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

Chapter 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., MA, MB, 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 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 125 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 PZN-7%PT and PZN8%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 126 (a) (b) d2 d1 d3 (c) Figure 8.1 d4 (d) Room temperature HR-XRD (002) RSMs of unpoled (annealed) (a) and (b) PZN-7%PT, and (c) and (d) PZN-8%PT single crystals 127 sample of PZN-7%PT and PZN-8%PT, respectively, but of the same wafers as in Figures 8.1(a) and (c) These 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 MB or MC phase When referred to the pc coordinates, we have for the MB system, cpc < apc ≈ bpc (≈ (am/2)2 + (bm/2)2) and that cpc, apc, and bpc are possibly degenerated As for MC phase, cm((≈ cpc) > bm (≈ bpc) > am (≈ apc) and while cm and apc may be degenerated, bm does not These predicted diffraction patterns are consistent with the diffraction patterns shown in Figure 8.1(d) Three plausible assignments of the 128 diffractions shown in Figure 8.1(d) are thus: (i) (R+T) phase mixture, (ii) MB phase, or (iii) MC 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 MB 129 phase as MB-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 MB has been ruled out, the peak must be diffractions from the MC phase if the emerging new phase is indeed a true M phase However, this again violates the crystal group theory as the R-MC transformation is forbidden Ruling out both MB and MC 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 roomtemperature 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 130 (a) 25ºC (b) 80ºC ∆ω Figure 8.2 (c) 95ºC ∆ω ∆ω 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 131 (a) 25 ºC (b) 90 ºC (a) (b) 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 132 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 133 1.1 1.0 0.9 ∆V/V(%) 0.8 0.7 0.6 0.5 0.4 0.3 %PT Figure 8.4 Volume expansion associated with T-R transformation in PZNPT single crystals Note that the abrupt increase in volume associated with the transformation when x > 0.07 134 2 in perovskite structure Since geometrically d R(110) = 2a R(100) ≈ a T(100) + c T(001) , the 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 to 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 to the population ratio of (100)T to (001)T 135 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 136 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 to 1, which agrees with our previous discussion The detection 137 Tσ R Tσ [001] R R [100] Figure 8.5 [010] 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 138 Figure 8.6 Geometry of the {110}R//{110}T interface (in blue) and domain arrangement in the mixture of R and Tσ phases The {110}R//{110}T interface is either (a) perpendicular to or (b) lying at 45° to the (001) diffracting plane 139 Figure 8.7 Schematic illustrations of the two-phase coexistence, R and Tσ after fracturing (a) For {110}R//{110}T interface perpendicular to the (001) diffracting plane, the effect of stress relaxation produced by fracture is not as significant Thus, the Tσ phase remains metastable and both R and (100)T can be detected from the fractured surface (b) For slant {110}R//{110}T interface, the constraints produced by the neighbouring R phase in the crystal is removed by fracturing, causing the Tσ phase to transformed to the R phase in the surface layer Thus, only R diffraction can be detected from the fractured surface For x-ray of low energy as in the present work, the diffraction profile thus depends on the penetration depth in the (see text for details) 140 ... and (b) PZN- 7% PT, and (c) and (d) PZN- 8%PT single crystals 1 27 sample of PZN- 7% PT and PZN- 8%PT, respectively, but of the same wafers as in Figures 8.1(a) and (c) These peaks lie in two different... Ruling out both MB and MC 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... (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