Acta Materialia 149 (2018) 132e141
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Complex morphotropic phase transformations and high piezoelectric properties in new ternary perovskite single crystals Zenghui Liu a, b, Hua Wu a, c, **, Wei Ren b, ***, Zuo-Guang Ye a, b, * a
Department of Chemistry and 4D LABS, Simon Fraser University, Burnaby, British Columbia, V5A 1S6, Canada Electronic Materials Research Laboratory, Key Laboratory of the Ministry of Education & International Center for Dielectric Research, Xi'an Jiaotong University, Xi'an, 710049, China c Department of Applied Physics, Donghua University, Ren Min Road 2999, Songjiang, Shanghai, 201620, China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 6 November 2017 Received in revised form 8 February 2018 Accepted 10 February 2018 Available online 15 February 2018
In order to understand the complex phase symmetry and phase transitions, and to illustrate the microscopic mechanisms of high piezoelectricity, single crystals of a new ternary complex perovskite system, Pb(Mg1/3Nb2/3)O3-Bi(Zn2/3Nb1/3)O3-PbTiO3, are grown by the high temperature solution growth method and their domain structure, dielectric and ferro-/piezoelectric properties, and phase transformation behavior are investigated by various techniques. Different phase symmetries including the rhombohedral, tetragonal and monoclinic are found in these crystals, indicating that the composition of the crystals is close to the morphotropic phase boundary (MPB) region. Most interestingly, unusual phase transformation sequences of rhombohedral / monoclinic / cubic, and monoclinic / cubic are directly observed by polarized light microscopy and confirmed by the dielectric and birefringence results. Moreover, an ultrahigh piezoelectric coefficient d33 z 2000 pC/N is obtained in these crystals, making these crystals useful for applications as electromechanical transducers. The unusual phase transformation sequences and the high piezoelectric response are explained from the polarization rotation mechanism, which has been evidenced in this work. Based on these results, a temperature-composition phase diagram is established, which illustrates the complex phases present and their transformation behavior. These studies provide new insights into the intricate morphotropic phase symmetry and phase components in complex perovskite solid solutions, and a better understanding of the microscopic mechanisms of high piezoelectric response in relaxor-based piezocrystals, which in turn will be helpful for designing better piezoelectric single crystals. © 2018 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
Keywords: Ternary complex perovskite single crystals PMN-PT-BZN Morphotropic phase boundary Polarized light microscopy Phase transformations Piezoelectric and ferroelectric properties
1. Introduction Piezo-/ferroelectrics are essential materials for electromechanical sensors and actuators and energy harvesters in a wide range of technological applications. Therefore, there have been continued efforts in enhancing the performance of piezoelectrics to meet ever-increasing demands for more advanced devices. Relaxorbased single crystals of binary solid solution (1-x)Pb(Mg1/3Nb2/3) O3-xPbTiO3 (PMN-PT) and (1-x)Pb(Zn1/3Nb2/3)O3-xPbTiO3 (PZN-PT)
have attracted considerable attention in the last decade due to their ultrahigh piezoelectric coefficient (d33 > 2000 pC/N) and large electromechanical coupling factor (k33 > 90%) for the compositions near the morphotropic phase boundary (MPB) region, which entitle these crystals most promising materials for the next generation of electromechanical transducers, actuators and sensors for medical ultrasonic imaging and therapy, underwater sonar, and many other applications [1e6]. Recent experimental and theoretical studies on the crystal
* Corresponding author. Department of Chemistry and 4D LABS, Simon Fraser University, Burnaby, British Columbia, V5A 1S6, Canada ** Corresponding author. Department of Chemistry and 4D LABS, Simon Fraser University, Burnaby, British Columbia, V5A 1S6, Canada and Department of Applied Physics, Donghua University, Ren Min Road 2999, Songjiang, Shanghai, 201620, China *** Corresponding author. Electronic Materials Research Laboratory, Key Laboratory of the Ministry of Education & International Center for Dielectric Research, Xi'an Jiaotong University, Xi'an, 710049, China E-mail addresses:
[email protected] (H. Wu),
[email protected] (W. Ren),
[email protected] (Z.-G. Ye). https://doi.org/10.1016/j.actamat.2018.02.017 1359-6454/© 2018 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
Z. Liu et al. / Acta Materialia 149 (2018) 132e141
structure and phase transformation in relaxor-based piezo-/ferroelectric systems have revealed that the presence of MPB is responsible for the ultrahigh piezoelectric coefficient. In the MPB region, different phases were detected by various techniques. For example, synchrotron X-ray powder diffraction study identified that the MPB in the PMN-PT system is located in the composition region of x ¼ 0.30e0.35 with a monoclinic MC phase bridging the rhombohedral and tetragonal phase [7]. Evidences for the existence of other types of monoclinic phases, i.e., MA and MB, were also reported from high-resolution powder (X-ray and neutron) diffraction [8e11] and polarized light microscopy (PLM) [12e15] studies. Likewise, in the PZN-PT system, an orthorhombic phase and various monoclinic phases were found in the composition range between the rhombohedral and tetragonal phases [10,16,17]. Based on various reported data, a “universal phase diagram” was proposed for PbZr1-xTixO3 (PZT) and PZN-PT solid solutions, including the six symmetries: rhombohedral, tetragonal, orthorhombic and monoclinic MA, MB and MC [18]. The existence of these intermediate phases enables the continuous rotation of the polarization vectors in these phases in response to the poling and driving fields [1,19], which was believed to be responsible for the high piezoelectric properties in relaxor-based single crystals. In the MPB region, various phase transformations were found as the temperature varies. Upon heating, the PMN-PT binary system undergoes a phase transformation sequence of rhombohedral/monoclinic / tetragonal / cubic phases, which arises from a significantly curved MPB top line [7]. Correspondingly, the piezoelectric coefficient in this system shows an acute drop as the crystal is heated through the rhombohedral or monoclinic / tetragonal phase transformation temperature (referred to as TMPB or TR-T, which is around 60e95 C), causing the crystals to depole [6,7]. This low depoling temperature, together with the low Curie temperature (TC ~ 130 C) and the low coercive field (EC ~ 2.5 kV/ cm), makes the PMN-PT crystals unsuitable for applications at high temperatures and under high electric fields [4,6,7,20]. Moreover, despite the recent progresses, the mechanisms underlying the high piezoelectricity in these piezocrystals are still a puzzling issue, which prevents the development of better materials. One of the main reasons is the lack of understanding of the complex phase symmetry, phase components and phase transitions in these materials, especially in the crystals with the MPB compositions. Recently, in order to develop new high-performance piezoelectric materials, we have incorporated Bi(Zn2/3Nb1/3)O3 (BZN), a Bi-based complex perovskite with ferroelectrically active Zn2þ and Nb5þ ions occupying the perovskite B site, into PbTiO3 (PT) [21] and PMN-PT [22,23] and successfully prepared the binary PT-BZN single crystals and the ternary PMN-PT-BZN ceramics which exhibit excellent piezoelectric properties. In this work, the phase transformations, domain structure and electric properties of the PMN-PT-BZN single crystals are investigated and a pseudobinary temperature-composition phase diagram is established. The phase transformation sequences of rhombohedral / monoclinic / cubic, and monoclinic / cubic phases without going through the tetragonal phase are observed by PLM. These unusual phase transformation sequences are found to be directly related to the enhanced piezo-/ferroelectric properties.
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composition of 0.60PMN-0.35PT-0.05BZN (solute). The mixture of PbO and B2O3 was used as flux with the solute to flux molar ratio of 1:1. The mixed chemicals were homogenously ground and loaded into a 50-mL platinum crucible and then placed into a crystal growth furnace, where it was melted at 1180 C and then slowly cooled down at a rate of 2e5 C/h until the end of crystallization at 700 C. The grown crystals were separated from the solidified flux by leaching in a diluted hot HNO3 solution. Finally, large crystals with sizes of 5e20 mm were obtained, as pictured in Fig. 1. Three single crystal platelets (Samples #1, #2 and #3) were cut parallel to the naturally grown (001)C facet from the grown crystal block and carefully polished to the thickness of 300e380 mm. The polished (001)C crystal surfaces were then painted with silver paste and attached with gold wires for electrical measurements. The ferroelectric hysteresis loops were displayed by means of a Radiant RT-66 standardized ferroelectric testing system under 10 Hz at room temperature. Then the crystal platelets were poled at room temperature under 10e20 kV/cm for 20 min to perform piezoelectric measurements. After poling, the attached gold wires were removed and a quasi-static piezoelectric d33 meter (ZJ-6B, Institute of Acoustics, CAS, China) was used to measure the piezoelectric coefficient of these crystals. After these measurements, the crystals were polished down to less than 100 mm in thickness with mirror surfaces in order to minimize the domain overlapping. After polishing, the crystals were annealed at 600 C for 0.5 h to eliminate the residual stresses possibly introduced during the polish process. Then the domain structure and phase transformations were examined at various temperatures by PLM using Olympus BX60 polarizing microscopy equipped with a Linkam THMS600 optical heating/ cooling stage. After the PLM studies, the crystals were painted with electrodes once again for dielectric measurements. The dielectric constant and loss tangent were measured as a function of temperature and frequency by using a Novocontrol Alpha high-resolution broadband dielectric analyzer equipped with a computer-controlled heating/cooling system. To estimate the composition of each crystal, energy dispersive X-ray spectroscopy (EDX) was performed on a FEI Quanta 250 FEG scanning electron microscope.
2. Experimental Single crystals of Pb(Mg1/3Nb2/3)O3-PbTiO3-Bi(Zn2/3Nb1/3)O3 ternary solid solution were grown by the high-temperature solution growth method. The starting raw chemicals of PbO (99.9%), Bi2O3 (99.9%), MgO (99.9%), ZnO (99.9%), Nb2O5 (99.9%) and B2O3 (99.9%) were mixed according to the stoichiometry of a nominal
Fig. 1. As-grown single crystals with a nominal composition of 0.60PMN-0.35PT0.05BZN after leaching (the smallest scale is in mm).
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Table 1 Extinction angles characteristic of crystal symmetries when observed on a (001)C platelet. “✓” stands for “allowed” and “7” stands for “forbidden”.
Cubic (C) Rhombohedral (R) Tetragonal (T) Monoclinic (MA) Monoclinic (MB) Monoclinic (MC) Orthorhombic (O) Triclinic (Tr)
q ¼ 0/90
q ¼ ±45
0 < jqj < 45
✓ 7 ✓ 7 7 ✓ ✓ 7
✓ ✓ 7 ✓ ✓ 7 ✓ 7
✓ 7 7 ✓ ✓ ✓ 7 ✓
crossed polarizers. In this paper, we define the angle between the [100]C direction and the analyzer when the (001)C cut crystal platelet is at extinction as the extinction angle. The extinction angles characteristic of different crystal symmetries when observed on a (001)C platelet are listed in Table 1 as reference. In this work, three crystal platelets (#1, #2 and #3) were selected and prepared from three different crystals, which represent three different phase features at room temperature, i.e. rhombohedral, tetragonal and MPB phases, respectively. Their domain structure, phase transformations and properties are investigated below. 3.1. Rhombohedral crystal
3. Results and discussion Studies of various ferro-/piezoelectric crystals with MPB compositions by PLM were reported by several groups [12,14,15,24e26] and the detailed optical and crystallography principles can be found in Refs. [24] and [12]. Here we present a brief description on the principles for understanding and interpreting the domain structures observed by PLM in this work. The rhombohedral phase has its spontaneous polarizations along one of the C directions and the optical axis is also parallel to the C directions. So, for the (001)C cut rhombohedral crystal platelets, the extinction occurs when the [001]C edges are at an angle of 45 to the crossed polarizer and analyzer. For the (001)C cut tetragonal crystals, the extinction occurs with the [001]C edges at an angle of 0 or 90 to the polarizer or analyzer. For the monoclinic or triclinic phase of lower symmetry, the extinction occurs with the [001]C edges at an angle between 0 and 45 to the polarizer or analyzer. For crystals with cubic symmetry, the extinction occurs in all directions under
The ferroelectric polarization-electric field P(E) hysteresis loop of Sample #1 (300 mm in thickness) was displayed in Fig. 2 (a). The coercive field EC and remnant polarization Pr are determined to be 5.8 kV/cm and 30 mC/cm2, respectively, from this loop. The piezoelectric coefficient d33 is found to be about 500 pC/N for the crystal poled under 10 kV/cm, which is smaller than the PMN-PT crystals with MPB composition, suggesting that the composition of this crystal is away from the MPB. Fig. 3 shows the domain structures of Sample #1 observed under PLM after it was further mirror polished to 100 mm thick. The extinction angle of the birefringent domain in this crystal is determined to be 45 at room temperature for the whole crystal area, suggesting a typical rhombohedral symmetry, similar to the rhombohedral PMN-PT single crystals [24]. Upon heating, the crystal becomes optical isotropic at around TF-R ¼ 95 C (TF-R refers to the phase transition temperature from the ferroelectric phase to the relaxor state), indicating a phase transformation from the
Fig. 2. Ferroelectric hysteresis loops of (a) Sample #1 with rhombohedral symmetry, (b) Sample #2 with tetragonal symmetry, and (c) Sample #3 with MPB phases, displayed at room temperature under ±25 kV/cm.
Fig. 3. Domain patterns of Sample #1 (rhombohedral crystal) observed under polarized light microscope at various temperatures (upon heating).
Z. Liu et al. / Acta Materialia 149 (2018) 132e141
Fig. 4. Temperature dependence of the birefringence (Dn ¼ nE - nO) for Sample #1 (rhombohedral crystal). Thermal hysteresis between heating and cooling runs indicates the first-order nature of the phase transformation from the rhombohedral ferroelectric phase to the relaxor state with a macroscopically cubic symmetry.
Fig. 5. Temperature dependence of dielectric constant and loss tangent measured at various frequencies during (a) zero-field-heating (ZFH) and (b) by zero-field-cooling (ZFC) after ZFH of Sample #1 (rhombohedral crystal) poled at room temperature under an electric field of 10 kV/cm.
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(TR-F refers to the phase transition temperature from the relaxor state to the ferroelectric state) upon cooling, as shown in Fig. 4. The discontinuous variation of birefringence as a function of temperature and the observed thermal hysteresis indicate the first-order phase transition, which is consistent with the behavior reported for the PMN-PT crystals [27]. It should be noted that the roomtemperature birefringence is small (Dn z 1.2 103), indicating that the rhombohedral distortion is relatively weak, corresponding to a primitive long-range ferroelectric phase like on the PMN-rich side of PMN-PT [24]. The dielectric measurements are performed on Sample #1 after being poled under 10 kV/cm at room temperature. The temperature dependences of the dielectric constant and loss tangent of Sample #1 measured upon zero-field-heating (ZFH) at various frequencies are illustrated in Fig. 5 (a). A dielectric anomaly independent of frequency at around TF-R ¼ 87 C is visible, indicating a phase transformation from the ferroelectric phase to the ergodic relaxor phase which will be confirmed later. Upon heating, the long-range macroscopic domains in ferroelectric state are disrupted and evolve into polar nanodomains in relaxor state, giving rise to a round and flat dielectric curve in the higher temperature range around Tm. The phase transformation temperature obtained by this dielectric measurement is very close to the TF-R obtained by the PLM study for the unpoled sample. After ZFH process, the dielectric constant is measured upon zero-field-cooling (ZFC), as shown in Fig. 5 (b). A high and wide peak in the temperature dependence of the dielectric constant is observed during the subsequent ZFC process with the temperature Tm shifting to higher temperature with increasing frequency, indicating the characteristics of relaxor behavior. However, the shift of Tm from 114 C at 100 Hz to 120 C at 100 kHz is relatively small compared with canonical relaxor PMN [28], indicating a relatively weak relaxor behavior. Below Tm, no further distinct dielectric anomaly could be observed due to the weak rhombohedral distortion, as revealed in the birefringence measurement, confirming that Sample #1 undergoes a transformation from an ergodic relaxor phase with cubic symmetry to a primitive ferroelectric phase with rhombohedral symmetry upon cooling. To estimate the chemical composition of the crystals, EDX measurements were performed and the results obtained are listed in Table 2. The composition of Sample #1 is estimated to be 0.71PMN-0.26PT-0.03BZN approximately, which is consistent with the composition estimated according to the composition dependence of the Curie temperature in the PMN-PT-BZN ceramics [22,23], further confirming the rhombohedral structure of the crystal.
3.2. Tetragonal crystal rhombohedral ferroelectric phase to the high temperature relaxor phase with a macroscopically cubic symmetry. (This transition temperature differs slightly in different parts of the crystal due to a small composition fluctuation.) This phase transformation is also revealed by the temperature dependence of the birefringence (Dn ¼ nE - nO), which undergoes a discontinuous vanishing at TF R ¼ 103 C upon heating and a more gradual increase at TR-F ¼ 85 C
The ferroelectric P(E) hysteresis loop of Sample #2 (380 mm thick) is displayed in Fig. 2 (b), which indicates a coercive field EC ¼ 17 kV/cm and a remnant polarization Pr ¼ 25 mC/cm2. The EC obtained in this crystal is much larger than that in the rhombohedral crystal Sample #1. As a result, the P(E) loop is not fully saturated at E ¼ ±25 kV/cm. This behavior is typical of the
Table 2 Elemental analysis and calculated chemical composition of Sample#1, Sample#2 and Sample#3 based on the EDX measurements. Sample No. Pb2þ
Bi3þ
Mg2þ
Zn2þ
Ti4þ
Nb5þ
O2-
Normalized Composition
Atomic % Error % Atomic % Error % Atomic % Error % Atomic % Error % Atomic % Error % Atomic % Error % Atomic % Error % #1 #2 #3
17.29 16.50 17.20
6.6 7.0 6.6
0.96 0.58 0.83
39.1 63.2 43.7
5.83 5.45 4.50
16.9 17.6 19.8
0.34 0.20 0.25
74.6 89.2 77.8
8.09 12.67 6.74
24.4 22.8 26.6
9.88 6.74 8.77
9.0 11.5 9.5
57.62 60.78 61.71
12.6 12.3 12.2
0.71PMN-0.26PT-0.03BZN 0.61PMN-0.37PT-0.02BZN 0.65PMN-0.32PT-0.03BZN
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Fig. 6. Domain patterns of Sample #2 (tetragonal crystal) observed under polarized light microscope at various temperatures (upon heating).
196 C. These two variations indicate the first-order nature of the phase transition at TC. It is found that the room temperature birefringence of this tetragonal crystal (Dn z 9.4 103) is almost 8 times larger than that of the rhombohedral crystal, confirming the larger structural distortion and enhanced long-range ferroelectric order in it. In addition, no frequency dispersion can be found in the peak temperatures of the dielectric constant (namely Tm ¼ TC), as shown in Fig. 8, which confirms that Sample #2 has a normal ferroelectric phase below TC. The composition of Sample #2 is deduced to be 0.61PMN0.37PT-0.02BZN based on the EDX studies, as listed in Table 2. It is clear that the concentration of Ti4þ of Sample #2 is higher than that in the rhombohedral crystal of Sample #1. The composition determined from the EDX data agrees well with the composition estimated from the phase transition temperature TC as a function of composition estimated in the PMN-PT-BZN ceramics [22,23], further confirming that the composition of Sample #2 falls in the tetragonal phase region.
tetragonal phase in the PMN-PT-based solid solutions. The high coercive field can be associated with enhanced crystal distortion arising from a higher concentration of strongly ferroelectrically active Ti4þ on the B-site compared with Sample #1 (The evidence will be given later). The piezoelectric measurements were performed on the crystal poled under 20 kV/cm at room temperature. The piezoelectric coefficient d33 is found to be about 300 pC/N. This relatively smaller value of piezoelectric coefficient compared with the reported d33 for MPB composition of the PMN-PT crystals [1e6] is probably associated with the following two factors: the composition of this crystal is away from the MPB region and the poling is not complete. PLM investigation was performed on the sample mirrorpolished to 48 mm. Large birefringent domains with the axes of the optical indicatrix section parallel to [100]/[010] directions are observed under PLM, as labeled in Fig. 6. The extinction occurs along the C directions (the edges of the platelet) and the domain walls align along the [110]C direction, which indicates that the crystal possesses in a tetragonal symmetry. Upon heating, the crystal transforms from the anisotropic tetragonal phase to an isotropic cubic phase at about 200 C (Fig. 6), which is confirmed by the birefringence (Fig. 7) and dielectric (Fig. 8) measurements. Fig. 7 shows that the birefringence gradually decreases upon heating up to 200 C where it undergoes a discontinuous drop to zero, corresponding to the tetragonal to cubic phase transformation at TC. Fig. 8 shows an abrupt increase of dielectric constant that peaks at
Sample #3 (330 mm thick) exhibits a quite unusual phase transition behavior. A well saturated ferroelectric hysteresis loop is found in this crystal with a coercive field EC ¼ 4.7 kV/cm and a remnant polarization Pr ¼ 27 mC/cm2, as displayed in Fig. 2(c). The
Fig. 7. Temperature dependence of birefringence (Dn ¼ nE - nO) measured on Sample #2 (tetragonal crystal). The discontinuous drop at 200 C indicates the first-order ferroelectric to paraelectric phase transformation upon heating.
Fig. 8. Temperature dependence of dielectric constant and loss tangent measured at various frequencies for Sample #2 (tetragonal crystal), showing the tetragonal to cubic phase transition at TC ¼ 196 C upon heating. The data were collected on an unpoled crystal.
3.3. MPB crystal
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reason for the smaller coercive field in this crystal compared with the rhombohedral or the tetragonal crystals will be discussed in detail in a later part. Nevertheless, this coercive field obtained from the ternary single crystal is almost 2 times as large as that of the PMN-PT binary crystal with the composition in the MPB region [6]. The average piezoelectric coefficient of Sample #3 (poled under 10 kV/cm for 20 min at room temperature) is found to be about 2000 pC/N (ranging from 1700 pC/N to 2200 pC/N in different parts of the crystal). The high piezoelectric coefficient found in this crystal indicates that the composition of this crystal is in the MPB region. In order to determine the structural origin of the high piezoelectric properties of this crystal, we further polish this crystal down to 45 mm in thickness for crystal optical study. Fig. 9 (a) shows the domain patterns of Sample #3 at room temperature. Four areas with different extinction positions (which will be discussed in the subsequent section) are clearly observed and marked as R1, R2, M1 and M2, respectively. After several heating/cooling cycles between room temperature and 300 C, the extinction behaviors remain almost unchanged, suggesting that all of these domains are stable and the extinction features are intrinsic. Some parts of the crystal (R1 and R2) become totally dark with an extinction angle of 45 /45 , indicating a rhombohedral symmetry. Further analysis with the assistance of an adjustable compensator reveals that the optical indicatrix sections of R1 and R2 are perpendicular to each other as marked in Fig. 9(b), giving rise to the broad (or unshaped) domain boundaries separating R1 and R2 domains, as illustrated in Fig. 9(b).
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For other parts marked as M1 and M2, the extinction angles are 25 and 25 , respectively, at room temperature, with their indicatrix configurations indicated in Fig. 9(c), suggesting the presence of a lower symmetry phase that coexists along with the rhombohedral phase and confirming the MPB feature of this crystal. It is shown in Fig. 10 (a) that the extinction angles of R1 and R2 remain unchanged upon heating until about 47 C (45e55 C for different parts of the crystal), at which they abruptly change to 35 and 19 , respectively. At the same time, the M1 and R2 regions merge at this temperature, as illustrated in Fig. 9 (d), indicating a phase transformation from rhombohedral phase to another phase of lower symmetry upon heating. It is shown in Table 1 that any of the monoclinic MA, MB, MC phases or a triclinic phase could demonstrate the extinction behavior mentioned above with an extinction angle 0 < jqj < 45 . Therefore, it may be difficult to determine unambiguously the exact crystal symmetry merely through the PLM studies. However, when combining the PLM results with the results of theoretical analysis and structural study on the PMN-PT system [7,11,18,29,30], we can conclude that this low symmetry phase is most likely a monoclinic MA phase. Firstly, monoclinic phases have been found in the piezoelectric single crystals such as PZT, PMN-PT and PZN-PT based systems, which act as a bridging phase linking the rhombohedral and tetragonal phases in the MPB region. Secondly, among all the monoclinic phases (MA, MB and MC), MA phase is more energetically favorable and easier to form from the rhombohedral phase. The spontaneous polarization of MA phase, which lies within the {110}C
Fig. 9. Optical domain structure and phase transformation sequence in Sample #3. (a) Four areas with different extinction behaviors observed at room temperature. (b) Two areas, R1 and R2, with both 45 extinction angle but perpendicular optical indicatrix sections. (c) Two areas, M1 and M2, with extinction angle of ±25 , suggesting the coexistence of monoclinic and rhombohedral phases at room temperature. (d) Monoclinic phase in areas R1 and R2, and merging areas of R2 and M1 at 55 C, suggesting a rhombohedral to monoclinic phase transformation. (e) Appearance of an isotropic phase at 160 C, indicating a monoclinic to cubic phase transformation.
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Fig. 10. (a) Variations of the extinction angle and (b) the birefringence (Dn ¼ nE-nO) as a function of temperature for areas R1, R2, M1 and M2 of Sample #3 (MPB crystal). (c) The values of extinction angles and birefringence as a function of temperature in areas R1 and R2 exhibit discontinuous decrease upon the phase transformation from rhombohedral phase to monoclinic phase.
plane of the unit cell, is crystallographically free to rotate within the plane between the C and C directions [Fig. 10 (c)] and energetically, this is the most favorable way to allow the polarization to rotate between the rhombohedral and tetragonal phases. In addition, the first-principles calculations on a single-domain crystal of BaTiO3 showed that the lowest free-energy pathway for the polarization to rotate under an external electric field is from the rhombohedral phase (Ps//C) to the MA phase and then to the tetragonal phase (Ps//C) within the {110}C plane, rather than through the MB phase to an orthorhombic phase, and the MC phase [19]. Based on the eighth-order Devonshire theory analysis of perovskite ferroelectrics [30], a phase diagram was established in the space of the dimensionless parameters a and b which reflect the importance of the coefficients of the fourth, sixth and eighth order terms in the free energy expansion. It was found in the phase diagram that the areas covered by the MC and MB regions are much smaller than the area covered by the MA phase, which suggests that the MC and MB phases are more difficult to form than the MA phase in real materials [30]. So, the rhombohedral to MA phase transformation is more favorable than the rhombohedral to MB phase transformation. Thirdly, to the best of our knowledge, the phase transition from rhombohedral to MC phase usually does not happen unless it has MB as an intermediate phase [31]. Based on the above
analysis, we conclude that this low symmetry phase is most likely of MA symmetry. With the further increase of temperature from around 50 C, as illustrated in Fig. 10 (a), the values of the extinction angles of regions R1, R2, M1 and M2 gradually decrease until around 160 C, at which they all suddenly drop to zero [see Fig. 9(e)], indicating the disappearance of the anisotropic feature and the appearance of the isotropic paraelectric phase. The monotonous and smooth decrease of the extinction angles between 50 C and 160 C reflects the continuous polarization rotation within the monoclinic planes, as shown in Fig. 10 (c). The two phase-transformations of these regions, i.e. from rhombohedral to monoclinic (R / M) and from monoclinic to cubic (M / C), are also demonstrated by the variation of birefringence as a function of temperature which shows two discontinuities at around 50 C and 160 C, respectively [Fig. 10 (b)]. It is worth noting that the room-temperature birefringence (Dn z 5.1 102) of Sample #3 is much larger than that of the rhombohedral (Sample #1) or tetragonal (Sample #2) crystal, indicating a stronger anisotropy that may contribute to the high piezoelectric coefficient in this crystal. It is interesting that there are no other phase transitions (e.g. monoclinic to tetragonal) before reaching the cubic paraelectric phase for both R1/R2 and M1/M2 regions, suggesting unusual, direct rhombohedral / monoclinic / cubic (R / M / C), and monoclinic / cubic (M / C) phase transformation sequences in the PMN-PT-BZN ternary single crystals, which are clearly shown in Figs. 9 and 10. The unusual R / M / C phase transition sequence is worthy being discussed as the crystal symmetry does not increase monotonously with the increasing temperature. It is well known that the thermodynamic free energies of the different phases are very close in the MPB system and the symmetry of a crystal is determined by various factors such as temperature, electric field, mechanical stress and chemical potential, etc [32,33]. In a complex perovskite system like the currently studied PMN-PT-BZN, due to the delicate energy balance and the specific domain structures, together with the possible defects which would involve large microstrains at the MPB region, the different phases around and within the MPB region will maintain an elusive stability to stay in a local minimum of the equilibrium free energy landscape and will transform spontaneously under external stimulation. Though the R-M-C phase transition is an unusual experimentally observed sequence compared with most of the reported PMN-PT- and PZNPT-based systems, this finding can be explained within the framework of Landau-Devonshire theory [30] which predicts the existence of monoclinic phase(s) and the possible transformations between other different phases. A similar phase transition sequence of R / M / C phase transition sequence was also found in the C-cut PMN-PT crystal [15]. Taking a closer look at the R / M and M / C phase transformations in individual domains by investigating the PLM images at variable temperature, we find that the region with a lower R / M phase transformation temperature (TR-M) has a higher M / C transition temperature (TM-C). Fig. 11 demonstrates the evolution of the domain structure and phase components undergoing the R / M and M / C phase transformations upon heating. It is found that the low temperature R / M phase transformation proceeds with the inter-phase boundary moving to the left side [from R2 towards R1, see Fig. 9(a)] as shown in Fig. 11(a), while the high temperature M / C phase transformation takes place in the reverse path [Fig. 11 (b)], which indicates that TR-M (R2) < TR-M (R1), and TM-C (R1) < TM-C (R2). The observed different phase transition temperatures in different regions indicate a slight composition gradient in the crystal. The ZFH dielectric measurement was performed on the crystal poled under 10 kV/cm. Two anomalies are observed in the
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Fig. 11. The progressing of the phase transformation in Sample #3 upon heating through (a) the rhombohedral to monoclinic, and (b) the monoclinic to cubic, phase sequences. Two opposite directions on the sketches indicate that a lower rhombohedral to monoclinic phase transformation temperature corresponds to a higher monoclinic to cubic transition temperature.
temperature dependence of the dielectric constant, as shown in Fig. 12(a), corresponding to the R / M phase transformation at around 50 C and the M / C transition at around 160 C. The dielectric behavior of the depoled crystal was measured by performing the ZFC after the ZFH process, in which a significant thermal hysteresis between TM-R ¼ 36 C and TR-M ¼ 51 C can be found [Fig. 12(b)], suggesting the first-order nature of the rhombohedral to monoclinic phase transformation, which is consistent with the theoretical analysis [30]. The thermal hysteresis feature and the broad anomalies at the structural phase transition temperatures prove the coexistence of the rhombohedral and monoclinic phase in a wide temperature range. The coexistence of the rhombohedral and monoclinic MA phases in this crystal at room temperature could explain the low EC value found in this crystal [Fig. 2(c)]. The coexistence of the multi-phases, allows for different possible polarization states (domain variants) and makes the polarization easily reorient under an applied electric field, giving rise to a lower coercive field [34,35]. At the same time, the existence of the monoclinic phase which facilitates the polarization rotation among different domain states, leads to the ultrahigh piezoelectricity obtained from this crystal.
The EDX measurements carried out on different parts of Sample #3 show no significant difference in composition across the crystal within the measurement error, which is indicative of a relatively weak composition fluctuation in this crystal which is beyond the detecting limit of the EDX technique. The average composition of Sample #3 is determined to be around 0.65PMN-0.32PT-0.03BZN (Table 2), which lies in the MPB region determined for the PMN-PTBZN ceramics [22,23].
Fig. 12. Temperature dependence of dielectric constant and loss tangent measured at various frequencies for Sample #3 (MPB crystal): (a) after being poled under an electric field of 10 kV/cm at room temperature, and (b) by zero-field-cooling (ZFC) after ZFH.
Fig. 13. Proposed MPB phase diagram for the pseudo-binary (1-x)(PMN-BZN)-xPT crystal system, demonstrating the unusual rhombohedral to monoclinic and monoclinic to cubic phase transformations.
3.4. Morphotropic phase diagram The three crystals of PMN-PT-BZN studied in this work show different phase symmetries, phase components and phase transition sequences. This results from composition segregation phenomenon that occurs inevitably in solid solution crystal such as in the PMN-PT system [24,36e38]. Given the relatively small amount of BZN in this ternary system, it is reasonable to believe that it is primarily the different PT concentration that leads to the various symmetries and phase behaviors. The rhombohedral symmetry of Sample #1 suggests that its PT concentration is the lowest, while the tetragonal crystal (Sample #2) should have the highest PT content. This reasoning is supported by the results of the compositional analysis by EDX, which show that the PT concentration is 26, 32 and 37 mol% for Samples #1, #3 and #2, respectively (Table 2). Based on the results on the phase symmetries and phase transitions of these three crystals, a temperature-composition phase diagram has been established for the pseudobinary system
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of (1-x)(PMN-BZN)-xPT around the MPB region, as illustrated in Fig. 13. This phase diagram delimits the various phases and shows the unusual rhombohedral to monoclinic to cubic, and monoclinic to cubic phase transformations. Since the MPB and related phase transformation behaviors in complex perovskites are quite sensitive to various factors such as chemical composition, defects, processing parameters, external electric field and mechanical stress, further investigations are still underway to explore the underlying mechanisms of this unusual phase transformation and to clarify the relationship between the microstructure and macroscopic electromechanical properties. 4. Conclusions Single crystals of a novel ternary Pb(Mg1/3Nb2/3)O3-Bi(Zn2/3Nb1/ solid solution system were grown by the high temperature solution growth method. The domain structure, electric properties and phase transformations were investigated on three selected (001)C-cut crystal platelets, which exhibit a rhombohedral phase (#1), a mixture of morphotropic phases (#3), and a tetragonal phase (#2), respectively, at room temperature, as a result of increasing PT concentration across the morphotropic phase boundary region. Most interestingly, unusual phase transformation sequences from rhombohedral to monoclinic and to cubic phases, and from monoclinic to cubic phases were observed in the MPB crystal by PLM, which were further confirmed by birefringence and dielectric measurements as a function of temperature. A temperature-composition phase diagram is established for the pseudobinary system of (1-x)(PMN-BZN)-xPT, showing the various phases and their transformations. Moreover, the results of temperature-dependent optical domain studies demonstrate a continuous polarization rotation within the {110}C plane of the unit cell upon heating, which is believed to contribute to the development of the unusual phase transformation sequences and to result in significantly high piezoelectricity (d33 > 2000 pC/N) and enhanced ferroelectric properties in the MPB crystal. These high piezo-/ferroelectric properties suggest that the ternary PMN-PTBZN single crystals constitute a new family of ferroelectric and piezoelectric materials potentially useful for high power electromechanical transducer applications. These studies provide new insights into the complex MPB phase symmetry, phase components and phase transformations, and prove experimentally the polarization rotation mechanism that indeed leads to enhanced piezoelectricity in relaxor-based ferro-/piezoelectric crystals with complex perovskite structure. This work provides a better understanding of the microscopic mechanisms of high piezoelectric response in piezocrystals, and will be helpful in designing new and better piezoelectric single crystals. It will stimulate further studies on this and related subjects both experimentally and theoretically. 3)O3-PbTiO3
Acknowledgments This work was supported by the U. S. Office of Naval Research (Grants No. N00014-12-1-1045 and N00014-16-1-3106), the Natural Sciences and Engineering Research Council of Canada (NSERC, Grant No. 203773), the National Natural Science Foundation of China (Grant Nos. 51332003 and 51202184), the International Science and Technology Cooperation Program of China (Grant No. 2011DFA51880), and the “111 Project” of China (Grant No. B14040). ZL would like to acknowledge the China Scholarship Council for supporting his studies at SFU. The EDX work was carried out at International Center for Dielectric Research (ICDR), Xi'an Jiaotong University. The authors would like to thank Dr. Yijun Zhang and Zeng Luo for their help in the experiments.
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