Effect of Li2CO3 addition on the structural, optical, ferroelectric, and electric field induced strain of lead free BNKT based ceramics

Author’s Accepted ManuscriptEffect of Li2CO3 addition on the structural, optical, ferroelectric, and electric-field-induced strain of lead-free BNKT-based ceramics Nguyen Van Quyet, Luon

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

PII: S0022-3697(15)00125-0

DOI: http://dx.doi.org/10.1016/j.jpcs.2015.05.010

Reference: 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 electric- field-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

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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,*

1 School of Materials Science and Engineering, University of Ulsan, Ulsan 680-749,

Republic of Korea

2 School of Engineering Physics, Ha Noi University of Science and Technology,

1 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 5 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 5 mol% Li- addition BNKT ceramics, respectively The

maximum Smax/Emax value was 567 pm/V at 2 mol% Li2CO3 access We expected this

work will help to understand the role of A-site dopant in lead-free ferroelectric BNKT

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

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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 3 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/Emaxvalues 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 5 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

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0.970[Bi0.5(Na0.78K0.22)0.5TiO3]-0.030BiAlO3at 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

3 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

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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 2 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 5 mol %

Li-added The maximum polarization decreased from 49.66 C/cm2 to 27.11 C/cm2 as

increasing the Li concentration from 0 to 5 mol% The Smax/Emax value was 567 pm/V

with 5 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

2 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

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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 1 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 1 in the range 2 from 39.0 to 41.0 and 44.0 to 48.0 are shown in Fig 2 (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 3 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 3 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

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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 1 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 3 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 0 to 5 mol% This results were consisted with our recently reported for effect of Li-modified BNKT-based ceramics on the optical

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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 1 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 5 (b) From Fig 5 (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 0 to 5 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

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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<<2, the materials belongs to relaxor ferroelectrics; and when =2, the materials belongs to ideal relaxor ferroelectric Fig 6 (a)-(f) shows ln[(m-)/] as a function of ln(T-Tm) for BNKTCZ-xLi ceramics with x=0.00, 0.01, 0.02, 0.03, 0.04 and 0.05, respectively, at 1 kHz The

results indicated that a linear relationship is observer in all un-doped and Li-added BNKTCZ ceramics The  and  values were extracted from Fig 6 by linear fitting The values of  and  as function of Li-dopant BNKTCZ ceramics were shown in Fig

7 The  values were in range from 1.79 to 2.00 indicating that our samples exhibited relaxor ferroelectric feature The  values were 1.86 for Li-undoped

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BNKTCZ specimen, and it increased close to 2.00 for 3 mol% Li addition However, the  values trend to decrease to 1.79 for 5 mol% Li addition A similar trend has been found for  values that it increased from 108.5 K to 254.6 K for un-doped and 2 mol% Li addition and then decreased to 94.4 K for 5 mol% Li access Santos et al proposed that the higher  values displayed a higher diffusivity during study in comparing the dielectric properties of Sr0.61Ba0.39Nb2O6 and PbMg1/2Nb2/3O6 ceramics [39]

The dielectric constant and dielectric loss as function of temperature with

various frequencies were shown in Fig 8(a)-(f) for x = 0-0.05, respectively The

dielectric curves of Li-addition lead-free piezoelectric BNKTCZ ceramics

exhibit broad transition peaks around Td and Tm , which shows the characteristics of diffuse phase transition In addition, frequency dispersion

around Td and Tm indicate that Li-addition lead-free piezoelectric BNKTCZ samples exhibit relaxor characteristics Our result was in agreement with report of Samara for the relaxational properties of compositionally disordered

ABO3 perovskites [40] Setter et al reported that diffused phase transition of

Pb(Sc 0.5 Ta 0.5 )O 3 ceramics system is considered due to random distribution of

Sc 3+ and Ta 5+ cations at B-site [41] Rahman et al reported that the observed

frequency dependent of the BaZrO 3 -modified 0.935Bi 0.5 Na 0.5 TiO 3 –0.065BaTiO 3

ceramics dispersed diffuseness at Tm could be attributed to the disordering of

A- as well as B-sites cations [42] Zheng et al reported that the observation the

frequency dispersion is considered due to random distribution of La 3+ and

Ba 2+ at A-site in La-modified Bi0.5 Na 0.5 TiO 3 -BaTiO 3 ceramics [43] Yang et al

reported that the coexistence of Na + , K + , Li + and Bi 3+ disordering at A-site

causes the reason for observation of the relation behavior [44] This is in

temperature indicated that all specimens were typical ferroelectric materials The

effective of Li concentration dopants to maximum polarization (Pm), remnant polarization (Pr) and coercive field (EC) showed in Fig 9 (b) The both Pm and Pr

values were found to be decease from 49.7 C/cm2 to 27.1 C/cm2, and 22.9

C/cm2 to 5.4 C/cm2, respectively, as the Li amount increased from 0 to 5 mol% In

addition, the HC values were also obtained to significantly decrease from 1.7 to 0.9 kV/mm within the corresponding composition range Recently, the reduction of Pm and Pr values were widely reported in BNKT-modified with Zr, Y, Nb etc cause of

phase transition from polar to non-polar via dopants and/or promotion via oxygen vacancies [4-10] As we mentioned above that the origin of phase transitions were still debated However, our results indicated that no phase transition existed There were instated by distorted structure when Li added BNKTCZ ceramics Thomas et al

reported that the manifestation of ferroelectric properties in ABO3-perovskite were strongly relative with the BO6 oxygen octahedral [32] Therefore, the degree of the coupling between neighboring BO6 octahedral will be significantly weakened by

introducing defects that acts to break the translational invariance of polarization,

result in a decrease in the coupling of ferroelectrically active BO6 octahedral [32, 34,

45, 46] Grinberg et al predicted that the B-cation displacement was highest in case

of Ti which were expected for strong ferroelectric activation [47] Moreover, Yuan et

al reported that both A-site and B-site vacancies decreased the degree of coupling between dipoles in ABO3-type perovskite resulting in reduced the coupling of

Ti4+-site possible and resulted in rapidly decreasing both Pm and Pr values This

results were consisted with our recently reported for Li-modified BNKT-based materials [26]

Fig 10 (a) shows the bipolar electric-field-induced strain curves of the BNKTCZ-xLi ceramics All the ceramics showed butterfly-shaped curves that are distinct features

of ferroelectric materials The effect of Li-doped BNKTCZ on the maximum strain

(Smax) and negative strain (Sneg) values as a function of Li addition showed in Fig 10 (b) The Li-undoped BNKTCZ exhibited a butterfly-shaped curve with a Smax of 0.21% and Sneg of 0.15% Noted that the negative strain denoted the difference

between zero field strain and the lowest strain [13] The Li-added BNKTCZ displayed

increased Smax values up to 0.34% at 2 mol% then decreased to 0.15% at 5 mol%

In addition, the Sneg values slightly increased to 0.17% at 1 mol% then gradually

decreased to 0.004% which corresponded to 5 mol% Li-added BNKTCZ The

estimation of Smax/Emax values was 350 pm/V and 567 pm/V for undoped and 2 mol%

Li-added BNKTCZ ceramics specimen, respectively The results were solid evident for enhancement of electrical induced strain in lead-free BNKTCZ via Li-addition

The maximum Smax/Emax value was 567 pm/V which was higher than that of pure

BNKT ceramics with Smax/Emax around 230 pm/V or only CZ-modified BNKT with

Smax/Emax value around 500 pm/V as reported by Hong et al [17, 49] Our observation was consisted with recently reported for Li-doped BNKT-modified with Zr

[26] Moreover, the observation maximum Smax/Emax values were presented in

12

comparison with previously reported for lead-free piezoelectric BNKT-based materials, as shown in Fig 11 Our results indicated that Li-modified BNKTCZ

ceramics resulted in enhancement of Smax/Emax values which overcome the pure

BNKT [49] or Cu-[50], Y-[6], Zn-[51], Hf-[7] and (Cu,Nb)-[49] modified BNKT

ceramics However, the Smax/Emax values were smaller than that single element

Zr-[8], Sn-[19], La-[4], Nb-[52], Ta-[5], or co-doped-(Li,Sn)-[10], (Li,Ta)-[9] modified BNKT ceramics materials

VI Conclusion

The distorted structures of the BNKTCZ ceramics samples were happened when Li amount was added up to 5 mol% The multisite Li ions occurred was observation The band gap values decreased with increasing Li concentration The both maximum polarization and remanent polarization values was reduced from 49.66

C/cm2 to 27.11 C/cm2 and from 22.93 C/cm2 to 5.35 C/cm2 respectively as Li

concentration increased from 0 to 5 mol% The maximum Smax/Emax value was

enhancement up to 567 pm/V We expected this work will help to understand the role

of A-site dopant in lead-free ferroelectric BNKT materials

Acknowledgments

This work was financially supported by the Ministry of Education and Training, Vietnam, under project number B 2013.01.55

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