High resolution x ray diffraction study of phase and domain structures and thermally induced phase transformations in PZN (4 5 9)%PT 7

Chapter 8 Rhombohedral and Tetragonal Micro/Nanotwins Mixture and Thermally-induced Phase Transformations in Unpoled PZN-6-8%PT 8.1 Introduction As mentioned in Chapter 7, while direc

Chapter 8 Rhombohedral and Tetragonal Micro/Nanotwins Mixture and Thermally-induced Phase Transformations in Unpoled

PZN-(6-8)%PT

8.1 Introduction

As mentioned in Chapter 7, while direct evidence of nanotwin domains has

only been reported by TEM [49, 51, 52], support for the M phases (i.e., M A , M B, and

MC) has been elicited by high-resolution x-ray diffraction study [30-32, 34-36, 38-40,

66, 93] However, the reported evidence of M or O phase is still in dispute Firstly, the

soft elastic constants of R phase which inherently infer a large piezoelectric distortion would result in a strained or distorted R phase instead of the M phases [41] Secondly, Wang [47, 48] have pointed out that the reported M phases could indeed be a result of volume average of the R and T micro- and nanotwin structures

As described in Chapters 6 and 7, neither M nor O phase was detected in upoled PZN-9%PT and PZN-4.5%PT single crystal A purely R phase was evident at room temperature in PZN-4.5%PT, while, a mixture phase of (R+T) is reported in

PZN-9%PT

In this chapter, the phases and domain structures of unpoled PZN-PT single

crystals of compositions closer to the MPB, i.e., PZN-(6-8)%PT, are investigated by means of HR-XRD and dielectric and thermal current measurements, to ascertain

whether or not M and/or O phases are present in these crystal compositions.

8.2 Room temperature phases of PZN-(7-8)%PT

The room-temperature HR-XRD (002) RSMs of unpoled (annealed) (001)-oriented PZN-7%PT and PZN-8%PT single crystals are shown in Figures 8.1(a) to (d)

Figures 8.1(a) and (c), taken at room temperature from 7%PT and PZN-8%PT respectively, show a single extremely broad peak at 2θ ≈ 44.64° Bragg’s

position This single extremely broad R peak is in good agreement with the projection

on (002) RSM as discussed in Section 7.2 It is the convoluted peak of the {100}-type

and/or {110}-type R micro- and/or nanotwin domains, a result of the large diffraction

half-width associated with the fine domain structure and the extreme compliant nature

of the R phase In addition to the broad R convoluted peak, a weak peak at 2θ ≈ 44.90° could be detected in Figure 8.1(a) This peak is likely to arise from phases other than

the R We shall discuss the origin of this weak peak later

In contrast, four distinguishable diffraction peaks, marked d1 to d4, were detected in the RSMs shown in Figures 8.1(b) and (d), which were taken from another

Figure 8.1 Room temperature HR-XRD (002) RSMs of unpoled (annealed)

(a) and (b) PZN-7%PT, and (c) and (d) PZN-8%PT single crystals

(a)

(b)

d1

d2

d3 d4

sample of PZN-7%PT and PZN-8%PT, respectively, but of the same wafers as in Figures 8.1(a) and (c) These 4 peaks lie in two different Bragg’s positions, with d1 and

d2 at 2θ ≈ 44.95º and d3 and d4 at 2θ ≈ 44.60º, all outside the ω = 0º plane As discussed in the previous chapter, peaks d3 and d4 can be assigned to the degenerated

{100}-type R* domains with ∆ω ≈ 0.1º, which agrees with the projection shown in Figure 7.3(c) However, the sources for peaks d1 and d2 remain to be ascertained The

Bragg’s position of the latter two peaks, being at 2θ ≈ 44.90-44.95º, strongly indicates

that they may be the (100)T diffractions One plausible room temperature structure of

PZN-(7-8)%PT single crystal is thus a (R+T) phase mixture

Alternatively, in the context of the more recent works in which M phases in

PZN-PT have been reported, one may also assigned the various diffractions in Figures

8.1(b) and (d) to that of a suitable M phase Table 6.1 gives the relationships between the m axes and the pc axes of the various M phases and the O phase Judging from the

nature of splitting, these diffraction peaks in Figure 8.1(d), say, may be assigned to the

either the M B or M C phase When referred to the pc coordinates, we have for the M B

system, cpc < a pc ≈ bpc (≈ (a m/2)2 + (b m/2)2) and that cpc, apc, and bpc are possibly

degenerated As for M C phase, c m((≈ cpc ) > b m (≈ b pc ) > a m (≈ a pc ) and while c m and a pc

may be degenerated, b m does not These predicted diffraction patterns are consistent with the diffraction patterns shown in Figure 8.1(d) Three plausible assignments of the

diffractions shown in Figure 8.1(d) are thus: (i) (R+T) phase mixture, (ii) M B phase, or

(iii) M C phase For the time being, we may assign the room temperature phase of the unpoled PZN-8%PT as the “low-temperature” phase(s), or “LT” phase(s) for short

To ascertain whether the room temperature phase in PZN-(7-8)%PT single

crystals is a (R+T) mixture or a true M phase, the following investigations were carried

out The PZN-8%PT sample was heated up to higher temperatures and the RSMs were taken to follow the changes induced by the thermal effect

The results of the heating experiment are given in Figure 8.2 It shows that there are no significant changes as the temperature was increased from 25 °C to about

80 °C At 80 °C (Figure 8.2b), two new peaks superimposing onto the existing diffraction peaks were noted on the (002) mapping, indicating the emergence of a new phase

With increasing temperature, the peaks gradually shifted towards the ω = 0° plane At 95 °C (Figure 8.2c), only two peaks at 2θ ≈ 44.37° and 2θ ≈ 44.86° remained The two peaks are identified as the T phase since both peaks lie in the ω = 0° plane The heating experiment showed that both the “LT” and T phases coexisted over the temperature range of 80-95 °C (Figures 8.2b-c), while only T phase persisted above 95

°C (which remained the only phase detected up to 160 °C) The above result shows

that the “LT” phase transformed to the T phase upon heating It thus cannot be the M B

phase as M B -T transformation is forbidden by the crystal group theory

Figure 8.3 shows the result of another heating experiment with a PZN-8%PT

sample of predominantly R domains to begin with (Figure 8.3a) On heating to 90°C, the characteristic M peaks at 2θ ≈ 44-90-45.0° appeared As described earlier, since the

presence of M B has been ruled out, the peak must be diffractions from the M C phase if

the emerging new phase is indeed a true M phase However, this again violates the crystal group theory as the R-M C transformation is forbidden

Ruling out both M B and M C phases being the viable phase, the peak(s) at 2θ ≈ 44-90-45.0° must arise from the (100)T domains We may thus conclude that “LT”

phase in PZN-8%PT (and hence PZN-7%PT as well) is a (R+T) phase mixture

8.3 Nature of rhomboedral and tetragonal micro-/nanotwin mixture in

PZN-(7-8)%PT at room temperature

In the previous section, it has been shown that the (R+T) mixture exist in

PZN-(7-8)%PT at room temperature However, careful examination of the room-temperature RSMs revealed that only (100)T diffractions were detected but not the (001)T diffractions In this section, we shall examine the nature of the T phase in

PZN-(7-8)%PT and provide an explanation for the absence of the (001)T diffractions in these crystal

Figure 8.2 Temperature dependent (002) RSMs taken from fractured surface

of annealed PZN-8%PT crystal obtained at (a) 25 ºC, (b) 80 ºC, and (c) 95 ºC The intensity contours are on log scale

∆ω

∆ω

(c) 95ºC

∆ω

Figure 8.3 Temperature dependent (002) RSMs taken from fractured surface of

another annealed PZN-8%PT crystal of predominantly R phase to

begin with at room temperature: (a) 25 ºC, and (b) 90 ºC The intensity contours are on log scale

In the crystal growth process, the C-T-R phase transformations occur during

cooling of the crystal to room condition involving structural changes in the crystal Accompanying the phase transformations are volume and shape changes and

associated transformation stresses The % of volume increase accompanying the T-R

phase transformation for PZN-PT single crystals has been estimated from the crystal data obtained in the present work The results are shown in Figure 8.4 as a function of

PT content It is evident that at high PT contents (i.e., >7%PT), the % volume expansion is quite significant, being ~1.0% at 8%PT; so must be the magnitude of residual stresses produced by the transformation Since the resultant overall volume

change increases with the amount of transformed R phase, with more and more

transformation events, the residual stress in the crystal builds up accordingly This, in

turn, would retard further T-R transformation Despite the R phase being elastically

soft, it is also possible for the cooling and transformation stresses to be relaxed via

twinning of both the untransformed T and the transformed R phases Thus, at room condition, the mixture of R and T micro- and/or nanotwins may coexist in PZN-PT

single crystals of PT contents closer to the MPB It should be noted that the room

temperature Tσ manifested by the phase in this case is metastable which is stabilized by

the stresses present in the crystal

Let us examine the likely R and T micro- and nanotwin domain configurations

V/V

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

1.1

Figure 8.4 Volume expansion associated with T-R transformation in

PZN-PT single crystals Note that the abrupt increase in volume

associated with the transformation when x > 0.07

in perovskite structure Since geometrically 2

T(001)

2 T(100) R(100)

interfaces between the R and T phases are likely to be the {110} R//{110}T type The

likely domain configuration of R* and T* mixture with {110} R//{110}T interface is depicted schematically in Figure 8.5, in which twinning serves to relax the residual stresses in the perovskite structure Note that micro- and/or nanotwin domains exist in

both the R and T phases although only those in the T phase (distinguished by the

direction of the arrows which are meant to denote the c-axis and hence polarization

direction of respective T domains) are depicted in this figure for purposes which will

clear later

For diffractions from the {100}pc family planes, two different R/T domain configurations are possible on the diffracting surface depending on the orientation of the {110}R//{110}T interface One such orientation is when the {110}R//{110}T

interface is lying at 45° to the (100)pc diffracting planes and the other when such an interface is lying perpendicular to the (100)pc diffracting planes, as shown in Figures 8.6(a) and (b) respectively Both figures show that under normal conditions, both (100)T and (001)T diffractions should be present and that their intensity ratio should be about 2 to 1 assuming equal probability of occurrence So, why should only (100)T

diffractions be detected but not (001)T diffractions? To answer this observation, we shall study below what fracturing may do to the population ratio of (100)T to (001)T

domains in the exposed surface layer

It should be noted that upon fracturing, the constraints on T-R transformation imposed by the transformed R phase are relaxed to various degrees depending on the

orientation of the {110}R//{110}T interface relative to the fracture (hence diffracting) surface This may be more vividly seen from Figure 8.7

Figure 8.7(a) shows the case of perpendicular {110}R//{110}T interface relative to the (100)pc diffracting plane As evident from this figure, such a R/T domain

configuration will produce only R and (100) T diffractions Note that on fracturing, although the stress normal to the fracture surface is relaxed (for the surface layer), the

lateral constraint from the neighbouring R phases remains In other words, most of the residual stresses remain and so does the metastable T phase

Figure 8.7(b) shows the case of slant {110}R//{110}T interface relative to the (100)pc diffracting plane Under unrelaxed conditions, diffractions from such a domain structure will produce {100}R, (100)T and (001)T peaks However, upon fracturing

along the (001) plane, the constraints for the T domains in the surface layer is effectively relaxed, as shown in schematically in Figure 8.7(b) As discussed, the Tσ

phase is metastable stabilized by the residual stress in the material Upon effective

relaxation of the residual stress by fracturing, the T phase in the surface layer is no longer stable and would transform to the more stable R phase, as depicted

schematically in the figure Thus, only R diffractions can be detected from the fracture surface This is especially true when only T microtwin domains are present in the

material Due to the lack of penetration of the x-ray radiation used (of 8.048 keV in energy and with an estimated penetration depth of only a few µm thick), the underlying (001)T domains could not be detected by the x-ray This would explain the observation made in the present work

One deduction from the above is that under the condition when T nanotwin

domains exist in the crystal and/or when the x-ray radiation used is of high energy, and hence of larger penetration depth, it is likely that the (001)T could be revealed In this connection, it should be mentioned that in addition to the {100}R and the (001)T

diffractions, (100)T diffraction was detected in an “as-grown (unpoled)” PZN-6%PT single crystal sample at 25 ºC As shown in Figure 8.8, in addition to the {100}R peaks

at 2θ ≈ 44.60º (peaks d3 and d5 which are likely to arise from the {100}-type and

{110}-type R nanotwins as discussed in the previous section) and the (100) T

diffractions at 2θ ≈ 44.80º (peak d1), there is an extra diffraction at 2θ ≈ 44.47º (peak

d2) This new diffraction can be assigned to that of the (100)T domains Note that both the (100)T and (001)T diffractions lie out of the ω = 0º plane, suggesting that they are both twinned diffractions Note also that the intensity ratio of (100)T and (001)T

diffractions is about 2 to 1, which agrees with our previous discussion The detection

Figure 8.5 Domain configurations of coexistence R* and T* domain structures

The arrows represent the directions of the polar axis in T phase The two polar directions are joined by the {110}-type T* as indicated by

the red solid lines The {110}R//{110}T interface are indicated the by

blue solid lines Note that the Tσ phase is metastable in this case,

stabilized by the residual stress in the crystal

R

R

R

[001]

[010]

[100]