High resolution x ray diffraction study of phase and domain structures and thermally induced phase transformations in PZN (4 5 9)%PT 9

For instance, while the MPB appeared in the first phase diagram of PZN-PT single crystal [10] has been represented by a steep near-vertical boundary separating the R and T phases, more

Chapter 9 Revised Phase Diagram for PZN-PT and Other Observations

9.1 Revised phase diagram of PZN-PT system

The phase diagrams of PZN-PT system has been studied by earlier researchers

The most remarkable feature of relaxor ferroelectric PZN-PT and PMN-PT single

crystals is that near the MPBs, the poled crystals exhibit excellent dielectric and

piezoelectric properties [1, 12, 89-91, 103, 104] Despite decades of studies, there have

been debates about the nature of MPBs in relaxor single crystals For instance, while

the MPB appeared in the first phase diagram of PZN-PT single crystal [10] has been

represented by a steep near-vertical boundary separating the R and T phases, more

recent PZN-xPT phase diagrams portray the MPB as a narrow region with 8% ≤ x ≤

11%, which is occupied by either the M or O phase [27, 59] It is also interesting to

note that neither M nor O phases were observed in the present work in unpoled

PZN-(4.5-9)%PT single crystals (see Chapters 6 to 8 for details)

Based on the new results obtained in the present work, the phase diagram of

PZN-xPT system has been amended as shown in Figure 9.1 In the revised phase

diagram, the MPB consists of a (R+T) two-phase mixture spanning from x ≈ 0.09 to x

≈ 0.10 at room temperature In this MPB region, both the R and the T phases are

thermodynamically stable phases of comparable Gibb’s free energies

More interestingly, the present work has shown that due to the presence of

residual stresses in the crystal, the MPB, in effect, has been extended to much lower

PT level For instance, a mixture of (R+Tσ) phases are detected in PZN-(6-8)%PT at

room temperature, where Tσ denotes metastable T micro- and nanotwin domains

stabilized by the residual stress in the crystal This lower PT boundary of the extended

MPB is shaded in red in Figure 9.1 for easy reference

It should be noted since the Tσ phase is stabilized by the residual stress in the

crystal, on pulverizing the crystal into powders of sufficiently smaller particle sizes,

the residual stress is removed and the Tσ phase may no longer be present This may

explain why the extended MPB region is missing in the phase diagrams constructed by

earlier researchers who used the powder samples in their investigations

In the present work, neither M nor O phase has been detected in unpoled

PZN-(4.5-10.5)%PT single crystals Although out-of plane diffractions at ∆ω ≠ 0º were

detected, they have been found to arise from either R* or T* domains in the crystal

The formation of micro- and nanotwin domains in perovskite structure may serve as a

means to relax the transformation stress in the crystal

Another interesting finding of the present work is the (T+C) two phase

coexistence region at high temperatures, shaded in blue in Figure 9.1 This high

Figure 9.1 The revised phase diagram of PZN-PT system with extended

(R+T) MPB region The extended (R+T) MPB region can be divided into two regions In the lower PT region, 6% ≤ x ≤ 8%, the T phase is metastable, denoted by T σ. The Tσ is stabilized by

the residual stresses in the crystal In the high PT region, 9% ≤ x

≤ 10.5%, both the R and T are thermodynamically stable at room temperature A two-phase (T+C) coexistence region was

detected at high temperature before the crystal transforms to a

single C phase

- present work - Ref [10]

%PT

o C)

40 60 80 100

120

140

160

180

200

R

C

T

R+Tσ

T+C

R+T

temperature (T+C) two-phase region has not been reported thus far

It should be noted that over the entire MPB (standard + extended) region where

(R+T) coexist, the crystal exhibits extremely high piezoelectric properties This

suggests that the presence of (R+T) phase mixture in the form of micro/nanotwin

domain structure play an important role to the superior electromechanical properties of

PZN-PT and other relaxor crystals

9.2 Room temperature phase of PZN-PT single crystals of different PT contents

Figure 9.2 shows the (002) RSMs of PZN-(4.5-8)%PT single crystals at room

temperature (i.e., 25 °C) as a function of PT content The mappings exhibit a broad

single diffraction peak lying in the ω = 0 º plane at 2θ ≈ 44.58º, ≈44.64º, and ≈44.65º

for PZN-4.5%PT, PZN-7%PT and PZN-8%PT, respectively The dominant phase at

room temperature in these crystals is thus the R phase of micro/nanotwin structure

Samples showing similar RSMs were ground into powder of <38 µm in

particle size and standard x-ray diffraction was performed on it The result confirmed

that the room temperature phase for PZN-(4.5-8)%PT is indeed the R phase The lattice

constants of the R phase was deduced from the peak positions of the powdered samples

as a function of PT content The results are given in Table 9.1

Table 9.1 Deduced lattice constants for the R phase at room conditions

PT Content Lattice constant,

Figure 9.2 HR-XRD diffraction patterns of unpoled (a) PZN-4.5%PT, (b)

PZN-7%PT, and (c) PZN-8%PT at room temperature The aR and

It is interesting to note from Figure 9.2 that the FWHM of the R for

PZN-8%PT is a lot smaller than the corresponding peak for PZN-(4.5-7)%PT crystals As

discussed in Section 7.2, due to the tilting of the four degenerate R domain variants

both in the 2θ and ω planes, the actual diffractions of the domain variants are outside

the detection range of the high-resolution diffractometer used In fact, the diffractions

detected in the HR-XRD are either: (a) the convoluted peaks of the {100}-type R

micro- and nanotwin if they lie out of the ω = 0° plane, (b) the convoluted peaks as

well as nanotwin diffractions (i.e., the constructive interference peak of the streaked

nanotwin peaks) of the {110}-type R nanotwins if the diffraction is of considerably

smaller FWHM and lies in the ω = 0°, or (c) the convoluted peak of both {100}-type

and {110}-type R micro- and nanotwin mixture if the diffraction is a very broad single

peak lying in the ω = 0° plane The RSMs shown in Figure 9.2 strongly indicates that

with lower PT contents (i.e., 0.045 ≤ x ≤ 0.07), the R phase has a mixed micro- and

nanotwin structure of both {100}-type and {110}-type In contrast, at high PT content

(i.e., x ≈ 0.08), the R phase is made up predominantly of {110}-type micro/nanotwin,

as manifested by the single R peak of much smaller FWHM

9.3 Effect of poling on rhombohedral and tetragonal domains in PZN-PT

Figure 9.3(a) shows the (002) room temperature HR-XRD RSM of

PZN-4.5%PT and PZN-7%PT single crystals after optimal poling (at 0.5 and

0.7 kV/mm at room temperature, respectively) Both figures reveal a broad peak at 2θ

≈ 44.57-44.65° lying in the ω = 0º plane This broad diffraction pertains to that of the R

phase On comparing to the unpoled samples (Figures 9.3c and d), the R (002)

diffraction of the poled samples appeared slightly elongated in certain crystal

directions This, with the slight tilting of the R diffraction out of the ω = 0° plane

indicates that both crystals existed in a strained state as the result of poling induced

stresses This finding is supported by the work Liu and Lynch [105], who noted that

the crystal is in a strained state after poling This result shows that, on being optimally

poled, PZN-(4.5-8)%PT single crystals remain in the R state but possess a higher

residual strain The effect of poling on the domain switching in PZN-9%PT is shown

in Figure 9.4 It shows that both the (R+T) phases coexisted prior to the E-field

application Upon increasing the E-field along the (001)pc crystal direction, the

intensity of (001)T domains increased accordingly (Figure 9.4b) Meanwhile, the (100)T

domains faded away and disappeared at E = 0.8 kV/mm (Figure 9.4c) The above

phenomenon can be understood readily Since the poling field direction is normal to

(001)pc plane, switching of the (100)T domains to the (001)T domains is promoted by

the applied E-field, causing the crystal to become poled (i.e., with all the domains

aligned in the [001]pc E-field direction) eventually

Figure 9.3 Room temperature (002) RSMs of annealed-and-optimally-poled

(a) PZN-4.5%PT and (b) PZN-7%PT single crystals The poling field is normal to the (001)pc diffraction plane The intensity contours are on log scale (c) and (d) show the RSMs of their unpoled counterparts for comparison purposes

∆ω

∆ω

∆ω

∆ω

Figure 9.4 E-field dependent (002) RSMs taken at (a) 0 kV/mm, (b) 0.5

kV/mm, and (c) 0.8 kV/mm from the fractured surface of

[001]-poled PZN-9%PT The E-field dependent RSM reveals an (R+T)–

T transformation under an E-field application along [001]pc

direction The intensity contours are on log scale

(a) E = 0 kV/mm

(b) E = 0.5 kV/mm

(c) E = 0.8 kV/mm

9.4 Depolarization phenomenon in poled PZN-PT single crystals

In this work, dynamic depolarization behaviors and structural evolution as a

function of temperature below 140 °C were studied to provide a better understanding

of the depolarization and R-T phase transformation events in poled PZN-PT single

crystals PZN-4.5%PT and PZN-7%PT single crystals, of [100]L×[010]W×[001]T in

orientation and 6L×2W×0.35T mm3 in dimensions, were investigated They were first

annealed at 257 °C for 1 h and then poled at room temperature to optimum properties

(i.e., at 0.5-0.7 kV/mm) Prior-poling-zero-field-heating (PP-ZFH) thermal current

measurements were carried out The results are given in Figure 9.5 No apparent

current signals were detected until about 105 ºC for PZN-4.5%PT and until 92 ºC for

PZN-7%PT, above which a strong current signal was detected in both crystals This is

followed by a string of much weaker current signals over the next 10 ºC to 15 ºC

The above-described current activities indicate changes in the state of bound

surface and volume charges arising from changes in local domain polarizations [13-16,

71, 75, 79, 106, 107] Three phenomena that could lead to the changes in local domain

configurations and the observed thermal current activities are: (a) depolarization

without R-T phase phenomenon, i.e., from poled R domains to depolarized R domains,

(b) merely R-T transformation, i.e., from poled R to poled T state followed by

depolarization in the T state on further heating, and (c) R-T phase transformation

accompanied by simultaneous depolarization, i.e., from poled R domains to

depolarized T domains The “TR-T” of [001]-annealed-and-poled PZN-4.5%PT and

PZN-7%PT single crystals were determined conventionally from the ε’-temperature

plot shown in Figure 9.6, giving “TR-T” ≅ 105°C for 4.5%PT and ≅95°C for

PZN-7%PT A question arises as whether the “TR-T” determined by the above simple

technique corresponds to either depolarization or R-T transformation phenomenon, or

to both phenomena occurring simultaneously (As it will be shown later, the “TR-T”

determined using the technique described above is in fact the lower bound value of the

rhombohedral-to-tetragonal temperature, hereafter referred to as TR-T(L))

The poled samples were further fractured to expose the relatively strain-free

(001) surfaces for HR-XRD measurements Figure 9.7 gives the (002) RSMs of

[001]-annealed-and- poled PZN-4.5%PT in the region of 100-135 ºC, while Figure 9.8 shows

the same but for poled PZN-7%PT in the region of 105-115 ºC The diffractions at 2θ ≈

44.50-44.60° are the the main R peak in both crystals In addition to this main peak,

indications of possible T diffractions (at 2θ ≈ 44.45° and 2θ ≈ 44.60° for PZN-4.5%PT

and at 2θ ≈ 44.83° for PZN-7%PT) could be vaguely detected when the samples were

heated to about 100-105 ºC The R-T transformation process continued over the next

10 to 35 ºC Only the T phase was detected at 135 ºC for PZN-4.5%PT (Figure 9.7c)

and at 115 ºC for PZN-7%PT (Figure 9.8c) The present work indicates that the R-T