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

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Handbook of dielectric, piezoelectric and ferroelectric materials340

using the extinction pattern of Fig. 12.2. First, one of the crossed P/A pair

axes at reading 0° was aligned with one sample edge, such as [110] or [100].

The extinction angles therefore were measured with respect to the [110] or

the [100] directions. The solid crosses within the symbols in Fig. 12.2 represent

the orientation of the extinction. For instance, if the (001)-cut sample’s [110]

edge is aligned with P/A: 0°, R phase domains represented by triangles with

solid crosses thus have extinction at 0° (or 90°). T phase domains represented

by squares with solid crosses have extinction at 45°. T phase domains with

polarization P along the [001] axis represented by the solid black square in

the center have extinction at every orientation of the crossed P/A pair and are

written as ‘T

001

’ in the following discussion. The O phase domains represented

by circles with solid crosses have extinctions at 0° (or 90°) or 45°, whereas

R domains only give extinctions at 0°. Any extinctions at angles other than

0° (or 90°) and 45° must be from the M or Tri phase domains. Note that all

the phases except Tri phase have been reported in various PMN–PT crystals.

The large observed variation in extinction angle with E field or temperature

in some of our experiments indicates M domains whose polarization P can

vary with E field or temperature through a large angle, whereas the polarization

directions are nearly fixed for R, T, or O domains as E field or temperature

varies.

12.3

Relation between the optical extinction orientations

corresponding to the ferroelectric polarization directions for various

phases and domains projected on the (110) plane (adapted from

Chien

et al

., 2005b).

[111]

[110] [111]

[001]

[111]

[001][111]

[111]

[110]

[100]

[110]

[010]

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Electric field-induced domain structures and phase transitions 341

12.4 Thermal stability for various PMN–PT

compositions

12.4.1 (001)-cut PMN–

PT crystals (

= 0.24, 0.26, 0.27,

0.29, 0.35, 0.38)

To enhance piezoelectric performance, a prior E-field poling process has

usually been done before employing these materials in applications. However,

how prior E-field poling affects phase thermal stability still remains unclear.

In this section, dielectric permittivity, electric polarization, domain structure,

and X-ray spectrum were measured as functions of Ti content, temperature,

and poling strength for (001)-cut crystals before and after a dc E-field poling.

Figure 12.5(a) shows dielectric results of ZFH and FC-ZFH for PMN–

24%PT. The maximum temperature T

was shifted a few degrees higher in

the FC-ZFH. Besides a broad maximum near 390K an extra peak appears

near 370K in the FC-ZFH, whose position (but not the amplitude) is

independent of frequency as seen in the inset. The P

also exhibits an abrupt

decline near 370K. The coercive field E

is ~3.8kV/cm at room temperature.

12.4

Relation between the optical extinction orientations

corresponding to the ferroelectric polarization directions for various

phases and domains projected on the (111) plane (adapted from Tu

et al

., 2003b).

[111]

[001]

[101]

[111]

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Handbook of dielectric, piezoelectric and ferroelectric materials342

ZFH domain structures after FC are shown in Fig. 12.6(a). ‘N’ indicates

the area without ITO films. When observing the (001)-cut sample along

[001] between a crossed P/A pair with the <110> sample edge aligned with

P/A: 0°, the optical extinction angle of the R phase is 0°. As seen in the inset

which shows extinction at 0°, the symmetry of unpoled PMN-24%PT is

mostly R at 298K. After the FC, besides the R domains at 298K a small

fraction of the domain matrix exhibits extinction in the range of ~20–70°,

indicating M domains (perhaps mixed with T domains whose extinction

angle is 45°). As shown in Fig. 12.7(a), the unpoled <002> X-ray diffraction

taken at room temperature shows a single peak, indicating the R phase even

after poling at E = 6 kV/cm. Near 371K M domains associated with extinction

Temperature (K)

300 350 400 450

Real part of dielectric permittivity ε′ (×10

)

(d)

ZFH

FR-ZFH (2kV/cm)

29%

(e)

ZFH

FR-ZFH (6kV/cm)

35%

(K)

300 400

(µC/cm

)

(K)

180200 220 240

0.3

0.2

0.1

38%

35%

38%

FR-ZFH

(K)

300 350

(µC/cm

)

Temperature (K)

250 300 350 400

Real part of dielectric permittivity ε′ (×10

)

1.5

1.0

0.5

(K)

300 350 400

27%

(µC/cm

)

(K)

300 350 400

(b)

26%

(kV/cm)

(µC/cm

)

ZFH

= 25kV/cm

= 15kV/cm

= 5kV/cm

FC-ZFH

(K)

300

350 400

(µC/cm

)

(a)

ZFH FC-ZFH (6kV/cm)

24%

1.0

0.5

ε′ (×10

)

360

380 400

0.5

100kHz

12.5

Dielectric permittivity and remanent polarization for (001)-cut

PMN–

PT crystals,

= (a) 24%, (b) 26%, (c) 27%, (d) 29%, and (e)

35% (adapted from Tu

et al.

, 2004).

(4 kV/cm)

ε′ (×10

)

ZFH

38%

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Electric field-induced domain structures and phase transitions 343

<110>(a) 24% FC-ZFH (7kV/cm)

<110>

<100>

(b) 26% FR-ZFH (5kV/cm)

(d) 35% FR-ZFH (6kV/cm)

12.6

Domain structures. Angles of the P/A pair are with regard to the

[110] for PMN–24%PT, 26%PT, and 29%PT and [100] for 35%PT

crystals (adapted from Tu

et al

., 2004). A color version of the figure

can be found in Tu

et al.

, 2004.

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Handbook of dielectric, piezoelectric and ferroelectric materials344

angles at ~20–70° expand widely in the domain matrix. This anomaly was

also seen in the FR-ZFH (Fig. 12.6a), suggesting that FR and FC have the

same poling effect. The crystal becomes cubic phase near 383K. Thus, after

poling the PMN–24%PT undergoes a R(M)→M(R)→C transition sequence

near 370 and 383K upon heating. ‘R(M)’ represents that dominant R domains

coexist with a smaller fraction of M domains.

More complicated dielectric anomalies were seen in PMN–26%PT.

Compared with the ZFH, in the FR-ZFH two other anomalies, as shown in

Fig. 12.5(b), were observed near 370–376K and ~385K (as indicated by an

(a) 24%

(b) 26%

(d) 38%

<002>

Unpoled

= 6kV/cm

Unpoled

= 6kV/cm

Unpoled

= 5kV/cm

Unpoled

= 6kV/cm

2θ (degree)

45.545.044.544.043.0 43.5

Intensity (arb. units)

12.7

<002> X-ray diffraction spectra taken at room temperature for

unpoled and poled (001)-cut PMN–

PT crystals,

= (a) 24%, (b) 26%,

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Electric field-induced domain structures and phase transitions 345

arrow) which were shifted to lower temperatures with increasing E field.

The minimum E field to induce this behavior is ~1.0 kV/cm which is smaller

than room temperature E

~2.5kV/cm. As E field increases, the anomaly

seen near 370–376 K becomes a single peak and rather weaker. As shown in

Fig. 12.5(b), P

and E

also exhibit a peak near 370K. Similar dielectric

anomalies were observed in the FC–ZFH. ZFH domain structures after poling

are shown in Fig. 12.6(b). The unpoled sample shows optical extinction at 0°

at 298K, indicating R domains. In the FR-ZFH, besides the dominant R

domains, a very small fraction of M domain (with extinction angles of ~0-

10°) was observed at 298K. Near 378K the M domains dramatically expand

with extinction angles of ~0–15°. Near 388K some domains show extinction

at 45°, indicating T domains. The crystal becomes cubic near 393K. Similar

domain anomalies were seen in the FR-ZFH (10 kV/cm). It is important to

note that the ‘370 K’ anomaly was also observed in the pure ZFH domain

observation, but not as apparent as the FR-ZFH. It implies that a prior poling

can reveal a ‘hidden’ transition which is not obvious in an unpoled sample.

The unpoled <002> X-ray data (Fig. 12.7b) show a broad peak and a weak

shoulder, which are similar with the <002> synchrotron X-ray profile (Noheda

et al., 2002) and probably correspond to R and M phases respectively. The

M phase becomes more pronounced after poling at E = 5kV/cm. Thus, after

poling a R(M)→M(R)→M(T)→C transition sequence occurs in PMN–26%PT

near 370–376, 388, and 393K upon heating.

Similar dielectric anomalies were observed in PMN–27%PT as given in

Fig. 12.5(c). In the FC-ZFH two clear frequency-independent anomalies (as

marked by dashed lines) occurred near 370 and 392K. The P

also shows a

maximum peak and a rapid decline near 370 and 392 K. After poling the

PMN–27%PT probably goes through a R(M)→M(R)→M(T)→C transition

sequence near 370, 392, and 400K respectively upon heating.

Figure 12.5(d) shows ZFH and FR-ZFH dielectric results for PMN–29%PT.

Besides a broad maximum near 407K, two extra peaks appear near 358

and 368K after poling. The minimum E field to induce this anomaly is

~2.0kV/cm which is smaller than room temperature E

~3.3 kV/cm. The P

exhibits an abrupt decline near 365K. As given in Fig. 12.6(c), at 298K

domains show extinction in the range of ~40°–80°, indicating mostly M

domains. At 371K, besides a small fraction of M domains associated with

extinction at ~60–80°, most domains exhibit extinction at 45°, which indicates

T phase domains. In other words, an M(T)→T(M) transition occurs near

370 K. A similar anomaly was seen in the pure ZFH domain observation. An

extra peak that occurred near 358K and weak shoulders that appeared in the

region of 400–407K, as seen in Fig. 12.5(d), are probably due to phase

segregation.

Instead of a gradual climb up in the ZFH, the FR-ZFH of PMN–35%PT

in Fig. 12.5(e) exhibits a step-like decline near 200–210K. ZFH domain

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Handbook of dielectric, piezoelectric and ferroelectric materials346

structures observed after poling are shown in Fig. 12.5(d) for 198 and 208K.

Below 198K domains exhibit extinction from 0 to 10°, but near 208K the

extinction range becomes twice as wide, i.e. 0–20°. A similar anomaly was

seen in the pure ZFH domain observation. The 90° domain walls between

two domains with their T-phase polar axes along [100] and [010] were seen

before the FR process but disappeared after poling. T and M

-type M domains

have extinction at 0° with regard to [100]. The broad extinction angles (0–

20°) imply that polarization directions of M domains are close to [001] T

polarization. The crystal reaches mostly total extinction at E = 9kV/cm.

The unpoled <002> X-ray diffraction [Fig. 12.7c) for PMN–35%PT exhibits

at least four peaks. A dominant T phase mixed with M phase was found in

the unpoled PMN–35%PT ceramics (Noheda et al., 2002). Thus, these peaks

probably are associated with T and M phase domains. After poling, a strong

broad peak and a weak shoulder were observed, perhaps indicating two main

T and M phase domains. Based on the above evidence, a long-range

M(T)→T(M) transition probably takes place in the region of 200–210K

after poling. The crystal becomes cubic near 440K, where the P

exhibits a

rapid decline.

A step-like decline was seen near 210K in the FR–ZFH of PMN-38%PT

as shown in Fig. 12.5(e). This anomaly was not observed for E ≤ 5 kV/cm

and E

is ~6 kV/cm at room temperature. From the ZFH domain observation,

a coexistence of T and M domains was seen in a PMN–38%PT crystal in

which T domains increase rapidly from 200 to 300 K. Two peaks were seen

in the unpoled <002> X-ray diffraction of Fig. 12.7(d), perhaps indicating a

coexistence of T and M domains. After poling, only a broad peak was observed.

Note that the E field was along the [001] T polar direction. Thus, the dielectric

anomaly seen near 210K in the FR–ZFH most likely indicates a long-range

T(M)→T transition.

Briefly, phase thermal stability of (001)-cut PMN–PT crystals after a

prior E-field poling strongly depends on Ti content and field strength. The

‘370 K’ dielectric anomaly was seen in PMN–24%PT, PMN–26%PT, and

PMN-27%PT, and correlates to a R(M)→M(R) transition. FC and FR have

the same effect on phase transitions as seen upon subsequent ZFH. For a

differently oriented compound, similar dielectric anomalies after a prior

field poling were also seen in (110)-cut PMN–26%PT as shown in Fig. 12.8.

An extra dielectric anomaly ‘triggered’ by a prior field poling seems to be a

common phenomenon in PMN–PT (Chien et al., 2005b) and PZN–PT crystals.

However, this extra dielectric anomaly can be easily erased by thermal annealing

in the C phase.

Figure 12.9 shows a phase diagram based on the above evidence, in which

the phase boundaries may be changed with stronger E field or different

crystallographic orientation. It is very different from the phase diagram for

unpoled PMN–PT crystals, in which the M phase was not found. An M

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Electric field-induced domain structures and phase transitions 347

phase was found to coexist with R, T, or O phases in unpoled PMN–xPT

ceramics for 31 ≤ x ≤ 37 (Noheda et al., 2002). M

and M

phases were also

found in unpoled PMN–xPT ceramics for 27 ≤ x ≤ 30 and 31 ≤ x ≤ 34

respectively. Regarding the prior poling effect in PMN–PT crystals, more

details and references can be found in Tu et al. (2004).

Temperature (K)

250 300 350 400

ε′ (×10

)

(110) 26%

ZFH

FR-ZFH

= 0.75

= 12

= 20kV/cm

(K)

300 350 400

(µC/cm

)

12.8

Dielectric permittivity and remanent polarization for (110)-cut

PMN–26%PT crystal (adapted from Chien

et al.

, 2005b).

RMT

Ti concentration (%)

4035302520

Temperature (K)

450

400

350

300

250

200

12.9

Phase diagram of (001)-cut PMN–PT crystals after poling.

Triangle and square symbols represent R(M)→M(R) and M(T)→T(M)

transitions respectively. Lines are estimated boundaries for various

dominant phases (adapted from Tu

et al.

, 2004).

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Handbook of dielectric, piezoelectric and ferroelectric materials348

12.4.2 WO

doped PMN–PT crystals

Recently, an unpoled PMN–PT single crystal with 0.5 mole% WO

in the

growth solution exhibited an interesting wavelength-dependent photovoltaic

response after a prior E-field poling (Tu et al., 2006a).

In addition, WO

doped PMN–PT crystals show higher dielectric permittivities compared with

the pure PMN–PT crystals (without tungsten dopant). Figure 12.10 illustrates

temperature-dependent dielectric permittivities from different layers of the

same WO

-doped unpoled crystals. It shows obvious PT segregation with

different dielectric maximum temperature T

. Dielectric permittivity exhibits

a wide range of thermal stability (below the cubic phase) and can reach ∼10

at room temperature for some compounds, which are promising for piezoelectric

applications.

Figure 12.11 shows hysteresis loops (polarization vs. E field) from WO

doped PMN–PT crystals (#1 and #2) and an undoped PMN–PT compound.

The spontaneous and remanent polarizations and the coercive field (E

) are

smaller compared with the undoped crystal. Such significant reduction of

electric polarization may be due to a tendency toward a different structure

near tungsten ions. Pure WO

has a monoclinic ferroelectric phase (Jona and

Shirane, 1962).

1. 444K 36.2%

2. 439K 35.2%

3. 446K 36.6%

4. 440K 35.4%

5. 454K 38.2%

6. 451K 37.6%

7. 460K 39.4%

%PT

0.5 mole% WO

doped PMN–PT single crystals

Temperature (K)

450400350300250200150

ε′ (×10

)

12.10

Dielectric permittivities of 0.5 mole% WO

doped PMN–PT

crystals taken at

= 10kHz upon heating without any

-field poling.

Crystals #1–6 are from different layers of the same crystal. Crystal #7

is without WO

dopant.

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Electric field-induced domain structures and phase transitions 349

12.5 Field-dependent domain structures of various

PMN–PT compositions

12.5.1 (001)-cut PMN–24% PT crystal

Figure 12.12 shows the E-field-dependent domain structures taken at P/A:

45° in a PMN–24%PT crystal as a dc E field is applied along [001]. Without

E-field poling, the whole domain matrix exhibits extinction at P/A: 0° with

respect to the [110] direction as shown in the inset of Fig. 12.12(a). When

observing the (001)-cut sample along the [001] direction between a crossed

P/A pair, as shown in Fig. 12.2, the extinction angle is 0° (or 90°) for all R

domains. Thus, domains in the PMN–24%PT crystal are certainly R phase at

E = 0kV/cm. In addition, there is no evidence for the O phase in the PMN–

24%PT at zero E field.

Near E = 4kV/cm (Fig. 12.12b), the domain matrix begins to exhibit

change and some domains show extinction at all P/A angles, indicating a T

phase with polarization along the [001] direction labeled as T

001

in the

micrograph. This is consistent with the coercive field E

∼3.8kV/cm as shown

in Fig. 12.13. The T

001

domain corresponds to the black square in Fig. 12.2.

The rest of the domain matrix exhibits R phase with extinction at 0°. As the

field increases, the T

001

phases gradually expand in the domain matrix as

shown in Fig. 12.12(c) and (d). Above E∼30kV/cm (Fig. 12.12e), the T

001

phase rapidly spreads into the domain matrix and forms network-like [001]

T domain chains, indicating a long-range order of the [001] T phase. Most

domains still maintain extinction angle at 0° except for the network-like T

001

Without dopant

#1 WO

doped sample

#2 WO

doped sample

–15 –10 0 5 10 15

–20

–10

(kV/cm)

(µC/cm

)

12.11

Hysteresis loops of WO

-doped PMN–PT crystals (#1 and #2),

and the pure PMN–PT crystal taken at room temperature.

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