High resolution x ray diffraction study of phase and domain structures and thermally induced phase transformations in PZN (4 5 9)%PT 8

8.4 Thermally-induced phase transformations in unpoled PZN-8%PT 8.4.1 Temperature dependent polarization characteristics Figure 8.9a shows the temperature-dependent ε’ at various frequ

Figure 8.8 Room temperature HR-XRD (002) RSM of an as-grown (unpoled)

PZN-6%PT Peaks d3, d4 and d5 are R* diffractions, while peaks d1 and

d2 are the (100)T diffraction and (001)T microtwin diffractions, respectively (see text for details)

of (001)T twin diffractions in as-grown PZN-6%PT single crystal may be due to the

higher residual stress and/or the presence of a much higher portion of T nanotwin

domains in this crystal sample, or a combination of both factors

8.4 Thermally-induced phase transformations in unpoled PZN-8%PT

8.4.1 Temperature dependent polarization characteristics

Figure 8.9(a) shows the temperature-dependent ε’ at various frequencies (0.1 kHz – 1 MHz) of an unpoled (annealed) PZN-8%PT sample during the ZFH The ε’

increased gradually with temperature initially, followed by a weak anomaly over the temperature range of about 90-105 °C This anomaly can be more readily seen in the

1/ε’ versus temperature plot As shown in the inset of Figure 8.9(a), the value of 1/ε’ showed a steeper drop at about 90 °C In addition, a clear anomaly in ε’ (and a step-like decrease in 1/ε’) was observed over the temperature range of 160-173 °C The latter anomaly was followed by a frequency dependent ε’ maximum (at Tmax), which

may be attributed to the dynamic relaxation processes of polar nanoclusters in the material [95, 96]

Figure 8.9(b) shows the current signals of the unpoled PZN-8%PT single crystal as a function of temperature The unpoled sample displayed multiple but very weak current signals over the temperature range of 90-113 °C Of these activities, three

2 )

-0.010 -0.005 0.000 0.005 0.010 0.015 0.020

90 100 110

(b)

4 )

0 2 4

(R*+Tσ) (R*+Tσ+T)

(T*+C)

50 100 150 200

0.0 0.2 0.4

0.6

0.5

10

50 100 500

1 MHz

Figure 8.9 (a) ZFH ε’ and (b) ZFH J of unpoled (annealed) PZN-8%PT

crystal The sample thickness is 1.5 mm

more obvious current signals could be noted, at 97 ºC, 104 ºC, and 110 ºC, respectively (see inset) There followed a period of no current activities on further heating until about 165 ºC, at which a stronger current signal was detected, followed by a string of weak current activities spreading over the temperature range of 165-170 ºC

8.4.2 Structural studies

As described in Section 8.3, at 25 °C, the diffraction pattern of PZN-8%PT contains four diffraction peaks, marked d1 to d4 in the Figure 8.10(a) These four diffraction peaks are located at three different Bragg’s angles, i.e., d1 and d2 at 2θ ≈ 44.95° (with ∆ω ≈ 0.54°), and d3 and d4 at 2θ ≈ 44.60° (∆ω ≈ 0.24°) The room temperature structure of unpoled PZN-8%PT has been identified to be a (R+Tσ) phase mixture, in which the subscript σ denotes that the T phase is metastable stabilized by

the residual stress in the crystal

Upon increasing the temperature to 80 °C, the (002) diffraction pattern remained relatively unaffected At 80 °C (Figure 8.10b), two new peaks superimposing onto the existing diffraction peaks were noted, indicating the emergence of a new phase With increasing temperature, the peaks d1-d4 gradually shifted towards the ω = 0° plane At 95 °C (Figure 8.10c), only two peaks at 2θ ≈ 44.37° and 2θ ≈ 44.86° remained These two peaks are identified as the T phase since both peaks lie in the ω =

0° plane It should be noted that the T phase that appeared at 95 °C is a stable T phase, which is different from that of Tσ at room temperature from thermodynamic point of

view The stable T phase persisted above 95 °C which remained the only phase

detected up to 160 °C (Figures 8.10d-g)

Careful observation shows that the C phase, at 2θ ≈ 44.60º, started to emerge

at 160°C (Figure 8.10g) It is interesting to note from Figures 8.10(g)-(h) that the

emergence of the C phase caused the diffractions of the T phase to become increasingly tilted out of the ω = 0° plane The C phase, at 2θ ≈ 44.63º, remained the

dominant phase above 173 ºC (Figure 8.10i) The above-experiment was repeated with several other unpoled PZN-8%PT single crystal samples Similar results were obtained

It is also interesting to note that a string of current signals was detected over the temperature region of 90-113 °C (Figure 8.9b) As will be discussed in Chapter 9, one likely cause could be the compositional segregation in the crystal, leading to a

range of R-T transformation temperature Despite the thermal current signals, the

anomalies in the ε’ over the temperature range of 90-115 °C remained not obvious in

the unpoled sample As discussed previously, this phenomenon was attributed to the effect of the strong dynamic relaxation process of the polar nanoclusters in the material

which helps mask the anomalies in the ε’ displayed by the R-T transformation

Figure 8.10 Temperature dependent (002) RSMs taken from fractured surface of

annealed PZN-8%PT crystal obtained at (a) 25 ºC, (b) 80 ºC, (c) 95 ºC, (d) 120 ºC, (e) 140 ºC, (f) 150 ºC , (g) 160 ºC, (h) 165 ºC, and (i) 173 ºC The intensity contours are on log scale The PZN-8%PT undergoes a

transformation of (R*+Tσ)–(R*+Tσ+T)–T–(T+T*+C) –C upon heating

Figure 8.11 ZFH domain structures of annealed PZN-8%PT crystal observed by

the PLM at (a) 25 ºC, (b) 85 ºC, (c) 105 ºC, (d) 145 ºC, (e) 165 ºC, and (f) 168 ºC The sample thickness is about 100 µm The PLM observation is consistent with the HR-XRD results

(a) 25 ºC

P/A= 45º P/A= 0º

(c) 105 ºC

P/A= 45º

P/A= 0º (d) 145 ºC

P/A= 45º

P/A= 0º

(e) 165 ºC

P/A= 45º P/A= 0º

[100]

[010]

200 µm

(b) 85 ºC

P/A= 45º

P/A= 0º

C

C

Figure 8.11 shows the domain structures obtained by the PLM as a function

of temperature At 25 ºC, the crystal exhibited complete optical extinction at only P/A

= 45º (Figure 8.11a), indicating that the room temperature phase is the R phase The R

phase remained till 85 ºC before the first sign of phase transformation (Figure 8.11b) Gradual change in the extinction angle was observed above 85 ºC, suggesting that some structural changes must have occurred At 105 ºC, optical extinction was shown

to occur at P/A = 0o, which may arise from the T phase (Figure 8.11c) The T phase

persisted upon increasing to higher temperature (Figures 8.11d-e) At 168 ºC, scattered

areas started to exhibit extinction at most P/A angles, suggesting the occurrence of the

C phase (Figure 8.11f) Above Tmax, the sample exhibited a total extinction, indicating the C phase only The PLM observation is thus consistent with the HR-XRD results

presented above

8.5.1 Temperature dependent polarization characteristics

Figures 8.12(a) shows the temperature-dependent ε’ of the unpoled (annealed) PZN-7%PT sample measured at various frequencies (0.1 kHz – 1 MHz) The ε’

increased gradually with temperature initially, followed by a step-up like anomaly near

90 °C This anomaly can be more readily seen in the 1/ε’ versus temperature (see inset

Figure 8.12 (a) ZFH ε’ and (b) ZFH J of unpoled (annealed) PZN-7%PT

crystal The sample thickness is 1.0 mm

Temperature (oC)

40 60 80 100 120 140 160 180 200 220

2 )

-0.004 -0.002 0.000 0.002 0.004

4 )

0 2 4 6

8

TC

Tmax

TR-T

C T+T*

(T+T*+C)

(R*+Tσ)

(R*+Tσ+T*)

(a)

50 100 150 200

ε'(x10

-4 )

0.0 0.5

1.0 TR-T T

C

0.1 kHz

0.5 10

50

100

500

1 MHz

of Figure 8.12a) The ε’ remains relatively constant on further heating until about 160

°C Over the temperature region of 160-165 °C, the second anomaly (and a sudden

plunged in 1/ε’) accompanied with a broad frequency dependent maximum was observed at Tmax (i.e., temperature at ε’ maximum)

The temperature dependent current density obtained from the unpoled PZN-7%PT single crystal is shown in Figure 8.12(b) There were hardly any current signals until about 95 ºC where a string of current signals occurred in the region of 95-110 °C

Of these current activities, a clear current response appeared at 110 °C There followed

by a region where no major current activities were detected until at about 160 ºC, at which a new string of strong current activities was detected over the temperature range

of 160-165 ºC

8.5.2 Structural studies

Figure 8.13 shows typical (002) RSMs of the unpoled sample taken over the

temperature range of 25-175 °C At 25 °C (Figure 8.12a), a broad convoluted R single peak with 2θ ≈ 44.64° lying at ω = 0° plane was detected (see Section 7.2 for details) Besides the main R peak, a weak peak located at 2θ ≈ 44.90° and lying out of the ω = 0° plane indicates that a small amount of the T phase may be present The room temperature structure of PZN-7%PT is thus a mixture of (R+Tσ) in which the Tσ is

metastable stabilized by the residual stress in the crystal

With increasing temperature, the R phase remained as the stable phase with its broadness gradually reduced in both the 2θ and ω planes Figure 8.13(b) gives the (002) mapping taken at 95 °C It is interesting to note that the Tσ at 2θ ≈ 44.80°-44.85° persisted with the main R peak

At 100 °C (Figure 8.13c), subtle changes in the diffraction pattern can be noted, i.e., new diffractions at 2θ ≈ 44.40°-44.45° and 2θ ≈ 44.80° emerged At 105 °C,

in addition to the existing (R+Tσ) mixture phase, five new peaks (i.e., d1, d2, d5, d6, and

d7) became evident (Figure 8.13d) These peaks lie in three Bragg’s positions, with peaks d1, d2 and d3 lying at 2θ ≈ 44.80°, d4 at 2θ ≈ 44.60°, and d5, d6 and d7 at 2θ ≈

44.46° While peaks d2, d4 and d6 lie in the ω = 0° plane, the remaining peaks lie out of the ω = 0° plane

Upon increasing the temperature to 115 °C, six peaks were diffracted (Figure 8.13e) Of the six peaks, d1, d2, and d3 lie at 2θ ≈ 44.80° and d4, d5, and d6 lie at 2θ ≈

44.48° Note that peaks d1 and d3 at this temperature are separated by a significantly larger ∆ω (≈ 0.46°) values as opposed to peaks d1 and d3 at lower temperatures (with

∆ω ≈ 0.28°)

Upon heating to 155 °C, the six diffraction peaks persisted despite slight

changes in their 2θ positions, from the initial value of 2θ ≈ 44.80º to 2θ ≈ 44.75º for d1,

d2 and d3, and from 2θ ≈ 44.45º to 2θ ≈ 44.53º for d4, d5 and d6, with the off ω = 0° peaks shifting towards the ω = 0° plane (Figures 8.13f-g) The C phase, at 2θ ≈ 44.67º (ω = 0º), started to emerge at 160 ºC (Figure 8.13h) with further reduction of the

diffraction intensity of other remaining peaks Above 165 ºC, the C phase, at 2θ ≈

44.67º, remained as the dominant phase (Figure 8.13i)

Based on the analysis given in Table 6.1, the various diffraction peaks at 115

ºC (Figure 8.13e) may be assigned as (a) MB (d1, d3, d4, and d6) + T (d2 and d5), (b) MC

(d1, d2, d3, d4, and d6) + T (d2 and d5) or T* (d1, d3, d4, and d6) + T (d2 and d5) (see

Section 6.2.3 for details) The following reason helps to rule out the presence of the MC phase That is, typically apc ≠ bpc for the MC phase This means that the apc and bpc

diffractions for the MC should not lie on the same Bragg’s angle and that the bpc diffraction is non-degenerated These features are not borne out by the diffractions

shown in Figure 8.13(e) Furthermore, Figure 8.12 revealed only one anomaly in ε’

and one major thermal current signal in the range of 140-170 ºC concerned Based on these observations, we may conclude that the relevant diffraction peaks at 115 ºC are

those of the (T+T*) domains

It should be mentioned that upon heating from 115 ºC to 155 ºC (Figures 8.13e-g), the various diffraction peaks are of asymmetrical degeneracy This observation may be understood from the strained state and the unbalanced population

of R* and T* domains formed in the material near the R-T transformation temperature (TR-T)

Figure 8.14 shows the domain structures obtained by the PLM as a function of temperature At 25 ºC, the crystal exhibited complete optical extinction at only P/A =

45º (Figure 8.14a), confirming that the room temperature phase is the R phase The R

phase remained till 100 ºC before the first sign of phase transformation (Figure 8.14b) Gradual change in the extinction angle can be noticed to begin at 110 ºC (Figure 8.14c), suggesting that some structural changes must have occurred Above 125 ºC, scattered areas exhibit extinction at P/A = 0o, which may arise from the T phase (Figures 8.14d-e) On heating to Tmax ≅ 165 ºC, the scattered areas started to exhibit extinction at most

P/A angles, suggesting the occurrence of the C phase (Figure 8.14f) Above Tmax, the sample exhibited a total extinction indicating the C phase only The PLM observation

is thus consistent with the HR-XRD results presented above

8.6 Summary of main observations

(a) In addition to the R diffractions, T* domains could also be detected in

PZN-(6-8)%PT single crystals at room condition Our HR-XRD RSM results indicate

that the T phase is likely to be a metastable phase stabilized by the

transformation-cum-cooling stress in the crystal This is manifested by the stress

Figure 8.13 Temperature dependent (002) RSMs taken from fractured surface of

annealed PZN-7%PT crystal obtained at (a) 25 ºC, (b) 95 ºC, (c) 100

ºC, (d) 105 ºC, (e) 115 ºC , (f) 140 ºC, (g) 155 ºC, (h) 160 ºC and (i)

168 ºC The intensity contours are on log scale The PZN-7%PT

undergoes an (R*+Tσ)–(R*+Tσ+T*)–(T+T*)–(T+T*+C)–C phase

transformation sequence upon heating

MC (d1, d2, d3, d4, and d6) + T (d2 and d5), or (c) T* (i.e., d1, d3, d4, and d6) + T (i.e., d2 and d5) We may rule out possibilities (a) and (b) as transformations of the type MB-C and MC-C are not permitted by the crystal transformation theory [42, 98, see also

(h) 160ºC: (T+T*+C)

(a) 25 ºC

P/A= 45º

P/A= 0º (b) 100 ºC

P/A= 45º

P/A= 0º

(d) 125 ºC

P/A= 45º

P/A= 0º

P/A= 45º

R

T

T

C

C

T

T

(e) 145 ºC

P/A= 45º P/A= 0º

[100]

[010]

200 µm

Figure 8.14 ZFH domain structures of annealed PZN-7%PT crystal observed

by the PLM at (a) 25 ºC, (b) 100 ºC, (c) 110 ºC, (d) 125 ºC, (e) 145

ºC, and (f) 165 ºC The sample thickness is about 100 µm The PLM observation is thus consistent with the HR-XRD results

relaxation effect is especially pronounced for the (R+T) domains with their

{110}R//{110}T interface lying at about 45° to the (100)pc diffraction planes

(b) Strong evidence of T nanotwins, manifested by their streaked diffractions, has been detected in PZN-(4.5-8)%PT in the vicinity of TR-T phase transformation,

suggesting that its presence being promoted by the transformation stress, possibly as a means to relax the transformation stresses in the crystal