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Domain structures and phase transitions in diisopropylammonium bromide molecular ferroelectric crystal
Chemical Physics Letters 689 (2017) 174–178
Contents lists available at ScienceDirect
Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
Domain structures and phase transitions in diisopropylammonium bromide molecular ferroelectric crystal Yuanyuan Li a, Kai Li b,⇑, Juan He a a b
College of Chemistry, Chemical and Environmental Engineering, Henan University of Technology, Henan 450001, PR China College of Chemistry and Molecular Engineering, Zhengzhou University, Henan 450001, PR China
a r t i c l e
i n f o
Article history: Received 5 July 2017 In final form 11 October 2017 Available online 12 October 2017 Keywords: Molecular ferroelectric Domain structure Phase transition Diisopropylammonium bromide Temperature-dependent
a b s t r a c t Diisopropylammonium bromide (DIPAB) molecular ferroelectric single crystals were prepared. Temperature-dependent domain evolutions were observed in situ by polarized light microscopy. Fast color variations were found both in heating and cooling processes. The color-variation temperatures agreed well with the phase transition temperatures that took placed in the dielectric measurement, which demonstrated that the fast domain transitions were exactly the ferroelectric-parroelectric phase transitions. Quick changed birefringence of the crystal was the main cause for the color variation of the domains during the phase transitions. In addition, 180° domains were detected by piezoresponse force microscopy, which were proposed to minimize the depolarization field. Ó 2017 Elsevier B.V. All rights reserved.
1. Introduction Ferroelectrics, which could undergo series phase transitions under an applied field (electric, temperature or stress), have attracted much attention for the widely technological applications in capacitors, sensors, transducers, data storage and electrooptical devices [1,2]. Inorganic ferroelectrics, such as BaTiO3, Pb(Zr, Ti)O3, and Pb(Mg, Nb)O3-PbTiO3, dominate the ferroelectric fields over a long period of time due to their high phase transition temperature (Curie temperature, Tc) and excellent electrical properties [3,4]. However, high processing temperatures and environmentally unfriendly character of most of these inorganic ferroelectrics still compels people to search for new ferroelectric materials [5,6]. In the past few years, more and more outstanding molecular ferroelectrics were developed [7–12]. It is inspiring that some of these ferroelectrics possess satisfactory properties, which are even comparable to that of some common inorganic ferroelectrics. One of the best known is the molecular crystal diisopropylammonium bromide (DIPAB) [13,14]. DIPAB crystals can be easily obtained, and exhibit the advantage of lightweight, mechanical flexibility, and nontoxicity. The spontaneous polarization (Ps = 23 lC cm2) of the ferroelectric DIPAB crystal is 4 times higher than that of other molecular compounds and, notably, is comparable to that of BaTiO3. Furthermore, the Curie temperature is much higher than those of existing molecular ferroelectrics, and even exceed that of ⇑ Corresponding author. E-mail address: [email protected] (K. Li). https://doi.org/10.1016/j.cplett.2017.10.024 0009-2614/Ó 2017 Elsevier B.V. All rights reserved.
BaTiO3 [13]. These outstanding properties suggest that DIPAB might be a potential alternative to inorganic ferroelectrics in some practical applications. The key performances of ferroelectrics are closely related to polarization ordering and dipole coupling in the physics aspect of ferroelectrics. Domain structures of aligned dipoles, and the domain switching behaviors, play an essential role in interpreting this issue [15]. It is well known that in situ observation of ferroelectric domains and their dynamic behaviors has been employed in many inorganic ferroelectrics for understanding the phase transition behavior [16–19]. For the DIPAB molecular system, static and dynamic domains in film or microcrystals were studied by piezoresponse force microscopy (PFM) [20,21]. To the best of our knowledge, however, detailed investigation on the temperaturedependent domain evolution of millimeter-size DIPAB single crystal (especially during the paraelectric-ferroelectric phase transition around Tc) is still rare. Since domain structures are closely connected with crystal symmetry, and consequently reflect on the electrical properties, studies on the domain evolutions are expected to provide a fundamental knowledge for understanding the phase transition. In this work, domain structures of DIPAB single crystal were studied by polarized light microscopy (PLM) and PFM. Temperature-dependent domain evolutions were specially observed in situ by PLM, which gave exactly visualized reflections on the phase transition. Furthermore, the domain switching dynamics also provided a visual and micro insight into the performance jumps at the phase transition.
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on the surfaces of the crystal along b-direction and the measurements were perpendicular to it. The PFM experiments were carried out to characterize the ferroelectric domains using a commercial microscope (Asylum Research, MFP-3D).
3. Results and discussion
Fig. 1. Ferroelectric DIPAB single crystals.
2. Experimental DIPAB crystals were prepared first by slow evaporation from absolute methanol solution containing diisopropylamine and hydrobromic acid. Needle-shaped single crystals (Fig. 1) were then obtained by recrystallizing the above DIPAB crystals in another absolute methanol solution. As reported, the needle-shape samples would be ferroelectrically active at room temperature and grow along b-direction, which is also the polarization axis of the ferroelectric DIPAB crystal [13,22]. Careful investigations of the temperature-dependent domain structures were executed using a polarized light microscope (Jiangnan, XJZ-6). A relatively thin crystal (about 0.1 mm) was selected for the PLM observation. An optical heating stage (Linkam, THMS600) was equipped on the PLM for temperature control. Dielectric measurements were carried out using a LCR meter (Agilent, HP4294). Silver electrodes were coated
The setup of PLM for investigating domain structures of DIPAB single crystal is interpreted in Fig. 2. In this experiment, domains of ferroelectrics are characterized by the principle of birefringence difference under polarized light [23,24]. The extinction position can be determined by the vibration directions of the polarizer/analyzer and the slow/fast vibration directions of the domain. As shown in Fig. 2, the vibration directions of the polarizer/analyzer should be set to mutually perpendicular. Meanwhile, the domain’s slow/fast vibration directions, which are also the major/minor axes (Ng and Np) of the refractive index ellipse that formed on the crosssectional area of the optical indicatrix, are mutually perpendicular too. That means the extinction will occur when the slow/fast vibration directions of the domain is along the same vibration direction of the polarizer/analyzer (Fig. 2(a)). In this study, angles of the crossed polarizer/analyzer (P/A) pair are referred to the b-direction of the crystal, i.e., P/A:0° means that the angle between the polarizer and b-direction is 0°. As can be seen from the inset of Fig. 3(a), typical extinction was observed at P/A:0° in the whole crystal at room temperature. Three more extinctions were automatically found at P/A:90°, P/A:180°, P/A:270°, respectively. Apart from these angles, the crystal remained always bright with interference color, which could reach to the brightest level at P/A:45° and other three angles increased by degrees of 90° (Figs. 2 (b) and 3(a)). The extinction characteristic of the DIPAB crystal at P/A:0° suggests that one of the vibration directions of the domain is aligned along the b-direction of the crystal. More succinctly,
Fig. 2. The setup of PLM. (a) Extinction at P/A:0°, i. e., the major/minor axes of the ellipse (also b-direction of the crystal) are aligned along the polarizer/analyzer (P/A) pair. (b) Interference color at P/A:45°, i. e., the angle between one of the major/minor axes of the ellipse (also b-direction of the crystal) and the polarizer/analyzer (P/A) pair is 45°.
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Fig. 4. Dielectric constant (e) and dielectric loss (tan d) at the frequency of 100 kHz upon heating. The inset shows the corresponding data at the cooling stage.
Fig. 3. Temperature-dependent domain evolutions upon (a)–(e) heating and (f)–(h) cooling at P/A:45°. The inset in (a) shows the domain at P/A:0° at room temperature.
the symmetry axis of refractive index ellipse is collinear to the spontaneous polarization direction. Temperature-dependent domain evolutions were further observed in situ by PLM, which was equipped with an optical heating stage for precise temperature control. As can be seen from Fig. 3(b) at P/A:45°, the domain did not move or change, with only the color varying a little when the temperature increased. Once the temperature reached to 155 °C, a new color emerged fast and covered the whole crystal instantly (Fig. 3(c)). This sudden change of the domain suggested that there might be a phase transition in the crystal. The crystal then transferred to darker hue as the temperature increased for about one degree (Fig. 3(d)). After that another color appeared at 157 °C. With the temperature continued to increase, however, the domain barely changed, as shown in Fig. 3(e) at 160 °C, indicating that the crystal may remain in the same phase. In the cooling stage, the domain experienced a reversible evolution as found in Fig. 3(f)-(h). Transient domain with a dark blue color arose first (Fig. 3(f)). Then a very sharp variation around 152 °C was observed (Fig. 3(g)). After that a color similar to that below 150 °C appeared and almost kept at the same in the ferroelectric phase (Fig. 3(h)). The sharp domain transition and a thermal hysteresis (3 °C) indicated that the phase transition is of first-order. In order to clarify the domain switching behaviors during heating and cooling process, dielectric constants (e) and dielectric loss (tan d) of the DIPAB crystal were measured. As shown in Fig. 4, one dielectric anomaly, namely, a sharp maximum at 155 °C, was observed on the heating process. This anomaly can be attributed to the transition from ferroelectric to parraelectric phase, where the transition temperature is the Curie Point Tc. The phase transition likewise occurred during the cooling process (Fig. 4 inset), suggesting a reversible phase transition. Evidently, the phase-transition temperatures that took place in the dielectric
measurement just fitted well with the domain variation temperatures, which demonstrated that the fast domain transitions in 155 °C (heating process) and 152 °C (cooling process) were corresponding to the ferroelectric-parroelectric and in turn the parroelectric-ferroelectric phase transition, respectively. The sharp peak accompanied with the thermal hysteresis (about 3 °C) indicated that the ferroelectric-parraelectric phase transition is of first-order, again in accordance with what was observed in the domain evolution. Therefore, it can be concluded that the domain switching behaviors was exactly a visualized reflection of the phase transition. Since the ferroelectric domains are characterized by the principle of birefringence difference under polarized light [23,24], the birefringence difference may be the main cause for the color variations of the DIPAB crystal. Because the polarized lights passing through the crystal exhibit different vibration directions (Ng and Np) and different propagation rates, retardation occurs. Interference colors then appear due to the superposition or elimination of lights according to the retardation. In other words, the interference colors are always associated with certain retardations, which can be determined as follows [24]:
R ¼ DðNg Np Þ where R is the retardation, D is the thickness of the crystal, Ng and Np are the domain’s slow/fast vibration directions, (Ng-Np) represents the birefringence. According to the formula, the retardation depends on the thickness and the birefringence. As the thickness of the DIPAB crystal remains a constant, the retardation is directly proportional to the birefringence, which means that the interference colors vary along with the birefringence. For a wide temperature range between room temperature and Tc, as the crystal remained in ferroelectric phase without apparent birefringence change, the domain did not show significant change, which all showed similar fuchsia color (Fig. 3(a), (b), (h)). While during the phase transitions of the DIPAB crystal, along with the crystal transforming from a centrosymmetric paraelectric phase into ferroelectric phase when cooled (and vice versa when heated), the atoms would distort quickly, which cause the refractive index ellipse deflects thereby, giving rise to a quick changed birefringence. This deduction agreed well with what was proved by J. Przeslawski et. al. that a jump of birefringence occurred at the phase transitions in the DIPAB single crystal [25]. The quick change of birefringence then resulted in the fast color variations at 155 °C (heating) and 152 °C (cooling). It was noted that deep fuchsia color was observed in the parraelectric phase of 160 °C (Fig. 3(e)), which seemed to be
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Fig. 5. Piezoresponse force microscopy of the DIPAB crystal (a) phase image (b) amplitude image. The polarization direction is denoted by arrows.
similar to that in ferroelectric phase. According to the interference color chart [26,27], it is known that even though there are different birefringence, similar colors can still occurs, depending on the interference color orders. Thus, the analogous colors at 160 °C (Fig. 3(e)) and the ferroelectric phase (Fig. 3(a), (b), (h)) were attributed to the coincidence caused by the birefringence in different color orders in the interference color chart. As is known, sudden changes of electrical and thermal properties during the phase transition were also found in the DIPAB crystal [13]. The color variations revealed by the above PLM observation, may also provide a visual and micro insight into the sudden changes. During the cooling stage of first-order, ferroelectric nuclei arose in the metastable paraelectric (overcooled) phase firstly [28]. Instant transformation of the domain then quickly achieved at Tc. Accordingly, spontaneous polarization arose as a jump of 4Ps. The emergence of spontaneous polarization results in bound charges at the polar surfaces of the crystal and at local defects, which produces depolarization field 4EDm [28,29]. It is known that the depolarization field could be minimized by two competitive processes: creating domains of opposite polarization (180° domains) or polarization charge compensation [28]. In the DIPAB crystal, 180° domains were clearly detected by PFM at room temperature (Fig. 5). As shown, stripe domains with antiparallel polar axis were observed, which confirmed the ferroelectric feature of the DIPAB crystal. The existence of 180° domains demonstrated that the depolarization field in DIPAB crystal could be reduced by the formation of 180° domains. Accordingly, electrical, mechanical and thermal jumps occurred when the depolarization field was minimized. These jumps in crystal parameters and constants are considered to be the main internal factors that determining the domain structures [28]. The domain structures, in turn, can reflect the transition kinetics and the performance jumps intuitively, which was proven by the consistent process of dielectric properties (Fig. 4). 4. Conclusions In conclusion, DIPAB ferroelectric single crystals were studied in this work. Temperature-dependent domain evolutions were observed in situ by PLM, which showed distinct color changes around the temperature of 155 °C (heating process) and 152 °C (cooling process). These color-changed temperatures fitted well
with the phase transition temperatures that took placed in the dielectric measurement, demonstrating that the fast domain transitions were corresponding to the ferroelectric-parroelectric phase transitions. This consistency indicated that the domain dynamic behaviors of the crystal can give a visualized reflection on the phase transitions. The quick color variations were attributed to the quick change of birefringence around Tc. In addition, 180° domains were detected by PFM, which were proposed to minimize the depolarization field. Acknowledgments We are very grateful for the financial support from the National Natural Science Foundation of China (No. 51502079, 21501150 and 21575034), the Science and Technology Key Project of Henan Education Department (No. 18A150026), the Fundamental Research Funds for the Henan Provincial Colleges and Universities (No. 2015QNJH09), Science Foundation of Henan University of Technology (No. 2015BS004), the Scientific Research Foundation for Young Teacher of Zhengzhou University (No. 51099075). References [1] N. Luo, S. Zhang, L. Qiang, X. Chao, Z. Yang, Q. Yan, Y. Zhang, T.R. Shrout, New Pb (Mg1/3Nb2/3)O3-Pb(In1/2Nb1/2)O3-PbZrO3-PbTiO3 quaternary ceramics: morphotropic phase boundary design and electrical properties, ACS Appl. Mat. Interfaces 8 (2016) 15506–15517. [2] C. Hu, H. Tian, X. Meng, G. Shi, W. Cao, Z. Zhou, High-quality K0.47Na0.53NbO3 single crystal toward high performance transducer, RSC Adv. 7 (2017) 7003– 7007. [3] S. Zhang, F. Li, High performance ferroelectric relaxor-PbTiO3 single crystals: status and perspective, J. Appl. Phys. 111 (2012) 031301. [4] M.-Q. Cai, Z. Yin, M.-S. Zhang, Y.-Z. Li, First-principles study of ferroelectric and nonlinear optical property in bismuth titanate, Chem. Phys. Lett. 401 (2005) 405–409. [5] F.-Z. Yao, Q. Yu, K. Wang, Q. Li, J.-F. Li, Ferroelectric domain morphology and temperature-dependent piezoelectricity of (K, Na, Li)(Nb, Ta, Sb)O3 lead-free piezoceramics, RSC Adv. 4 (2014) 20062–20068. [6] D. Pang, Z. Yi, Ferroelectric, piezoelectric properties and thermal expansion of new Bi(Ni3/4W1/4)O3-PbTiO3 solid solutions, RSC Adv. 7 (2017) 19448–19456. [7] X. Jiang, H. Lu, Y. Yin, X. Zhang, X. Wang, L. Yu, Z. Ahmadi, P.S. Costa, A.D. DiChiara, X. Cheng, A. Gruverman, A. Enders, X. Xu, Room temperature ferroelectricity in continuous croconic acid thin films, Appl. Phys. Lett. 109 (2016) 102902. [8] S. Horiuchi, Y. Tokunaga, G. Giovannetti, S. Picozzi, H. Itoh, R. Shimano, R. Kumai, Y. Tokura, Above-room-temperature ferroelectricity in a singlecomponent molecular crystal, Nature 463 (2010) 789–792.
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Contents lists available at ScienceDirect
Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
Domain structures and phase transitions in diisopropylammonium bromide molecular ferroelectric crystal Yuanyuan Li a, Kai Li b,⇑, Juan He a a b
College of Chemistry, Chemical and Environmental Engineering, Henan University of Technology, Henan 450001, PR China College of Chemistry and Molecular Engineering, Zhengzhou University, Henan 450001, PR China
a r t i c l e
i n f o
Article history: Received 5 July 2017 In final form 11 October 2017 Available online 12 October 2017 Keywords: Molecular ferroelectric Domain structure Phase transition Diisopropylammonium bromide Temperature-dependent
a b s t r a c t Diisopropylammonium bromide (DIPAB) molecular ferroelectric single crystals were prepared. Temperature-dependent domain evolutions were observed in situ by polarized light microscopy. Fast color variations were found both in heating and cooling processes. The color-variation temperatures agreed well with the phase transition temperatures that took placed in the dielectric measurement, which demonstrated that the fast domain transitions were exactly the ferroelectric-parroelectric phase transitions. Quick changed birefringence of the crystal was the main cause for the color variation of the domains during the phase transitions. In addition, 180° domains were detected by piezoresponse force microscopy, which were proposed to minimize the depolarization field. Ó 2017 Elsevier B.V. All rights reserved.
1. Introduction Ferroelectrics, which could undergo series phase transitions under an applied field (electric, temperature or stress), have attracted much attention for the widely technological applications in capacitors, sensors, transducers, data storage and electrooptical devices [1,2]. Inorganic ferroelectrics, such as BaTiO3, Pb(Zr, Ti)O3, and Pb(Mg, Nb)O3-PbTiO3, dominate the ferroelectric fields over a long period of time due to their high phase transition temperature (Curie temperature, Tc) and excellent electrical properties [3,4]. However, high processing temperatures and environmentally unfriendly character of most of these inorganic ferroelectrics still compels people to search for new ferroelectric materials [5,6]. In the past few years, more and more outstanding molecular ferroelectrics were developed [7–12]. It is inspiring that some of these ferroelectrics possess satisfactory properties, which are even comparable to that of some common inorganic ferroelectrics. One of the best known is the molecular crystal diisopropylammonium bromide (DIPAB) [13,14]. DIPAB crystals can be easily obtained, and exhibit the advantage of lightweight, mechanical flexibility, and nontoxicity. The spontaneous polarization (Ps = 23 lC cm2) of the ferroelectric DIPAB crystal is 4 times higher than that of other molecular compounds and, notably, is comparable to that of BaTiO3. Furthermore, the Curie temperature is much higher than those of existing molecular ferroelectrics, and even exceed that of ⇑ Corresponding author. E-mail address: [email protected] (K. Li). https://doi.org/10.1016/j.cplett.2017.10.024 0009-2614/Ó 2017 Elsevier B.V. All rights reserved.
BaTiO3 [13]. These outstanding properties suggest that DIPAB might be a potential alternative to inorganic ferroelectrics in some practical applications. The key performances of ferroelectrics are closely related to polarization ordering and dipole coupling in the physics aspect of ferroelectrics. Domain structures of aligned dipoles, and the domain switching behaviors, play an essential role in interpreting this issue [15]. It is well known that in situ observation of ferroelectric domains and their dynamic behaviors has been employed in many inorganic ferroelectrics for understanding the phase transition behavior [16–19]. For the DIPAB molecular system, static and dynamic domains in film or microcrystals were studied by piezoresponse force microscopy (PFM) [20,21]. To the best of our knowledge, however, detailed investigation on the temperaturedependent domain evolution of millimeter-size DIPAB single crystal (especially during the paraelectric-ferroelectric phase transition around Tc) is still rare. Since domain structures are closely connected with crystal symmetry, and consequently reflect on the electrical properties, studies on the domain evolutions are expected to provide a fundamental knowledge for understanding the phase transition. In this work, domain structures of DIPAB single crystal were studied by polarized light microscopy (PLM) and PFM. Temperature-dependent domain evolutions were specially observed in situ by PLM, which gave exactly visualized reflections on the phase transition. Furthermore, the domain switching dynamics also provided a visual and micro insight into the performance jumps at the phase transition.
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on the surfaces of the crystal along b-direction and the measurements were perpendicular to it. The PFM experiments were carried out to characterize the ferroelectric domains using a commercial microscope (Asylum Research, MFP-3D).
3. Results and discussion
Fig. 1. Ferroelectric DIPAB single crystals.
2. Experimental DIPAB crystals were prepared first by slow evaporation from absolute methanol solution containing diisopropylamine and hydrobromic acid. Needle-shaped single crystals (Fig. 1) were then obtained by recrystallizing the above DIPAB crystals in another absolute methanol solution. As reported, the needle-shape samples would be ferroelectrically active at room temperature and grow along b-direction, which is also the polarization axis of the ferroelectric DIPAB crystal [13,22]. Careful investigations of the temperature-dependent domain structures were executed using a polarized light microscope (Jiangnan, XJZ-6). A relatively thin crystal (about 0.1 mm) was selected for the PLM observation. An optical heating stage (Linkam, THMS600) was equipped on the PLM for temperature control. Dielectric measurements were carried out using a LCR meter (Agilent, HP4294). Silver electrodes were coated
The setup of PLM for investigating domain structures of DIPAB single crystal is interpreted in Fig. 2. In this experiment, domains of ferroelectrics are characterized by the principle of birefringence difference under polarized light [23,24]. The extinction position can be determined by the vibration directions of the polarizer/analyzer and the slow/fast vibration directions of the domain. As shown in Fig. 2, the vibration directions of the polarizer/analyzer should be set to mutually perpendicular. Meanwhile, the domain’s slow/fast vibration directions, which are also the major/minor axes (Ng and Np) of the refractive index ellipse that formed on the crosssectional area of the optical indicatrix, are mutually perpendicular too. That means the extinction will occur when the slow/fast vibration directions of the domain is along the same vibration direction of the polarizer/analyzer (Fig. 2(a)). In this study, angles of the crossed polarizer/analyzer (P/A) pair are referred to the b-direction of the crystal, i.e., P/A:0° means that the angle between the polarizer and b-direction is 0°. As can be seen from the inset of Fig. 3(a), typical extinction was observed at P/A:0° in the whole crystal at room temperature. Three more extinctions were automatically found at P/A:90°, P/A:180°, P/A:270°, respectively. Apart from these angles, the crystal remained always bright with interference color, which could reach to the brightest level at P/A:45° and other three angles increased by degrees of 90° (Figs. 2 (b) and 3(a)). The extinction characteristic of the DIPAB crystal at P/A:0° suggests that one of the vibration directions of the domain is aligned along the b-direction of the crystal. More succinctly,
Fig. 2. The setup of PLM. (a) Extinction at P/A:0°, i. e., the major/minor axes of the ellipse (also b-direction of the crystal) are aligned along the polarizer/analyzer (P/A) pair. (b) Interference color at P/A:45°, i. e., the angle between one of the major/minor axes of the ellipse (also b-direction of the crystal) and the polarizer/analyzer (P/A) pair is 45°.
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Fig. 4. Dielectric constant (e) and dielectric loss (tan d) at the frequency of 100 kHz upon heating. The inset shows the corresponding data at the cooling stage.
Fig. 3. Temperature-dependent domain evolutions upon (a)–(e) heating and (f)–(h) cooling at P/A:45°. The inset in (a) shows the domain at P/A:0° at room temperature.
the symmetry axis of refractive index ellipse is collinear to the spontaneous polarization direction. Temperature-dependent domain evolutions were further observed in situ by PLM, which was equipped with an optical heating stage for precise temperature control. As can be seen from Fig. 3(b) at P/A:45°, the domain did not move or change, with only the color varying a little when the temperature increased. Once the temperature reached to 155 °C, a new color emerged fast and covered the whole crystal instantly (Fig. 3(c)). This sudden change of the domain suggested that there might be a phase transition in the crystal. The crystal then transferred to darker hue as the temperature increased for about one degree (Fig. 3(d)). After that another color appeared at 157 °C. With the temperature continued to increase, however, the domain barely changed, as shown in Fig. 3(e) at 160 °C, indicating that the crystal may remain in the same phase. In the cooling stage, the domain experienced a reversible evolution as found in Fig. 3(f)-(h). Transient domain with a dark blue color arose first (Fig. 3(f)). Then a very sharp variation around 152 °C was observed (Fig. 3(g)). After that a color similar to that below 150 °C appeared and almost kept at the same in the ferroelectric phase (Fig. 3(h)). The sharp domain transition and a thermal hysteresis (3 °C) indicated that the phase transition is of first-order. In order to clarify the domain switching behaviors during heating and cooling process, dielectric constants (e) and dielectric loss (tan d) of the DIPAB crystal were measured. As shown in Fig. 4, one dielectric anomaly, namely, a sharp maximum at 155 °C, was observed on the heating process. This anomaly can be attributed to the transition from ferroelectric to parraelectric phase, where the transition temperature is the Curie Point Tc. The phase transition likewise occurred during the cooling process (Fig. 4 inset), suggesting a reversible phase transition. Evidently, the phase-transition temperatures that took place in the dielectric
measurement just fitted well with the domain variation temperatures, which demonstrated that the fast domain transitions in 155 °C (heating process) and 152 °C (cooling process) were corresponding to the ferroelectric-parroelectric and in turn the parroelectric-ferroelectric phase transition, respectively. The sharp peak accompanied with the thermal hysteresis (about 3 °C) indicated that the ferroelectric-parraelectric phase transition is of first-order, again in accordance with what was observed in the domain evolution. Therefore, it can be concluded that the domain switching behaviors was exactly a visualized reflection of the phase transition. Since the ferroelectric domains are characterized by the principle of birefringence difference under polarized light [23,24], the birefringence difference may be the main cause for the color variations of the DIPAB crystal. Because the polarized lights passing through the crystal exhibit different vibration directions (Ng and Np) and different propagation rates, retardation occurs. Interference colors then appear due to the superposition or elimination of lights according to the retardation. In other words, the interference colors are always associated with certain retardations, which can be determined as follows [24]:
R ¼ DðNg Np Þ where R is the retardation, D is the thickness of the crystal, Ng and Np are the domain’s slow/fast vibration directions, (Ng-Np) represents the birefringence. According to the formula, the retardation depends on the thickness and the birefringence. As the thickness of the DIPAB crystal remains a constant, the retardation is directly proportional to the birefringence, which means that the interference colors vary along with the birefringence. For a wide temperature range between room temperature and Tc, as the crystal remained in ferroelectric phase without apparent birefringence change, the domain did not show significant change, which all showed similar fuchsia color (Fig. 3(a), (b), (h)). While during the phase transitions of the DIPAB crystal, along with the crystal transforming from a centrosymmetric paraelectric phase into ferroelectric phase when cooled (and vice versa when heated), the atoms would distort quickly, which cause the refractive index ellipse deflects thereby, giving rise to a quick changed birefringence. This deduction agreed well with what was proved by J. Przeslawski et. al. that a jump of birefringence occurred at the phase transitions in the DIPAB single crystal [25]. The quick change of birefringence then resulted in the fast color variations at 155 °C (heating) and 152 °C (cooling). It was noted that deep fuchsia color was observed in the parraelectric phase of 160 °C (Fig. 3(e)), which seemed to be
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Fig. 5. Piezoresponse force microscopy of the DIPAB crystal (a) phase image (b) amplitude image. The polarization direction is denoted by arrows.
similar to that in ferroelectric phase. According to the interference color chart [26,27], it is known that even though there are different birefringence, similar colors can still occurs, depending on the interference color orders. Thus, the analogous colors at 160 °C (Fig. 3(e)) and the ferroelectric phase (Fig. 3(a), (b), (h)) were attributed to the coincidence caused by the birefringence in different color orders in the interference color chart. As is known, sudden changes of electrical and thermal properties during the phase transition were also found in the DIPAB crystal [13]. The color variations revealed by the above PLM observation, may also provide a visual and micro insight into the sudden changes. During the cooling stage of first-order, ferroelectric nuclei arose in the metastable paraelectric (overcooled) phase firstly [28]. Instant transformation of the domain then quickly achieved at Tc. Accordingly, spontaneous polarization arose as a jump of 4Ps. The emergence of spontaneous polarization results in bound charges at the polar surfaces of the crystal and at local defects, which produces depolarization field 4EDm [28,29]. It is known that the depolarization field could be minimized by two competitive processes: creating domains of opposite polarization (180° domains) or polarization charge compensation [28]. In the DIPAB crystal, 180° domains were clearly detected by PFM at room temperature (Fig. 5). As shown, stripe domains with antiparallel polar axis were observed, which confirmed the ferroelectric feature of the DIPAB crystal. The existence of 180° domains demonstrated that the depolarization field in DIPAB crystal could be reduced by the formation of 180° domains. Accordingly, electrical, mechanical and thermal jumps occurred when the depolarization field was minimized. These jumps in crystal parameters and constants are considered to be the main internal factors that determining the domain structures [28]. The domain structures, in turn, can reflect the transition kinetics and the performance jumps intuitively, which was proven by the consistent process of dielectric properties (Fig. 4). 4. Conclusions In conclusion, DIPAB ferroelectric single crystals were studied in this work. Temperature-dependent domain evolutions were observed in situ by PLM, which showed distinct color changes around the temperature of 155 °C (heating process) and 152 °C (cooling process). These color-changed temperatures fitted well
with the phase transition temperatures that took placed in the dielectric measurement, demonstrating that the fast domain transitions were corresponding to the ferroelectric-parroelectric phase transitions. This consistency indicated that the domain dynamic behaviors of the crystal can give a visualized reflection on the phase transitions. The quick color variations were attributed to the quick change of birefringence around Tc. In addition, 180° domains were detected by PFM, which were proposed to minimize the depolarization field. Acknowledgments We are very grateful for the financial support from the National Natural Science Foundation of China (No. 51502079, 21501150 and 21575034), the Science and Technology Key Project of Henan Education Department (No. 18A150026), the Fundamental Research Funds for the Henan Provincial Colleges and Universities (No. 2015QNJH09), Science Foundation of Henan University of Technology (No. 2015BS004), the Scientific Research Foundation for Young Teacher of Zhengzhou University (No. 51099075). References [1] N. Luo, S. Zhang, L. Qiang, X. Chao, Z. Yang, Q. Yan, Y. Zhang, T.R. Shrout, New Pb (Mg1/3Nb2/3)O3-Pb(In1/2Nb1/2)O3-PbZrO3-PbTiO3 quaternary ceramics: morphotropic phase boundary design and electrical properties, ACS Appl. Mat. Interfaces 8 (2016) 15506–15517. [2] C. Hu, H. Tian, X. Meng, G. Shi, W. Cao, Z. Zhou, High-quality K0.47Na0.53NbO3 single crystal toward high performance transducer, RSC Adv. 7 (2017) 7003– 7007. [3] S. Zhang, F. Li, High performance ferroelectric relaxor-PbTiO3 single crystals: status and perspective, J. Appl. Phys. 111 (2012) 031301. [4] M.-Q. Cai, Z. Yin, M.-S. Zhang, Y.-Z. Li, First-principles study of ferroelectric and nonlinear optical property in bismuth titanate, Chem. Phys. Lett. 401 (2005) 405–409. [5] F.-Z. Yao, Q. Yu, K. Wang, Q. Li, J.-F. Li, Ferroelectric domain morphology and temperature-dependent piezoelectricity of (K, Na, Li)(Nb, Ta, Sb)O3 lead-free piezoceramics, RSC Adv. 4 (2014) 20062–20068. [6] D. Pang, Z. Yi, Ferroelectric, piezoelectric properties and thermal expansion of new Bi(Ni3/4W1/4)O3-PbTiO3 solid solutions, RSC Adv. 7 (2017) 19448–19456. [7] X. Jiang, H. Lu, Y. Yin, X. Zhang, X. Wang, L. Yu, Z. Ahmadi, P.S. Costa, A.D. DiChiara, X. Cheng, A. Gruverman, A. Enders, X. Xu, Room temperature ferroelectricity in continuous croconic acid thin films, Appl. Phys. Lett. 109 (2016) 102902. [8] S. Horiuchi, Y. Tokunaga, G. Giovannetti, S. Picozzi, H. Itoh, R. Shimano, R. Kumai, Y. Tokura, Above-room-temperature ferroelectricity in a singlecomponent molecular crystal, Nature 463 (2010) 789–792.
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