Electric field-induced phase transitions in Li-modified

Electric field-induced phase transitions in Li-modified Na0.5K0.5NbO3 at the polymorphic phase boundary Thanakorn Iamsasri, Goknur Tutuncu, Chunmanus Uthaisar, Supattra Wongsaenmai, Soodkhet Pojprapai, and Jacob L. Jones Citation: Journal of Applied Physics 117, 024101 (2015); doi: 10.1063/1.4905613 View online: http://dx.doi.org/10.1063/1.4905613 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/117/2?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Composition induced structure evolution and large strain response in ternary Bi0.5Na0.5TiO3-Bi0.5K0.5TiO3SrTiO3 solid solution J. Appl. Phys. 114, 164105 (2013); 10.1063/1.4825122 Phase transitions, relaxor behavior, and large strain response in LiNbO3-modified Bi0.5(Na0.80K0.20)0.5TiO3 lead-free piezoceramics J. Appl. Phys. 114, 044103 (2013); 10.1063/1.4816047 A monoclinic-tetragonal ferroelectric phase transition in lead-free (K0.5Na0.5)NbO3-x%LiNbO3 solid solution J. Appl. Phys. 111, 103503 (2012); 10.1063/1.4716027 Microstructure, phase transition, and electrical properties of ( K 0.5 Na 0.5 ) 1 − x Li x ( Nb 1 − y Ta y ) O 3 leadfree piezoelectric ceramics J. Appl. Phys. 102, 034102 (2007); 10.1063/1.2761852 Structure and electrical properties of K 0.5 Na 0.5 Nb O 3 – Li Sb O 3 lead-free piezoelectric ceramics J. Appl. Phys. 101, 074111 (2007); 10.1063/1.2715486

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JOURNAL OF APPLIED PHYSICS 117, 024101 (2015)

Electric field-induced phase transitions in Li-modified Na0.5K0.5NbO3 at the polymorphic phase boundary Thanakorn Iamsasri,1,2 Goknur Tutuncu,2 Chunmanus Uthaisar,3 Supattra Wongsaenmai,4 Soodkhet Pojprapai,3 and Jacob L. Jones1,2,a)

1 Department of Materials Science and Engineering, North Carolina State University, Raleigh, North Carolina 27695, USA 2 Department of Materials Science and Engineering, University of Florida, Gainesville, Florida 32611, USA 3 School of Ceramic Engineering, Institute of Engineering, Suranaree University of Technology, Nakorn Ratchasima 30000, Thailand 4 Program in Materials Science, Faculty of Science, Maejo University, Chiang Mai 50290, Thailand

(Received 16 July 2014; accepted 26 December 2014; published online 8 January 2015) The electric field-induced phase transitions in Li-modified Na0.5K0.5NbO3 at the polymorphic phase boundary (PPB) were observed using in situ X-ray diffraction. The ratio of monoclinic to tetragonal phase fraction was used as an indicator of the extent and reversibility of the phase transitions. The reversibility of the phase transition was greater in compositions further from the PPB. These results demonstrate that the field-induced phase transition is one of the origins of high piezoelectric propC 2015 AIP Publishing LLC. erties in lead-free ferroelectric materials. V [http://dx.doi.org/10.1063/1.4905613]

I. INTRODUCTION

Compositions of lead zirconate titanate (Pb(Zr,Ti)O3 or PZT) have been the dominant material in many piezoelectric applications including actuators and sensors. The search for a Pb-free replacement for PZT that is more biologically and environmentally friendly has resulted in a rigorous reinvestigation of solid solutions between Na0.5K0.5NbO3 (NKN), Na0.5Bi0.5TiO3 (NBT), and other perovskites.1–3 Solid solutions of NKN have been widely investigated because they have high piezoelectric properties that are comparable to Pbbased ferroelectrics.4–7 Several different mechanisms have been reported to play a role in achieving high piezoelectric properties in Pb-based ferroelectrics including polarization rotation,8–10 coexistence of mixed polymorphic phases,11–14 and domain wall motion.15–17 However, the origin of high piezoelectric properties in Pb-free materials is not well understood. Saito et al.4 conjectured that the origin of high piezoelectric properties in modified NKN compositions was due to an electric field-induced phase transition. From Landau-Ginsburg-Devonshire theory, external electric fields can change the free energy profile of each phase.18–20 Since two coexisting phases at the phase boundary have similar free energies, the field-induced phase transitions are more likely to occur in a mixed phase material than in a single phase material. In this work, Li-modified NKN (LNKN) at the polymorphic phase boundary (PPB) is used as a model system to investigate the contribution from field-induced phase transitions on piezoelectric properties in lead-free ferroelectrics. Piezoelectric properties of LNKN are enhanced at compositions containing approximately 6%–7% Li which is near the PPB.6,7 However, there is a lack of consensus on the a)

Author to whom correspondence should be addressed. Electronic mail: [email protected]

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structure of LNKN at the PPB. For example, from the phase diagram of LNKN by Klein et al.21 in 2007, the PPB separates structural polymorphs of orthorhombic and tetragonal symmetries. On the other hand, a recent study in 2012 by Ge et al.22 showed that 5% Li-modified NKN, which lies on the orthorhombic side of the PPB, has an intermediate monoclinic phase existing between orthorhombic and tetragonal compositions. The phase diagram of Na1-yKyNbO3 by Baker et al.23 also shows that the unmodified NKN composition is in close proximity to the orthorhombic Amm2 and the monoclinic Pm phase boundary. In this work, the field-induced phase transitions in LNKN are investigated using in situ high energy X-ray diffraction (XRD) because it can determine the structure of ferroelectrics during application of electric fields. High resolution XRD is also utilized to determine the crystal structure before application of electric field of LNKN near the PPB. The results show that a field-induced phase transition is observed only in compositions in close proximity to the PPB, where piezoelectric properties are maximized. The field-induced phase transition is therefore shown to be one of the origins of high piezoelectric properties. II. EXPERIMENT

(1–x)(Na0.5K0.5)NbO3–xLiNbO3 (100xLNKN) for x ¼ 0.055–0.07 samples were prepared through the conventional mixed oxide method, as described in more detail elsewhere.24 For the electrical measurements, gold electrodes were sputtered on both sides of the samples. The permittivity as a function of temperature was measured with an LCR meter (HP-4284A, Hewlett-Packard, Santa Clara, CA) in conjunction with an environmental chamber (9023, Delta Design, Poway, CA) using a heating rate of 2 C/min and frequency of 1 kHz. Polarization and strain were measured at room temperatures (25 C) with a standardized ferroelectric

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test system (RT-66A, Radiant Technologies, Albuquerque, NM). Bipolar triangular waveform of amplitude 3 kV/mm was applied for 5 cycles during the measurement. High resolution XRD patterns for 6LNKN and 6.5LNKN were measured at beamline 11-BM at the Advanced Photon Source at Argonne National Laboratory. Crushed powders of each composition were loaded into Kapton capillaries and spun to achieve good powder averaging statistics. XRD patterns were recorded at room temperature with a wavelength ˚ , using a scintillator detector range covering 2h of 0.413733 A approximately 3.5 –30 , 2h step of 0.001 , and time per step of 0.1 s. Diffraction patterns were analyzed using the Rietveld refinement program GSAS with the EXPGUI interface.25,26 The patterns were refined using profile function 4, which utilizes a convolution of pseudo-Voigt and asymmetric function with microstrain broadening terms. Ten parameters in the Shifted Chebyschev model were used to model the background intensity. Space groups were determined from the high resolution XRD patterns. In situ XRD patterns were measured during the application of electric fields using high energy X-rays at beamline 11-ID-C of the Advanced Photon Source, Argonne National ˚, Laboratory. The wavelength of the X-rays was 0.10798 A and the size of the beam was 0.5 mm 0.5 mm. The diffraction patterns were collected in forward scattering geometry using a Perkin Elmer area detector at a distance of approximately 2.25 m. In the first electric field cycle, the samples were subjected to an electric field of amplitude 2 kV/mm utilizing a triangular bipolar waveform with a frequency of 0.0125 Hz. The second electric field cycle was then applied at the same frequency using an increased amplitude of 2.24 kV/mm for the 6LNKN sample and 2.5 kV/mm for the 6.5LNKN sample. The diffracted intensity within the vertical sector of the two-dimensional XRD pattern, which measures scattering vectors approximately parallel to the electric field direction, was integrated by Fit2D software using 615 azimuthal angles.27,28 Similarly, the horizontal sector of two-dimensional XRD patterns, which measures scattering vectors perpendicular to the electric field direction, was integrated by the same method.

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FIG. 1. Dielectric permittivities of unpoled LNKN for x ¼ 0.055–0.07. Inset shows dielectric permittivities across the polymorphic phase transition temperature.

readily observed in the polarization or strain of single crystal ferroelectrics (e.g., see Refs. 29 and 30), field-induced phase transitions in polycrystalline materials are less obvious to evidence through such measurements. Polycrystalline ferroelectrics can exhibit field-induced phase transitions and changes to phase fractions of co-existing phases that are readily apparent through in situ X-ray or neutron diffraction measurements, though polarization measurements show no unexpected discontinuities (e.g., see Refs. 14 and 31). This motivates in situ structural studies such as in situ XRD to characterize field-induced structural changes. Nevertheless, some observations can be made from the polarization measurements in Fig. 2. For both compositions, the polarization shows contributions from both reversible and irreversible processes. With respect to irreversible processes, we note that polarization decreases as the number of cycles increase. This suggests that the samples experience some irreversible processes during the cyclic application of electric fields,

III. RESULTS AND DISCUSSION

The dielectric permittivities of unpoled 100xLNKN for x ¼ 0.055–0.07 are shown in Fig. 1 as a function of temperature. As the amount of Li substitution increases, the polymorphic phase transition temperature decreases toward room temperature as shown in the inset of Fig. 1. High resolution XRD patterns of LNKN, presented below, indicate that 6LNKN and 6.5LNKN have mixed phases. Therefore, 6LNKN and 6.5LNKN compositions are in close proximity to the PPB, which is in agreement with prior work.6,7 These mixed phase compositions were selected in this study to investigate electric field-induced phase transitions. Polarization hysteresis loops for 6LNKN and 6.5LNKN are shown in Fig. 2. Polarization hysteresis loops of these polycrystalline ferroelectric materials do not exhibit abrupt changes that would immediately indicate a field-induced phase transition. While field-induced transitions can be

FIG. 2. Polarization hysteresis loops for (a) 6LNKN and (b) 6.5LNKN.


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FIG. 3. Measured and calculated diffraction pattern of 6.5LNKN using a mixture of Pm and P4mm space groups. Asterisks indicate reflections from the secondary phase K3Li2Nb5O15.

which may be related to irreversible field-induced phase transitions. The structure of 6LNKN and 6.5LNKN was determined using high resolution XRD. A representative refinement of high resolution XRD patterns of 6.5LNKN is shown in Fig. 3. A secondary phase K3Li2Nb5O15 is observed in the high resolution XRD patterns. This secondary phase has been previously observed in modified NKN samples.32,33 The {200}PC region, shown in the inset of Fig. 3, consists of five reflections. A mixed phase of orthorhombic Amm2 and tetragonal P4mm would result in a total of four reflections within this region, which is not enough to explain observed five peaks in the XRD patterns. Therefore, other phases and phase combinations are considered next. The unmodified NKN composition is near the phase boundary of

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orthorhombic Amm2 and monoclinic Pm,23 and the intermediate monoclinic phase was reported to fit the structure of 5LNKN.22 Since a mixture of tetragonal and monoclinic phases would result in a total of five reflections for the {200}PC profile, it is reasonable to use a combination of tetragonal P4mm and monoclinic Pm space groups in the refinement. The refined monoclinic and tetragonal structures produce calculated peak positions that match the peaks from the high resolution XRD pattern. Unfortunately, the combination of P4mm and Pm space groups results in too many parameters to obtain a reliable refinement of atomic positions. However, since the atomic positions alter only the peak intensities and not the lattice parameters, it is concluded and confirmed that the structure of LNKN at the PPB is a mixed phase of monoclinic and tetragonal (Fig. 3). Fig. 4 shows the {200}PC reflections with scattering vectors perpendicular (Fig. 4(a)) and parallel (Fig. 4(b)) to the electric field direction during the application of the bipolar triangular waveform for the 6LNKN sample. When electric fields are applied, the intensity of each reflection changes simultaneously for both perpendicular and parallel scattering vectors. From Fig. 4, at field amplitudes of approximately 1.5 kV/mm during the first electric field cycle, the intensity of (002)T reflection increases, while the intensities of (200)M and (002)M reflections decrease for both scattering vectors. Since the intensity is proportional to the volume fraction of a certain domain orientation, an increase in intensity for both scattering vectors indicates an increase of the overall volume fraction of the phase.24 This demonstrates an electric fieldinduced phase transition that is evidenced by an increase in the tetragonal phase fraction. Here, it should be noted that the intensity change mentioned above is different from that is observed as a result of domain reorientation. For domain reorientation, intensity changes from the parallel scattering vector would be opposite to those from the perpendicular scattering vector.

FIG. 4. The {200}PC reflections with scattering vectors (a) perpendicular and (b) parallel to the electric field direction during the application of triangular bipolar waveforms on 6LNKN.


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FIG. 5. The {200}PC reflections with scattering vectors (a) perpendicular and (b) parallel to the electric field direction during the application of triangular bipolar waveforms on 6.5LNKN.

Similarly, Fig. 5 shows the {200}PC profile with scattering vectors perpendicular (Fig. 5(a)) and parallel (Fig. 5(b)) to the electric field direction for a 6.5LNKN sample. At a field amplitude of approximately 1.5 kV/mm, the intensities of the (200)M and (002)M reflections increase, and the intensity of the (002)T reflection decreases. This again evidences an electric field-induced phase transition. The reversibility of the phase transition is different in 6LNKN and 6.5LNKN and is discussed later. Peak profile fitting was used to further analyze the fieldinduced phase transitions. Diffraction patterns were modeled by adjusting profile shape parameters to minimize the difference between the modeled and measured patterns. This refinement procedure provides the integrated intensity and position of each individual peak. For the mixture of monoclinic and tetragonal phases, there are five expected reflections for the {200}PC profile, i.e., (002)T, (200)M, (002)M, (200)T, and (020)M. Due to similar d-spacings of the (200)T and the (020)M reflections, corresponding reflections are difficult to distinguish. Moreover, the (200)M and the (002)M reflections are broad because of diffuse scattering between the Bragg peaks. Therefore, the integrated intensity of individual reflections is difficult to obtain with a high degree of certainty. To overcome this difficulty, the integrated intensities of individual {200}PC reflections were extracted by fitting the measured intensity with five Pearson VII profiles constrained by the same full width at half maximum and variance. Thus, the four peaks represent the (002)T, (200)M, (002)M, and the total intensity of (200)T and (020)M reflections, respectively. A fifth peak was introduced to fit the diffuse scattering intensity that results from ferroelastic domain walls,34,35 and it was positioned between the (200)M and the (002)M reflections. Since the intensity between the (200)M and the (002)M reflections does not belong to the intensity from the Bragg peaks, it is attributed to the diffuse scattering. Although the intensity from diffuse scattering may

overlap with the intensity from the (200)M and the (002)M reflections, it is fitted with a single peak for simplicity in this study, which is a reasonable first order approximation for modeling this scattering. The intensity for each corresponding peak was calculated by integrating the diffracted intensity under the profile between the centers of adjacent profiles. Fig. 6 shows a representative fitting result using the five profile shape functions for 6.5LNKN at an electric field amplitude of 2.5 kV/mm. As seen in Fig. 6, the difference curve shows minimal variation between the measured and modeled data, evidencing a good quality fit. Fig. 7 shows the results of the peak fitting procedure, including the change in intensities of (002)T, (200)M, and (002)M reflections and the 2h position of the (200)T/(020)M profile during the application of electric fields on 6LNKN.

FIG. 6. Measured intensity, Pearson VII model, and difference between the measured intensity and the model of {200}PC reflections of 6.5LNKN at an electric field amplitude of 2.5 kV/mm.


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FIG. 7. Normalized intensities of (002)T, (200)M, and (002)M reflections and 2h position of the (200)T/(020)M profile during the application of electric fields on 6LNKN.

The intensity change is shown on a normalized scale, which is calculated by the ratio of integrated intensity during the application of fields to that of the unpoled sample. When the normalized intensity is greater than one, the phase fraction is increased relative to the unpoled state. Similarly, when the normalized intensity is less than one, the phase fraction is decreased relative to the unpoled state. Since the (200)T and (020)M reflections are difficult to discriminate throughout the field application due to the fact that they have similar positions, the peak position that represents these two reflections is used as an indirect indicator of changes in phase fraction. When the 2h position of the (200)T/(020)M profile decreases compared to the unpoled state, it implies that the intensity of the (200)T reflection increases and the (020)M reflection decreases. Similarly, when the 2h position of the (200)T/(020)M profile increases compared to the unpoled state, it indicates that the intensity of the (200)T reflection decreases and the (020)M reflection increases. Such an inference is based on the positions of these peaks as determined from the analysis of the high resolution XRD pattern, where the (200)T peak is at a slightly lower 2h position than the (020)M peak. In Fig. 7, at the field amplitude of approximately 1.5 kV/mm in the first cycle, the intensity of the tetragonal phase increases, the intensity of the monoclinic phase decreases, and the position of (200)T/(020)M profile decreases. These results indicate a field-induced phase transition that increases the tetragonal phase fraction. After the field amplitude is decreased to approximately 1.5 kV/mm in the first cycle, the tetragonal phase fraction decreases to a

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value similar to the unpoled state. At the subsequent field amplitude of approximately 1.5 kV/mm in the first cycle, the intensity of tetragonal decreases, the intensity of monoclinic increases, and the position of (200)T/(020)M profile increases. These are an indication of a field-induced phase transition that increases the monoclinic phase fraction under negative field amplitudes. Similar field-induced phase transitions also occur in the second cycle. By observing the behavior of intensities and positions shown during the initial positive electric fields in Fig. 7, the field-induced phase transition that increases the tetragonal phase fraction can be said to be reversible. In contrast, the induced monoclinic phase remains after removal of applied fields in the subsequent negative polarity of the cycle. Therefore, the field-induced monoclinic phase fraction is observed to be irreversible. It is further noted that the use of in situ diffraction during application of electric fields provides insight into the fieldinduced tetragonal polymorph that would otherwise be absent in an ex situ investigation. Fig. 8 shows the results from profile fitting of diffraction patterns of 6.5LNKN during application of electric fields. From Fig. 8, the normalized intensities of (200)M and (002)M reflections increase, while the intensity of the (002)T reflection decreases. Also, the 2h position of the (200)T/(020)M profile increases. These results indicate that a field-induced phase transition is occurring in which the monoclinic phase fraction increases. In the second cycle, in which the amplitude of the electric field is larger than the first cycle, the monoclinic phase fraction increases to an even greater extent. The increased monoclinic phase fraction also remains

FIG. 8. Normalized intensities of (002)T, (200)M, and (002)M reflections and 2h position of the (200)T/(020)M profile during the application of electric fields on 6.5LNKN.


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after removal of electric fields. Therefore, the increased monoclinic phase fraction is observed to be irreversible, consistent with the results on 6LNKN. In contrast to 6LNKN, however, the field-induced tetragonal phase is not observed in 6.5LNKN. A different field-induced phase change behavior in 6LNKN and 6.5LNKN is attributed to different proximities to the PPB. Field-induced phase transitions between tetragonal and monoclinic phases have been observed in prior work. In 2012, Zuo et al.36 reported the field-induced phase transition from the tetragonal phase to the monoclinic phase in Li, Sb, and Ta-modified NKN polycrystalline ceramics. In 2013, Guo et al.37 reported the field-induced phase transition from the tetragonal phase to the monoclinic phase in Li and Sbmodified NKN samples using in situ TEM. In both experiments, the field-induced phase transition from the tetragonal phase to the monoclinic phase is irreversible, consistent with the results in the present work. A possible explanation for the field-induced phase transition from the tetragonal phase to the monoclinic phase is that the monoclinic phase has more degrees of freedom than the tetragonal phase.36,37 The monoclinic MC phase is a possible polarization rotation path between orthorhombic and tetragonal phases in the phase diagram by Vanderbilt and Cohen,38 and it is also the intermediate phase of LNKN at the PPB. When the electric field is applied, the monoclinic phase may therefore be more favorable than the tetragonal phase. Thus, the field-induced phase transition that increases the monoclinic phase fraction may be expected. The initial field-induced phase transition that increases the tetragonal phase fraction in 6LNKN is unexpected because the tetragonal phase has less possible polarization directions than the monoclinic phase. During the application of electric field to 6LNKN, the induced tetragonal phase initially increases, and then it decreases to a value similar to the unpoled state. After that the monoclinic phase fraction increases. These results imply that the tetragonal and monoclinic phases are unstable when external electric fields are applied. In compositions approaching phase boundaries, the free energy profiles of coexisting phases can flatten. As a result, the application of electric fields can change the phase stability. Phase instability has been hypothesized as an origin of high piezoelectric properties.39,40 In addition to the fieldinduced phase transition, this phase instability can contribute to the high piezoelectric properties in 6LNKN. In lead-free ferroelectrics, a field-induced phase transition has been hypothesized to be one of the origins of high piezoelectric properties at phase boundaries.4 For orthorhombic LNKN compositions in far proximity to phase boundaries, domain wall motion has been shown to occur, but fieldinduced phase transitions are not observed.24 Domain wall motion is therefore the primary extrinsic mechanism for electric field-induced strains in single phase orthorhombic LNKN. In the present work, field-induced phase transitions are observed in two compositions in closer proximity to the PPB: 6LNKN and 6.5LNKN. The field-induced phase transitions are therefore shown to be an additional extrinsic mechanism for field-induced strains in LNKN at the PPB (in addition to domain wall motion). Since the piezoelectric

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properties are enhanced at the PPB, it is concluded that the field-induced phase transition is one of the origins of high piezoelectric properties at the PPB. IV. CONCLUSIONS

Electric field-induced phase transitions in compositions of LNKN in close proximity to the PPB were observed using in situ XRD. The structure of LNKN at the PPB before the application of electric field is a mixed phase of tetragonal and monoclinic phases. After the application of electric field, the ratios of monoclinic and tetragonal phases change due to the field-induced phase transition. Compositions with different proximity from the PPB show different field-induced phase change behaviors. In compositions further from the PPB, the phase changes become more reversible. The results from this study show that the field-induced phase transition is one of the origins of high piezoelectric properties at the PPB. ACKNOWLEDGMENTS

The authors acknowledge Xiaoli Tan for helpful discussion. J.J. acknowledges support for this work from the Army Research Office through W911NF-09-1-0435. T.I. acknowledges support from the Development and Promotion of Science and Technology Talents Project, Royal Thai Government. S.P. acknowledges support from Thai Research Fund TRG5680095. S.W. acknowledges support from Thai Research Fund MRG5580029. Use of the Advanced Photon Source (APS), an Office of Science User Facility operated for the U.S. Department of Energy (DOE) Office of Science by Argonne National Laboratory, was supported by the U.S. DOE under Contract No. DE-AC0206CH11357. APS beamline scientists Matthew Suchomel, Yang Ren, and Guy Jennings are gratefully acknowledged for technical assistance with diffraction experiments. 1

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