Pseudocubic-based polymorphic phase boundary structures and their effect on the piezoelectric properties of (Li,Na,K)(Nb,Sb)O3-SrZrO3 lead-free ceramics

Accepted Manuscript Pseudocubic-based polymorphic phase boundary structures and their effect on the piezoelectric properties of (Li,Na,K)(Nb,Sb)O3-SrZrO3 lead-free ceramics Ku-Tak Lee, Dae-Hyeon Kim, Sung-Hoon Cho, Jeong-Seog Kim, Jungho Ryu, ChelWoo Ahn, Tae-Ho Lee, Gyeung-Ho Kim, Sahn Nahm PII:

S0925-8388(19)30011-8

DOI:

https://doi.org/10.1016/j.jallcom.2019.01.011

Reference:

JALCOM 49049

To appear in:

Journal of Alloys and Compounds

Received Date: 28 August 2018 Revised Date:

31 December 2018

Accepted Date: 2 January 2019

Please cite this article as: K.-T. Lee, D.-H. Kim, S.-H. Cho, J.-S. Kim, J. Ryu, C.-W. Ahn, T.-H. Lee, G.-H. Kim, S. Nahm, Pseudocubic-based polymorphic phase boundary structures and their effect on the piezoelectric properties of (Li,Na,K)(Nb,Sb)O3-SrZrO3 lead-free ceramics, Journal of Alloys and Compounds (2019), doi: https://doi.org/10.1016/j.jallcom.2019.01.011. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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ACCEPTED MANUSCRIPT

ACCEPTED MANUSCRIPT Pseudocubic-based polymorphic phase boundary structures and their effect on the piezoelectric properties of (Li,Na,K)(Nb,Sb)O3-SrZrO3 lead-free ceramics Ku-Tak Leea, Dae-Hyeon Kima, Sung-Hoon Choa, Jeong-Seog Kimc**, Jungho Ryud, Chel-

a

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Woo Ahne, Tae-Ho Leea, Gyeung-Ho Kimf and Sahn Nahma,b*

Department of Materials Science and Engineering, Korea University, 145 Anam-ro,

Seongbuk-gu, Seoul, 02841, Republic of Korea b

Department of Nano Bio Information Technology, KU-KIST Graduate School of Converging

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Science and Technology, Korea University, 145 Anam-ro, Seongbuk-gu, Seoul, 02841, Republic of Korea

Department of Materials Science and Engineering, Hoseo University, 20, Hoseo-ro 79

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c

beongil, Baebang-eup, Asan-si, Chungcheongnam-do 31499, Republic of Korea d

School of Materials Science and Engineering, Yeungnam University, Gyeongsan,

Gyeongbuk 38541, Republic of Korea e

Functional Ceramics Department, Powder & Ceramics Division, Korea Institute of Materials

Science (KIMS), Changwon, Gyeongnam 641-831, Republic of Korea f

Republic of Korea

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Advanced Analysis Center, Korea Institute of Science and Technology, Seoul, 02792,

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*Corresponding author. E-mail address: [email protected]

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**Corresponding author. E-mail address: [email protected]

ACCEPTED MANUSCRIPT ABSTRACT CuO-added 0.96(LixNa0.5-xK0.5)(Nb1-ySby)O3-0.04SrZrO3 ceramics were sintered at 1020oC for 6 h. Various crystal structures were synthesized in these specimens by controlling the Li2CO3 (x) and Sb2O5 (y) contents: pseudocubic, orthorhombic-pseudocubic polymorphic

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phase boundary (PPB), tetragonal-pseudocubic PPB, orthorhombic-tetragonal-pseudocubic PPB, and orthorhombic-tetragonal PPB structures. The pseudocubic structure developed in these specimens was similar to the R3m rhombohedral structure instead of the Pm3m cubic structure because the specimens with a pure pseudocubic structure showed good ferroelectric

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and piezoelectric properties. The piezoelectric properties of the specimens were influenced by their crystal structures. The specimen with the tetragonal-pseudocubic PPB structure showed

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the best piezoelectric properties because this structure was similar to the tetragonalrhombohedral morphotropic phase boundary structure developed in Pb(Zr1-xTix)O3-based ceramics. In particular, the specimen with the tetragonal-pseudocubic PPB structure corresponding to x = 0.05 and y = 0.065 showed the largest d33 and kp values of 431 pC/N

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and 0.43, respectively.

Keywords: Pseudocubic structure, Polymorphic phase boundary structure, Piezoelectric

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properties, (Li,Na,K)(Nb,Sb)O3-SrZrO3-based lead free piezoelectric ceramics

ACCEPTED MANUSCRIPT 1. Introduction

Owing to their promising piezoelectric properties, (Na1-xKx)O3(NKN)-based lead-free piezoelectric ceramics have gained tremendous interest as an alternative to Pb(Zr1-xTix) (PZT)-based piezoelectric ceramics [1-6]. The NKN ceramic has an orthorhombic structure

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with Amm2 space group at room temperature (RT) and exhibits three phase transition temperatures: the Curie temperature (TC) at 410oC, an orthorhombic-tetragonal phase transition temperature (TO-T) at 200oC, and a rhombohedral-orthorhombic phase transition temperature (TR-O) at -100oC [7,8]. The piezoelectric strain constant (d33) and

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electromechanical coupling factor (kp) of the NKN ceramic are reported to be approximately 120 pC/N and 0.35, respectively [8,9]. Most of the studies carried out so far for improving the

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piezoelectric properties of NKN-based ceramics have focused on shifting one of their phase transition temperatures to RT and developing a polymorphic phase boundary (PPB) structure, which would lead to the coexistence of its constituent structures at RT [10-12]. Previously, the NKN-ATiO3 (A: Ca, Sr, and Ba) ceramics were reported to display the orthorhombictetragonal PPB structure, with the TO-T at RT, and they generally exhibited slightly increased d33 and kp values of 230 pC/N and 0.37, respectively [9,13-16]. Recently, large d33 values

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(300 - 450 pC/N) were also obtained for the NKN-based ceramics with the orthorhombictetragonal PPB structure by controlling the grain size, domain structure, and lattice distortion [5,17-20].

Sb2O5 is added to NKN ceramics to increase their TR-O to RT and develop the

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rhombohedral-orthorhombic PPB structure [11,21]. The d33 and kp values of Sb2O5-added NKN ceramics have been reported as 230 pC/N and 0.43, respectively [21,22]. Moreover, the

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rhombohedral-tetragonal transition temperature (TR-T) was adjusted to RT by simultaneously decreasing the TO-T and increasing the TR-O. The orthorhombic structure is eliminated through this process. Thus, the rhombohedral-tetragonal PPB structure is developed at RT in NKNbased ceramics [23-34]. The NKN-based ceramics with the rhombohedral-tetragonal PPB structure generally exhibit a large d33 value of approximately 450 pC/N [24,26,29,32,35]. This can be attributed to the similarity between the rhombohedral-tetragonal PPB structure developed in these ceramics and the rhombohedral-tetragonal morphotropic phase boundary (MPB) structure developed in PZT-based ceramics [36,37]. In particular, (Na,K)(Nb,Sb)O3BiFeO3-BiNaZrO3 ceramics having nanodomains with tetragonal-rhombohedral PPB structure have been reported to show a very high d33 value of 550 pC/N [3]. Therefore, the

ACCEPTED MANUSCRIPT synthesis of NKN-based piezoelectric ceramics with rhombohedral-tetragonal PPB structure has gained immense attention. Moreover, the NKN-based ceramic with orthorhombic structure was reported to exhibit excellent piezoelectric properties (d33 of 512 pC/N and kp of 0.54), because its TR-O and TO-T are close to RT [4]. The piezoelectric properties of the NKNbased ceramics with the rhombohedral-orthorhombic-tetragonal PPB structure were also

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investigated [38,39]. Thermal stability of the piezoelectric properties is important for the application of these materials in devices. Therefore, many investigations were conducted to improve the thermal stability by increasing the TC and forming the diffused rhombohedraltetragonal phase boundary [5,19,23,27,39]. Furthermore, the domain structure, grain size, and

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lattice distortion of the specimens, which were also reported to influence the piezoelectric properties of NKN-based ceramics, were also investigated [4,5,18,24].

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NKN ceramics display a pseudocubic structure near TC, and this structure can be stabilized at temperatures close to RT by decreasing the TC as well as by broadening the TC peak. In addition, if the TC peak is broad, then TC does not need to be close to RT for the pseudocubic structure to stabilize near RT. Furthermore, this pseudocubic structure can possibly coexist with the orthorhombic (or tetragonal) structure, resulting in the formation of the orthorhombic-pseudocubic (or tetragonal-pseudocubic) PPB structures at RT. In general,

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NKN ceramics with a pseudocubic structure are considered to have a large dielectric constant (εT33/εo). Moreover, they are expected to display a remnant polarization (Pr) because they are not purely cubic. According to the conventional Landau-Devonshire relation, the d33 value can be expressed as follows: d33 = 2 × Q11 × εT33 × Pr, where Pr is the remnant polarization

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and Q11 is the electrostriction coefficient [40]. In general, since Q11 is constant, it can be considered that the d33 value is proportional to εT33 × Pr [36,41,42]. Moreover, kp is

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proportional to the d33 × g33 value, where g33 is a piezoelectric voltage constant. Therefore, it is reasonable to speculate that the specimen with the pseudocubic-based PPB structure can have large d33 and kp values. Recently, a specimen with the orthorhombic-pseudocubic PPB structure was reported, and it revealed promising piezoelectric properties: d33 and kp values of 325 pC/N and 0.42, respectively [43]. Therefore, it is interesting to study specimens with pseudocubic-based PPB structures systematically for the first time. In this work, specimens with various pseudocubic-based PPB structures, such as tetragonal-pseudocubic PPB and orthorhombic-tetragonal-pseudocubic PPB structures, were synthesized, and their structural and piezoelectric properties were investigated. In particular, the crystal structures developed in these specimens were analyzed in detail by using Rietveld analysis and transmission

ACCEPTED MANUSCRIPT electron microscopy (TEM).

2. Experimental procedures

CuO-added

0.96(LixNa0.5-xK0.5)(Nb1-ySby)-0.04SrZrO3

[C(LxN0.5-xK0.5)(N1-ySy)-SZ]

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ceramics with 0.0 ≤ x ≤ 0.07 and y = 0.055 and 0.065 were synthesized by using the conventional solid-state method. Li2CO3, K2CO3, Na2CO3, Nb2O5, Sb2O5, SrCO3, and ZrO2 (> 99%, High Purity Chemicals, Saitama, Japan) were weighed and mixed for 24 h by using anhydrous ethanol as the solvent in a nylon jar with zirconia balls and then dried. The dried

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powders were calcined at 850oC for 3 h. The calcined powders were re-milled for 48 h with 1.0 mol% of CuO as an additive for sintering and then dried. Finally, the calcined powders

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were pressed into disc-shaped pellets under a pressure of 100 MPa and sintered at 1020oC for 6 h in air. The density of the specimens was measured using Archimedes’ method. X-ray diffraction (Rigaku D/Max-RC, Tokyo, Japan) with CuKα radiation was used to determine the crystal structure of the specimens. Moreover, the XRD reflections at 45.5o and 66.5o were obtained by slow-speed scanning of the specimens, and these reflections were deconvoluted by using the Voigt function. The rate and resolution of the slow-speed scanning were

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0.3o/min and 0.01o, respectively. The XRD patterns of the specimens were analyzed by Rietveld refinement by using FullProf suite program. Field-emission transmission electron microscopy (FE-TEM; Tecnai F20, FEI, Netherlands) was used to study the crystal structure of the specimens. The microstructure of the specimens was investigated by scanning electron

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microscopy (SEM; Hitachi S-4800, Osaka, Japan). The Raman spectra were obtained by using a dispersive Raman microscope (Thermo Fisher Scientific, DXR Raman Microscope,

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Waltham, MA, USA) with an argon-laser (532 nm). The excitation power and the size of the spot focused on the specimens were 10 mW and approximately 2 µm, respectively. The specimens coated with silver electrodes were poled by a DC field of 4 kV/mm for 60 min at various temperatures. The d33 values of the specimens were measured by using a d33 meter (Micro-Epsilon Channel Product DT-3300, Raleigh, North Carolina). To measure the d33 value at various temperatures, a specimen was heated to a specific temperature on a hot plate and maintained at that temperature for 10 min. Then, the specimen was quickly moved into the d33-meter to measure its d33 value at a specific temperature. The relative permittivity (εT33/εo), loss (tan δ), kp, and mechanical quality factor (Qm) of the specimens were measured by using an impedance analyzer (Agilent Technologies HP 4194A, Santa Clara, California).

ACCEPTED MANUSCRIPT The εT33/εo and tan δ were obtained as functions of temperature by using an LCR meter (Agilent Technologies HP 4284) 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 by using a modified Sawyer-Tower circuit and measured in

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silicon oil to prevent electrical flashover at the fixed frequency of 1 Hz.

3. Results and discussion

The C(LxN0.5-xK0.5)(N0.935S0.065)-SZ (y = 0.065) ceramics with 0.0 ≤ x ≤ 0.07 sintered at

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1020oC showed a homogeneous perovskite structure without any secondary phase, as shown in Figs. S1a-h in Supporting Information 1. These XRD patterns indicate that all the

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specimens have a pseudocubic structure, implying that it was difficult to investigate the detailed crystal structure of these specimens by using these patterns. Therefore, the XRD reflections at 66.5o were measured by slow-speed scanning and deconvoluted by using the Voigt function to investigate the crystal structure of the specimens with 0.0 ≤ x ≤ 0.07. Figs. 1a and b show the reflections observed from the specimens with x ≤ 0.01: they are interpreted as the rhombohedral (220)R and (2-20)R reflections. Therefore, it can be suggested that the

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pseudocubic structure formed in these specimens is similar to the rhombohedral structure. It is interesting to note that this reflection can also be analyzed as the cubic (220)C reflection, as shown in Figs. S2a and b in Supporting Information 2, which indicates that the pseudocubic structure developed in these specimens is similar to the cubic structure. However, the

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specimens with x ≤ 0.01 show good ferroelectric and piezoelectric properties, which will be discussed later. Therefore, it is reasonable to suggest that the pseudocubic structure developed

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in the specimens with x ≤ 0.01 is similar to the rhombohedral structure instead of the cubic structure. Note that the subscripts T, O, and R are the abbreviations of tetragonal, orthorhombic and rhombohedral structures, respectively. The reflection observed from the specimen with x = 0.02 can be considered as a mixture of the rhombohedral (220)R and (2-20)R and the orthorhombic (004)O, (400)O, and (220)O reflections (see Fig. 1c), indicating that this specimen has the orthorhombic-rhombohedral PPB structure and that the pseudocubic structure developed in this specimen is also similar to the rhombohedral structure. For the specimen with x = 0.04, the tetragonal (202)T and (220)T reflections were observed, as shown in Fig. 1d. Therefore, this specimen is considered to have the orthorhombic-tetragonal-rhombohedral PPB structure, though the amount of the

ACCEPTED MANUSCRIPT orthorhombic structure is small. Finally, the orthorhombic reflections disappeared for the specimens with 0.05 ≤ x ≤ 0.07, as shown in Figs. 1e and f, suggesting that these specimens have the tetragonal-rhombohedral PPB structure. The above results show that the pseudocubic structure formed in the specimens with x ≤ 0.01 is similar to the rhombohedral structure, though it can be interpreted as the cubic structure, as shown in Figs. S2a-f in

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Supporting Information 2. Moreover, this rhombohedral structure changed to orthorhombicrhombohedral, orthorhombic-tetragonal-rhombohedral, and tetragonal- rhombohedral PPB structures with the increase in x. Identical results were obtained on studying the slowscanning XRD reflections observed near 45.5o, as revealed in Figs. S3a-f in Supporting

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Information 3. Additionally, the results of the Rietveld analysis confirmed that the rhombohedral, orthorhombic-tetragonal-rhombohedral PPB, and tetragonal-rhombohedral

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PPB structures existed in the specimens with x = 0.0, x = 0.04, and x = 0.05, respectively, as shown in Figs. S4a-e in Supporting Information 4.

A TEM analysis was also carried out on the C(LxN0.5-xK0.5)(N0.935S0.065)-SZ (y = 0.065) ceramics with x = 0.0 and 0.05 to investigate their crystal structures. Figs. 2a and b show the electron diffraction pattern and the high-resolution TEM (HRTEM) image of the C(LxN0.5xK0.5)(N0.935S0.065)-SZ

ceramic with x = 0.0, respectively. The inset of Fig. 2b shows the fast

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Fourier transform (FFT) pattern obtained from the HRTEM image. The electron diffraction pattern shows the rhombohedral (110)R and (102)R reflections. The zone axis of this electron diffraction pattern corresponds to the rhombohedral [-221]R direction. The HRTEM image shows the lattice fringes of the rhombohedral (110) and (102) planes. Moreover, the FFT

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pattern obtained from the HRTEM image is identical to the electron diffraction pattern. Therefore, it can be suggested that the crystal structure of the specimen with x = 0.0 is similar

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to the rhombohedral structure.

The electron diffraction pattern and the HRTEM image of the specimen with x = 0.05 and y = 0.065 are shown in Figs. 2c and d, respectively. The electron diffraction pattern shows the rhombohedral (101)R and (0-11)R reflections, which are identical to the tetragonal (010)T and (100)T reflections, respectively (see Fig. 2c). The zone axis of this electron diffraction pattern could have been either the rhombohedral [1-1-1]R or the tetragonal [001]T direction. The HRTEM image of this specimen shows lattice fringes of the rhombohedral (101) and (0-11) planes, which correspond to the tetragonal (010) and (100) planes. Moreover, the FFT pattern obtained from this HRTEM image is consistent with the electron diffraction pattern. Therefore, the TEM results also reveal that the specimen with x = 0.05 has the

ACCEPTED MANUSCRIPT tetragonal-rhombohedral PPB structure. Moreover, the TEM results are consistent with the XRD results. The XRD patterns of the specimens with x = 0.0, 0.02, 0.04, and 0.05 were obtained at various temperatures to investigate the variation of the crystal structure with respect to temperature and construct a schematic phase diagram of the C(LxN0.5-xK0.5)(N0.935S0.065)-SZ

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ceramics with 0.0 ≤ x ≤ 0.07. The εT33/εo vs. temperature curve and the XRD patterns obtained at various temperatures for the specimens with x = 0.0 and 0.02 are shown in Figs. S5a and b in Supporting Information 5. Fig. 3a-i shows the εT33/εo vs. temperature curve for the specimen with x = 0.04. The TC of this specimen is approximately 155oC. The broad peak

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at 28oC is defined as a phase transition temperature (TO-T-R), at which the tetragonalrhombohedral (or pseudocubic) PPB structure begins to transform to the orthorhombic-

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tetragonal-rhombohedral (or pseudocubic) PPB structure, as shown in Fig. 3a-i. Moreover, it is considered that a tetragonal structure could form below 75oC because the εT33/εo vs. temperature curve is a relatively constant around 75oC. The XRD patterns of the specimen with x = 0.04 recorded at various temperatures are shown in Fig. 3a-ii, and similar XRD patterns were obtained between 100oC and 175oC. Since the TC of this specimen is 155oC, it has a cubic structure at 175oC and a rhombohedral (or pseudocubic) structure at 100oC.

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Moreover, the reflection at 45.5o starts broadening when it is recorded at 50oC (see Fig. 3a-ii), which could be related to the formation of the tetragonal structure. Therefore, based on the εT33/εo vs. temperature curve and the XRD results, it can be assumed that the tetragonal structure begins to form at 75oC, suggesting that the tetragonal and rhombohedral structures

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coexist near 50oC (or between 30oC and 75oC). According to the XRD patterns obtained through slow-scanning measurements and Reitveld analysis, the orthorhombic-tetragonal-

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rhombohedral PPB structure develops at RT. Therefore, it can be assumed that the orthorhombic structure begins to form near 30oC, although the XRD patterns obtained at RT and -25oC are similar to that obtained at 50oC (Fig. 3a-ii). Based on the above results, it can be suggested that the specimen with x = 0.04 has a rhombohedral (or pseudocubic) structure between 75oC and 155oC, a tetragonal-rhombohedral PPB structure between 30oC and 75oC, and an orthorhombic-tetragonal-rhombohedral PPB structure between 30oC and -25oC, as shown in Fig. 3a-i. The εT33/εo vs. temperature curve and the XRD patterns measured at various temperatures for the specimen with x = 0.05 are shown in Figs. 3b-i and b-ii, respectively. The TC and TO-T of this specimen are approximately 167oC and 19oC, respectively (see Fig. 3b-i). This

ACCEPTED MANUSCRIPT specimen has a cubic structure at 175oC because the TC is 167oC. The XRD patterns recorded at 175oC and 150oC are very similar (see the reflection at 45.5o in Fig. 3b-ii), indicating that this specimen has a rhombohedral (or pseudocubic) structure at 150oC. Moreover, the shape of the reflection peak at 45.5o changed when the XRD pattern was measured at 100oC, and a similar XRD pattern was also observed at 50oC. According to the XRD pattern obtained by

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the slow-scanning measurement and Rietveld analysis (see Figs. 1e and S4d in Supporting Information 4), the tetragonal- rhombohedral PPB structure forms at RT, indicating that the tetragonal structure begins to form in this specimen at temperatures lower than 100oC. Therefore, it can be suggested that this specimen has the tetragonal-rhombohedral PPB

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structure between RT and 100oC. For the XRD pattern recorded at 0oC, the reflection at 45.5o broadened. A similar XRD pattern was obtained at -25oC, indicating that the orthorhombic

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structure appeared near 0.0oC (see Fig. 3b-ii). Therefore, it can be suggested that the specimen with x = 0.05 has a rhombohedral (or pseudocubic) structure between 100oC and 167oC, the tetragonal-rhombohedral PPB structure between RT and 100oC, and the orthorhombic-tetragonal PPB structure between -25oC and 20.0oC, as shown in Fig. 3b-i. In addition, the εT33/εo vs. temperature curves for the specimens with 0.0 ≤ x ≤ 0.07 are shown in Figs. S6a-f in Supporting Information 6. Finally, a schematic phase diagram of the

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C(LxN0.5-xK0.5)(N0.935S0.065)-SZ ceramics with 0.0 ≤ x ≤ 0.07 was constructed, as shown in Fig. 3c. The εT33/εo vs. temperature curves, the XRD patterns measured at various temperatures, the slow-scan XRD measurements, TEM analysis, and the results of Rietveld analysis were used to construct this schematic phase diagram. However, further investigation is required to

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clearly determine the phase diagram of these specimens. Raman analysis was performed to investigate the effect of Li+ and Sb5+, and Fig. 4a

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shows the Raman spectra of the specimens with 0.0 ≤ x ≤ 0.07 and y = 0.065. The vibrations of the BO6 (B = Nb5+ and Sb5+) octahedra, including A1g (v1) + Eg (v2) + 2F1u (v3, v4) + F2g(v5) + F2u(v6) can be observed in Fig. 4a; the A1g (v1) + Eg (v2) + F1u (v3) modes correspond to the stretching vibrations, whereas the F1u (v4) + F2g(v5) + F2u(v6) modes correspond to the bending vibrations [46,47]. The v1 stretching and v5 bending modes shift to higher frequencies with an increase in x. In particular, when x exceeds 0.05, the shift in these peaks is large. The peak shift to a higher frequency can be explained by the decreasing distance between the B ions and their coordinated oxygen ions in the BO6 octahedra due to the incorporation of the small Li ions into the perovskite unit cell [46-48]. Raman analysis was also conducted on the specimens with 0.0 ≤ x ≤ 0.05 and y = 0.055, as shown in Fig. 4b; the amount of Sb5+ in these

ACCEPTED MANUSCRIPT specimens is less than that in the specimens with y = 0.065. The v1 stretching and v5 bending modes also shift to higher frequencies with an increase in x, and the peak shift observed in these specimens is similar to that of the specimens with y = 0.065. Furthermore, the lattice parameters of the specimens with y = 0.065 and 0.055 increased with increases in the Li contents, as shown in Figs. S7a and b of Supporting Information 7.

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Figs. 5a-g show the P-E hysteresis curves of the C(LxN0.5-xK0.5)(N0.935S0.065)-SZ (y = 0.065) ceramics with 0.0 ≤ x ≤ 0.07, and the variations in the Pr and EC of these specimens are shown in Fig. 5h. The specimens with x = 0.0 and 0.01, which have a rhombohedral (or pseudocubic) structure, reveal normal hysteresis curves with a large Pr of approximately 15.0

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µC/cm2 and an EC of approximately 0.5 kV/mm, as shown in Figs. 5a and b, indicating that these specimens exhibit excellent ferroelectric properties. Moreover, the specimen having a

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rhombohedral structure generally exhibits a high Pr owing to the presence of 71o domains [36,49,50]. Therefore, the structure formed in these specimens could be similar to the R3m rhombohedral structure instead of the Pm3m cubic structure, as revealed by the XRD results. Similar Pr values were obtained for the specimens with 0.02 ≤ x ≤ 0.04, implying that these specimens contained a fraction of the rhombohedral structure. However, as the value of x increased beyond 0.04, the Pr value decreased because of the predominance of a tetragonal

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structure with 90o domains [51,52]. The specimen with x = 0.0 showed a relatively low EC of approximately 0.5 kV/mm, which increased slightly with an increase in x. This can be attributed to the increased proportion of the tetragonal structure [51]. Fig. 6a shows the relative density, εT33/εo, d33, kp, and Qm values of the C(LxN0.5(y = 0.065) ceramics with 0.0 ≤ x ≤ 0.07. All the specimens showed

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xK0.5)(N0.935S0.065)-SZ

large relative densities (≥ 95% of the theoretical densities) because of their dense

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microstructures, as observed in Figs. S8a-d in Supporting Information 8. The specimen with x = 0.0 showed a low εT33/εo value of 1553, and it increased with an increase in x because of an increase in the amount of the tetragonal structure (or orthorhombic structure). The specimens with x = 0.0 and 0.01, which have the rhombohedral (or pseudocubic) structure, showed d33 values of 150 pC/N and 185 pC/N, respectively. These values are greater than that of the pure NKN ceramic. Therefore, these specimens display relatively good piezoelectric properties, indicating that the structure developed in these specimens could be similar to the R3m rhombohedral structure instead of the Pm3m cubic structure. The d33 value increased with an increase in x, and the specimen with x = 0.05 showed a maximum d33 value of 431 pC/N. According to the XRD and TEM analyses, this specimen has the tetragonal-rhombohedral

ACCEPTED MANUSCRIPT structure. According to previous work, the specimen with the tetragonal-rhombohedral PPB structure revealed a large d33 value because this structure is similar to the tetragonalrhombohedral MPB structure developed in PZT-based ceramic [53-55]. Therefore, the large d33 value observed in the specimen with x = 0.05 can be explained by the presence of the tetragonal-rhombohedral PPB structure, which is similar to the tetragonal-rhombohedral

the increase in the proportion of the tetragonal structure.

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MPB structure. The d33 value decreases as x increases beyond 0.05. This can be attributed to The d33 value is mostly influenced by the Pr and εT33 values of the specimens [56-58]. Fig. 6b shows the variation in the Pr × εT33 value with respect to x, which reveals a very similar

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trend to the d33 value. The kp values also exhibit a similar trend. The specimen with x = 0.0 revealed a low kp of 0.24, which increased with an increase in x. The maximum kp value of

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0.43 was obtained for the specimen with x = 0.05. The Qm values of the specimens with 0.0 ≤ x ≤ 0.02 were low because they contained a large proportion of the pseudocubic (or rhombohedral) structure. The Qm value increased with an increase in x because of an increase in the proportion of the tetragonal structure. However, this variation was not significant. These results show that the piezoelectric properties such as the d33 and kp values of the specimens were influenced by their crystal structures: the specimen with the rhombohedral

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structure (x = 0.0) showed d33 and kp values of 151 pC/N and 0.24, respectively, and the specimen with the orthorhombic-rhombohedral PPB structure (x = 0.03) showed slightly increased d33 (298 pC/N) and kp (0.34) values; the specimen with the orthorhombictetragonal-rhombohedral PPB structure showed large d33 (355 pC/N) and kp (0.4) values, and

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the highest d33 (431 pC/N) and kp (0.43) values were observed for the specimen with the tetragonal-rhombohedral PPB structure (x = 0.05). Therefore, the specimen having the

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tetragonal-rhombohedral PPB structure showed excellent piezoelectric properties. In general, εT33/εo = (dP/dE)E=0 [59,60]. The (dP/dE)E=0 of the orthorhombicrhombohedral PPB structure is smaller than that of the tetragonal- rhombohedral PPB structure, as shown in Figs. S9a and b in Supporting Information 9. Therefore, the εT33/εo of the orthorhombic-rhombohedral PPB structure (2107) is much smaller than that of the tetragonal-rhombohedral PPB structure (2503). Moreover, the εT33/εo of the orthorhombictetragonal-rhombohedral PPB structure (2142) is smaller than that of the tetragonalrhombohedral PPB structure, but larger than that of the orthorhombic-rhombohedral PPB structure. Furthermore, the orthorhombic structure reveals a 60o domain, whereas the tetragonal structure exhibits a 90o domain. Thus, the decrease in the polarization after the

ACCEPTED MANUSCRIPT removal of the electric field is smaller in the orthorhombic structure than in the tetragonal structure. Therefore, the Pr value of the orthorhombic-rhombohedral PPB structure (15.27 µC/cm2) is slightly larger than that of the tetragonal-rhombohedral PPB structure (13.53 µC/cm2). In addition, the Pr value of the orthorhombic-tetragonal-rhombohedral PPB structure (14.84 µC/cm2) is slightly larger (smaller) than that of the tetragonal-rhombohedral

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(orthorhombic-rhombohedral) PPB structures. The d33 value is proportional to Pr × εT33 [41,42,61]. However, the difference in the Pr values between the orthorhombic-rhombohedral and tetragonal--rhombohedral PPB structures is not large, indicating that the d33 value is primarily influenced by the εT33 value. Therefore, the tetragonal-rhombohedral PPB structure,

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which displays the largest εT33 value with a slightly smaller Pr value, exhibits the highest d33 value, as shown in Figs. 6a and b. Moreover, it can be suggested that the tetragonal-

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rhombohedral PPB structure has the best dielectric and piezoelectric properties. For the PZT-based ceramics, the specimens with the triple-point structure, containing the three structures, namely rhombohedral, pseudocubic, and tetragonal, showed the best piezoelectric properties [36,37]. Therefore, specimens with the orthorhombic-tetragonalrhombohedral PPB structure are expected to display excellent piezoelectric properties. However, the piezoelectric properties of such a specimen (x = 0.04) are inferior than those of

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the specimen (x = 0.05) with the tetragonal-rhombohedral PPB structure. Therefore, more investigation is required to evaluate the piezoelectric properties of the specimens with the orthorhombic-tetragonal-rhombohedral PPB structure. It was reported previously that the C(N0.5K0.5)(N0.945S0.055)-SZ ceramic (x = 0.0 and y = 0.055) has the orthorhombic-

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rhombohedral PPB structure [43]. Therefore, the specimens with the orthorhombictetragonal-rhombohedral

PPB

structure

can

be

synthesized

from

Li2CO3-added

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C(N0.5K0.5)(N0.945S0.055)-SZ ceramics, because the addition of Li2CO3 generally increases the tetragonal structure stable temperature region of NKN-based ceramics by increasing their TC [62].

The C(LxN0.5-xK0.5)(N0.945S0.055)-SZ (y = 0.055) ceramics with 0.0 ≤ x ≤ 0.05 exhibit a homogeneous perovskite structure without any secondary phase, as shown in Figs. S10a-f in Supporting Information 10. According to the analysis of the slow-scan XRD reflections, the orthorhombic-rhombohedral, orthorhombic-tetragonal-rhombohedral, and orthorhombictetragonal PPB structures existed in the specimens with x ≤ 0.01, 0.02 ≤ x ≤ 0.04, and x = 0.05, respectively, as shown in Figs. S11a and b in Supporting Information 11. Rietveld analysis was also performed for these specimens to investigate their crystal

ACCEPTED MANUSCRIPT structures. Figs. 7a-c show the XRD patterns of the specimens with x = 0.0, 0.03, and 0.05 that were analyzed by the Rietveld method, and the lattice parameters of all the specimens are listed in Table S2 in Supporting Information 4. The crystal structure of the specimen with x = 0.0 was identified to be a mixture of the R3m rhombohedral (Rb= 3.1 and Rf = 1.0) (46%) and Amm2 orthorhombic (Rb = 3.3 and Rf = 2.9) (Rp=7.8 and Rwp = 10.5) (54%) structures, as

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shown in Fig. 7a. Moreover, the crystal structure of the specimen with x = 0.03 was considered as a three-phase model involving the R3m rhombohedral (Rb = 2.6 and Rf = 4.5) (19%), Amm2 orthorhombic (Rb = 3.5 and Rf = 3.6) (36%), and P4mm tetragonal (Rb = 3.3 and Rf = 3.3) (45%) structures (see Fig. 7b). The orthorhombic-tetragonal-rhombohedral PPB

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structure of this specimen can also be observed in the TEM results (see Figs. S12a and b in Supporting Information 12). Finally, the crystal structure of the specimen with x = 0.05 was

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identified as a mixture of the P4mm tetragonal (Rb = 5.3 and Rf = 4.1) (69%) and Amm2 orthorhombic (Rb = 5.8 and Rf = 4.9) (31%) (Rp = 7.0 and Rwp = 9.0) structures, as shown in Fig. 7c. Thus, the results of the Rietveld analysis confirmed that the orthorhombicrhombohedral, orthorhombic-tetragonal-rhombohedral, and orthorhombic-tetragonal PPB structures existed in the specimens with x ≤ 0.01, 0.02 ≤ x ≤ 0.04, and x = 0.05, respectively. Figs. 8a-f show the εT33/εo vs. temperature curves of the C(LxN0.5-xK0.5)(N0.945S0.055)-SZ

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ceramics with 0.0 ≤ x ≤ 0.05. The TC of the specimen with x = 0.0 is 156oC, which increased to 212oC with an increase in x for the specimen with x = 0.05. The specimens with x = 0.0 and 0.01 showed the orthorhombic-rhombohedral PPB structure at RT. Therefore, the broad peaks observed at 84oC and 73oC for the specimens with x = 0.0 and 0.01, respectively,

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correspond to their TO-R (see Figs. 8a and b). Furthermore, the broad peak observed at a temperature slightly higher than RT in the specimens with 0.02 ≤ x ≤ 0.04 corresponds to the

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TO-T-R (see Figs. 8c-e), because these specimens displayed the orthorhombic-tetragonalrhombohedral PPB structure at RT. Finally, the broad peak observed in the specimen with x = 0.05 at approximately 28oC corresponds to the TO-T because this specimen showed the orthorhombic-tetragonal PPB structure at RT. In other words, the TO-R changed to the TO-T-R and TO-T with an increase in x. The relative density, εT33/εo, d33, kp, and Qm values of the C(LxN0.5-xK0.5)(N0.945S0.055)-SZ ceramics with 0.0 ≤ x ≤ 0.05 are shown in Fig. 9. All the specimens revealed high relative densities (> 95% of the theoretical densities) because of the formation of a dense microstructure, as shown in Figs. S13a-d in Supporting Information 13. The εT33/εo value of the specimen with x = 0.0 was approximately 1740, while the specimens with 0.01 ≤ x ≤ 0.02

ACCEPTED MANUSCRIPT exhibited a slightly lower εT33/εo value, probably due to the increase in TC with an increase in x. However, when x exceeded 0.02, the εT33/εo value increased with an increase in x. This can be explained by the increase in the amount of the tetragonal structure with an increase in x, because a specimen with tetragonal structure shows a large εT33/εo value [63]. The specimen with x = 0.0 that displayed the orthorhombic-rhombohedral PPB structure showed a large d33

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value of 325 pC/N. This result is consistent with the previous results [43]. Furthermore, the specimen with the orthorhombic-tetragonal-rhombohedral PPB structure (x = 0.03) exhibited the largest d33 value of 331 pC/N. A similar result was also obtained for the specimen with x = 0.04. Finally, the specimen with the orthorhombic-tetragonal PPB structure (x = 0.05)

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showed a low d33 value of 260 pC/N. The variation in the d33 value was similar to that in the Pr x εT33 value, as shown in Fig. S14h in Supporting Information 14. The Pr of these

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specimens was obtained from the P-E hysteresis curves, as shown in Figs. S14a-f in Supporting Information 14. The specimen with x = 0.0 showed a large kp value of 0.41. The specimen with x = 0.03, which had the orthorhombic-tetragonal-rhombohedral PPB structure, also exhibited a large kp value of 0.4. Moreover, the specimen with the orthorhombictetragonal PPB structure (x = 0.05) showed a relatively small kp value of 0.38, which is consistent with that of the NKN-based piezoelectric ceramics with the orthorhombic-

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tetragonal PPB structure [64,65]. Therefore, the variation in the kp value with the increase in x was similar to the variation in the d33 value. In addition, all the specimens showed small Qm values, which did not vary significantly as a function of x. These results showed that the specimen with the orthorhombic-rhombohedral PPB structure (x = 0.0) and that with the

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orthorhombic-tetragonal-rhombohedral PPB structure (x = 0.03) exhibited similar piezoelectric properties. In both the cases, a d33 value of approximately 330 pC/N and a kp

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value of 0.4 were observed. However, the d33 and kp values of the specimen with the orthorhombic-tetragonal-rhombohedral PPB structure (or orthorhombic-rhombohedral PPB structure) were lower than those of the specimen with the tetragonal-rhombohedral structure. Therefore, it can be suggested that the specimen with the tetragonal-rhombohedral PPB structure exhibits superior piezoelectric properties. However, further investigation is required to elucidate the relationship between the crystal structure of NKN-based ceramics and their piezoelectric properties. Fig. 10 shows the d33 values of the C(LxN0.5-xK0.5)(N0.935S0.065)-SZ (y = 0.065) ceramics with 0.0 ≤ x ≤ 0.07 measured at various temperatures. The d33 value of the specimen with x = 0.0 remained constant at 171 pC/N up to 90oC and started decreasing as the measurement

ACCEPTED MANUSCRIPT temperature exceeded 90oC. This is because this specimen exhibits a low TC of 112oC. The d33 value of the specimen with x = 0.05, which showed a maximum d33 value of 431 pC/N, decreased slightly with an increase in the measurement temperature, and a large d33 value of 352 pC/N was obtained at 150oC. This value decreased considerably as the measurement temperature increased beyond 150oC, because its TC is approximately 167oC. The specimen

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with x = 0.07 showed a d33 value of 268 pC/N at RT. This value remained constant up to 180oC because of the high TC (207oC) of this specimen. Therefore, it can be stated that the piezoelectric properties of the specimen with x = 0.05 are stable up to approximately 150oC. In addition, the d33 values of the C(LxN0.5-xK0.5)(N0.945S0.055)-SZ (y = 0.055) ceramics with 0.0

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≤ x ≤ 0.05 were measured at various temperatures, as shown in Fig. S15 in Supporting Information 15. The piezoelectric properties of the specimen with x = 0.03 and y = 0.055,

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which had the orthorhombic-tetragonal-rhombohedral PPB structure, were stable up to approximately 170oC.

4. Conclusions

C(LxN0.5-xK0.5)(N0.935S0.065)-SZ (y = 0.065) ceramics with 0.0 ≤ x ≤ 0.07 were sintered at

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1020oC, and showed a homogeneous perovskite phase without any secondary phases. All the specimens revealed dense microstructures. The pseudocubic structure developed in the specimens with x ≤ 0.01 was similar to the R3m rhombohedral structure, and the specimens with 0.02 ≤ x ≤ 0.03 exhibited the orthorhombic-rhombohedral PPB structure. The

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orthorhombic-tetragonal-rhombohedral PPB structure was observed in the specimen with x = 0.04. Finally, the specimens with 0.05 ≤ x ≤ 0.07 exhibited the tetragonal-rhombohedral PPB

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structure. The piezoelectric properties of the specimens were influenced by the crystal structure. Similar results were also observed for C(LxN0.5-xK0.5)(N0.945S0.055)-SZ (y = 0.055) ceramics with 0.0 ≤ x ≤ 0.05. The specimens with orthorhombic-rhombohedral PPB and orthorhombic-tetragonal-rhombohedral PPB structures showed similar d33 and kp values. The best piezoelectric properties were obtained for the specimen with the tetragonalrhombohedral structure. In particular, the C(L0.05N0.45K0.5)(N0.935S0.065)-SZ (x = 0.05 and y = 0.065) ceramic, which had the tetragonal-rhombohedral structure, showed the largest d33 (431 pC/N) and kp (0.43) values. Moreover, the piezoelectric properties of these specimen were stable up to 170oC.

ACCEPTED MANUSCRIPT Acknowledgements

This research was supported by the National Research Council of Science & Technology (NST) grant from the Korean government (MSIP) (grant number CAP-17-04-KRISS). The

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authors also thank the KU-KIST Graduate School Program of Korea University.

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ACCEPTED MANUSCRIPT List of figure captions Fig. 1. XRD reflections at 66.5o measured by slow-speed scanning for C(LxN0.5xK0.5)(N0.935S0.065)-SZ

(y = 0.065) ceramics with 0.0 ≤ x ≤ 0.07 sintered at 1020oC for 6 h: (a)

x = 0.0, (b) x = 0.01, (c) x = 0.02, (d) x = 0.04, (e) x = 0.05, and (f) x = 0.07.

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Fig. 2. (a) Electron diffraction pattern and (b) HRTEM image of C(N0.5K0.5)(N0.935S0.065)-SZ ceramic (x = 0.0, y = 0.065). (c) Electron diffraction pattern and (d) HRTEM image of C(L0.05N0.45K0.5)(N0.935S0.065)-SZ ceramic (x = 0.05 and y = 0.065). The insets of (b) and (d) show the FFT patterns obtained from the corresponding HRTEM images.

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Fig. 3. (a-i) εT33/εo vs. temperature curve and (a-ii) XRD patterns measured at various temperatures for the specimen with x = 0.04. (b-i) εT33/εo vs. temperature curve and (b-ii)

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XRD patterns measured at various temperatures for the specimen with x = 0.05. (c) Schematic phase diagram of C(LxN0.5-xK0.5)(N0.935S0.065)-SZ ceramics with 0.0 ≤ x ≤ 0.07. Fig. 4. Raman spectra of the specimens with (a) 0.0 ≤ x ≤ 0.07 and y = 0.065, and (b) 0.0 ≤ x ≤ 0.05 and y = 0.055.

Fig. 5. P-E curves of C(LxN0.5-xK0.5)(N0.935S0.065)-SZ ceramics: (a) x = 0.0, (b) x = 0.01, (c) x of these specimens.

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= 0.02, (d) x = 0.03, (e) x = 0.04, (f) x = 0.05, and (g) x = 0.07. (h) Variations in the Pr and EC Fig. 6. (a) Relative density, εT33, d33, kp, and Qm values of C(LxN0.5-xK0.5)(N0.935S0.065)-SZ ceramics and (b) the variations in their Pr × εT33 and d33 values. Fig. 7. Rietveld refinement XRD profiles of C(LxN0.5-xK0.5)(N0.945S0.055)-SZ ceramics sintered

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at 1020oC for 6 h: (a) x = 0.0, (b) x = 0.03, and (c) x = 0.05. Fig. 8. εT33/εo vs. temperature curves of C(LxN0.5-xK0.5)(N0.945S0.055)-SZ ceramics obtained at

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in the range -50oC to 300oC: (a) x = 0.0, (b) x = 0.01, (c) x = 0.02, (d) x = 0.03, (e) x = 0.04, and (f) x = 0.05.

Fig. 9. Relative density, εT33/εo, d33, kp, and Qm values of C(LxN0.5-xK0.5)(N0.945S0.055)-SZ ceramics.

Fig. 10. d33 values of C(LxN0.5-xK0.5)(N0.935S0.065)-SZ ceramics measured at various temperatures.

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Fig. 1. XRD reflections at 66.5o measured by slow-speed scanning for C(LxN0.5xK0.5)(N0.935S0.065)-SZ

(y = 0.065) ceramics with 0.0 ≤ x ≤ 0.07 sintered at 1020oC for 6 h: (a)

x = 0.0, (b) x = 0.01, (c) x = 0.02, (d) x = 0.04, (e) x = 0.05, and (f) x = 0.07.

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Fig. 2. (a) Electron diffraction pattern and (b) HRTEM image of C(N0.5K0.5)(N0.935S0.065)-SZ ceramic (x = 0.0, y = 0.065). (c) Electron diffraction pattern and (d) HRTEM image of C(L0.05N0.45K0.5)(N0.935S0.065)-SZ ceramic (x = 0.05 and y = 0.065). The insets of (b) and (d) show the FFT patterns obtained from the corresponding HRTEM images.

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Fig. 3. (a-i) εT33/εo vs. temperature curve and (a-ii) XRD patterns measured at various temperatures for the specimen with x = 0.04. (b-i) εT33/εo vs. temperature curve and (b-ii) XRD patterns measured at various temperatures for the specimen with x = 0.05. (c) Schematic phase diagram of C(LxN0.5-xK0.5)(N0.935S0.065)-SZ ceramics with 0.0 ≤ x ≤ 0.07.

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Fig. 4. Raman spectra of the specimens with (a) 0.0 ≤ x ≤ 0.07 and y = 0.065, and (b) 0.0 ≤ x ≤ 0.05 and y = 0.055. 31

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Fig. 5. P-E curves of C(LxN0.5-xK0.5)(N0.935S0.065)-SZ ceramics: (a) x = 0.0, (b) x = 0.01, (c) x = 0.02, (d) x = 0.03, (e) x = 0.04, (f) x = 0.05, and (g) x = 0.07. (h) Variations in the Pr and EC of these specimens.

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Fig. 6. (a) Relative density, εT33, d33, kp, and Qm values of C(LxN0.5-xK0.5)(N0.935S0.065)-SZ ceramics and (b) the variations in their Pr × εT33 and d33 values. 33

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Fig. 7. Rietveld refinement XRD profiles of C(LxN0.5-xK0.5)(N0.945S0.055)-SZ ceramics sintered at 1020oC for 6 h: (a) x = 0.0, (b) x = 0.03, and (c) x = 0.05. 34

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Fig. 8. εT33/εo vs. temperature curves of C(LxN0.5-xK0.5)(N0.945S0.055)-SZ ceramics obtained in the range -50oC to 300oC: (a) x = 0.0, (b) x = 0.01, (c) x = 0.02, (d) x = 0.03, (e) x = 0.04, and (f) x = 0.05.

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Fig. 9. Relative density, εT33/εo, d33, kp, and Qm values of C(LxN0.5-xK0.5)(N0.945S0.055)-SZ ceramics.

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Fig. 10. d33 values of C(LxN0.5-xK0.5)(N0.935S0.065)-SZ ceramics measured at various temperatures.

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Highlights

- The specimen with x = 0.05 has a tetragonal-rhombohedral PPB structure.

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- A schematic phase diagram of the (Li,Na,K)(Nb,Sb)O3-SrZrO3 ceramics was constructed.

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- This specimen maintains a high d33 value of 400 pC/N at 150oC.

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- This specimen showed the highest piezoelectric properties (d33=431 pC/N and kp=0.43).