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

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Handbook of dielectric, piezoelectric and ferroelectric materials50

from the cluster nucleation on (001) faces (Fig. 2.8a) followed by platelet

growth (Fig. 2.7a) to <111> corner and <001> edge nucleation (Fig. 2.8b)

followed by microscopic (001) layer growth (Fig. 2.7b) takes place.

Since the corner and edge nucleation mechanisms are favoured in the

growth of ionic crystals from solutions of simple ions (Van Hook, 1961), the

addition of B

must have helped break up existing Ti

–O

2–

-based complexes

into simpler or smaller ones comprising, possibly, Ti

, B

and O

2–

ions.

This, in turn, gives rise to a change in the crystal growth mechanism observed.

Note also that when this occurs, the diffusivity of the simpler or smaller

ionic complexes in the solution is substantially enhanced. This, in turn,

would lead to reduced compositional gradients in the grown crystal.

Figure 2.9 shows two large PMN–PT single crystals grown from optimal

fluxes of PbO + zB

, where z ≥ 0.10. The colour of as-grown PMN–PT

crystals varies from brownish-to-greenish yellow. High-quality crystals are

translucent when viewed against the light. Unlike PZN–PT single crystals,

the surfaces of as-grown PMN–PT crystals often show characteristic <001>

domain wall traces criss-crossing one another at right angles (Fig. 2.9, see

also Section 2.6.8).

mapping was obtained with the array dot-electrode technique from

successive (001) wafers cut from a number of large (≥25 mm edge length)

PMN–PT crystals, grown at relatively slow cooling rates of ≤0.8°C/h to

ensure that the growth condition is near equilibrium (Lim, 2004; Lim et al.,

2005). Figure 2.10(a) shows the wafers cut from one of such crystal which

2.9

Two large PMN–PT single crystals grown from PbO + B

complex fluxes.

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Flux-growth and characterization of PZN–PT and PMN–PT 51

was grown from a starting charge composition of 34mol% PT. Figure 2.10(b)

gives the T

distribution in three selected wafers, i.e. W3, W9 and W12. It

shows that the T

distribution is fairly uniform over a large part of the wafers

adjacent to the base of the crystal, i.e. adjacent to the nucleation point (arrowed

in Fig. 2.10b). The T

values for this part of the crystal vary around 140 ±

3°C, corresponding to a composition of about PMN–32% PT. The PT content

in this uniform part of the grown crystal is thus about 2mol% less than that

in the initial charge.

Figure 2.10(b) further shows that the T

values for the outer part of the

wafers, hence the crystal, increases appreciably. This implies that although

the initial part of the grown crystal has a relatively uniform PT content, the

outer part shows a steep increase in PT content. Similar observations were

also made for the two other crystals grown with the same initial charge

composition of 34%PT, of which one was grown under a much lower cooling

rate of about 0.2 °C/h.

146.8

150.3

158.6

143.2

145.5

150.4

142.2

144.1

147.8

141.3

142.2

145.4

140.4

142.2

145.0

150.6

151.6

145.9 145.9 148.5 152.6

142.3 138.6 141.0 141.8

142.2

138.5 141.9 143.1

151.4

139.0 141.3 140.5 148.9

140.3 140.0 140.6

153.1

146.3

–

145.7

149.9

140.1

139.2

140.3

139.6

140.7

152.8

141.3

139.9

147.4

138.6

139.5

139.6

141.1

(b)

(a)

10 mm

2.10

Mapping of ferroelectric phase transition temperature: (a)

0.5mm thick wafers sliced parallel to the prevalent (001) layer growth

plane of a grown PMN–PT crystal; (b)

distributions in wafer nos.

W3, W9 and W12 shown in (a). The dot electrodes are 5mm in

centre-to-centre separation.

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Handbook of dielectric, piezoelectric and ferroelectric materials52

Since the composition remains relatively uniform over a large initial portion

of the grown crystals which is independent of the growth rate used, flux

growth of PMN–PT crystals from optimum PbO–B

fluxes is likely to

occur under equilibrium or near-equilibrium conditions, at least during the

initial to intermediate stage of the growth. Furthermore, since the difference

between the PT content in the initial charge and the grown crystal is not

affected by the growth rate used, it is likely that this difference, i.e. about 2.0

mol% PT, is retained in the solution to maintain the equilibrium of the

complex flux formation reaction. This being the case, one can anticipate that

the grown crystal always has a lower PT content than the charge, regardless

of the initial charge composition and the growth rate. This holds as long as

equilibrium is attained for the complex flux reaction.

The above hypothesis was checked by altering the charge composition to

30% and 32% PT while maintaining the optimum B

content in the flux.

Our aim was to grow PMN–28%PT and PMN-30% PT crystals as our intended

crystal compositions. After the growth, examination of the distribution of T

of the wafers cut from the grown crystals revealed that the crystals were

indeed of 28% PT (T

≈ 125°C) and 30% PT (T

≈ 135°C) as per the

experimental plan. The T

values of the flux-grown PMN–PT single crystals

agree reasonably well with those obtained by Choi et al. (1989) and Noblanc

et al. (1996) on PMN–PT ceramics of controlled compositions.

The above result confirms that the actual fluxes for the growth of PMN–

PT are indeed complex fluxes of PbO + z(B

PT), where

is a function

of B

content in the flux. Furthermore, it suggests that with the established

complex flux, the growth path is nearly vertical under equilibrium or near-

equilibrium growth conditions. This is illustrated schematically in Fig. 2.11.

The present work shows that with optimum B

content in the flux, such

that the large ionic complexes are broken up into simpler ions, high uniformity

PMN–PT single crystals can be grown with PbO-based fluxes when the

growth is allowed to proceed in a near-equilibrium manner. However, as the

growth temperature reduces, the ratio of B

-to–PT in the solution increases

appreciably and the B

-based ionic complexes may reform in the solution.

When this happens, the viscosity of the solution would increase considerably.

The (001) layer growth may then become kinetics-controlled. As a result, the

uniformity in the crystal composition degrades accordingly, and the PT content

increases appreciably in the last-to-grow part of the crystal. This is illustrated

schematically in Fig. 2.11.

2.6 Other commonly encountered problems

In addition to phase-separated flux inclusions and compositional segregation

problems discussed above, other commonly encountered problems during

flux growth of PZN–PT and PMN–PT single crystals include multiple

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Flux-growth and characterization of PZN–PT and PMN–PT 53

nucleation at the cooling point, formation of satellite and parasitic crystals,

side-wall nucleation, flux trappings, cracks, PT–rich surface layer. They are

briefly described below (see also Fig. 2.1b).

2.6.1 Multiple nucleation and satellite crystals

The use of local point cooling arrangement in flux growth does not always

guarantee single crystal nucleation and growth. Instead, multiple crystal

nucleation at the intended cooling point is common. Even if single crystal

nucleation is successful, satellite crystals may nucleate later in the course of

slow cooling, destroying an otherwise perfect crystal growth run. Under less

controlled conditions, the situation could be a lot worse and the end result is

crystals many millimetres in size. The problem of satellite crystal formation

has been traced to limited convection in the solution coupled with a high

L =

PMNT

L +

PbO

PMNT

PMN

PMNT

+ L

Charge

compositions

PbO +

+ δPT)

(PMNT)

xtal

, of δPT lower

than (PMNT)

charge

2.11

Modified phase diagram and growth path for flux growth of

PMN–PT single crystals from PbO–B

-based fluxes. The actual flux

should be PbO +

+ δPT), because a fixed amount of PT,

determined by the amount of B

in the flux, is needed to maintain

the equilibrium of the complex flux formation reaction in the solution

(marked ‘1’ in the figure). The growth path is relatively vertical at

sufficiently high growth temperatures under near-equilibrium growth

conditions (marked ‘2’). As a result, the composition of the initial part

of the grown crystal is about 2mol%PT less than that of the initial

charge (marked ‘3’). The growth path deviates from the vertical line

at low growth temperatures when the growth becomes kinetics

controlled, as indicated by the dashed curve (marked ‘4’).

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Handbook of dielectric, piezoelectric and ferroelectric materials54

cooling rate. Such problems can be alleviated, to a large degree, by improving

the point cooling arrangement and lowering the cooling rate used.

2.6.2 Parasitic crystals

Unlike satellite crystals which nucleate at the periphery of the main crystal

at the base of the Pt crucible, parasitic crystals nucleate and grow onto the

main growing crystal. The cause for the formation of parasitic crystals is the

same as for satellite crystals except that a higher degree of supersaturation is

required for their formation. Similar to satellite crystals, the formation of

parasitic crystals can be suppressed by lowering the cooling rate.

2.6.3 Side-wall nucleation

In conventional flux growth of relaxor–PT single crystals, profuse spontaneous

nucleation on the Pt crucible wall along the meniscus ring often occurs later

in the crystal growth run (Mulvihill et al., 1996; Park et al., 1997; Kumar et

al., 2000, Zhang et al., 2000. Although this may not affect the quality of the

main crystal growing from the base of the Pt crucible, it deprives the latter

of solute that is needed to feed its growth. Suppression of side-wall nucleation

is crucial for the growth of large relaxor–PT single crystals by the flux

growth technique. Side-wall nucleation can be eliminated by ensuring the

top surface of the solution always experiences a temperature greater than

that of the bottom of the crucible (Lim et al., 2004, 2005; Babu et al., 2006).

2.6.4 Pyrochlore crystals

Pyrochlore crystals were observed when inappropriate flux compositions

and/or flux-to-solute ratios were used. They are fairly common features

when the crystal growth run is allowed to proceed to a much lower temperature,

say, <900°C. Under the said condition, the solution becomes exceedingly

enriched in PbO flux, giving rise to conducive conditions for their formation.

They are also found when point nucleation has failed such that a pool of

solution is trapped inside the inner periphery of a ring of interconnected

satellite crystals. They may also form when the crystallization rate is too

high such that the crystal formed may contain a high degree of imperfections.

When this occurs, there is a tendency for the defect-containing crystal to

transform to the pyrochlore phase during the subsequent slow-cooling stage.

Formation of pyrochlore crystals can be avoided when the crystal growth

conditions are carefully controlled to ensure that they are not far from ideal

and when satellite crystal formation has been avoided. Ending the crystal

growth run at a higher temperature also helps in this regard. The use of B

in PbO flux should be avoided in the growth of PZN–PT as it promotes the

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Flux-growth and characterization of PZN–PT and PMN–PT 55

formation of the pyrochlore phase. This agrees with the findings of earlier

researchers (Bokov and Myl’nikova, 1961; Gentil et al., 2000; Yamashita,

quoted in Zhang et al., 2000) although Zhang et al. (2000) reported a different

result.

2.6.5 Flux trappings

Even with optimal flux composition such that the problem of phase-separated

flux inclusion problem is avoided, as-grown relaxor–PT single crystals may

also contain abundant flux inclusions. An example is given in Fig. 2.12(a).

It should be noted that under normal growth conditions, multiple growth

sectors often operate concurrently on a given crystal. At sufficiently high

growth rates, the boundaries where such growth sectors meet are potential

sites for flux trapping. When uncontrolled nucleation and growth on (001)

crystal faces occurs due to a high degree of supersaturation in the solution,

the flux can be easily trapped at the growth front of the solidifying solid. A

vivid example of such is given in Fig. 2.12(b).

2.6.6 Cracks

Relaxor single crystals are brittle and crack easily. A large as-grown single

crystal of relaxor compositions may crack along its periphery if it experiences

mild thermal shock during the crystal growth or retrieval process. When

satellite and/or parasitic crystals are present, the stress concentrations associated

with anisotropic thermal expansion produced by cooling may also induce

cracking of the larger crystal, which often initiates at the apex of the smaller

embedded crystal. The presence of pyrochlore crystals is another main cause

for crack formation, due probably to the large difference in thermal expansion

coefficients between the perovskite PZN–PT and the non-perovskite pyrochlore

2.12

Trapping of flux (a) at growth sector boundary (arrowed) and (b)

at the growth front at high growth rates in PZN–9%PT.

(a)

(b)

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Handbook of dielectric, piezoelectric and ferroelectric materials56

phase during cooling. Cracks in relaxor–PT single crystals may or may not

be crystallographic, especially for periphery cracks, which at times can assume

a smooth hemispherical shape. Crystal cracking problems can be largely

eliminated by avoiding the pyrochlore phase and the formation of satellite

and parasitic crystals during the crystal growth, and by handling the crystal

with care to avoid unnecessary thermal and/or mechanical shock.

2.6.7 Pt inclusions

Although not always, Pt inclusions were at times observed in flux-grown

PMN–PT single crystals produced in our laboratory. They often manifested

as a shining deposit layer on the surface of internal cracks and/or growth

sector boundaries. Their presence may be attributed to the presence of B

in the solution and the higher growth solution temperature used in this case.

In contrast, Pt inclusions were seldom detected in the growth of PZN–PT

single crystal when pure PbO flux and a significantly lower solution temperature

were employed.

2.6.8 PT–rich surface layer with fragile domain walls

As described in Section 2.5, severe segregation occurs at the later stage of

flux growth of PMN–PT single crystals. When growing PMN–PT single

crystals of near-morphotropic phase boundary compositions, this may lead

to the formation of a tetragonal-rich surface layer. This layer is manifested

by the characteristic long and straight <100> domain wall traces criss-crossing

one another at right angles. As shown in Fig. 2.13. When viewed against the

2.13

PT-rich surface layer with prominent but fragile (001) domain

walls (arrowed) in an as-grown PMN–30%PT single crystal.

10 mm

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Flux-growth and characterization of PZN–PT and PMN–PT 57

light, some of these domain walls are highly reflective, suggesting that they

may be cracked. Those intact walls remain fairly fragile and are potential

sites of cracking during subsequent crystal cutting and polishing. Similar

surface layer and characteristic domain walls were seldom observed in flux-

grown PZN–PT single crystals when the starting charge composition was

kept below 8 mol% PT.

2.7 Properties of flux-grown PZN–PT and

PMN–PT single crystals

PZN–xPT and PMN–yPT single crystals with x = 4.5–8.0% and y = 28–32%,

being the final crystal composition determined using their Curie temperatures

(cf. works of Kuwata et al., 1981; Shrout et al., 1990; Noblanc et al., 1996),

were sliced and diced into [100]

/[010]

/[001]

– and k

-plates and k

–

bars, of 7

×2

× 0.5

, 5

× 5

×0.5

and 3

× 3

× 9

dimensions, respectively. To check the effect of length orientation on k

properties, [110]

/[1–10]

/[001]

-plates were also prepared. As for [011]-

poled samples, k

-mode plates of 9.0

× 3.0

× 0.5

in dimensions

were diced from (011) wafers with two different length orientations, i.e.

[100]

/[0–11]

/[011]

and [0–11]

/[100]

/[011]

. The samples were coated

with nichrome–gold/palladium electrodes on appropriate faces. To establish

the optimum poling fields for both the plate and bar samples, the samples

were poled progressively in silicone oil at room temperature from 0.2 to

2.0kV/mm for the k

- (or k

-) and k

-plates and from 0.2 to 0.5 kV/mm for

the k

-bars. After each poling step, the dielectric constant (K

, measured at

1kHz) and electromechanical coupling factors (k

, k

and k

, measured

with the resonance technique) were determined using an Agilent 4294

impedance network analyzer and the piezoelectric coefficients (d

, d

and d

)

were obtained with a Berlincourt-type meter. The poling field giving the best

property values was taken as the optimum poling condition. X-ray diffraction

studies and polarized light microscopy were carried out to identify the phases

present in the poled crystals (Shanthi et al., 2005b; Rajan et al., 2007).

For [001]-poled PZN–4.5%PT and PMN–28%PT single crystals, which

are compositionally far from the MPBs, the poled domain structure is largely

rhombohedral (Kuwata et al., 1981; Shrout et al., 1990; La-Orauttapong et

al., 2002; Lu et al., 2002; Noheda et al., 2002; Rajan et al., 2007). The poled

properties of these crystals are given in columns 2 and 3 of Table 2.1. Such

crystals show respectable properties with K

≈ 5000, d

≈ 2200pC/N, d

≈

−(1000–1100)pC/N, k

≈ 0.90–0.92, k

≈ 0.50 for PZN–4.5%PT; and K

≈

4500–5000, d

≈ 2000 pC/N, d

≈ −1000pC/N, k

≈ 0.90–0.92, k

≈ 0.50

for PMN–28%PT. Most interestingly, insignificant degradation of properties

was observed when the above crystals were poled to 2.0kV/mm. Crystals of

the above two compositions are thus ideal for high field applications. For

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Handbook of dielectric, piezoelectric and ferroelectric materials58

Table 2.1

Properties of flux-grown [001]-poled PZN–PT and PMN–PT single crystals (poled at 0.7–0.8kV/mm for

- and

- plates and 0.4–

0.6 kV/mm for

-bars)

Properties Optimallypoled Over–poled

PZN–4.5%PT PMN–28%PT PZN–(6–7)%PT PMN–30%PT PZN–8%PT PZN–8%PT PMN–32%PT

(°C) ≈156 ≈125 ≈164 ≈135 ≈170 ≈170 ≈145

(°C) ≈ 110 ≈ 95 ≈ 100 ≈ 85 ≈90 70–80 ≈ 80

4500–5500 4500–5500 6800–8000 7500–9000 5500–6500 3300–4500 4500–5500

tanδ <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01

0.90–0.92 0.90–0.92 0.92–0.94 0.92–0.94 0.92–0.94 0.88–0.91 0.91–0.94

0.50–0.52 0.55–0.58 0.52–0.53 0.58–0.62 0.53–0.57 0.53 0.57–0.60

[110]

– – 0.80–0.85 0.80–0.85 – – 0.79–0.81

(

in pC/N) –(1100–1400) (–900) –(750–1000)

(pC/N) 2000–2500 1900–2100 2500–3000 2200–2500 2500–2900 1800–2200 1600–1800

[010]

–1140 –(900–1100) –(1400–1800) –(1100–1400) – – –(350–800)

(pC/N) (

≈ 0.50) (

≈ 0.55) (

≈ 0.50) (

≈ 0.50)

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Flux-growth and characterization of PZN–PT and PMN–PT 59

[011]-poled single crystal plates of [100]

length, we obtained K

≈ 3000,

≈ −1400pC/N and k

= 0.88 for PZN–4.5%PT, which again show negligible

sign of property degradation on being poled to 2.0kV/mm.

Columns 4 and 5 of Table 2.1 give the properties of [001]-poled PZN–(6–

7)%PT and PMN–30%PT in the optimally poled state, which are 3–4 mol%PT

away from the MPBs but have a predominantly rhombohedral domain structure

(Lim and Rajan, 2004; Rajan et al., 2004, 2005, 2007; Lim et al., 2005;

Shanthi et al., 2005a). They exhibit extremely high dielectric and

electromechanical properties, with K

≈ 6800–8000, d

≈ 2800pC/N, d

≈

−(1400–1800)pC/N, k

≈ 0.93–0.94, k

[110]

≈ 0.80–0.85; k

≈ 0.50–0.55

for PZN–(6–7)%PT; and K

=7500–9000, d

=2200–2500pC/N,

= −(1100–1400)pC/N, k

≈ 0.92–0.94, k

[110]

≈ 0.80–0.85; k

≈ 0.58–

0.62 for PMN–(30–31)%PT.

As for [011]-poled crystals, superior dielectric and piezoelectric properties

were obtained for the [100]

-length cut samples, with K

≈ 4800,

= −3000pC/N and k

= 0.91 for PZN-6%PT and K

≈ 5000–6000,

= −(3200–4000)pC/N and k

= 0.92–0.96 for PZN–7%PT. These properties

pertain to the optimally poled samples. Our X-ray and polarized light

microscopy studies further revealed that the superior dielectric and piezoelectric

properties of the [011]-optimally poled PZN–PT can be attributed to the

presence of 10–15% of orthorhombic phase among a rhombohedral matrix

in the material (Rajan et al., 2007). This observation indicates that the

coexistence of both rhombohedral and a metastable phase is responsible for

the superior piezoelectric properties of relaxor–PT single crystals, despite

the metastable phase being a minor phase.

As described above, PZN–PT and PMN–PT may become over-poled at

high poling fields with significant amounts of induced monoclinic or

orthorhombic phase in the material (Rajan et al., 2005; Shanthi et al., 2005a).

This is especially pronounced for [011]-poled crystals of compositions close

to the morphotropic phase boundary (Shanthi et al., 2005a; Rajan et al.,

2007). The [011] poling responses of PZN–PT single crystals of different

compositions are given in Fig. 2.14. It shows that when optimally poled, the

measured properties of k

plates are much superior to the [001]-poled plates.

For instance, [011]-optimally poled PZN–7%PT exhibits d

≈ −4000 pC/N,

which compares favourably to the value of −1800pC/N for [001]-poled

crystals. When poled above 0.6 kV/mm, however, PZN–7%PT single crystal

plates become over-poled and their d

values drop to −(600–700)pC/N

instead. For [011]-poled crystal, the overpoled structure corresponds to a

predominantly orthorhombic state (Lu et al., 2001, 2002; Priya et al., 2002;

Guo et al., 2002a,b; Shanthi et al., 2005a,b; Rajan et al., 2007). The last two

columns of Table 2.2 give the measured properties of PZN–7%PT and PMN–

32%PT in the over-poled state. It is evident that the over-poled properties are

much inferior to those of the optimally poled samples.

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