High resolution x ray diffraction study of phase and domain structures and thermally induced phase transformations in PZN (4 5 9)%PT 10

transformation occurs over a range of temperature involving concurrent formation of R * and T* domains.. The effect of heating to higher temperature on the dielectric behaviour and the r

transformation occurs over a range of temperature involving concurrent formation of

R * and T* domains

The effect of heating to higher temperature on the dielectric behaviour and

the room temperature piezoelectric properties after each heating-cooling cycle were

then investigated The results showed that, for PZN-4.5%PT, perceptible sign of

dielectric hysteresis loop could be noted (Figure 9.9) and that the resultant room

temperature piezoelectric properties (i.e., dielectric constant (K T) and

electromechanical constant (k 31)) started to degrade after the sample was heated above

105°C (Figure 9.10) From this figure, the T DP of PZN-4.5%PT is estimated at about

105°C above which perceptible property degradation begins The T DP of PZN-7%PT

was determined similarly, giving T DP ≅ 95 °C

The T DP values for PZN-4.5%PT and PZN-7%PT determined as described

above agree with the T R-T (L) obtained from the first anomaly in ε’ given in Figure 9.6

It also corroborates with our HD-XRD results to within 10 °C As shown in Figure

9.7(b), emergence of (100)T domains were noted at about 105 °C The emergence of

(100)T domains not only indicates R-T transformation but also concurrent thermal

depolarization of the crystal Should R-T transformation occur without thermal

depoling, as stated in case (b) above, then only (001)T domains would be detected in

the (002) mappings instead This is because in the poled condition, c-axis hence (001)T

J

2 )

-2 -1 0 1 2 3

100 110 120

40 60 80 100 120 140 160 180 200 220

2 )

-1 0 1 2

80 90 100 110 120 b)

Figure 9.5 PP-ZFH J of the [001]-annealed-and-poled (a) PZN-4.5%PT

and (b) PZN-7%PT single crystals

Figure 9.6 PP-ZFH ε’ curves of the [001]-annealed-and-poled PZN-4.5%PT

and PZN-7%PT single crystals from which the “T R-T” was determined from the first anomaly

ε' (x1

4 )

0 1 2 3 4 5

TR -T(L )

PZ N -7% PT

PZ N -4.5% PT

Figure 9.7 Temperature dependent (002) RSMs taken from fractured surfaces

of the [001]-annealed-and-poled PZN-4.5%PT single crystal: (a)

100 ºC, (b) 125 ºC, and (c) 135 ºC The intensity contours are in

log scale T# indicates the vague T diffractions T NT denotes the T

at ∆ω ≠ 0º

(c) 135ºC: T+T*

(b) 125ºC: R*+T+T*

Figure 9.8 Temperature dependent (002) RSMs taken from fractured surfaces

of the [001]-annealed-and-poled PZN-7%PT single crystal: (a)

105 ºC, (b) 110 ºC, and (c) 115 ºC The intensity contours are in

log scale T# indicates the vague T diffractions T NT denotes the T

at ∆ω ≠ 0º

Figure 9.9 Dielectric hysteresis behaviors of [001]-poled PZN-4.5%PT

recorded during the heating-cooling cycles to (a) 100 ºC, (b) 105 ºC, (c) 115 ºC, and (d) 125 ºC, respectively After heating to 105 ºC, PZN-4.5%PT showed clear signs of hysteresis on cooling to room temperature Above this temperature, the area of the dielectric hysteresis increases with increasing heating temperature The heating and cooling rate is 1.5 ºC/min

4 )

4 )

0 1

2 (a) 100oC

Temperature (oC)

20 40 60 80 100 120 140

4 )

0 1 2

3 (d) 125oC

(b) 105oC

4 )

0 1 2

0 1 2

3 (c) 115oC

Figure 9.10 (a) K T and (b) k 31 of four plate samples of [001]-poled

PZN-4.5%PT taken at room temperature after cooling from the

temperatures indicated on the x-axis Both the K T and k 31 of PZN-4.5%PT started to degrade after it was heated to 105 ºC

T (x10

3 )

2 4

k 31

0.2 0.4

domains are parallel to the E-field direction which is normal to the fractured surface

being x-rayed

Our results thus indicate that thermal depolarization occurs with the

emergence of R-T phase transformation in PZN-PT single crystals, giving T R-T (L) ≅

T DP ≅105-110 °C for PZN-4.5%PT and ≅90-100 °C for PZN-7%PT These results,

including the upper bound values of T R-T , i.e., T R-T (U) or the temperature at which

complete R-T transformation was detected from our HR-XRD studies, are summarized

in Table 9.2 The slight difference in the various temperature values can be attributed

to the different temperature measurement instruments used in the tests

In summary, the R-T transformation in relaxor single crystals occurs over a

temperature range, manifested by the coexistence of R* and T* domains, a string of

thermal current signals, and continued degradation of K T and k 31 The T R-T determined

from the conventional ε’-temperature curve corresponds to T R-T (L), which marks the

beginning of R-T phase transformation with concurrent thermal depolarization The T

R-T (L) can thus be taken as the T DP of the crystal above which perceptible property

degradation begins Thermal current and HR-XRD results suggest T R-T≅ 100-135 °C

for [001]-annealed-and-poled PZN-4.5%PT and 95-115 °C for [001]-annealed-and-

poled PZN-7%PT single crystals

Table 9.2 T DP , T R-T (L), and T R-T (U) of [001]-poled PZN-PT single crystals

PT (%) T max (oC) T DP (oC) T RT (L) (oC) T RT (U)(oC)

[a] [b] [a] [c] [d] [c] [d]

4.5 ≅155 ≅105 ≅105 ≅107 ≅100 ≅120 ≅135

7 ≅165 ≅95 ≅90 ≅92 ≅105 ≅115 ≅115

[a] Determined from the ε’-temperature plot of [001]-poled crystals T R-T (L) is the temperature corresponding to the first dielectric anomaly of the plot; T max is the

temperature at which the ε’ is at its maximum.

[b] Determined by means of the temperature cycling tests of Honoso et al [77, 78] and Rajan et al [104] T DP is defined as the temperature above which perceptible degradation of dielectric and electromechanical properties begins

[c] Determined from the thermal current density data of [001]-poled crystals

[d]Determined by means of HR- XRD results

9.5 Summary of main observations

(a) A revised phase diagram for the PZN-PT system has been constructed Two new

evident features of this revised phase diagram are: (a) the expanded (R+T)

two-phase MPB region, and (b) a (T+C) two-two-phase region at high temperature before

the crystal transforms completely into the single C phase

(c) The expanded (R+T) MPB region can be further divided into two regions In the

lower PT region, 0.06 ≤ x ≤ 0.08, the T phase is metastable stabilized by the

residual stress in the crystal In the high PT region, 0.09 ≤ x ≤ 0.10, both the

(R+T) phase are thermodynamically stable phases at room temperature

(d) The FWHM suggests 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 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

(e) In lower PT contents (i.e., 0.045 ≤ x ≤ 0.07), the room temperature R phase of

poled crystals is under strained in which the structure appeared to elongate along

a certain crystal direction on comparing to the unpoled samples of broad R phase

The strained state of the poled crystals is the result of poling induced stresses in

(f) In the event of PP-ZFH in PZN-(4.5-7)%PT, the T R-T occurs in a temperature

range of 10-35 ºC The beginning of R-T transformation with concurrent thermal

depolarization shows perceptible property degradation This suggests that the T

R-T (L) corresponds to the T DP Thermal current and HR-XRD results suggest T RT ≅ 100–135 °C for [001]-poled 4.5%PT and ≅90–115 °C for [001]-poled

PZN-7%PT