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

Chapter 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 to 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 157 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 158 200 180 C Temperature (oC) 160 T+C 140 120 T 100 80 R 60 R+Tσ 40 R+T 10 %PT - present work Figure 9.1 - Ref [10] 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 Tσ 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 159 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 = 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