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Crystal structure and piezoelectric characteristics of various phases near the triple-point composition in PZ-PT-PNN system
Journal of the European Ceramic Society xxx (xxxx) xxx–xxx
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Original Article
Crystal structure and piezoelectric characteristics of various phases near the triple-point composition in PZ-PT-PNN system Tae-Gon Leea, Sun-Woo Kimb, Eun-Ji Kimb, Sang Jin Leeb,c, Hyun-Gyu Hwanga, Youn-Woo Hongd, Jeong Seog Kime, Keun Hwa Chaef, Ji-Won Choig, Chong-Yun Kanga,g, Sahn Nahma,b,* a Department of Nano Bio Information Technology, KU-KIST Graduate School of Converging Science and Technology, Korea University, 141 Anam-ro, Seongbuk-gu, Seoul, 02841, Republic of Korea b Department of Materials Science and Engineering, Korea University, 141 Anam-ro, Seongbuk-gu, Seoul, 02841, Republic of Korea c Electronic Materials and Device Research Center, Korea Electronics Technology Institute, Seongnam, Gyeonggi-do, Republic of Korea d Korea Institute of Ceramic Engineering and Technology, 101, Soho-Ro, Jinju-Si, Gyeongsangnam-do 660-031, Republic of Korea e Department of Materials Science and Engineering, Hoseo University, 20, Hoseo-ro 79 beongil, Baebang-eup, Asan-si, Chungcheongnam-do 31499, Republic of Korea f Advanced Analysis Center, Korea Institute of Science and Technology (KIST), Seoul 02792, Republic of Korea g Center for Electronic Materials, Korea Institute of Science and Technology (KIST), Seoul, 02791, Republic of Korea
A R T I C LE I N FO
A B S T R A C T
Keywords: Piezoelectricity Ceramic material Phase diagram Nanodomains Landau theory
Crystal structures and piezoelectric properties of PbZrO3-PbTiO3-Pb(Ni1/3Nb2/3)O3 ceramics near the triple point composition, particularly characteristics of the pseudocubic phase, were investigated. The pseudocubic phase, which formed near the triple point composition, disappeared with increase in the PbZrO3 content. The pseudocubic phase had the Pm3m cubic structure. The tetragonal-pseudocubic morphotropic phase boundary (MPB) structure was developed during the tetragonal-to-cubic phase transformation. However, the rhombohedral phase directly transformed to the cubic phase because the structure of pseudocubic phase was similar to the rhombohedral structure. The specimens with pseudocubic phase and the specimens near pseudocubic phase exhibited nano-sized domains and small coercive electric fields, revealing their low domain wall energies. These specimens exhibited second-order ferroelectric-to-paraelectric phase transition and low Curie temperatures, confirming their low domain wall energies. The enhanced dielectric and piezoelectric properties of these specimens could be attributed to their low domain wall energies.
1. Introduction Pb(Zr,Ti)O3 (PZT)-based piezoelectric materials have been used in various electrical devices including multilayer actuators, sensors, transformers, and piezoelectric energy harvesters because of superior piezoelectric properties [1–8]. In particular, the interest in the piezoelectric actuators and energy harvesters has been increased because of the development of portable haptic and mobile devices [9–12]. For these device applications, piezoelectric ceramics should exhibit large piezoelectric constants (dij), piezoelectric voltage constants (gij), and displacements. PbZrO3-PbTiO3-Pb(Ni1/3Nb2/3)O3 (PZ-PT-PNN) ceramics have been investigated for these device applications because of their excellent piezoelectric properties [13–17]. Moreover, low-temperature sintering of these ceramics has been also studied for
piezoelectric multilayer device [18–20]. Recently, a schematic phase diagram of these ceramics was reported, and the structural and piezoelectric properties of the PZ-PT-PNN ceramics near the triple point composition were investigated [21]. Moreover, the application of these ceramics for the development of piezoelectric energy harvesters has been explored [22]. Most of the previous studies were focused on the rhombohedral-tetragonal (R–T) morphotropic phase boundaries (MPBs) in PZ-PT-PNN ceramics. However, the structural and piezoelectric property changes occurring in these ceramics during the tetragonal (or rhombohedral)-to-cubic phase transformation have not been investigated. In particular, the properties of the pseudocubic phase have not been studied in detail. In this study, four groups of PZ-PT-PNN ceramics were fabricated to study the changes in the structural and piezoelectric properties near the
⁎ Corresponding author at: Department of Nano Bio Information Technology, KU-KIST Graduate School of Converging Science and Technology, Korea University, 141 Anam-ro, Seongbuk-gu, Seoul, 02841, Republic of Korea. E-mail address: [email protected] (S. Nahm).
https://doi.org/10.1016/j.jeurceramsoc.2019.12.063 Received 12 August 2019; Received in revised form 27 December 2019; Accepted 30 December 2019 0955-2219/ © 2019 Published by Elsevier Ltd.
Please cite this article as: Tae-Gon Lee, et al., Journal of the European Ceramic Society, https://doi.org/10.1016/j.jeurceramsoc.2019.12.063
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Fig. 1. Schematic phase diagram of PZ-PT-PNN ceramics near the triple point composition.
monochromatic XRD (Rigaku D/Max-RC, Tokyo, Japan) analysis was performed with the Cu Kα1 radiation. In addition, synchrotron XRD measurements were performed on the 5A XRS KIST-PAL beamline at PLS (Pohang, Korea). A monochromatic radiation (1.0716 Å) was obtained using a double crystal Si (111) monochromator. Powder diffraction patterns were recorded in the Bragg-Brentano geometry in the angular interval 20°–60° (2θ) with a constant 0.005° step and a counting time of 1 s per step. These XRD patterns were analyzed by Rietveld refinement using the Rietan software to determine the structure of the pseudocubic phase. Field-emission transmission electron microscopy (FETEM; Tecnai F20, FEI, Netherlands) was used to investigate the domain structures of the specimens. The specimens with silver electrodes were poled by a DC field of 3 kV/mm for 15 min at room temperature (RT). The d33 values of the specimens were measured using a d33 meter (Micro-Epsilon Channel Product DT-3300, Raleigh, NC, USA). The relative permittivity (εT33/εo), dielectric loss (tan δ), and coupling factor (kp) of the specimens were measured with an impedance analyzer (Agilent Technologies HP 4194A, Santa Clara, California). The εT33/εo and tan δ values were obtained as functions of temperature using an LCR meter (Agilent Technologies HP 4194A, Santa Clara, CA, USA) in an automated temperature-controlled furnace equipped with a computer interface for data acquisition. The polarization vs. electric field (P-E) hysteresis loops were obtained using a modified Sawyer-tower circuit by performing the measurements in silicon oil to prevent electrical flashover at the fixed frequency of 1 Hz
triple point composition, occurring during the tetragonal (or rhombohedral)-to-cubic phase transformation. Fig. 1 shows a schematic phase diagram of the PZ-PT-PNN ceramics near the triple point composition; the four groups of specimens are marked on four lines in this figure. The 0.1PZ-(0.9-x)PT-xPNN ceramics with 0.52 ≤ x ≤ 0.65 are marked on Line 1 in Fig. 1 and these specimens were selected to investigate the structural and piezoelectric property changes accompanying the tetragonal-to-cubic phase transformation. The (0.72-x)PZ-0.28PT-xPNN ceramics with 0.52 ≤ x ≤ 0.65 (Line 2 in Fig. 1) were chosen to investigate the property variations occurring during the rhombohedralto-cubic structural transformation. Pseudocubic phases were detected in these two groups of specimens. The (0.9-x)PZ-0.1PT-xPNN ceramics with 0.48 ≤ x ≤ 0.56 (Line 4 in Fig. 1) were used to study the crystal structure and piezoelectric property changes occurring during the rhombohedral to cubic structure transformation. The pseudocubic phase was not detected in these specimens. In addition, the specimens marked on Line 3 in Fig. 1 with compositions (0.8-x)PZ-0.2PT-xPNN (0.50 ≤ x ≤ 0.60) were used to study the various properties of the specimens near the triple point composition. In particular, the characteristics of the pseudocubic phase, such as the domain structure, domain wall energy, and ferroelectric-to-paraelectric phase transition, were investigated. 2. Experimental Four groups of PZ-PT-PNN ceramics (marked in Fig. 1) were produced by the conventional ceramic process. Typically, PbO (99.5 %, Dansuk Industry, Korea), ZrO2 (98 %, Z-Tech, USA), TiO2 (99.2 %, Sakai Kagaku, Japan), NiO (99.5 %, Nikko Rika, Japan), and Nb2O5 (99.8 %, Jiujiang, China) were weighed and mixed with zirconia balls for 24 h in a nylon jar; anhydrous ethanol was used as the solvent. Next, the mixed powders were dried and calcined at 880 °C for 4 h. The calcined powders were then re-milled for 24 h and dried. Finally, the dried powders were sieved to obtain fine ceramic powders. These ceramic powders were pressed into disc-shaped pellets under a pressure of 9.8 MPa and sintered at 1150 °C for 2 h in air. The densities of the specimens were measured by Archimedes’ method. The structural properties of the specimens were investigated by Xray diffraction (XRD; Rigaku D/Max-RC, Tokyo, Japan) with the Cu Kα radiation, and scanning electron microscopy (SEM; Hitachi S-4800, Osaka, Japan). The XRD reflections at 65.5° were obtained by lowspeed scanning of the specimens and then deconvoluted using the Voigt function to identify the crystal structures. The rate and resolution of low-speed scanning were 0.3°/min and 0.01°, respectively. Moreover, to obtain the XRD patterns of the specimens with pseudocubic phases,
3. Results and discussion 3.1. Crystal structure analysis All specimens investigated in this study exhibit homogeneous perovskite structures, without any secondary phases, as shown in Fig. S1(a)-(d). Fig. 2(a)-(d) shows the SEM images of the fractured surfaces of the specimens selected from each group. As observed, they exhibit dense microstructures with similar average grain sizes (approximately 2.0 μm). Therefore, the effect of microstructure on the piezoelectric properties is expected to be the same for all the specimens. It was difficult to identify the exact crystal structures of the specimens using the normal XRD patterns. Therefore, the XRD reflections at 65.5° were recorded by low-speed scanning and deconvoluted using the Voigt function for a detailed analysis of the crystal structures of the specimens. Fig. 3(a–i)-(a–v) shows the XRD reflections at 65.5° of 0.1PZ-(0.9-x) PT-xPNN ceramics with 0.54 ≤ x ≤ 0.65, which are marked on Line 1 in Fig. 1. The specimen with x = 0.54 exhibits tetragonal (202)T and 2
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Fig. 2. SEM images of PZ-PT-PNN ceramics: (a) 0.1PZ-0.32PT-0.58PNN, (b) 0.14PZ-0.28PT-0.58PNN, (c) 0.25PZ-0.2PT-0.55PNN, and (d) 0.38PZ-0.10PT-0.52PNN.
0.57 ≤ x ≤ 0.63 were determined as the pseudocubic phase owing to their piezoelectric properties whereas the specimens with x ≥ 0.64 were concluded as the cubic phase due to their absence of piezoelectric properties (Fig. 4(b)). Since the structure of pseudocubic phase also can be interpreted by the rhombohedral structure, the XRD reflections of the specimens with x = 0.57 and 0.63 were deconvoluted to the (220)P and (2–20)P reflections, as shown in Fig. S3. In addition, the XRD pattern of the specimen with x = 0.56 can be analyzed as a rhombohedral structure only while that of the specimen with x = 0.57 can be analyzed as a cubic structure as well as a rhombohedral structure. Therefore, the crystal structures of the specimens with x = 0.56 and 0.57 are considered to have rhombohedral and pseudocubic phases, respectively. Interestingly, the rhombohedral phase directly transforms into the pseudocubic phase, probably because the structure of pseudocubic phase is similar to the rhombohedral structure. Variation in the crystal structures of the specimens marked on Line 3 in Fig. 1 was also studied, as shown in Fig. S4(a)-(e). Crystal structures of these specimens change from rhombohedral to pseudocubic, and then, to cubic with increase in x, confirming that the rhombohedral structure directly changes to the pseudocubic phase structure. In addition, the structural properties of the specimens marked on Line 4 in Fig. 1 were investigated (Figs. S5(a)-(c)). Crystal structures of these specimens change from rhombohedral to cubic. These results show that the pseudocubic phase exists near the triple point composition, and it disappears with increase in the PZ content; however, it remains in the PT region [21]. In order to determine the exact crystal structure of pseudocubic phase, Rietveld XRD analyses and Raman spectra analysis were conducted and their results were discussed in Supplementary Information 6–8 in detail. The results of the Rietveld analysis using monochromatic XRD indicate that the crystal structure of the pseudocubic phase is a rhombohedral R3m structure as well as a cubic Pm3m structure, as shown in Fig. S6. Furthermore, the Raman spectra results also show that the pseudocubic phase can be explained by the rhombohedral
(220)T reflections (Fig. 3(a–i)); similar results are observed for the specimens with 0.52 ≤ x ≤ 0.54, indicating that these specimens have tetragonal structures. Pseudocubic (220)P reflection is observed for the specimens with x = 0.55 and 0.6, as shown in Fig. 3(a–ii) and (a–iii), respectively. Therefore, the specimens with 0.55 ≤ x ≤ 0.6 have tetragonal-pseudocubic MPB (T-P MPB) structure. The specimen with x = 0.61 shows only pseudocubic (220)P reflection (Fig. 3(a-iv)); similar results are observed for the specimens with x = 0.62 and 0.63. The specimens with x ≥ 0.64 also exhibit cubic structures, as indicated by the XRD pattern of the specimen with x = 0.64 (Fig. 3(a–v)). Although the specimens with x ≥ 0.61 exhibit a single (220) XRD reflection indicating the cubic structure, only the specimens with 0.61 ≤ x ≤ 0.63 were determined as a pseudocubic phase because they showed the piezoelectric properties (Fig. 4(a)). On the other hand, the specimens with x ≥ 0.64 were concluded as a cubic phase due to their absence of piezoelectric properties (Fig. 4(a)). In addition, the pseudocubic (220)P reflection observed for the specimens with 0.61 ≤ x ≤ 0.63 also can be interpreted as rhombohedral structure of (220)P and (2–20)P reflections, as shown in Fig. S2, revealing that the structure of pseudocubic phase can be considered as a the rhombohedral structure; this will be discussed later using the Rietveld analysis results. Therefore, the crystal structure of the pseudocubic phase can be explained by the cubic structure as well as the rhombohedral structure. However, the crystal structure of the rhombohedral phase can be explained only by the rhombohedral structure. Variation in the crystal structures of (0.72-x)PZ-0.28PT-xPNN ceramics with 0.52 ≤ x ≤ 0.65, which are marked on Line 2 in Fig. 1, was also investigated (Fig. 3(b–i)-(b–v)). Specimens with x = 0.52 and 0.56 show the rhombohedral (220)R and (-220)R reflections (Fig. 3(b–i) and (b–ii)), revealing the development of a rhombohedral structure in them. On the other hand, the specimens with x ≥ 0.57 exhibit a single (220) XRD reflection, indicating the cubic structure, and XRD reflections of the specimens with x = 0.57, 0.63 and 0.64 were expressed in Fig. 3(b–iii)-(b–v), respectively. Here, the specimens with 3
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relaxor ceramic has a polar nano-region [26,27]. The specimens (0.52 ≤ x ≤ 0.54) with tetragonal structures exhibit a kp value of 0.52, and the specimen with x = 0.55 exhibits a higher kp value of 0.56 owing to the presence of the T-P MPB structure. The value considerably decreases when x exceeds 0.6 because of the formation of a pure pseudocubic phase. Moreover, it was difficult to measure the kp values of specimens with 0.64 ≤ x ≤ 0.65 due to the formation of cubic structure. The maximum phase angle (θmax) was measured at frequencies between the antiresonance and resonance frequencies of specimens. θmax is close to 90° for an ideal ferroelectric ceramic and -90° for a paraelectric ceramic with a cubic structure [28,29]. The specimens with tetragonal and T-P MPB structures (0.52 ≤ x ≤ 0.6) exhibit large θmax values raging between 82° and 67°, and it decreases when x exceeds 0.6 because of the formation of a pure pseudocubic phase. Moreover, the specimens with x = 0.64 and 0.65 exhibit very low θmax values (close to -83°) owing to the formation of paraelectric cubic structure. In addition, with increase in x, the εT33/εo value continuously increases because of the decrease in TC. Fig. 4(b) shows the variations in the d33, kp, θmax, and εT33/εo values of the (0.72-x)PZ-0.28PT-xPNN ceramics with 0.52 ≤ x ≤ 0.65, which are marked on Line 2 in Fig. 1. The d33 value of the specimen slightly increases with increase in x. The maximum d33 value of the 490 pC/N is observed for the specimen with a rhombohedral structure (x = 0.56). When x exceeds 0.56, the d33 value starts decreasing because of the formation of the pseudocubic phase. However, the specimens with pseudocubic phase (0.58 ≤ x ≤ 0.6) show large d33 values ranging between 475 and 365 pC/N. However, very low d33 values are observed for specimens with cubic structures (x ≥ 0.64). The specimens with rhombohedral structures (0.52 ≤ x ≤ 0.56) show relatively large kp values (0.44-0.48). The value decreases when x exceeds 0.56 because of the formation of the pseudocubic phase. Finally, it was very difficult to measure kp values of specimens with x ≥ 0.64, confirming the formation of cubic structures in these specimens. The variation in θmax is similar to that in kp. The specimens with rhombohedral structures (0.52 ≤ x ≤ 0.56) show large θmax values; the θmax value decreases when x exceeds 0.56 because of the formation of the pseudocubic phase. Finally, the specimens with x ≥ 0.64 exhibit a θmax value close to -83°, indicating the formation of cubic structures in these specimens. In addition, the εT33/εo values continuously increase with increase in x because of the decrease in TC. The variations in d33, kp, θmax, and εT33/εo values of (0.8-x)PZ-0.2PTxPNN ceramics with 0.5 ≤ x ≤ 0.6 are shown in Fig. 4(c). These specimens are marked on Line 3 in Fig. 1. The d33 value of the specimen with x = 0.5 is approximately 265 pC/N and it increases with increase in x. The maximum d33 value of 388 pC/N is observed for the specimen with x = 0.55. The d33 value decreases when x exceeds 0.56, probably because of the formation of the pseudocubic phase. Finally, the specimen with x = 0.6 shows a very small d33 value because of the formation of the cubic structure. Variation in the kp value with respect to x is similar to that in the d33 value. The specimens with rhombohedral structures exhibit relatively large kp values; however, the specimens with pseudocubic phases exhibit low kp values. Notably, it was difficult to measure the kp value for the specimen with a cubic structure. Moreover, the specimens with rhombohedral structures exhibit large θmax values, indicating that they possess ferroelectric properties. Nonetheless, the specimens with pseudocubic phases exhibit considerably low θmax values. Finally, the specimen with a cubic structure (x = 0.6) shows a θmax value of -81°, indicating that it is a paraelectric material. In addition, the εT33/εo value increases with increase in x because of the decrease in TC, and the specimen with a cubic structure (x = 0.6) exhibits a very large εT33/εo value of 8870. The piezoelectric properties of (0.9-x)PZ-0.1PT-xPNN ceramics with 0.48 ≤ x ≤ 0.56 were also investigated (Fig. 4(d)) and these specimens are marked on Line 4 in Fig. 1. The d33 values of the specimens with 0.48 ≤ x ≤ 0.52 are relatively small (ranging between 150 pC/N and 159 pC/N) and it starts decreasing at x = 0.53. Finally, the specimens
Fig. 3. XRD patterns showing peaks at 65.5° obtained by slow-speed scanning for 0.1PZ-(0.9-x)PT-xPNN ceramics: (a–i) x = 0.54, (a-ii) x = 0.55, (a–iii) x = 0.60, (a–iv) x = 0.61, and (a–v) x = 0.64. XRD patterns showing peaks at 65.5° obtained by slow-speed scanning for (0.72-x)PZ-0.28PT-xPNN ceramics: (b–i) x = 0.52, (b–ii) x = 0.56, (b–iii) x = 0.57, (b-iv) x = 0.63, and (b–v) x = 0.64.
phase and the cubic phase, as shown in Fig. S7(a) and (b), respectively. However, according to the Rietveld analysis using a synchrotron XRD, the pseudocubic phase is expected to have a Pm3m cubic structure, as shown in Fig. S8(a)-(c) and Table S2. Therefore, the crystal structure of pseudocubic phase is finally concluded as a Pm3m cubic structure.
3.2. Piezoelectric properties of the specimens The d33, kp, θmax, and εT33/εo values of 0.1PZ-(0.9-x)PT-xPNN ceramics with 0.52 ≤ x ≤ 0.65 were investigated (Fig. 4(a)) and these specimens are marked on Line 1 in Fig. 1. The d33 value of the specimen increases with increase in x. Specimens with 0.55 ≤ x ≤ 0.6 exhibit larger d33 values because of the formation of T-P MPB structures in these specimens. When x exceeds 0.6, the d33 value decreases because of the formation of a pure pseudocubic phase. The specimen with the pseudocubic phase (x = 0.61) has a relatively large d33 value of 330 pC/N even though its crystal structure is the Pm3m cubic structure as shown in Fig. 3(a-iv). Previous studies reported that cubic relaxor ceramics, which have a non-ergodic characteristic, exhibit piezoelectric properties because their structure changed from cubic to noncubic after electric field application [23–25]. Therefore, the pseudocubic phase developed in this study is considered to have the non-ergodic characteristic. On the other hand, specimens with x ≥ 0.64 exhibit small d33 values owing to the formation of cubic structure. The specimens with cubic structures exhibit small d33 values probably because the PNN 4
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Fig. 4. The d33, kp, θmax, and εT33/εo values of the PZ-PT-PNN ceramics: (a) 0.1PZ-(0.9-x)PT-xPNN ceramics with 0.52 ≤ x ≤ 0.65, (b) (0.72-x)PZ-0.28PT-xPNN ceramics with 0.52 ≤ x ≤ 0.65, (c) (0.8-x)PZ-0.2PT-xPNN ceramics with 0.5 ≤ x ≤ 0.6, and (d) (0.9-x)PZ-0.1PT-xPNN ceramics with 0.48 ≤ x ≤ 0.56.
with x ≥ 0.55 show a very small d33 value of 48 pC/N. The kp and θmax values show similar variations with respect to x. Therefore, these results show that the specimens with 0.48 ≤ x ≤ 0.54 have rhombohedral structures, and in the specimens with x ≥ 0.55, cubic structures are developed. Finally, the εT33/εo values continuously increase with increase in x because of the decrease in TC.
domains (100 nm × 5 nm). Therefore, these results show that nanosized domains are formed in specimen with pseudocubic phase and the specimens close to the pseudocubic phase. Furthermore, TEM analysis was conducted on specimens that are far away from the pseudocubic phase. Fig. 6(a) shows a TEM bright field image of the 0.19PZ-0.4PT-0.41PNN ceramic with a tetragonal structure (T2, Fig. 1). This bright-field image was obtained by slightly tilting the specimen from the [1 2 0] zone axis (see inset in Fig. 6(a)). This specimen comprises domains with sizes 1.3 μm × 100 nm, which are much larger than those of specimen T1. Moreover, 90° and 180° domain walls are observed in this specimen, as indicated by green and red lines, respectively, in Fig. 6(a) [30–34]. Domain structure of the 0.27PZ0.32PT-0.41PNN ceramic with a rhombohedral structure (R2, Fig. 1) was also investigated by TEM. Fig. 6(b) shows a bright-field image of this specimen obtained by slightly tilting the specimen from the [0 2 1] zone axis. This specimen exhibits large lamella-shaped domains (400 nm × 25 nm) with 71° and 109° domain walls, as indicated by green and red dotted lines, respectively, in Fig. 6(b) [30–35]. Furthermore, TEM analysis was conducted on the 0.38PZ-0.1PT-0.52PNN ceramic with a rhombohedral structure, as shown in Fig. 6(c). This specimen is close to the cubic phase (R3, Fig. 1). The R3 specimen exhibits large domains (300 nm × 50 nm). Moreover, the domains of R2 and R3 ceramics are much larger than those of the R1 ceramic. Therefore, the TEM results show that domains of the specimens close to pseudocubic phase are much smaller than those of the domains in the
3.3. TEM analysis for domain structure TEM analysis was carried out to identify the domain structures of specimens with the pseudocubic phase (or specimens close to pseudocubic phase). Fig. 5(a) shows a bright-field TEM image of the 0.12PZ0.28PT-0.6PNN ceramic with a pseudocubic phase (P1, Line 2 in Fig. 1). This bright-field image was obtained by slightly tilting the sample from the [1-1-1] zone axis to clearly identify its domain structure (see inset in Fig. 5(a)). This specimen shows several small domains with sizes of a few nanometers, revealing the formation of nanodomains in this specimen. TEM analysis was also conducted on the 0.1PZ-0.36PT-0.54PNN ceramic (Fig. 5(b)) with a tetragonal structure (T1, Line 1 in Fig. 1). This specimen comprises small strip-shaped domains (80 nm × 20 nm), revealing the formation of nanodomains in this specimen, as well. Moreover, the domain structure of the 0.16PZ-0.28PT-0.56PNN ceramic with a rhombohedral structure was analyzed using TEM, as shown in Fig. 5(c). This specimen is denoted as R1 on Line 2 in Fig. 1 and it is close to pseudocubic phase. This specimen also exhibits small 5
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Fig. 5. TEM bright-field images of specimens with various structures: (a) 0.12PZ-0.28PT-0.60PNN (P1) with pseudocubic phase, (b) 0.10PZ-0.36PT-0.54PNN (T1) with tetragonal structure, and (c) 0.16PZ-0.28PT-0.56PNN (R1) with rhombohedral structure. Inset in each figure shows the corresponding electron diffraction pattern; TEM bright-image of each figure has been taken using the corresponding diffraction pattern.
to the pseudocubic phase because of their low domain wall energies (small EC values). Interestingly, the specimens with pseudocubic phases exhibit large εT33/ε0, as shown in Fig. 4(a)-(c). Notably, εT33/ε0 is nearly proportional to (∂P/∂E)E=0 [21,37,38]. In general, if the domain rotation is easy (small ED), then the (∂P/∂E)E=0 value will be large [37,39–42]. Therefore, the large εT33/ε0 values of the specimens with pseudocubic phases can be attributed to their low ED values, resulting from the existence of nanodomains. Furthermore, the specimens with pseudocubic phases, have relatively large d33 values (≥ 300 pC/N), as shown in Fig. 4(a)-(c). The d33 is proportional to εT33 × Pr and the specimens with pseudocubic phases exhibit large εT33 and Pr values because of their low ED values [21,37,40,43]. Therefore, the large d33 values of the specimens with pseudocubic phases can also be attributed to the existence of nanodomains. It is worth mentioning that the piezoelectric properties of rhombohedral and tetragonal phases near the pseudocubic phase are generally larger than those of the pseudocubic phase because the structure of the pseudocubic phase is a cubic structure. Moreover, the piezoelectric properties of the pure pseudocubic phase (d33 = 100–480 pC/N) can be smaller than those of the PZ-PT-PZT ceramics with R–T MPB compositions (d33 = 550–830 pC/N) [21,22]. However, the piezoelectric properties of the triple-point composition in the PZPT-PZT system (d33 = 850–1000 pC/N), which is composed of tetragonal, rhombohedral, and pseudocubic structures, are larger than those of the R–T MPB composition [21]. These larger piezoelectric properties of the triple-point composition can be explained by the presence of the pseudocubic phase with nanodomains.
specimens far away from the pseudocubic phase. 3.4. Domain wall energy analysis using P-E curves In general, the domain size (TD) is related to the domain wall energy (ED), as described by the following equation [32,33,36]: TD ∝ ED1/2
(1)
Therefore, the specimen with small domains exhibits a low domain wall energy, indicating facile domain rotation in this specimen. According to the TEM analysis, nanodomains are developed in specimens with pseudocubic phase or the specimens close to the pseudocubic phase. Therefore, domain rotation is expected to be facile in these specimens because of their small ED values. To investigate the effect of domain size on the domain rotation, P-E hysteresis curves of the specimens were analyzed. Fig. 7(a)-(c) shows the P-E hysteresis curves of specimens P1, R1, and T1, which contain nanodomains. The coercive electric field (EC) values of P1, R1, and T1 specimens are 0.30, 0.42, and 0.63 kV/mm, respectively. Moreover, the EC values of specimens R2, R3, and T2 with large domains are 0.65, 0.55, and 1.08 kV/mm, respectively, as shown in Fig. 7(d)-(f). Thus, the specimen with the pseudocubic phase exhibits a very small EC. Furthermore, in the case of two specimens with the same crystal structure, the specimen with nanodomains will have a smaller EC than that with large domains; thus, as expected, EC of R1 is smaller than those of R2 and R3 (Fig. 7(b), (d) and (e)) and EC of T1 is smaller than that of T2 (Fig. 7(c) and (f)). In particular, R3 is close to cubic phase however, its EC value and domains are larger than those of R1. Therefore, it can be concluded that domain rotation is facile in specimens with pseudocubic or the specimens close
Fig. 6. TEM bright-field images of specimens with various structures: (a) 0.19PZ-0.4PT-0.41PNN (T2) with tetragonal structure, (b) 0.27PZ-0.32PT-0.41PNN (R2) with rhombohedral structure, and (c) 0.38PZ-0.1PT-0.52PNN (R3) with rhombohedral structure. Inset in each figure shows the corresponding electron diffraction pattern; TEM bright-image of each figure has been taken using the corresponding diffraction pattern. 6
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Fig. 7. P-E hysteresis loops of various ceramics: (a) P1, (b) R1, (c) T1, (d) R2, (e) R3, and (f) T2.
second-order phase transition. The free energy of this specimen can be described by the plot in Fig. 8(a) because its TC is low (approximately 75 °C) and it undergoes second-order phase transition. The free energy curve reveals that ED of P1 is relatively small. The εT33/εo and tan δ values were measured with respect to temperature near TC during heating and cooling for R1 (Fig. 9(b)). ΔT is zero for R1, indicating that second-order phase transition occurs in this specimen. In addition, R1 exhibits a low TC (approximately 87 °C). Therefore, the free energy curve of R1 can be described by the plot in Fig. 8(a), indicating that R1 has a small ED. Similarly, T1 undergoes second-order phase transition (Fig. 9(c)) and exhibits a relatively low TC (109 °C), revealing that its ED is small. These results indicate that P1, R1, and T1 undergo secondorder ferroelectric-to-paraelectric phase transition at relatively low TC values; their ED values are small and their free energy curves are similar to the one shown in Fig. 8(a). Moreover, the free energy curves indicate that the specimens with pseudocubic phase and the specimens near pseudocubic phase have low domain wall energies. The variations in εT33/εo and tan δ with respect to temperature (near TC) were also investigated for R3 (Fig. 9(d)). TC of this specimen is low and its ΔT is 5 °C, indicating that R3 undergoes strong first-order phase transition (β < < 0). Therefore, the free energy curve of R3 is similar to that presented in Fig. 8(b) and its ED is expected to be high. The variations in εT33/εo and tan δ with respect to temperature (near TC) for R2 and T2 are presented in Figs. 9(e) and (f), respectively. The TC values of these specimens are high (> 150 °C) and they undergo firstorder phase transition because they exhibit nonzero ΔT values. Therefore, the free energy curves of these specimens are similar to those given in Fig. 8(d), indicating that their ED values are large. Moreover, these results show that specimens, which are far away from the pseudocubic phase, have high domain wall energies In the PZT system, second-order ferroelectric-to-paraelectric phase transition occurs in specimens with compositions between two tricritical points [36,51]. However, first-order phase transition occurs in specimens whose compositions do not exist between two tricritical points [36,51]. Moreover, it has been reported that for a ceramic of the PZT system, an MPB forms between two tricritical points, as shown in Fig. 10 [36,51]. Therefore, materials of the PZT system undergo secondorder ferroelectric-to-paraelectric phase transition near the MPB region. When PNN is added to PZT, the resulting PZ-PT-PNN ceramic shows a low TC because the TC of PNN is low. In addition, the tricritical points, which exist at TC, decrease with increase in the PNN content. Finally,
3.5. Interpretation of domain wall energy using Landau theory ED is influenced by TC and the type of ferroelectric-to-paraelectric phase transition [36,39]. According to the Landau theory, free energy (G) of a specimen can be expressed by following equation [36,39,44–47]; G(c, T, n, P) = α(c, T)P2 + β(c, T, n)P4 + γ(c, T, n)P6
(2)
where c is composition, T is temperature, P is polarization, and n is a unit vector of spontaneous polarization; n = [111]/√3 and [100] for rhombohedral and tetragonal structures, respectively. α is a constant influenced by TC and state of the specimen (ferroelectric or paraelectric). For a ferroelectric ceramic, α is negative; α is a small negative value (close to zero) when TC is low (≤100 °C) and a large negative value when TC is high (> 150 °C) [47,48]. On the other hand, for a paraelectric ceramic, α is always positive. β is a constant determined by the type of ferroelectric-to-paraelectric phase transition; for a specimen with first-order phase transition, β is negative, and for the one with second-order phase transition, β is positive [36,39,44]. γ is a constant with a small positive value [36,39,44]. For a ferroelectric specimen with a low TC and second-order phase transition, α ≤ 0 and β > 0 [36,39,40,44]. Therefore, its free energy can be schematically expressed as shown in Fig. 8(a), indicating that the energy barrier between the spontaneous polarization states is small. For a ferroelectric specimen with a low TC and strong first-order phase transition, α and β are small negative and large negative values, respectively [36,39,40,44]. Therefore, this specimen will have a relatively large energy barrier, as shown in Fig. 8(b). On the other hand, for a ferroelectric specimen with a high TC, α is a large negative value [47,48], and its free energy for the second- and first-order transitions can be expressed by the plots in Fig. 8(c) and (d), respectively. Therefore, the energy barriers between spontaneous polarization states for these specimens are high irrespective of the type of ferroelectric-toparaelectric phase transition [40,46,49,50]. The energy barrier between two spontaneous polarization states can be regarded as ED. Therefore, ED is influenced by TC and the type of ferroelectric-to-paraelectric phase transition. Fig. 9(a) shows the variation in εT33/εo and tan δ with respect to temperature (near TC) for P1, which comprises a pseudocubic phase. The difference in TC values (ΔT), which were measured during heating and cooling processes, is zero, revealing that the specimen undergoes 7
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Fig. 8. Polarization versus free energy curves corresponding to various conditions at RT: (a) α ≤ 0 and β > 0, (b) α ≤ 0 and β < < 0, (c) α < < 0 and β > 0, and (d) α < < 0 and β < 0.
thus it exhibits excellent ferroelectric and piezoelectric properties [21]. In addition, the specimens in this region are expected to have small domains because of their low ED values.
for specimens with large amounts of PNN, the tricritical points exist near RT because PNN has a cubic structure. Therefore, the two tricritical points for the PZT system can be expressed by two tricritical lines in the PT-PZ-PNN phase diagram, as shown in Fig. 10. The specimens existing between these two tricritical lines are expected to undergo second-order ferroelectric-to-paraelectric phase transition. Moreover, the specimens with pseudocubic phase and the specimens near pseudocubic phase, which exist between two tricritical lines (yellow-colored region in Fig. 10), are expected to have low TC values because they are close to the cubic phase. Furthermore, since these specimens undergo second-order phase transition with low TC values, they are expected to have low domain wall energies, as shown in Fig. 8(a). Therefore, the specimens in the yellow-colored region are expected to possess enhanced ferroelectric and piezoelectric properties owing to their low ED values. P1, R1, and T1 ceramics exhibit large εT33 and d33 values (Fig. 4(a) and (b)) because they exist in this region. Moreover, the specimen with the triple point composition exists in this region, and
4. Conclusions The tetragonal-to-cubic structural transition involved the formation of the pseudocubic phase. Moreover, the tetragonal-to-pseudocubic phase transformation was accompanied by the T-P MPB structure formation. In addition, in specimens without large amounts of PZ, the rhombohedral-to-cubic structural transition involved the formation of the pseudocubic phase. However, in specimens with large amounts of PZ, the rhombohedral structure directly changed to the cubic structure without formation of pseudocubic phase. The structure of pseudocubic phase formed in the PZ-PT-PNN ceramics was considered to be a Pm3m cubic structure, and specimens with pseudocubic phases showed the piezoelectric and ferroelectric
Fig. 9. Variations in εT33/εo and tan δ with respect to temperature for various ceramics: (a) P1, (b) R1, (c) T1, (d) R3, (e) R2, and (f) T2. 8
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Fig. 10. Schematic phase diagram for the PZ-PT-PNN ternary system showing the ferroelectric-to-paraelectric phase transition. Inset is the magnified schematic phase diagram of the PZT system showing two tricritical points.
properties probably owing to their non-ergodic characteristic. Because the structure of pseudocubic phase had a similarity to the rhombohedral structure, the rhombohedral phase directly changed to the pseudocubic phase. The specimens far away from the pseudocubic phase exhibited large domains and large EC values because of their high domain wall energies. On the other hand, nanodomains were observed in specimens with pseudocubic phase and the specimens close to the pseudocubic phase, and these specimens exhibited low EC values. Therefore, these specimens were expected to have low domain wall energies. Moreover, these specimens exhibited second-order ferroelectric-to-paraelectric phase transition at low TC values, confirming their low domain wall energies. The enhanced ferroelectric and piezoelectric properties of these specimens could be attributed to their low domain wall energies.
[3]
[4]
[5] [6]
[7] [8] [9]
Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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Acknowledgments
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This research was supported by the National Research Council of Science & Technology (NST) grant by the Korean government (MSIP; No. CAP-17-04-KRISS) and the authors also thank the KU-KIST Graduate School Program of Korea University.
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Appendix A. Supplementary data
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Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.jeurceramsoc.2019.12. 063.
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Original Article
Crystal structure and piezoelectric characteristics of various phases near the triple-point composition in PZ-PT-PNN system Tae-Gon Leea, Sun-Woo Kimb, Eun-Ji Kimb, Sang Jin Leeb,c, Hyun-Gyu Hwanga, Youn-Woo Hongd, Jeong Seog Kime, Keun Hwa Chaef, Ji-Won Choig, Chong-Yun Kanga,g, Sahn Nahma,b,* a Department of Nano Bio Information Technology, KU-KIST Graduate School of Converging Science and Technology, Korea University, 141 Anam-ro, Seongbuk-gu, Seoul, 02841, Republic of Korea b Department of Materials Science and Engineering, Korea University, 141 Anam-ro, Seongbuk-gu, Seoul, 02841, Republic of Korea c Electronic Materials and Device Research Center, Korea Electronics Technology Institute, Seongnam, Gyeonggi-do, Republic of Korea d Korea Institute of Ceramic Engineering and Technology, 101, Soho-Ro, Jinju-Si, Gyeongsangnam-do 660-031, Republic of Korea e Department of Materials Science and Engineering, Hoseo University, 20, Hoseo-ro 79 beongil, Baebang-eup, Asan-si, Chungcheongnam-do 31499, Republic of Korea f Advanced Analysis Center, Korea Institute of Science and Technology (KIST), Seoul 02792, Republic of Korea g Center for Electronic Materials, Korea Institute of Science and Technology (KIST), Seoul, 02791, Republic of Korea
A R T I C LE I N FO
A B S T R A C T
Keywords: Piezoelectricity Ceramic material Phase diagram Nanodomains Landau theory
Crystal structures and piezoelectric properties of PbZrO3-PbTiO3-Pb(Ni1/3Nb2/3)O3 ceramics near the triple point composition, particularly characteristics of the pseudocubic phase, were investigated. The pseudocubic phase, which formed near the triple point composition, disappeared with increase in the PbZrO3 content. The pseudocubic phase had the Pm3m cubic structure. The tetragonal-pseudocubic morphotropic phase boundary (MPB) structure was developed during the tetragonal-to-cubic phase transformation. However, the rhombohedral phase directly transformed to the cubic phase because the structure of pseudocubic phase was similar to the rhombohedral structure. The specimens with pseudocubic phase and the specimens near pseudocubic phase exhibited nano-sized domains and small coercive electric fields, revealing their low domain wall energies. These specimens exhibited second-order ferroelectric-to-paraelectric phase transition and low Curie temperatures, confirming their low domain wall energies. The enhanced dielectric and piezoelectric properties of these specimens could be attributed to their low domain wall energies.
1. Introduction Pb(Zr,Ti)O3 (PZT)-based piezoelectric materials have been used in various electrical devices including multilayer actuators, sensors, transformers, and piezoelectric energy harvesters because of superior piezoelectric properties [1–8]. In particular, the interest in the piezoelectric actuators and energy harvesters has been increased because of the development of portable haptic and mobile devices [9–12]. For these device applications, piezoelectric ceramics should exhibit large piezoelectric constants (dij), piezoelectric voltage constants (gij), and displacements. PbZrO3-PbTiO3-Pb(Ni1/3Nb2/3)O3 (PZ-PT-PNN) ceramics have been investigated for these device applications because of their excellent piezoelectric properties [13–17]. Moreover, low-temperature sintering of these ceramics has been also studied for
piezoelectric multilayer device [18–20]. Recently, a schematic phase diagram of these ceramics was reported, and the structural and piezoelectric properties of the PZ-PT-PNN ceramics near the triple point composition were investigated [21]. Moreover, the application of these ceramics for the development of piezoelectric energy harvesters has been explored [22]. Most of the previous studies were focused on the rhombohedral-tetragonal (R–T) morphotropic phase boundaries (MPBs) in PZ-PT-PNN ceramics. However, the structural and piezoelectric property changes occurring in these ceramics during the tetragonal (or rhombohedral)-to-cubic phase transformation have not been investigated. In particular, the properties of the pseudocubic phase have not been studied in detail. In this study, four groups of PZ-PT-PNN ceramics were fabricated to study the changes in the structural and piezoelectric properties near the
⁎ Corresponding author at: Department of Nano Bio Information Technology, KU-KIST Graduate School of Converging Science and Technology, Korea University, 141 Anam-ro, Seongbuk-gu, Seoul, 02841, Republic of Korea. E-mail address: [email protected] (S. Nahm).
https://doi.org/10.1016/j.jeurceramsoc.2019.12.063 Received 12 August 2019; Received in revised form 27 December 2019; Accepted 30 December 2019 0955-2219/ © 2019 Published by Elsevier Ltd.
Please cite this article as: Tae-Gon Lee, et al., Journal of the European Ceramic Society, https://doi.org/10.1016/j.jeurceramsoc.2019.12.063
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Fig. 1. Schematic phase diagram of PZ-PT-PNN ceramics near the triple point composition.
monochromatic XRD (Rigaku D/Max-RC, Tokyo, Japan) analysis was performed with the Cu Kα1 radiation. In addition, synchrotron XRD measurements were performed on the 5A XRS KIST-PAL beamline at PLS (Pohang, Korea). A monochromatic radiation (1.0716 Å) was obtained using a double crystal Si (111) monochromator. Powder diffraction patterns were recorded in the Bragg-Brentano geometry in the angular interval 20°–60° (2θ) with a constant 0.005° step and a counting time of 1 s per step. These XRD patterns were analyzed by Rietveld refinement using the Rietan software to determine the structure of the pseudocubic phase. Field-emission transmission electron microscopy (FETEM; Tecnai F20, FEI, Netherlands) was used to investigate the domain structures of the specimens. The specimens with silver electrodes were poled by a DC field of 3 kV/mm for 15 min at room temperature (RT). The d33 values of the specimens were measured using a d33 meter (Micro-Epsilon Channel Product DT-3300, Raleigh, NC, USA). The relative permittivity (εT33/εo), dielectric loss (tan δ), and coupling factor (kp) of the specimens were measured with an impedance analyzer (Agilent Technologies HP 4194A, Santa Clara, California). The εT33/εo and tan δ values were obtained as functions of temperature using an LCR meter (Agilent Technologies HP 4194A, Santa Clara, CA, USA) in an automated temperature-controlled furnace equipped with a computer interface for data acquisition. The polarization vs. electric field (P-E) hysteresis loops were obtained using a modified Sawyer-tower circuit by performing the measurements in silicon oil to prevent electrical flashover at the fixed frequency of 1 Hz
triple point composition, occurring during the tetragonal (or rhombohedral)-to-cubic phase transformation. Fig. 1 shows a schematic phase diagram of the PZ-PT-PNN ceramics near the triple point composition; the four groups of specimens are marked on four lines in this figure. The 0.1PZ-(0.9-x)PT-xPNN ceramics with 0.52 ≤ x ≤ 0.65 are marked on Line 1 in Fig. 1 and these specimens were selected to investigate the structural and piezoelectric property changes accompanying the tetragonal-to-cubic phase transformation. The (0.72-x)PZ-0.28PT-xPNN ceramics with 0.52 ≤ x ≤ 0.65 (Line 2 in Fig. 1) were chosen to investigate the property variations occurring during the rhombohedralto-cubic structural transformation. Pseudocubic phases were detected in these two groups of specimens. The (0.9-x)PZ-0.1PT-xPNN ceramics with 0.48 ≤ x ≤ 0.56 (Line 4 in Fig. 1) were used to study the crystal structure and piezoelectric property changes occurring during the rhombohedral to cubic structure transformation. The pseudocubic phase was not detected in these specimens. In addition, the specimens marked on Line 3 in Fig. 1 with compositions (0.8-x)PZ-0.2PT-xPNN (0.50 ≤ x ≤ 0.60) were used to study the various properties of the specimens near the triple point composition. In particular, the characteristics of the pseudocubic phase, such as the domain structure, domain wall energy, and ferroelectric-to-paraelectric phase transition, were investigated. 2. Experimental Four groups of PZ-PT-PNN ceramics (marked in Fig. 1) were produced by the conventional ceramic process. Typically, PbO (99.5 %, Dansuk Industry, Korea), ZrO2 (98 %, Z-Tech, USA), TiO2 (99.2 %, Sakai Kagaku, Japan), NiO (99.5 %, Nikko Rika, Japan), and Nb2O5 (99.8 %, Jiujiang, China) were weighed and mixed with zirconia balls for 24 h in a nylon jar; anhydrous ethanol was used as the solvent. Next, the mixed powders were dried and calcined at 880 °C for 4 h. The calcined powders were then re-milled for 24 h and dried. Finally, the dried powders were sieved to obtain fine ceramic powders. These ceramic powders were pressed into disc-shaped pellets under a pressure of 9.8 MPa and sintered at 1150 °C for 2 h in air. The densities of the specimens were measured by Archimedes’ method. The structural properties of the specimens were investigated by Xray diffraction (XRD; Rigaku D/Max-RC, Tokyo, Japan) with the Cu Kα radiation, and scanning electron microscopy (SEM; Hitachi S-4800, Osaka, Japan). The XRD reflections at 65.5° were obtained by lowspeed scanning of the specimens and then deconvoluted using the Voigt function to identify the crystal structures. The rate and resolution of low-speed scanning were 0.3°/min and 0.01°, respectively. Moreover, to obtain the XRD patterns of the specimens with pseudocubic phases,
3. Results and discussion 3.1. Crystal structure analysis All specimens investigated in this study exhibit homogeneous perovskite structures, without any secondary phases, as shown in Fig. S1(a)-(d). Fig. 2(a)-(d) shows the SEM images of the fractured surfaces of the specimens selected from each group. As observed, they exhibit dense microstructures with similar average grain sizes (approximately 2.0 μm). Therefore, the effect of microstructure on the piezoelectric properties is expected to be the same for all the specimens. It was difficult to identify the exact crystal structures of the specimens using the normal XRD patterns. Therefore, the XRD reflections at 65.5° were recorded by low-speed scanning and deconvoluted using the Voigt function for a detailed analysis of the crystal structures of the specimens. Fig. 3(a–i)-(a–v) shows the XRD reflections at 65.5° of 0.1PZ-(0.9-x) PT-xPNN ceramics with 0.54 ≤ x ≤ 0.65, which are marked on Line 1 in Fig. 1. The specimen with x = 0.54 exhibits tetragonal (202)T and 2
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Fig. 2. SEM images of PZ-PT-PNN ceramics: (a) 0.1PZ-0.32PT-0.58PNN, (b) 0.14PZ-0.28PT-0.58PNN, (c) 0.25PZ-0.2PT-0.55PNN, and (d) 0.38PZ-0.10PT-0.52PNN.
0.57 ≤ x ≤ 0.63 were determined as the pseudocubic phase owing to their piezoelectric properties whereas the specimens with x ≥ 0.64 were concluded as the cubic phase due to their absence of piezoelectric properties (Fig. 4(b)). Since the structure of pseudocubic phase also can be interpreted by the rhombohedral structure, the XRD reflections of the specimens with x = 0.57 and 0.63 were deconvoluted to the (220)P and (2–20)P reflections, as shown in Fig. S3. In addition, the XRD pattern of the specimen with x = 0.56 can be analyzed as a rhombohedral structure only while that of the specimen with x = 0.57 can be analyzed as a cubic structure as well as a rhombohedral structure. Therefore, the crystal structures of the specimens with x = 0.56 and 0.57 are considered to have rhombohedral and pseudocubic phases, respectively. Interestingly, the rhombohedral phase directly transforms into the pseudocubic phase, probably because the structure of pseudocubic phase is similar to the rhombohedral structure. Variation in the crystal structures of the specimens marked on Line 3 in Fig. 1 was also studied, as shown in Fig. S4(a)-(e). Crystal structures of these specimens change from rhombohedral to pseudocubic, and then, to cubic with increase in x, confirming that the rhombohedral structure directly changes to the pseudocubic phase structure. In addition, the structural properties of the specimens marked on Line 4 in Fig. 1 were investigated (Figs. S5(a)-(c)). Crystal structures of these specimens change from rhombohedral to cubic. These results show that the pseudocubic phase exists near the triple point composition, and it disappears with increase in the PZ content; however, it remains in the PT region [21]. In order to determine the exact crystal structure of pseudocubic phase, Rietveld XRD analyses and Raman spectra analysis were conducted and their results were discussed in Supplementary Information 6–8 in detail. The results of the Rietveld analysis using monochromatic XRD indicate that the crystal structure of the pseudocubic phase is a rhombohedral R3m structure as well as a cubic Pm3m structure, as shown in Fig. S6. Furthermore, the Raman spectra results also show that the pseudocubic phase can be explained by the rhombohedral
(220)T reflections (Fig. 3(a–i)); similar results are observed for the specimens with 0.52 ≤ x ≤ 0.54, indicating that these specimens have tetragonal structures. Pseudocubic (220)P reflection is observed for the specimens with x = 0.55 and 0.6, as shown in Fig. 3(a–ii) and (a–iii), respectively. Therefore, the specimens with 0.55 ≤ x ≤ 0.6 have tetragonal-pseudocubic MPB (T-P MPB) structure. The specimen with x = 0.61 shows only pseudocubic (220)P reflection (Fig. 3(a-iv)); similar results are observed for the specimens with x = 0.62 and 0.63. The specimens with x ≥ 0.64 also exhibit cubic structures, as indicated by the XRD pattern of the specimen with x = 0.64 (Fig. 3(a–v)). Although the specimens with x ≥ 0.61 exhibit a single (220) XRD reflection indicating the cubic structure, only the specimens with 0.61 ≤ x ≤ 0.63 were determined as a pseudocubic phase because they showed the piezoelectric properties (Fig. 4(a)). On the other hand, the specimens with x ≥ 0.64 were concluded as a cubic phase due to their absence of piezoelectric properties (Fig. 4(a)). In addition, the pseudocubic (220)P reflection observed for the specimens with 0.61 ≤ x ≤ 0.63 also can be interpreted as rhombohedral structure of (220)P and (2–20)P reflections, as shown in Fig. S2, revealing that the structure of pseudocubic phase can be considered as a the rhombohedral structure; this will be discussed later using the Rietveld analysis results. Therefore, the crystal structure of the pseudocubic phase can be explained by the cubic structure as well as the rhombohedral structure. However, the crystal structure of the rhombohedral phase can be explained only by the rhombohedral structure. Variation in the crystal structures of (0.72-x)PZ-0.28PT-xPNN ceramics with 0.52 ≤ x ≤ 0.65, which are marked on Line 2 in Fig. 1, was also investigated (Fig. 3(b–i)-(b–v)). Specimens with x = 0.52 and 0.56 show the rhombohedral (220)R and (-220)R reflections (Fig. 3(b–i) and (b–ii)), revealing the development of a rhombohedral structure in them. On the other hand, the specimens with x ≥ 0.57 exhibit a single (220) XRD reflection, indicating the cubic structure, and XRD reflections of the specimens with x = 0.57, 0.63 and 0.64 were expressed in Fig. 3(b–iii)-(b–v), respectively. Here, the specimens with 3
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relaxor ceramic has a polar nano-region [26,27]. The specimens (0.52 ≤ x ≤ 0.54) with tetragonal structures exhibit a kp value of 0.52, and the specimen with x = 0.55 exhibits a higher kp value of 0.56 owing to the presence of the T-P MPB structure. The value considerably decreases when x exceeds 0.6 because of the formation of a pure pseudocubic phase. Moreover, it was difficult to measure the kp values of specimens with 0.64 ≤ x ≤ 0.65 due to the formation of cubic structure. The maximum phase angle (θmax) was measured at frequencies between the antiresonance and resonance frequencies of specimens. θmax is close to 90° for an ideal ferroelectric ceramic and -90° for a paraelectric ceramic with a cubic structure [28,29]. The specimens with tetragonal and T-P MPB structures (0.52 ≤ x ≤ 0.6) exhibit large θmax values raging between 82° and 67°, and it decreases when x exceeds 0.6 because of the formation of a pure pseudocubic phase. Moreover, the specimens with x = 0.64 and 0.65 exhibit very low θmax values (close to -83°) owing to the formation of paraelectric cubic structure. In addition, with increase in x, the εT33/εo value continuously increases because of the decrease in TC. Fig. 4(b) shows the variations in the d33, kp, θmax, and εT33/εo values of the (0.72-x)PZ-0.28PT-xPNN ceramics with 0.52 ≤ x ≤ 0.65, which are marked on Line 2 in Fig. 1. The d33 value of the specimen slightly increases with increase in x. The maximum d33 value of the 490 pC/N is observed for the specimen with a rhombohedral structure (x = 0.56). When x exceeds 0.56, the d33 value starts decreasing because of the formation of the pseudocubic phase. However, the specimens with pseudocubic phase (0.58 ≤ x ≤ 0.6) show large d33 values ranging between 475 and 365 pC/N. However, very low d33 values are observed for specimens with cubic structures (x ≥ 0.64). The specimens with rhombohedral structures (0.52 ≤ x ≤ 0.56) show relatively large kp values (0.44-0.48). The value decreases when x exceeds 0.56 because of the formation of the pseudocubic phase. Finally, it was very difficult to measure kp values of specimens with x ≥ 0.64, confirming the formation of cubic structures in these specimens. The variation in θmax is similar to that in kp. The specimens with rhombohedral structures (0.52 ≤ x ≤ 0.56) show large θmax values; the θmax value decreases when x exceeds 0.56 because of the formation of the pseudocubic phase. Finally, the specimens with x ≥ 0.64 exhibit a θmax value close to -83°, indicating the formation of cubic structures in these specimens. In addition, the εT33/εo values continuously increase with increase in x because of the decrease in TC. The variations in d33, kp, θmax, and εT33/εo values of (0.8-x)PZ-0.2PTxPNN ceramics with 0.5 ≤ x ≤ 0.6 are shown in Fig. 4(c). These specimens are marked on Line 3 in Fig. 1. The d33 value of the specimen with x = 0.5 is approximately 265 pC/N and it increases with increase in x. The maximum d33 value of 388 pC/N is observed for the specimen with x = 0.55. The d33 value decreases when x exceeds 0.56, probably because of the formation of the pseudocubic phase. Finally, the specimen with x = 0.6 shows a very small d33 value because of the formation of the cubic structure. Variation in the kp value with respect to x is similar to that in the d33 value. The specimens with rhombohedral structures exhibit relatively large kp values; however, the specimens with pseudocubic phases exhibit low kp values. Notably, it was difficult to measure the kp value for the specimen with a cubic structure. Moreover, the specimens with rhombohedral structures exhibit large θmax values, indicating that they possess ferroelectric properties. Nonetheless, the specimens with pseudocubic phases exhibit considerably low θmax values. Finally, the specimen with a cubic structure (x = 0.6) shows a θmax value of -81°, indicating that it is a paraelectric material. In addition, the εT33/εo value increases with increase in x because of the decrease in TC, and the specimen with a cubic structure (x = 0.6) exhibits a very large εT33/εo value of 8870. The piezoelectric properties of (0.9-x)PZ-0.1PT-xPNN ceramics with 0.48 ≤ x ≤ 0.56 were also investigated (Fig. 4(d)) and these specimens are marked on Line 4 in Fig. 1. The d33 values of the specimens with 0.48 ≤ x ≤ 0.52 are relatively small (ranging between 150 pC/N and 159 pC/N) and it starts decreasing at x = 0.53. Finally, the specimens
Fig. 3. XRD patterns showing peaks at 65.5° obtained by slow-speed scanning for 0.1PZ-(0.9-x)PT-xPNN ceramics: (a–i) x = 0.54, (a-ii) x = 0.55, (a–iii) x = 0.60, (a–iv) x = 0.61, and (a–v) x = 0.64. XRD patterns showing peaks at 65.5° obtained by slow-speed scanning for (0.72-x)PZ-0.28PT-xPNN ceramics: (b–i) x = 0.52, (b–ii) x = 0.56, (b–iii) x = 0.57, (b-iv) x = 0.63, and (b–v) x = 0.64.
phase and the cubic phase, as shown in Fig. S7(a) and (b), respectively. However, according to the Rietveld analysis using a synchrotron XRD, the pseudocubic phase is expected to have a Pm3m cubic structure, as shown in Fig. S8(a)-(c) and Table S2. Therefore, the crystal structure of pseudocubic phase is finally concluded as a Pm3m cubic structure.
3.2. Piezoelectric properties of the specimens The d33, kp, θmax, and εT33/εo values of 0.1PZ-(0.9-x)PT-xPNN ceramics with 0.52 ≤ x ≤ 0.65 were investigated (Fig. 4(a)) and these specimens are marked on Line 1 in Fig. 1. The d33 value of the specimen increases with increase in x. Specimens with 0.55 ≤ x ≤ 0.6 exhibit larger d33 values because of the formation of T-P MPB structures in these specimens. When x exceeds 0.6, the d33 value decreases because of the formation of a pure pseudocubic phase. The specimen with the pseudocubic phase (x = 0.61) has a relatively large d33 value of 330 pC/N even though its crystal structure is the Pm3m cubic structure as shown in Fig. 3(a-iv). Previous studies reported that cubic relaxor ceramics, which have a non-ergodic characteristic, exhibit piezoelectric properties because their structure changed from cubic to noncubic after electric field application [23–25]. Therefore, the pseudocubic phase developed in this study is considered to have the non-ergodic characteristic. On the other hand, specimens with x ≥ 0.64 exhibit small d33 values owing to the formation of cubic structure. The specimens with cubic structures exhibit small d33 values probably because the PNN 4
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Fig. 4. The d33, kp, θmax, and εT33/εo values of the PZ-PT-PNN ceramics: (a) 0.1PZ-(0.9-x)PT-xPNN ceramics with 0.52 ≤ x ≤ 0.65, (b) (0.72-x)PZ-0.28PT-xPNN ceramics with 0.52 ≤ x ≤ 0.65, (c) (0.8-x)PZ-0.2PT-xPNN ceramics with 0.5 ≤ x ≤ 0.6, and (d) (0.9-x)PZ-0.1PT-xPNN ceramics with 0.48 ≤ x ≤ 0.56.
with x ≥ 0.55 show a very small d33 value of 48 pC/N. The kp and θmax values show similar variations with respect to x. Therefore, these results show that the specimens with 0.48 ≤ x ≤ 0.54 have rhombohedral structures, and in the specimens with x ≥ 0.55, cubic structures are developed. Finally, the εT33/εo values continuously increase with increase in x because of the decrease in TC.
domains (100 nm × 5 nm). Therefore, these results show that nanosized domains are formed in specimen with pseudocubic phase and the specimens close to the pseudocubic phase. Furthermore, TEM analysis was conducted on specimens that are far away from the pseudocubic phase. Fig. 6(a) shows a TEM bright field image of the 0.19PZ-0.4PT-0.41PNN ceramic with a tetragonal structure (T2, Fig. 1). This bright-field image was obtained by slightly tilting the specimen from the [1 2 0] zone axis (see inset in Fig. 6(a)). This specimen comprises domains with sizes 1.3 μm × 100 nm, which are much larger than those of specimen T1. Moreover, 90° and 180° domain walls are observed in this specimen, as indicated by green and red lines, respectively, in Fig. 6(a) [30–34]. Domain structure of the 0.27PZ0.32PT-0.41PNN ceramic with a rhombohedral structure (R2, Fig. 1) was also investigated by TEM. Fig. 6(b) shows a bright-field image of this specimen obtained by slightly tilting the specimen from the [0 2 1] zone axis. This specimen exhibits large lamella-shaped domains (400 nm × 25 nm) with 71° and 109° domain walls, as indicated by green and red dotted lines, respectively, in Fig. 6(b) [30–35]. Furthermore, TEM analysis was conducted on the 0.38PZ-0.1PT-0.52PNN ceramic with a rhombohedral structure, as shown in Fig. 6(c). This specimen is close to the cubic phase (R3, Fig. 1). The R3 specimen exhibits large domains (300 nm × 50 nm). Moreover, the domains of R2 and R3 ceramics are much larger than those of the R1 ceramic. Therefore, the TEM results show that domains of the specimens close to pseudocubic phase are much smaller than those of the domains in the
3.3. TEM analysis for domain structure TEM analysis was carried out to identify the domain structures of specimens with the pseudocubic phase (or specimens close to pseudocubic phase). Fig. 5(a) shows a bright-field TEM image of the 0.12PZ0.28PT-0.6PNN ceramic with a pseudocubic phase (P1, Line 2 in Fig. 1). This bright-field image was obtained by slightly tilting the sample from the [1-1-1] zone axis to clearly identify its domain structure (see inset in Fig. 5(a)). This specimen shows several small domains with sizes of a few nanometers, revealing the formation of nanodomains in this specimen. TEM analysis was also conducted on the 0.1PZ-0.36PT-0.54PNN ceramic (Fig. 5(b)) with a tetragonal structure (T1, Line 1 in Fig. 1). This specimen comprises small strip-shaped domains (80 nm × 20 nm), revealing the formation of nanodomains in this specimen, as well. Moreover, the domain structure of the 0.16PZ-0.28PT-0.56PNN ceramic with a rhombohedral structure was analyzed using TEM, as shown in Fig. 5(c). This specimen is denoted as R1 on Line 2 in Fig. 1 and it is close to pseudocubic phase. This specimen also exhibits small 5
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Fig. 5. TEM bright-field images of specimens with various structures: (a) 0.12PZ-0.28PT-0.60PNN (P1) with pseudocubic phase, (b) 0.10PZ-0.36PT-0.54PNN (T1) with tetragonal structure, and (c) 0.16PZ-0.28PT-0.56PNN (R1) with rhombohedral structure. Inset in each figure shows the corresponding electron diffraction pattern; TEM bright-image of each figure has been taken using the corresponding diffraction pattern.
to the pseudocubic phase because of their low domain wall energies (small EC values). Interestingly, the specimens with pseudocubic phases exhibit large εT33/ε0, as shown in Fig. 4(a)-(c). Notably, εT33/ε0 is nearly proportional to (∂P/∂E)E=0 [21,37,38]. In general, if the domain rotation is easy (small ED), then the (∂P/∂E)E=0 value will be large [37,39–42]. Therefore, the large εT33/ε0 values of the specimens with pseudocubic phases can be attributed to their low ED values, resulting from the existence of nanodomains. Furthermore, the specimens with pseudocubic phases, have relatively large d33 values (≥ 300 pC/N), as shown in Fig. 4(a)-(c). The d33 is proportional to εT33 × Pr and the specimens with pseudocubic phases exhibit large εT33 and Pr values because of their low ED values [21,37,40,43]. Therefore, the large d33 values of the specimens with pseudocubic phases can also be attributed to the existence of nanodomains. It is worth mentioning that the piezoelectric properties of rhombohedral and tetragonal phases near the pseudocubic phase are generally larger than those of the pseudocubic phase because the structure of the pseudocubic phase is a cubic structure. Moreover, the piezoelectric properties of the pure pseudocubic phase (d33 = 100–480 pC/N) can be smaller than those of the PZ-PT-PZT ceramics with R–T MPB compositions (d33 = 550–830 pC/N) [21,22]. However, the piezoelectric properties of the triple-point composition in the PZPT-PZT system (d33 = 850–1000 pC/N), which is composed of tetragonal, rhombohedral, and pseudocubic structures, are larger than those of the R–T MPB composition [21]. These larger piezoelectric properties of the triple-point composition can be explained by the presence of the pseudocubic phase with nanodomains.
specimens far away from the pseudocubic phase. 3.4. Domain wall energy analysis using P-E curves In general, the domain size (TD) is related to the domain wall energy (ED), as described by the following equation [32,33,36]: TD ∝ ED1/2
(1)
Therefore, the specimen with small domains exhibits a low domain wall energy, indicating facile domain rotation in this specimen. According to the TEM analysis, nanodomains are developed in specimens with pseudocubic phase or the specimens close to the pseudocubic phase. Therefore, domain rotation is expected to be facile in these specimens because of their small ED values. To investigate the effect of domain size on the domain rotation, P-E hysteresis curves of the specimens were analyzed. Fig. 7(a)-(c) shows the P-E hysteresis curves of specimens P1, R1, and T1, which contain nanodomains. The coercive electric field (EC) values of P1, R1, and T1 specimens are 0.30, 0.42, and 0.63 kV/mm, respectively. Moreover, the EC values of specimens R2, R3, and T2 with large domains are 0.65, 0.55, and 1.08 kV/mm, respectively, as shown in Fig. 7(d)-(f). Thus, the specimen with the pseudocubic phase exhibits a very small EC. Furthermore, in the case of two specimens with the same crystal structure, the specimen with nanodomains will have a smaller EC than that with large domains; thus, as expected, EC of R1 is smaller than those of R2 and R3 (Fig. 7(b), (d) and (e)) and EC of T1 is smaller than that of T2 (Fig. 7(c) and (f)). In particular, R3 is close to cubic phase however, its EC value and domains are larger than those of R1. Therefore, it can be concluded that domain rotation is facile in specimens with pseudocubic or the specimens close
Fig. 6. TEM bright-field images of specimens with various structures: (a) 0.19PZ-0.4PT-0.41PNN (T2) with tetragonal structure, (b) 0.27PZ-0.32PT-0.41PNN (R2) with rhombohedral structure, and (c) 0.38PZ-0.1PT-0.52PNN (R3) with rhombohedral structure. Inset in each figure shows the corresponding electron diffraction pattern; TEM bright-image of each figure has been taken using the corresponding diffraction pattern. 6
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Fig. 7. P-E hysteresis loops of various ceramics: (a) P1, (b) R1, (c) T1, (d) R2, (e) R3, and (f) T2.
second-order phase transition. The free energy of this specimen can be described by the plot in Fig. 8(a) because its TC is low (approximately 75 °C) and it undergoes second-order phase transition. The free energy curve reveals that ED of P1 is relatively small. The εT33/εo and tan δ values were measured with respect to temperature near TC during heating and cooling for R1 (Fig. 9(b)). ΔT is zero for R1, indicating that second-order phase transition occurs in this specimen. In addition, R1 exhibits a low TC (approximately 87 °C). Therefore, the free energy curve of R1 can be described by the plot in Fig. 8(a), indicating that R1 has a small ED. Similarly, T1 undergoes second-order phase transition (Fig. 9(c)) and exhibits a relatively low TC (109 °C), revealing that its ED is small. These results indicate that P1, R1, and T1 undergo secondorder ferroelectric-to-paraelectric phase transition at relatively low TC values; their ED values are small and their free energy curves are similar to the one shown in Fig. 8(a). Moreover, the free energy curves indicate that the specimens with pseudocubic phase and the specimens near pseudocubic phase have low domain wall energies. The variations in εT33/εo and tan δ with respect to temperature (near TC) were also investigated for R3 (Fig. 9(d)). TC of this specimen is low and its ΔT is 5 °C, indicating that R3 undergoes strong first-order phase transition (β < < 0). Therefore, the free energy curve of R3 is similar to that presented in Fig. 8(b) and its ED is expected to be high. The variations in εT33/εo and tan δ with respect to temperature (near TC) for R2 and T2 are presented in Figs. 9(e) and (f), respectively. The TC values of these specimens are high (> 150 °C) and they undergo firstorder phase transition because they exhibit nonzero ΔT values. Therefore, the free energy curves of these specimens are similar to those given in Fig. 8(d), indicating that their ED values are large. Moreover, these results show that specimens, which are far away from the pseudocubic phase, have high domain wall energies In the PZT system, second-order ferroelectric-to-paraelectric phase transition occurs in specimens with compositions between two tricritical points [36,51]. However, first-order phase transition occurs in specimens whose compositions do not exist between two tricritical points [36,51]. Moreover, it has been reported that for a ceramic of the PZT system, an MPB forms between two tricritical points, as shown in Fig. 10 [36,51]. Therefore, materials of the PZT system undergo secondorder ferroelectric-to-paraelectric phase transition near the MPB region. When PNN is added to PZT, the resulting PZ-PT-PNN ceramic shows a low TC because the TC of PNN is low. In addition, the tricritical points, which exist at TC, decrease with increase in the PNN content. Finally,
3.5. Interpretation of domain wall energy using Landau theory ED is influenced by TC and the type of ferroelectric-to-paraelectric phase transition [36,39]. According to the Landau theory, free energy (G) of a specimen can be expressed by following equation [36,39,44–47]; G(c, T, n, P) = α(c, T)P2 + β(c, T, n)P4 + γ(c, T, n)P6
(2)
where c is composition, T is temperature, P is polarization, and n is a unit vector of spontaneous polarization; n = [111]/√3 and [100] for rhombohedral and tetragonal structures, respectively. α is a constant influenced by TC and state of the specimen (ferroelectric or paraelectric). For a ferroelectric ceramic, α is negative; α is a small negative value (close to zero) when TC is low (≤100 °C) and a large negative value when TC is high (> 150 °C) [47,48]. On the other hand, for a paraelectric ceramic, α is always positive. β is a constant determined by the type of ferroelectric-to-paraelectric phase transition; for a specimen with first-order phase transition, β is negative, and for the one with second-order phase transition, β is positive [36,39,44]. γ is a constant with a small positive value [36,39,44]. For a ferroelectric specimen with a low TC and second-order phase transition, α ≤ 0 and β > 0 [36,39,40,44]. Therefore, its free energy can be schematically expressed as shown in Fig. 8(a), indicating that the energy barrier between the spontaneous polarization states is small. For a ferroelectric specimen with a low TC and strong first-order phase transition, α and β are small negative and large negative values, respectively [36,39,40,44]. Therefore, this specimen will have a relatively large energy barrier, as shown in Fig. 8(b). On the other hand, for a ferroelectric specimen with a high TC, α is a large negative value [47,48], and its free energy for the second- and first-order transitions can be expressed by the plots in Fig. 8(c) and (d), respectively. Therefore, the energy barriers between spontaneous polarization states for these specimens are high irrespective of the type of ferroelectric-toparaelectric phase transition [40,46,49,50]. The energy barrier between two spontaneous polarization states can be regarded as ED. Therefore, ED is influenced by TC and the type of ferroelectric-to-paraelectric phase transition. Fig. 9(a) shows the variation in εT33/εo and tan δ with respect to temperature (near TC) for P1, which comprises a pseudocubic phase. The difference in TC values (ΔT), which were measured during heating and cooling processes, is zero, revealing that the specimen undergoes 7
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Fig. 8. Polarization versus free energy curves corresponding to various conditions at RT: (a) α ≤ 0 and β > 0, (b) α ≤ 0 and β < < 0, (c) α < < 0 and β > 0, and (d) α < < 0 and β < 0.
thus it exhibits excellent ferroelectric and piezoelectric properties [21]. In addition, the specimens in this region are expected to have small domains because of their low ED values.
for specimens with large amounts of PNN, the tricritical points exist near RT because PNN has a cubic structure. Therefore, the two tricritical points for the PZT system can be expressed by two tricritical lines in the PT-PZ-PNN phase diagram, as shown in Fig. 10. The specimens existing between these two tricritical lines are expected to undergo second-order ferroelectric-to-paraelectric phase transition. Moreover, the specimens with pseudocubic phase and the specimens near pseudocubic phase, which exist between two tricritical lines (yellow-colored region in Fig. 10), are expected to have low TC values because they are close to the cubic phase. Furthermore, since these specimens undergo second-order phase transition with low TC values, they are expected to have low domain wall energies, as shown in Fig. 8(a). Therefore, the specimens in the yellow-colored region are expected to possess enhanced ferroelectric and piezoelectric properties owing to their low ED values. P1, R1, and T1 ceramics exhibit large εT33 and d33 values (Fig. 4(a) and (b)) because they exist in this region. Moreover, the specimen with the triple point composition exists in this region, and
4. Conclusions The tetragonal-to-cubic structural transition involved the formation of the pseudocubic phase. Moreover, the tetragonal-to-pseudocubic phase transformation was accompanied by the T-P MPB structure formation. In addition, in specimens without large amounts of PZ, the rhombohedral-to-cubic structural transition involved the formation of the pseudocubic phase. However, in specimens with large amounts of PZ, the rhombohedral structure directly changed to the cubic structure without formation of pseudocubic phase. The structure of pseudocubic phase formed in the PZ-PT-PNN ceramics was considered to be a Pm3m cubic structure, and specimens with pseudocubic phases showed the piezoelectric and ferroelectric
Fig. 9. Variations in εT33/εo and tan δ with respect to temperature for various ceramics: (a) P1, (b) R1, (c) T1, (d) R3, (e) R2, and (f) T2. 8
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Fig. 10. Schematic phase diagram for the PZ-PT-PNN ternary system showing the ferroelectric-to-paraelectric phase transition. Inset is the magnified schematic phase diagram of the PZT system showing two tricritical points.
properties probably owing to their non-ergodic characteristic. Because the structure of pseudocubic phase had a similarity to the rhombohedral structure, the rhombohedral phase directly changed to the pseudocubic phase. The specimens far away from the pseudocubic phase exhibited large domains and large EC values because of their high domain wall energies. On the other hand, nanodomains were observed in specimens with pseudocubic phase and the specimens close to the pseudocubic phase, and these specimens exhibited low EC values. Therefore, these specimens were expected to have low domain wall energies. Moreover, these specimens exhibited second-order ferroelectric-to-paraelectric phase transition at low TC values, confirming their low domain wall energies. The enhanced ferroelectric and piezoelectric properties of these specimens could be attributed to their low domain wall energies.
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Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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Acknowledgments
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This research was supported by the National Research Council of Science & Technology (NST) grant by the Korean government (MSIP; No. CAP-17-04-KRISS) and the authors also thank the KU-KIST Graduate School Program of Korea University.
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Appendix A. Supplementary data
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Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.jeurceramsoc.2019.12. 063.
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