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Microstructure and microwave dielectric characteristics of magnesium fluoride additive to MgTiO3-(Ca0.8Sr0.2)TiO3 ceramics
Materials Letters 252 (2019) 191–193
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/mlblue
Microstructure and microwave dielectric characteristics of magnesium fluoride additive to MgTiO3-(Ca0.8Sr0.2)TiO3 ceramics Zhenpeng Xu, Lingxia Li ⇑, Shihui Yu ⇑, Mingkun Du, Weijia Luo School of Microelectronics and Key Laboratory for Advanced Ceramics and Machining Technology of Ministry of Education, Tianjin 300072, China
a r t i c l e
i n f o
Article history: Received 22 April 2019 Received in revised form 22 May 2019 Accepted 29 May 2019 Available online 30 May 2019 Keywords: Ceramics Temperature stable Fluoride Microstructure
a b s t r a c t In this paper, the MgF2-doped (1 x)MgTiO3-x(Ca0.8Sr0.2)TiO3 (x = 0.01–0.09) ceramics with high-quality factor (Q f) and near-zero temperature coefficient of the resonant frequency (sf) were synthesized by solid-state reaction method. The effects of MgF2 addition on the phase composition, sintering behavior, and microwave dielectric properties in this system were completely investigated. Furthermore, the sintering temperature of 3 wt% MgF2-doped 0.95MgTiO3-0.05(Ca0.8Sr0.2)TiO3 ceramics can be lowered to 1225 °C. Then, the best microwave dielectric properties of er 19.99, Q f 90,100 GHz (at 8.72 GHz) and sf 2.9 ppm/°C were obtained. It provides a solid foundation for widespread applications of microwave dielectric substrates, filters, resonators, and patch antennas in modern wireless communication equipment. Ó 2019 Elsevier B.V. All rights reserved.
1. Introduction In recent years, a series of microwave dielectric ceramics have been widely used as dielectrics in filters and resonators for global positioning systems, antenna, oscillators, diplexer, and radar systems operating at microwave frequencies [1–3]. With the deployment of 5G mobile communication technology, microwave devices are developing in the direction of devices in highfrequency, miniaturization, low loss, and high reliability. Considering the application of microwave devices in the high-frequency environment, high-quality factor (Q f) of microwave dielectric ceramics is the crux to improve the frequency selectivity of microwave devices and reduce the loss of devices. Besides, a relatively high dielectric constant (er 20) is a key indicator to realize device miniaturization [3,4]. Finally, to ensure the good stability of microwave devices, a near-zero sf is a primarily requirement [5]. MgTiO3 ceramics with ilmenite-type structure have excellent microwave performances: er 17, Q f 160,000 GHz (at 7 GHz) and sf 50 ppm/°C [6]. To improve the temperature stability of MgTiO3 ceramics, Ca0.8Sr0.2TiO3 ceramic: er 181, Q f 8,300 GHz, and sf 991 ppm/°C has been studied as sf compensator by many scholars [7,8]. Huang et al. reported 0.94MgTiO3-0.06(Ca0.8Sr0.2)TiO3 ceramics possessed a good combination of dielectric properties: er 21.42, Q f 83,700 GHz, and sf 1.8 ppm/°C [9]. However, the densification sintering temper⇑ Corresponding authors. E-mail addresses: [email protected] (L. Li), [email protected] (S. Yu). https://doi.org/10.1016/j.matlet.2019.05.136 0167-577X/Ó 2019 Elsevier B.V. All rights reserved.
ature of this system is as high as 1300 °C. Zhang et al. reported the substitution of F for O2– weakened oxygen bond strength which effectively lowered the sintering temperature of ceramics [10]. As a widely studied fluoride, MgF2 was chosen as the sintering aid in this research. In this paper, we firstly reported the effect of MgF2 additions on sintering temperatures and the microwave dielectric properties of (1-x)MgTiO3-x(Ca0.8Sr0.2)TiO3 ceramics. The addition of MgF2 improved the microwave dielectric properties of ceramics by affecting density and micromorphology. Ultimately, the MgF2-doped (1-x)MgTiO3-x(Ca0.8Sr0.2)TiO3 ceramics with the low-cost, high Q f value, and near-zero sf was obtained. 2. Experimental procedure The samples involved in this study were prepared by a conventional solid-state reaction method from high-purity powders of MgO (99.8%), CaCO3(99%), SrCO3(99%), TiO2 (99.8%), and MgF2 (99.99%). First of all, the powders were separately prepared according to the desired stoichiometry MgTiO3 and (Ca0.8Sr0.2)TiO3 with ZrO2 balls in deionized water for 6 h, then dried and calcined at 900 °C/1100 °C for 4 h. Moreover, the MgTiO3 powders were mixed with different molar fraction of (Ca0.8Sr0.2)TiO3 powders and 3 wt% MgF2. Then the mixture was ball-milled in deionized water with ZrO2 balls for 12 h. After drying and sieving, the sample powder and binder (5 wt% polyvinyl alcohol) were uniaxially pressed into cylinders with a diameter of 10 mm and a thickness of 4–5 mm under the pressure of 200 MPa. Furthermore, the pellets were sintered in the temperature range of 1200 °C1275 °C for 4 h in air.
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Z. Xu et al. / Materials Letters 252 (2019) 191–193
XRD analysis (D/MAX-2500 X-ray diffractometer; Rigaku, Tokyo, Japan) was used for the measurement of the crystalline phases of the sintered ceramics. The scanning electron microscope (SEM) (FE-SEM, S-4800; Hitachi, Ltd., Tokyo, Japan) with an attached energy dispersive spectrometer (EDS) was carried out to reflect the microstructure and distribution of elements. The microwave dielectric performance was measured by the Hakki-Coleman’s method using TE011 mode using the network analyzer (8720ES; Agilent, Santa Clara, CA) [11]. The sf value was measured in an invar cavity and calculated from the resonant frequencies obtained at 25 and 85 °C using the formula:
f 25 sf ¼ f 25f 85ð8525Þ 106 .
3. Results and discussions Table 1 demonstrates the microwave dielectric properties of 3 wt% MgF2-doped (1-x)MgTiO3-x(Ca0.8Sr0.2)TiO3 ceramics sintered at 1225 °C for 4 h. It is obvious that the regular variation in sf values is due to the different compositional ratio of (Ca0.8Sr0.2)TiO3. Table 1 Microwave dielectric properties of 3 wt% MgF2-doped (1-x)MgTiO3-x(Ca0.8Sr0.2)TiO3 (x = 0.01–0.09) ceramics sintered at 1225 °C for 4 h. x value
er
Q f (GHz)
sf (ppm/°C)
0.01 0.03 0.05 0.07 0.09
17.09 18.73 19.99 21.66 22.96
116,300 104,500 90,100 60,700 47,600
44.1 35.5 2.9 7.1 27.6
Fig. 1. X-ray diffraction patterns of 3 wt% MgF2-doped 95MCST ceramics sintered at different temperatures for 4 h.
Besides, 3 wt% MgF2-doped 0.95MgTiO3-0.05(Ca0.8Sr0.2)TiO3 (hereafter referred to as 3 wt% MgF2-doped 95MCST) ceramics has nearzero sf and excellent dielectric properties, so further research on 3 wt% MgF2-doped 95MCST ceramics is carried out. XRD patterns of the 3 wt% MgF2-doped 95MCST ceramics sintered in the temperature range of 1200 °C1275 °C for 4 h are shown in Fig. 1. In all samples, MgTiO3 (ICDD #00–006-0494), with ilmenite structure can be observed as the main crystal phase, and the second minor phase (Ca0.8Sr0.2)TiO3 (ICDD #01-070-0584) is clearly identified. The formation of the two-phase system is due to the large difference in radius between Ca2+ (1.00 Å)/Sr2+ (1.18 Å) and Mg2+ (0.72 Å), which limits the formation of solid solution. Fig. 2 shows the typical SEM micrographs of the 3 wt% MgF2doped 95MCST ceramics sintered in the temperature range of 1200 °C1275 °C for 4 h. All the samples exhibit a dense microstructure. However, due to the incomplete growth of grains, a small number of pores can be found in Fig. 2(a). With the increase of sintering temperature, the grain size increases gradually and the optimum dense microstructures are observed in Fig. 2(b). What’s more, Energy Dispersive Spectrometer (EDS) analysis was used in conjunction with SEM to identify each grain in 3 wt% MgF2doped 95MCST ceramics sintered at 1225 °C, as shown in Fig. 2 (b). The grain morphology of the sample could be divided into two types: large grains (Spots A) were MgTiO3 and small grains (Spots B) were (Ca0.8Sr0.2)TiO3. Fig. 3 displays the bulk densities, the dielectric constant (er), Q f, and sf values of the 3 wt% MgF2-doped 95MCST ceramics sintered in the temperature range of 1200 °C1275 °C for 4 h. With the increase of sintering temperature, bulk density increases gradually and reaches its maximum at 1225 °C, then decreases slightly. The increase in bulk density is contributed by grain growth and the liquid phase sintering of MgF2, which improves the effect of pores on dielectric properties of ceramics [12]. Furthermore, the evaporation of F at high temperature and abnormal grain growth lead to a decrease in bulk density [13]. The reduction of densification sintering temperature to 1225 °C is due to crystal defects caused by the substitution of small radius F for O2–. The substitution and defect are beneficial to facilitate the diffusion process and reduce the intrinsic sintering temperature in the sintering process of ceramics [14]. Besides, the variation trend of er also corresponds to the temperature of densification sintering, the 3 wt% MgF2doped 95MCST ceramics sintered at 1225 °C for 4 h shows the maximum er value of 19.99. In microwave dielectric ceramics, the dielectric loss is dominated by phase composition, grain sizes, porosity and so on [15]. In this study, the phase composition of all samples has not a significant difference. Therefore, the change of Q f value with temperature is mainly due to the increase of grain size and the decrease of porosity. Furthermore, the decrease of the grain boundary area reduces the lattice imperfections and thus improve the Q f value. After the Q f value reaches the maximum of 90,100 GHz at 1225 °C, the Q f value decreases
Fig. 2. SEM micrographs of 3 wt% MgF2-doped 95MCST ceramics sintered at (a) 1200 °C, (b) 1225 °C, (c) 1250 °C, and (d) 1275 °C for 4 h.
Z. Xu et al. / Materials Letters 252 (2019) 191–193
193
Fig. 3. Bulk densities, dielectric constant, Q f and sf values of 3 wt% MgF2-doped 95MCST ceramics as a function of the sintering temperature.
slightly with the rising sintering temperature. The same trend as bulk density is decided by abnormal grain growth and defects caused by F volatilization. The sf value is mainly determined by the composition of the ceramic. Since there is no compositional variation involved in this experiment, the sf value of 3 wt% MgF2doped 95MCST ceramics varies in a small range from 8.5 to 2.9 ppm/°C. 4. Conclusion (1-x)MgTiO3-x(Ca0.8Sr0.2)TiO3 (x = 0.01–0.09) ceramics were prepared by the conventional solid-state reaction method with 3 wt% MgF2 additions. The addition of MgF2 lowers the sintering temperature to 1225 °C. The microwave dielectric properties of 3 wt% MgF2-doped 95MCST ceramics show a strong relationship with density and micromorphology. In this work, 3 wt% MgF2doped 95MCST ceramics sintered at 1225 °C for 4 h exhibits the optimal microwave dielectric properties: er 19.99, Q f 90,100 GHz (at 8.72 GHz) and sf 2.9 ppm/°C. It provides a new reference for low-loss modern wireless communication devices. Declaration of Competing Interest None.
Acknowledgments This work was supported by the Joint fund of the equipment pre Research Ministry of Education (Grant No. 6141A02022412). References [1] W. Wersing, Curr. Opin. Solid. St. 1 (5) (1996) 715–731. [2] K.T. Santhosh, P. Gogoi, A. Perumal, J. Am. Ceram. Soc. 97 (4) (2014) 1054– 1059. [3] K. Wakino, T. Nishikawa, Y. Ishikawa, Br. Ceram. Trans. J. 89 (2) (1990) 39–43. [4] U. Ullah, W.F.F.W. Ali, M.F. Ain, Mater. Design. 85 (2015) 396–403. [5] F.F. Gu, G.H. Chen, X.Q. Li, Mater. Chem. Phys. 167 (2015) 354–359. [6] K. Wakino, Ferroelectrics. 91 (1) (1989) 69–86. [7] P.L. Wise, I.M. Reaney, W.E. Lee, T.J. Price, D.M. Iddles, D.S. Cannell, J. Eur. Ceram. Soc. 21 (10–11) (2001) 1723–1726. [8] W.B. Li, D. Zhou, D. Guo, J. Alloy. Compd. 694 (2017) 40–45. [9] J.Y. Chen, C.L. Huang, Mater. Lett. 64 (23) (2010) 2585–2588. [10] Y.Z. Hao, H. Yang, G.H. Chen, J. Alloy. Compd. 552 (2013) 173–179. [11] F.F. Gu, G.H. Chen, C.L. Yuan, J. Mater. Sci: Mater. Electron. 26 (2015) 360–368. [12] W.E. Counts, R. Roy, E.F. Osborn, J. Am. Ceram. Soc. 36 (1) (1953) 12–17. [13] Z.G. Ye, R. Von Der Mühll, J. Ravez, J. Mater. Res. 3 (1) (1988) 112–115. [14] J.L. Ma, Z.F. Fu, P. Liu, W. Bing, L. Yang, Mater. Sci. Eng., B. 204 (2016) 15–19. [15] B.D. Silverman, Phys. Rev. 125 (6) (1962) 1921–1930.
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/mlblue
Microstructure and microwave dielectric characteristics of magnesium fluoride additive to MgTiO3-(Ca0.8Sr0.2)TiO3 ceramics Zhenpeng Xu, Lingxia Li ⇑, Shihui Yu ⇑, Mingkun Du, Weijia Luo School of Microelectronics and Key Laboratory for Advanced Ceramics and Machining Technology of Ministry of Education, Tianjin 300072, China
a r t i c l e
i n f o
Article history: Received 22 April 2019 Received in revised form 22 May 2019 Accepted 29 May 2019 Available online 30 May 2019 Keywords: Ceramics Temperature stable Fluoride Microstructure
a b s t r a c t In this paper, the MgF2-doped (1 x)MgTiO3-x(Ca0.8Sr0.2)TiO3 (x = 0.01–0.09) ceramics with high-quality factor (Q f) and near-zero temperature coefficient of the resonant frequency (sf) were synthesized by solid-state reaction method. The effects of MgF2 addition on the phase composition, sintering behavior, and microwave dielectric properties in this system were completely investigated. Furthermore, the sintering temperature of 3 wt% MgF2-doped 0.95MgTiO3-0.05(Ca0.8Sr0.2)TiO3 ceramics can be lowered to 1225 °C. Then, the best microwave dielectric properties of er 19.99, Q f 90,100 GHz (at 8.72 GHz) and sf 2.9 ppm/°C were obtained. It provides a solid foundation for widespread applications of microwave dielectric substrates, filters, resonators, and patch antennas in modern wireless communication equipment. Ó 2019 Elsevier B.V. All rights reserved.
1. Introduction In recent years, a series of microwave dielectric ceramics have been widely used as dielectrics in filters and resonators for global positioning systems, antenna, oscillators, diplexer, and radar systems operating at microwave frequencies [1–3]. With the deployment of 5G mobile communication technology, microwave devices are developing in the direction of devices in highfrequency, miniaturization, low loss, and high reliability. Considering the application of microwave devices in the high-frequency environment, high-quality factor (Q f) of microwave dielectric ceramics is the crux to improve the frequency selectivity of microwave devices and reduce the loss of devices. Besides, a relatively high dielectric constant (er 20) is a key indicator to realize device miniaturization [3,4]. Finally, to ensure the good stability of microwave devices, a near-zero sf is a primarily requirement [5]. MgTiO3 ceramics with ilmenite-type structure have excellent microwave performances: er 17, Q f 160,000 GHz (at 7 GHz) and sf 50 ppm/°C [6]. To improve the temperature stability of MgTiO3 ceramics, Ca0.8Sr0.2TiO3 ceramic: er 181, Q f 8,300 GHz, and sf 991 ppm/°C has been studied as sf compensator by many scholars [7,8]. Huang et al. reported 0.94MgTiO3-0.06(Ca0.8Sr0.2)TiO3 ceramics possessed a good combination of dielectric properties: er 21.42, Q f 83,700 GHz, and sf 1.8 ppm/°C [9]. However, the densification sintering temper⇑ Corresponding authors. E-mail addresses: [email protected] (L. Li), [email protected] (S. Yu). https://doi.org/10.1016/j.matlet.2019.05.136 0167-577X/Ó 2019 Elsevier B.V. All rights reserved.
ature of this system is as high as 1300 °C. Zhang et al. reported the substitution of F for O2– weakened oxygen bond strength which effectively lowered the sintering temperature of ceramics [10]. As a widely studied fluoride, MgF2 was chosen as the sintering aid in this research. In this paper, we firstly reported the effect of MgF2 additions on sintering temperatures and the microwave dielectric properties of (1-x)MgTiO3-x(Ca0.8Sr0.2)TiO3 ceramics. The addition of MgF2 improved the microwave dielectric properties of ceramics by affecting density and micromorphology. Ultimately, the MgF2-doped (1-x)MgTiO3-x(Ca0.8Sr0.2)TiO3 ceramics with the low-cost, high Q f value, and near-zero sf was obtained. 2. Experimental procedure The samples involved in this study were prepared by a conventional solid-state reaction method from high-purity powders of MgO (99.8%), CaCO3(99%), SrCO3(99%), TiO2 (99.8%), and MgF2 (99.99%). First of all, the powders were separately prepared according to the desired stoichiometry MgTiO3 and (Ca0.8Sr0.2)TiO3 with ZrO2 balls in deionized water for 6 h, then dried and calcined at 900 °C/1100 °C for 4 h. Moreover, the MgTiO3 powders were mixed with different molar fraction of (Ca0.8Sr0.2)TiO3 powders and 3 wt% MgF2. Then the mixture was ball-milled in deionized water with ZrO2 balls for 12 h. After drying and sieving, the sample powder and binder (5 wt% polyvinyl alcohol) were uniaxially pressed into cylinders with a diameter of 10 mm and a thickness of 4–5 mm under the pressure of 200 MPa. Furthermore, the pellets were sintered in the temperature range of 1200 °C1275 °C for 4 h in air.
192
Z. Xu et al. / Materials Letters 252 (2019) 191–193
XRD analysis (D/MAX-2500 X-ray diffractometer; Rigaku, Tokyo, Japan) was used for the measurement of the crystalline phases of the sintered ceramics. The scanning electron microscope (SEM) (FE-SEM, S-4800; Hitachi, Ltd., Tokyo, Japan) with an attached energy dispersive spectrometer (EDS) was carried out to reflect the microstructure and distribution of elements. The microwave dielectric performance was measured by the Hakki-Coleman’s method using TE011 mode using the network analyzer (8720ES; Agilent, Santa Clara, CA) [11]. The sf value was measured in an invar cavity and calculated from the resonant frequencies obtained at 25 and 85 °C using the formula:
f 25 sf ¼ f 25f 85ð8525Þ 106 .
3. Results and discussions Table 1 demonstrates the microwave dielectric properties of 3 wt% MgF2-doped (1-x)MgTiO3-x(Ca0.8Sr0.2)TiO3 ceramics sintered at 1225 °C for 4 h. It is obvious that the regular variation in sf values is due to the different compositional ratio of (Ca0.8Sr0.2)TiO3. Table 1 Microwave dielectric properties of 3 wt% MgF2-doped (1-x)MgTiO3-x(Ca0.8Sr0.2)TiO3 (x = 0.01–0.09) ceramics sintered at 1225 °C for 4 h. x value
er
Q f (GHz)
sf (ppm/°C)
0.01 0.03 0.05 0.07 0.09
17.09 18.73 19.99 21.66 22.96
116,300 104,500 90,100 60,700 47,600
44.1 35.5 2.9 7.1 27.6
Fig. 1. X-ray diffraction patterns of 3 wt% MgF2-doped 95MCST ceramics sintered at different temperatures for 4 h.
Besides, 3 wt% MgF2-doped 0.95MgTiO3-0.05(Ca0.8Sr0.2)TiO3 (hereafter referred to as 3 wt% MgF2-doped 95MCST) ceramics has nearzero sf and excellent dielectric properties, so further research on 3 wt% MgF2-doped 95MCST ceramics is carried out. XRD patterns of the 3 wt% MgF2-doped 95MCST ceramics sintered in the temperature range of 1200 °C1275 °C for 4 h are shown in Fig. 1. In all samples, MgTiO3 (ICDD #00–006-0494), with ilmenite structure can be observed as the main crystal phase, and the second minor phase (Ca0.8Sr0.2)TiO3 (ICDD #01-070-0584) is clearly identified. The formation of the two-phase system is due to the large difference in radius between Ca2+ (1.00 Å)/Sr2+ (1.18 Å) and Mg2+ (0.72 Å), which limits the formation of solid solution. Fig. 2 shows the typical SEM micrographs of the 3 wt% MgF2doped 95MCST ceramics sintered in the temperature range of 1200 °C1275 °C for 4 h. All the samples exhibit a dense microstructure. However, due to the incomplete growth of grains, a small number of pores can be found in Fig. 2(a). With the increase of sintering temperature, the grain size increases gradually and the optimum dense microstructures are observed in Fig. 2(b). What’s more, Energy Dispersive Spectrometer (EDS) analysis was used in conjunction with SEM to identify each grain in 3 wt% MgF2doped 95MCST ceramics sintered at 1225 °C, as shown in Fig. 2 (b). The grain morphology of the sample could be divided into two types: large grains (Spots A) were MgTiO3 and small grains (Spots B) were (Ca0.8Sr0.2)TiO3. Fig. 3 displays the bulk densities, the dielectric constant (er), Q f, and sf values of the 3 wt% MgF2-doped 95MCST ceramics sintered in the temperature range of 1200 °C1275 °C for 4 h. With the increase of sintering temperature, bulk density increases gradually and reaches its maximum at 1225 °C, then decreases slightly. The increase in bulk density is contributed by grain growth and the liquid phase sintering of MgF2, which improves the effect of pores on dielectric properties of ceramics [12]. Furthermore, the evaporation of F at high temperature and abnormal grain growth lead to a decrease in bulk density [13]. The reduction of densification sintering temperature to 1225 °C is due to crystal defects caused by the substitution of small radius F for O2–. The substitution and defect are beneficial to facilitate the diffusion process and reduce the intrinsic sintering temperature in the sintering process of ceramics [14]. Besides, the variation trend of er also corresponds to the temperature of densification sintering, the 3 wt% MgF2doped 95MCST ceramics sintered at 1225 °C for 4 h shows the maximum er value of 19.99. In microwave dielectric ceramics, the dielectric loss is dominated by phase composition, grain sizes, porosity and so on [15]. In this study, the phase composition of all samples has not a significant difference. Therefore, the change of Q f value with temperature is mainly due to the increase of grain size and the decrease of porosity. Furthermore, the decrease of the grain boundary area reduces the lattice imperfections and thus improve the Q f value. After the Q f value reaches the maximum of 90,100 GHz at 1225 °C, the Q f value decreases
Fig. 2. SEM micrographs of 3 wt% MgF2-doped 95MCST ceramics sintered at (a) 1200 °C, (b) 1225 °C, (c) 1250 °C, and (d) 1275 °C for 4 h.
Z. Xu et al. / Materials Letters 252 (2019) 191–193
193
Fig. 3. Bulk densities, dielectric constant, Q f and sf values of 3 wt% MgF2-doped 95MCST ceramics as a function of the sintering temperature.
slightly with the rising sintering temperature. The same trend as bulk density is decided by abnormal grain growth and defects caused by F volatilization. The sf value is mainly determined by the composition of the ceramic. Since there is no compositional variation involved in this experiment, the sf value of 3 wt% MgF2doped 95MCST ceramics varies in a small range from 8.5 to 2.9 ppm/°C. 4. Conclusion (1-x)MgTiO3-x(Ca0.8Sr0.2)TiO3 (x = 0.01–0.09) ceramics were prepared by the conventional solid-state reaction method with 3 wt% MgF2 additions. The addition of MgF2 lowers the sintering temperature to 1225 °C. The microwave dielectric properties of 3 wt% MgF2-doped 95MCST ceramics show a strong relationship with density and micromorphology. In this work, 3 wt% MgF2doped 95MCST ceramics sintered at 1225 °C for 4 h exhibits the optimal microwave dielectric properties: er 19.99, Q f 90,100 GHz (at 8.72 GHz) and sf 2.9 ppm/°C. It provides a new reference for low-loss modern wireless communication devices. Declaration of Competing Interest None.
Acknowledgments This work was supported by the Joint fund of the equipment pre Research Ministry of Education (Grant No. 6141A02022412). References [1] W. Wersing, Curr. Opin. Solid. St. 1 (5) (1996) 715–731. [2] K.T. Santhosh, P. Gogoi, A. Perumal, J. Am. Ceram. Soc. 97 (4) (2014) 1054– 1059. [3] K. Wakino, T. Nishikawa, Y. Ishikawa, Br. Ceram. Trans. J. 89 (2) (1990) 39–43. [4] U. Ullah, W.F.F.W. Ali, M.F. Ain, Mater. Design. 85 (2015) 396–403. [5] F.F. Gu, G.H. Chen, X.Q. Li, Mater. Chem. Phys. 167 (2015) 354–359. [6] K. Wakino, Ferroelectrics. 91 (1) (1989) 69–86. [7] P.L. Wise, I.M. Reaney, W.E. Lee, T.J. Price, D.M. Iddles, D.S. Cannell, J. Eur. Ceram. Soc. 21 (10–11) (2001) 1723–1726. [8] W.B. Li, D. Zhou, D. Guo, J. Alloy. Compd. 694 (2017) 40–45. [9] J.Y. Chen, C.L. Huang, Mater. Lett. 64 (23) (2010) 2585–2588. [10] Y.Z. Hao, H. Yang, G.H. Chen, J. Alloy. Compd. 552 (2013) 173–179. [11] F.F. Gu, G.H. Chen, C.L. Yuan, J. Mater. Sci: Mater. Electron. 26 (2015) 360–368. [12] W.E. Counts, R. Roy, E.F. Osborn, J. Am. Ceram. Soc. 36 (1) (1953) 12–17. [13] Z.G. Ye, R. Von Der Mühll, J. Ravez, J. Mater. Res. 3 (1) (1988) 112–115. [14] J.L. Ma, Z.F. Fu, P. Liu, W. Bing, L. Yang, Mater. Sci. Eng., B. 204 (2016) 15–19. [15] B.D. Silverman, Phys. Rev. 125 (6) (1962) 1921–1930.