Author’s Accepted Manuscript Effect of Li2CO3 addition on the structural, optical, ferroelectric, and electric-field-induced strain of lead-free BNKT-based ceramics Nguyen Van Quyet, Luong Huu Bac, Dang Duc Dung www.elsevier.com/locate/jpcs PII: DOI: Reference: S0022-3697(15)00125-0 http://dx.doi.org/10.1016/j.jpcs.2015.05.010 PCS7542 To appear in: Journal of Physical and Chemistry of Solids Received date: 22 January 2015 Revised date: 30 April 2015 Accepted date: 10 May 2015 Cite this article as: Nguyen Van Quyet, Luong Huu Bac and Dang Duc Dung, Effect of Li2CO3 addition on the structural, optical, ferroelectric, and electricfield-induced strain of lead-free BNKT-based ceramics, Journal of Physical and Chemistry of Solids, http://dx.doi.org/10.1016/j.jpcs.2015.05.010 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 galley proof before it is published in its final citable 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 Effect of Li2CO3 addition on the structural, optical, ferroelectric, and electric-field-induced strain of lead-free BNKT-based ceramics Nguyen Van Quyet1, Luong Huu Bac2, and Dang Duc Dung2,* School of Materials Science and Engineering, University of Ulsan, Ulsan 680-749, Republic of Korea School of Engineering Physics, Ha Noi University of Science and Technology, Dai Co Viet Road, Ha Noi, Viet Nam Abstract In this work, we reported the effect of Li2CO3 addition on the structural, optical, ferroelectric properties and electric-field-induced strain of Bi0.5(Na,K)0.5TiO3 (BNKT) solid solution with CaZrO3 ceramics Both rhombohedral and tetragonal structures were distorted after adding Lithium (Li) The band gap values decreased from 2.91 to 2.69 eV for mol% Li-addition The maximum polarization and remanent polarization decreased from 49.66 C/cm2 to 27.11 C/cm2 and from 22.93 C/cm2 to 5.35 C/cm2 for un-doped and mol% Li- addition BNKT ceramics, respectively The maximum Smax/Emax value was 567 pm/V at mol% Li2CO3 access We expected this work will help to understand the role of A-site dopant in lead-free ferroelectric BNKT materials Keywords: A Ceramics; B Crystal growth; D Optical properties; D Ferroelectricity; D Piezoelectricity *) Corresponding e-mail: dung.dangduc@hust.edu.vn I Introduction The Pb(Ti1-xZrx)O3 (PZT)-based piezoceramics currently dominate the electronic industry, however the search for an appropriate lead-free replacement due to environmental and human health concerns continues [1] Among various lead-free systems, modified-Bi0.5(Na,K)0.5TiO3 (BNKT) ceramics seem to be a candidate for real application in piezoelectric devices due to giant electric field-induced strain (EFIS) [2] Recently, the current development BNKT-based indicated that the dynamic coefficient (Smax/Emax) could be compared with soft PZT-based materials [3] The dynamic coefficient in BNKT ceramics was around 225 pm/V and can be enhanced when the mostly A-site and/or B-site were modified [4-8] Dinh et al reported the enhancement Smax/Emax up to 715 pm/V due to replace Bi3+ by mol% La3+ as A-site [4] Do et al reported that trivalent Y3+ and alionvalent Ta5+ modified Ti4+ resulted in increasement the Smax/Emax values of 278 pm/V and 566 pm/V, respectively [5, 6] Similarly, Hussain et al obtained the enhancement of Smax/Emax to 475 and 614 pm/V by replacement of isovalent ions Hf4+ and Zr4+ for Ti4+ at B-site, respectively [7, 8] Recently, Nguyen et al archived strongly enhancement of the electric-field-induced strain due to the co-substitution in both A-site (Li+ substituted Na+) and B-site (Ta5+ or Sn4+ substituted Ti4+) [9, 10] In addition, the solid solutions of secondary ferroelectric perovskite materials with lead-free BNKT-based were also found to be enhanced Smax/Emax values In fact, the solid solution of A’B’O3 perovskite materials to BNKT ceramics which could consider as co-dopants at both A- and B-site, with similar concentration, because of diffuse element during sintering process Thank to well solid-solution with lead-free Bi0.5(Na,K)0.5TiO3-based ceramics, Ullah et al reported the highest value of Smax/Emax of 391 pm/V for mol% BiAlO3 solid solution in Bi0.5(Na0.8K0.2)0.5TiO3 which resulted from phase transition from the coexistence of rhombohedral and tetragonal into pseudocubic phase [11] Interestingly, Ullah et al pointed out that the Smax/Emax value was 533 pm/V in 0.975[Bi0.5(Na0.78K0.22)0.5TiO3]–0.025BiAlO3 due to the tetragonal side of the mophotropic phase boundary composition and 592 pm/V in 0.970[Bi0.5(Na0.78K0.22)0.5TiO3]-0.030BiAlO3 at near the tetragonal-pseudocubic phase boundary [12, 13] However, Fu et al reported that only distorted structure was obtained and phase transition did not happen in BiAlO3 solid solution with Bi0.5(Na0.82K0.18)0.5TiO3 [14] In fact, co-modifications at A-site and B-site in BNKT ceramics were further enhancement the Smax/Emax up to 579 pm/V when the tetragonal structure of lead-free 0.99Bi0.5(Na0.78K0.22)0.5TiO3–0.01(Bi0.5La0.5)AlO3 composition was distorted [15] Lee et al obtained the normal strain of 549 pm/V for mol.% Ba0.8Ca0.2ZrO3-modified BNKT [16] Moreover, the solute solution of CaZrO3 into BNKT was found to display in larger Smax/Emax values than BaZrO3 modification of BNKT [17, 18] The explanation for different enhancement of Smax/Emax in BNKT solid solutionoriginated from phase transition from polar to non-polar due to expansion tolerance factor and/or promotion of oxygen vacancies [4, 9, 19] In fact, the tolerance factors just only evaluated the perovskite or non-perovskite and it could not show the relationship between tolerance factors with structure symmetry [20-22] Therefore, these results were important to point out that: i) the mechanism in enhancement Smax/Emax values were still unclear in their research, and ii) the modification of A-site were more sensitive to Smax/Emax values than that of only modified at B-site The A-site modification by ion Li+ in lead-free BNKT-based ceramics have been reported with interesting phenomena and attractive results The Li+ ions were found to be suppressed formation of the second phase and Ti3+/4+ valence transitions when it substituted at Na-site in BNKT [17, 18] Co-doped Li+ (at Na+-site) and Ta5+ (at Ti+site) ions in BNKT ceramics were found to be strongly enhanced the Smax/Emax value up to 727 pm/V which resulted from transition from coexist of rhombohedral and tetragonal phase to pseudocubic phase [9] Unlikely, the co-doped Li+ (at Na+-site) and Sn4+ (at Ti+-site) ions caused phase transition from pseudocubic to tetragonal phase with Smax/Emax value of 646 pm/V [10, 25] Recently, we reported that the distorted tetragonal and rhombohedral structure due to Li+ ions modified Bi0.5(Na0.78K0.22)0.5Ti0.97Zr0.03O3 lead-free piezoceramics which were possible to increase the Smax/Emax from 600 pm/V to 643 pm/V for mol% Li+-added [26] In this work, we reported the effect of Li2CO3 addition on the structural, optical, ferroelectric properties and electric-field-induced strain of BNKT solid solution with CaZrO3 ceramics The both rhombohedral and tetragonal structures distorted after adding Li The band gap (Eg) values decreased from 2.91 to 2.69 eV with mol % Liadded The maximum polarization decreased from 49.66 C/cm2 to 27.11 C/cm2 as increasing the Li concentration from to mol% The Smax/Emax value was 567 pm/V with mol Li-added II Experiment The 0.97Bi0.5Na0.4-xLixK0.1TiO3-0.03CaZrO3 (BNKTCZ-xLi) (x = 0.00, 0.01, 0.02, 0.03, 0.04, and 0.05) ceramics were prepared by a conventional solid state reaction route The raw materials were Bi2O3, K2CO3, TiO2, Li2CO3, CaCO3 (99.9%, Kojundo Chemical), Na2CO3 (99.9%, Ceramic Specialty Inorganics) and ZrO2 (99.9%, Cerac Specialty Inorganics) The details of fabrication processing were found in elsewhere [26] The green compacts were sintered in a covered alumina crucible at 1180 °C for h in ambient condition The surface morphology was observed with a field emission scanning electron microscope (FE-SEM) The crystalline structures of the samples were characterized by X-ray diffraction (XRD) The optical properties were studied by UV-VIS spectroscopy The temperature dependence of the dielectric properties was measured using an impedance analyzer The polarization-electric fields (P-E) and electric field-induced strain hysteresis loops were measured in silicon oil using a modified Sawyer–Tower circuit and linear variable differential transducer system, respectively III Result and discussion Fig shows the X-ray diffraction pattern of BNKTCZ-xLi with x=0.00, 0.01, 0.02, 0.03, 0.04 and 0.05 The all samples show the single perovskite structure without impurity phase, indicating that Li+ ions were successfully diffused to lattice The magnifications of Fig in the range 2 from 39.0 to 41.0 and 44.0 to 48.0 are shown in Fig (a) and (b), respectively The results show that the peaks were unsymmetrical with shoulder peak which revealed the overlap of multi-peaks The each peak was carefully fitted by using the Lorentzian as shown in the red dash line The peaks were indexed as (003)/(021) and (002)/(200) in the range from 39.0 to 40.5 and 44.0 to 48.0, respectively, indicating that both rhombohedral and tetragonal phases were coexisted In addition, the peaks position trended to shift to higher angle when Li was added with content up to mo%, indicating that Li+ ions gave the local compression strain when it filled at Na+ site The result can be understood based on the different ionic radii between Li+ (0.092 nm in 8-fold coordination) and Na+ (0.136 nm in 12-fold coordination) [27] Interestingly, the peaks position was shifted back to lower angle when Li+ concentration was higher than mol% This phenomenon was suggested that the Li+ ions can be also filled at the octahedral site when Li+ addition was higher a threshold value and consequently resulted in expansion the lattice constants because the ionic radius of Ti4+ (0.0605 nm in 6-fold coordination) was larger than that ionic radius of Li+ (0.076 nm in 6-fold coordination) [27] The effect of multisite Li+ was well known in both lead-based and lead-free piezoelectric material [25, 26, 28, 29] We recently obtained the effect of multisite Li+ occurred in BNKT-modified with Sn or Zr piezoelectric materials [25, 26] Fig 3(a)–(f) shows FE-SEM micrographs of the BNKTCZ–xLi ceramics with x =0.00, 0.01, 0.02, 0.03, 0.04 and 0.05, respectively A dense microstructure with some distinct pores is observed for the BNKTCZ ceramic and mol% Li-added BNKTCZ, as seen in Fig 3(a) and (b) The compact structures were obtained when Li+ ions further added, as shown in Fig 3(c)-(f) indicating that the samples exhibited dense and uniform grains We suggested that the effect of Li on the grain size resulted from liquid phase sintering process because of low melting point of Li2CO3 and/or promotion oxygen vacancies during sintering process [25, 26] The room temperature absorption spectra of the Li addition in BNKTCZ are shown in Fig 4(a) All of the specimens exhibited absorption in the visible light region The absorption spectra show a red shift slightly as the Li concentration increases, indicating that the Li+ ions addition modified the band gap of lead-free BNKTCZ piezoelectric specimen The Eg values was associated with the absorbance and photon energy by following equation αh ~ (h-Eg)n, where α is the absorbance coefficient, h the Planck constant,  the frequency, Eg the optical band gap and n a constant associated with different types of electronic transition (n=1/2, 2, 3/2 or for direct allowed, indirect allowed, direct forbidden and indirect forbidden transitions, respectively) [30] The Eg values of lead-free piezoelectric BNKTCZ–xLi ceramics were evaluated by extrapolating the linear portion of the curve or tail The band gap estimated with n=1/2 for direct transition as shown in Fig 4(b) The optical band gap decreased with increase in Li+ ions addition It decreased from 2.91 eV to 2.69 eV as Li+ ions concentration increased from to mol% This results were consisted with our recently reported for effect of Li-modified BNKT-based ceramics on the optical properties which Eg values decreased from 2.88 eV to 2.68 eV and 2.99 eV to 2.84 eV for Li-doped BNKT-modified with Zr and BNKT-modified with Sn, respectively [25, 26] Thus, we suggested that the reduction of band gap in Li-modified BNKTCZ ceramics is strongly related to the ionics bonding between A-site and oxygen and/or distorted structure Fig 5(a) shows the temperature dependence of the dielectric constant of lead-free BNKTCZ–xLi ceramics at frequencies of kHz Similar to the lead-free ferroelectric Bi0.5Na0.5TiO3 ceramics, the curves show two distinct anomalies for all samples which correspond to the depolarization temperature (Td) and maximum dielectric constant temperature (Tm), respectively [31] The curves for different samples look similar, all exhibiting two-phase transition at Td and Tm The two dielectric peaks can cause by the phase transitions from ferroelectric to antiferroelectric (Td) and antiferroelectric to paraelectric phase (Tm), which is consistent with the previous reports of lead-free BNKT-based ceramic [26, 32, 33] The variations in the values of Td and Tm with different amount of Li addition for BNKTCZ ceramics are presented in Fig (b) From Fig (b), it is found that both Td and Tm exhibited an obvious dependency on amount of Li+ cations dopants concentration However, the first phase transition temperature change in narrow range from 427 K to 405 K while the secondary phase transition temperature increased from 531 K to 607 K as Li content increased from to mol% Compared with BNKTCZ ceramics, the increase of Tm in Li-added BNKTCZ specimen as increasing the Li concentration is probably consequence of the enhancement of antiferroelectric phase stability by Li addition Dai et al proposed that the dielectric maximum around Tm is related to relaxation of tetragonal phase emerged from rhombohedral polar nanoregions [34] According to Zhu et al., the reversed dependence of Td and Tm on Li amount dopants concentration can be attributed to the lattice distortion [35] In addition, Zhou et al reported that the vacancies facilitate the movement of the ferroelectric domain and result in a decrease of depolarization temperature [36] Moreover, according to the theory of dielectric response of relaxor ferroelectric discovered by Thomas that the stability of ferroelectric domain decreases as the coupling reaction between A-site cation and BO6 octahedron decreases [32] Yang et al reported that the coupling reaction between A-site cation and BO6 octahedron is weakened and the Td moves to lower temperature region when A-site is vacancies [37] Therefore, we suggested that the decrease of depolarization temperature was strongly related with multisite occurrence of Li+ ions in BNKTCZ ceramics To better understanding the dielectric behavior, we used the modified Curie-Weiss law which is described as follows: m/=1+(T-Tm)/(22) where m is the peak value of the dielectric constant and Tm is the temperature at with  reaches the maximum,  is the degree of diffuseness, and  is peak broadening parameter that indicates the diffuseness degree [38, 39] When =1, the materials with this type of phase transition belongs to normal ferroelectrics; when 1