxide

(PbO) with additives of Pb

may also be used. The addition of Pb

assures an oxidizing atmosphere during crystallization and it has been reported

that this may help promote the stability of the perovskite phase during the

crystal growth (Park and Shrout, 1997a; Dong and Ye, 1999; Kania et al.,

2005). The addition of B

not only helps lower the crystallization temperature

of the relaxor–PT system but also increases the viscosity of the solution.

Both of these effectively prevent the loss of PbO through volatilization

during the crystal growth run, which is desirable for the growth of certain

relaxor–PT single crystals of relatively high solution temperature, such as

the PMN–PT system.

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Flux-growth and characterization of PZN–PT and PMN–PT 43

2.2(a)

A hypothetical phase diagram of the PMN–PT–(PbO + B

) system.

The vertical sections of PMN–PT single crystals grown with insufficient

and sufficient B

in PbO-based fluxes are given in (b) and (c),

respectively.

L +

PMNT

L +

PbO

PMNT

PMN

PMNT

+ L

Charge

compositions

PbO +

Charge

(PMNT)

(a)

(b)

(c)

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Handbook of dielectric, piezoelectric and ferroelectric materials44

The flux composition strongly affects the outcome of the crystal growth.

For instance, our experience has shown that, on the one hand, the addition of

to PbO flux promotes the formation of the pyrochlore phase in the

PZN–PT system, on the other hand, a suitable amount of B

flux is needed

in the growth of PMN–PT to ensure a good growth result.

In addition to flux composition, an appropriate flux-to-solute ratio is essential

in relaxor–PT single crystal growth. Figure 2.2(a) gives a hypothetical phase

diagram of the PMN–PT–PbO(B

) system, which serves to illustrate the

importance of flux composition and content in PMN–PT growth (Lim, 2004;

Lim et al., 2005). This figure shows that when nil or insufficient B

present in the solution, the grown PMN–PT crystals always contain a large

amount of flux inclusions, especially in the initially grown part of the crystals

(Fig. 2.2b). This can be attributed to the high crystallization temperature

associated with the growth solution, such that the crystallization temperature

is higher than the solidus of the binary PMN–PT system (Fig. 2.2a). Under

the said conditions, phase separation is expected; the solidified mass is thus

a mixture of a low-PT solid phase and a high-PT liquid phase.

The problem of phase-separated flux inclusion can be avoided by increasing

the flux content in the starting charge to ensure that crystal growth takes

place predominantly below the solidus of the corresponding binary relaxor–

PT system. This is shown schematically in Fig. 2.2(a). A vertical section of

a PMN–30%PT single crystal grown under such a condition is shown in Fig.

2.2(c). The obtained crystal is free of flux inclusions and fully dense.

The above results show that appropriate flux compositions and correct

solute-to-flux mole ratios play a critical role in relaxor–PT single crystal

growth. This is especially so for the growth of relaxor–PT single crystals of

high PT content, such as PMN–PT. As will be shown below, although typical

solute-to-flux ratios of 0.55:0.45 to 0.30:0.70 have been quoted, it may be

necessary to fine-tune the ratio based on the PT content in the solution to

ensure good crystal growth results.

2.4 Growth of relaxor single crystals of low PT

contents: PZN–(4–7)%PT

The PZN–xPT system has a morphotropic phase boundary at x ≈ 0.09–0.10

(Kuwata et al., 1981; La-Orauttapong et al., 2002). PZN–PT single crystals

of PT contents less than 8 mol% are sought after due to their reasonably

good thermal stability and excellent dielectric and piezoelectric properties

after poling. Compared with PMN–PT crystals, the growth of PZN–(4–7)%PT

single crystals appears to be much less complicated due to its lower PT

contents.

The flux used for the growth of PZN–PT single crystals was PbO. Typical

solute-to-flux mole ratio used was 0.55:0.45. When the degree of

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Flux-growth and characterization of PZN–PT and PMN–PT 45

supersaturation in the solution is low (i.e. at low cooling rates), the growth

of PZN–PT single crystals starts by nucleation at <111> corners followed by

spreading down adjacent <001> edges and (001) crystal faces, as evidenced

in Fig. 2.3(a). This is typical of crystal growth from solution of simple ionic

salts (Van Hook, 1961), suggesting that PZN–PT–PbO solutions with ≤7

mol%PT must be composed of simple ions. This remains so even when the

PT content in the solution is close to the morphotropic phase boundary

composition, i.e. about 9–10 mol% PT.

With an increased degree of supersaturation in the solution, growth of

respective single crystals via nucleation at <001> edges and on {001} faces

becomes feasible, as evidenced in Fig. 2.3(b) and (c). The growth remains

faceted in nature until the supersaturation is sufficiently high to promote

profuse nucleation and growth on {001} faces. When this occurs, (001)

growth facets are gradually replaced with smooth, non-crystallographic crystal

faces. An example of such is shown in Fig. 2.3(d).

2.3

(a) Natural (001) growth faces in flux-grown PZN–(4–9)%PT.

(b) and (c) Nucleation of (001) layer growth at <001> edges and (001)

faces, respectively. (d) Non-crystallographic crystal faces produced

by a fast cooling rate.

(a)

(b)

(c)

(d)

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Handbook of dielectric, piezoelectric and ferroelectric materials46

When first nucleated, PZN–PT single crystals are close to cubic in shape

(Kumar and Lim, 2000; Lim and Rajan, 2004). With favourable isotherms in

the solution, preferential nucleation at a certain or given [111] corner is

promoted such that the corner nucleation rate outpaces the spreading rate

onto adjacent {001} crystal faces. As a result, the small cube-shaped crystal

would evolve into various shapes as it grows, as shown in Fig. 2.4.

It is evident that one could engineer the isotherms in the solution to

control [111] corner nucleation while promoting (001) layer growth. The

crystal growth process in this case will be dominated by the layer growth on

certain (001) crystal planes. Since each (001) layer represents a layer of

material grown within a given time interval, and hence of reduced solute

segregation, wafers cut parallel to the prevalent (001) layer growth plane

would show improved homogeneity in composition. Besides, should the

crystal growth occur in an equilibrium or near-equilibrium condition, such

as with suitably low cooling and hence growth rates, compositional uniformity

between wafers can also be enhanced.

Figure 2.5 shows two PZN–PT single crystals grown using the above

scheme. As-grown PZN–PT single crystals are translucent and light yellow

to brownish-yellow in colour and exhibit prominent (001) facets. Wafers of

0.4mm thickness were sliced from the crystals and the distribution of Curie

temperature (T

) were determined by means of the array dot electrode technique

(Kumar et al., 2003). Figure 2.6(a) shows the distribution of T

within an as-

cut wafer, while Fig. 2.6(b) gives the statistical distribution obtained from

more than 140 trimmed wafers of >20mm edge length. These figures show

that for PZN–PT single crystals of <7mol% PT grown under optimal conditions,

the compositions of the crystals were fairly uniform such that ∆T

< ±2° C

for most part (>75% in volume) of the crystal. This corresponds to <±0.5

2.4

Evolution from (a) near-cubic to (b) star-shaped, (c) arrowhead-

shaped and (d) spearhead-shaped crystal due to fast growth along

certain <111> directions promoted by favorable isotherms in the

solution.

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Flux-growth and characterization of PZN–PT and PMN–PT 47

mol% PT variations in the crystals. The above results show that for relaxor–

PT single crystals of low PT contents, the improved flux growth technique

is a viable means of producing large crystals of high compositional

homogeneity.

2.5 Flux growth of relaxor single crystals of high

PT contents: PMN–(28–34)% PT

PMN–yPT system has a morphotropic phase boundary at y ≈ 0.33–0.35

(Choi et al., 1989; Noblanc et al., 1996). Small PMN–PT single crystals (of

PZN–PT Single Crystals

2.5

Two large PZN–PT single crystals grown from PbO flux.

2.6

Compositional uniformity check of flux-grown PZN–(6–7)%PT

single crystals: (a) distribution of Curie temperature (

) within an as-

cut wafer determined by the array dot electrode technique, the

separation between the dot electrodes is 5mm; (b) statistical

distribution of

obtained from more than 140 wafers of >20mm

edge length prepared from about 10 single crystals grown in the

present work.

161 162 163 164 165 166 167 168 169 170

All with ∆

< ± 2.5 °C within wafers

90% with ∆

< ± 2.0 °C within wafers.

162–167 °C (97%)

165.0

165.1

–

166.0

163.7

164.8

162.0

164.5

168.3

163.0

163.8

164.2

165.0

165.6

163.0

163.9

163.7

163.4

164.3

164.1

163.8

163.2

163.5

162.5

166.9

165.2

165.7

164.7

164.4

(a)

(b)

(°C)

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Handbook of dielectric, piezoelectric and ferroelectric materials48

a few millimetres edge length) can be readily grown from PbO-based fluxes.

They generally exhibit clear (001) growth facets, as shown in Fig. 2.7(a).

However, it may not be as straightforward when growing large PMN–PT

single crystals, because the high PT content of the system produces significant

complications to the flux growth of this material. Our experience has shown

that the growth of PMN–PT single crystals with pure PbO flux often produced

undesirable results (Lim, 2004; Lim et al., 2005). Thus, PbO + B

complex

fluxes were used instead. Even so, the growth results depended sensitively

on the B

content in the PbO solution, as will be detailed below.

First of all, as described earlier in Fig. 2.2, when there is nil or insufficient

in the solution, phase-separated flux inclusions are common features in

flux-grown PMN–PT crystals (Fig. 2.2b). With sufficient B

in the PbO

flux to bring the crystallization temperature below the solidus of the binary

PMN–PT system, the phase-separated flux inclusion problem can be

successfully avoided (Fig. 2.2c). Even so, the amount of B

was found to

significantly affect the compositional segregation of the crystal (Lim, 2004;

Lim et al., 2005). For instance, measurements of T

distributions from wafers

cut from as-grown PMN–PT crystals showed that with less-than-optimum

amounts of B

in PbO flux, the variations within wafers were acceptable

(i.e.

∆

≤±3.0°C) but were too large even between adjacent cuts of wafers

(of about 0.5mm thickness). On the other hand, with higher-than-optimum

amounts of B

, the reverse was observed instead.

The amount of B

in PbO flux also affects the growth mechanism of

PMN–PT crystals. Figures 2.7(b) and (c) show the general morphologies of

large PMN–PT single crystals (i.e. those ≥ 20mm edge length) grown in the

present work. Although all crystals exhibit apparent (001) growth facets,

2.7

Different (001) growth morphologies in flux-grown PMN–30%PT

single crystals: (a) small cubic-shaped crystal of prominent (001)

facets of up to 10mm edge length produced from PbO-based fluxes.

Note the evidence of <111> corner nucleation (arrowed) followed by

<011> edge and (001) layer growth; (b) (001) platelet growth in large

crystals (>20mm edge length) produced from pure PbO flux;

length) produced from PbO-based flux containing >5mol% B

(a)

(b)

(c)

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Flux-growth and characterization of PZN–PT and PMN–PT 49

those grown without or with insufficient B

in the PbO flux have a platelet

morphology (Fig. 2.7b). An obvious change from (001) platelet growth to

microscopic (001) layer growth is evident even with a few mol% of B

(i.e. about 5mol%) added to the PbO flux (Fig. 2.7c).

Figure 2.8 shows the change in nucleation mechanism during growth

from solution with increasing B

content, revealed by deliberately increasing

the cooling rate at the later stage of the crystal growth run. Apparently, with

a low B

content in the PbO flux, the (001) layer growth is initiated via the

nucleation and growth of (001)-oriented crystal blocks on the (001) growth

facets (Fig. 2.8a). On the other hand, with sufficient B

addition, (001)

layer growth occurs through <111> corner nucleation followed by spreading

down the adjacent <001> edges and (001) faces (Fig. 2.8b).

The above observations suggest that ionic complexes formed in the high

temperature solution of PMN–PT–PbO(B

) system play an important role

in the growth of PMN–PT single crystals. Owing to the strong affinity between

and O

2–

ions and the substantial amount of PT present in the PMN–yPT

system studied (y = 0.28–0.34), it is likely that Ti

and O

2–

ions may form

various large ionic complexes or clusters (possibly with some covalent nature)

in the solution. The growth of PMN–PT with insufficient B

addition is

thus controlled by cluster growth of Ti

–O

2–

based complexes. Owing to the

lower diffusivity of the latter in the solution, as opposed to other simpler

ions, the adjacent solution becomes enriched with Ti

–O

2–

complexes, leading

to a significant increase in the PT content of successively grown layers as the

crystal grows in size.

On the other hand, owing to the strong B

–O

2–

bonds and the valency

difference, addition of B

helps modify the nature of the Ti

–O

2–

-based

complexes in the solution. With sufficient B

in the PbO flux, a change

(a)

(b)

2.8

Different nucleation mechanisms in flux growth of PMN–PT single

crystals, revealed by higher cooling rates near the end of the growth

runs: (a) cluster nucleation on (001) facets with less-than-optimum

amounts of B

in PbO flux; (b) <111> corner and <001> edge

nucleation and growth with near-optimum amounts of B

in PbO

flux.

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