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CERAMICS
INTERNATIONAL
Ceramics International 41 (2015) S458–S463
www.elsevier.com/locate/ceramint
Comparison of structural, ferroelectric, and strain properties between A-site
donor and acceptor doped Bi1/2(Na0.82K0.18)1/2TiO3 ceramics
Thi Hinh Dinha, Mohammad Reza Bafandehb, Jin-Kyu Kanga, Chang-Hyo Hongc, Wook Joc,
Jae-Shin Leea,n
b
a
School of Materials Science and Engineering, University of Ulsan, Ulsan, Republic of Korea
Department of Materials Science and Engineering, Faculty of Engineering, University of Kashan, Kashan, Islamic Republic of Iran
c
School of Materials Science and Engineering, Ulsan National Institute of Science and Technology, Ulsan, Republic of Korea
Received 26 October 2014; accepted 14 March 2015
Available online 31 March 2015
Abstract
Effects of Li- and La-doping on the structural, ferroelectric, and strain properties of Bi1/2(Na0.82K0.18)1/2TiO3 (BNKT) ceramics were
compared. In this study, Li þ was selected as the A-site acceptor and La3 þ as the A-site donor. Li doping resulted in hardening of BNKT with an
improved mechanical quality factor (Qm) of 253 and an increased coercive field (Ec), while La doping brought about a softening effect that was
evidenced by an improved piezoelectric constant (d33) of 172 pC/N and decreased Ec. In addition, the large normalized bipolar strain (Smax/Emax)
was obtained up to 650 pm/V in 3 mol% La-doped BNKT.
& 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Keywords: B. Defects; C. Ferroelectric properties; C. Piezoelectric properties; Solid state reaction
1. Introduction
Recently, lead-free piezoelectric ceramics have been extensively
studied from both physical and technical points of view. Among
various lead-free materials, binary (Bi,Na)TiO3–(Bi,K)TiO3
(BNKT) solid solutions are considered as potential candidates
due to their excellent electromechanical properties near the
morphotropic phase boundary (MPB) [1–4]. A number of previous
studies have reported that electromechanical properties can be
improved by modification of the MPB composition with various
dopants or modifiers, such as BNKT–BiAlO3 [5], BNKT–LiSbO3
[6], Nb-doped BNKT [7], Ta-doped BNKT [8], Sn-doped BNKT
[9,10], Li- and Ta-codoped BNKT [11], BNKT–LaFeO3 [12], Ladoped BNKT [13], and Ta-doped BNKT–LiSbO3 [14].
It has been well known in Pb-based piezoelectric ceramics
that the hardening or softening through the addition of dopants
n
Correspondence to: University of Ulsan, P. O. Box 18, Nam-Ulsan,
680-749, Republic of Korea. Tel.: þ 82 52 259 2286; fax: þ82 52 259 1688.
E-mail address: jslee@ulsan.ac.kr (J.-S. Lee).
http://dx.doi.org/10.1016/j.ceramint.2015.03.150
0272-8842/& 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
is a key technique to tailor their properties [15–17]. The best
known examples are hard and soft Pb(Zr,Ti)O3 (PZT) ceramics, the most widely used piezoelectric materials. Hardening
can be induced by the addition of acceptor dopants [18]. A
better understanding of the hardening and softening in Pbbased ceramics has been presented by Chandrasekaran et al.
[19]. Oxygen vacancies in acceptor (Fe) doped PbTiO3 (PT)
are responsible for the formation of polar defect complexes.
Both of the “bulk effects” and the “domain-wall effects”
contribute similarly to the hardening phenomenon [19], while
donor (Nb) doping increased domain mobilities, resulting from
the lack of polar defect complexes. The effects of donor
(Nb5 þ , Ta5 þ ) doping on the electromechanical properties of
lead-free Bi0.5Na0.5TiO3 ceramics have been also reported
[20]. In 2012, Han et al. [21] also compared the effects of Bsite acceptor and B-site donor doping and they found that
donor-doping contributed to the destabilization of ferroelectric phases. More recently, the electromechanical properties of
Mn- or Fe-doped BNT–BKT–Bi0.5Li0.5TiO3 piezoelectric
ceramics were examined by Taghaddos et al. [22] in 2014. It
T.H. Dinh et al. / Ceramics International 41 (2015) S458–S463
was found that the acceptor dopants can slightly decrease the
optimum sintering temperature and enhance the mechanical
quality factor considerably.
Therefore, there is a great attraction from both scientific and
technical points of view to investigate the effects of A-site
acceptor as well as A-site donor on the piezoelectric properties
of lead-free BNKT ceramics. In BNKT system (near MPB),
due to difference between the ionic radius of doping elements
and lattice ions, there are limited acceptor ions that can occupy
the A-site ( e.g. Li þ , Rb þ , Cs þ ) or B-site ( e.g. Fe3 þ , Mn3 þ ,
Al3 þ ) and restricted donor ions that can substitute into A-site
(e.g. In3 þ , Tl3 þ , La3 þ ) or B-site ( e.g. Ta5 þ , Nb5 þ , Sb5 þ ).
This work compared the crystal structure, ferroelectric, and
electromechanical properties of Li- and La-doped BNKT
ceramics. It is expected to see a hardening effect in Li-doped
BNKT ceramics due to creation of oxygen vacancies when
Li þ ions occupy the A2 þ -sites. In contrast, a softening effect
is presumed in La-doped BNKT ceramics due to generation of
A-site vacancies when La3 þ ions substitute doubly charged
A2 þ -site ions. The oxygen vacancies or A-site vacancies
created in the lattice are compensated by the incorporated
acceptor or donor ions, respectively. This can be understood
by considering the Schottky defect reaction
0
LiA -LiA þ 1=2VO
ð1Þ
LaA -LaA þ 1=2V″A
ð2Þ
where VO and VA indicate oxygen and A-site vacancies,
respectively.
2. Experiments
Ceramic specimens with composition of [Bi1/2(Na0.82K0.18)1/
]
2 1 xAxTiO3 (A ¼ Li or La; x ¼ 0.00–0.05), abbreviated as
A100x, were synthesized using a conventional solid state
reaction route. Reagent grade Bi2O3, Na2CO3, K2CO3, TiO2,
Li2CO3, and La2O3 (99.9%, High Purity Chemicals, Japan)
powders were used as raw materials. These reagents were dried
S459
in an oven at 100 1C for 24 h and then weighed according to
the formula. The powders were mixed in ethanol with zirconia
balls by ball milling for 24 h, dried at 80 1C for 24 h, and
calcined at 850 1C for 2 h in an alumina crucible. After
calcination, the powder was mixed with polyvinyl alcohol as
a binder and then pressed into green discs with a diameter of
12 mm under a uniaxial pressure of 98 MPa. Green pellets of
Li100x and La100x were sintered at 1100 1C and 1175 1C,
respectively, in covered alumina crucibles for 2 h in air with a
heating rate of 5 1C/min.
For electrical measurements, a silver paste was screenprinted on both sides of each specimen, fired at 700 1C for
30 min, and then poled in silicone oil bath at 80 1C under a
direct electric field of 50 kV/cm for 15 min. The crystal
structures of the poled and unpoled samples were analyzed
with an X-ray diffractometer (XRD, RAD III, Rigaku, Japan)
using Cu Kα radiation. The relative density of the fired
specimen was determined by the Archimedes method. A
field-emission scanning electron microscope (FE-SEM, JEOL,
JSM-65OFF, Japan) was used to examine the surface morphology of samples. The electrical polarization (P) and
electromechanical strain (S) as a function of external electric
field (E) were recorded at 0.3 Hz with a 15 mF measurement
capacitance using a Sawyer-Tower circuit equipped with an
optical sensor (Philtec, Inc., Annapolis, MD, USA). The
piezoelectric constant d33 was measured using a Berlincourt
d33-meter after poling samples under a direct electric field of
50 kV/cm for 15 min in silicone oil kept at 80 1C. The planar
piezoelectric coupling coefficient (kp) and electromechanical
quality factor (Qm) were characterized by the resonanceantiresonance method.
3. Results and discussion
Field-emission scanning electron microscopic investigations
were carried out to study grain size and grain morphology of
Li100x and La100x, and the results are shown in Fig. 1. All
Fig. 1. Comparison of scanning electron micrographs between Li- and La-modified BNKT ceramics with different dopant levels.
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T.H. Dinh et al. / Ceramics International 41 (2015) S458–S463
samples show a similar grain morphology and dense microstructures like previous reports on modified BNKT ceramics
[7,8,9,23,24]. As a function of doping level, the grain size of
both Li100x and La100x ceramics were increased slightly. The
increase in grain size of KNN–LiTaO3 [25] and BNT–KNN [26]
by added Li þ were also presented. The increased sinterability was
also observed in BNKT ceramics doped with Zr [23], Hf [24], Nb
[7], and co-doped with Li and Ta [11]. In addition, the lower
sintering temperature for Li-doped BNKT was also reported on
(Na1 x yKxLiy)0.5Bi0.5TiO3 [27], which was sintered at 1040–
1120 1C in air. The acceptor dopants were known to enhance
interdiffusions during sintering via generation of O-vacancies [22].
XRD patterns of all sintered specimens in the 2θ range of
39–481 are presented in Fig. 2. A single phase perovskite
structure without any traces of secondary phases was observed
for all compositions. It was reported [5–13] that the difference
between rhombohedral and tetragonal structures in the perovskite materials can be easily distinguished by the {111} and
{200} profiles of XRD patterns.
As observed in Fig. 2(a) and (b), the addition of either Li or
La induced a significant change in the crystal structure. With
increasing x, Li100x and La100x revealed a phase transition
from the coexistence of rhombohedral and tetragonal to
pseudocubic phases, which might have been arisen from the
size difference between the substituent and lattice ions. The
occupation of Li þ and La3 þ ions at A-site tends to shrink the
lattice owing to the formation of oxygen and A-site vacancies
and a smaller ionic size of Li þ and La3 þ than that of K þ and
Na þ (ionic radii: 1.39, 1.64, 1.36, and 0.92 A1 for Na þ , K þ ,
La3 þ , and Li þ ; CN ¼ 12 [28]). The effects of Li and La
doping on the crystal structure of BNKT ceramics show
similar tendency to that of Zr [23], Nb [4], and Sn [9] doping.
In addition, the systematic XRD investigation of unpoled
(Fig. 2(a)) and poled (Fig. 2(c)) Li100x samples was carried
out. The phase transition from mixture of rhombohedral and
tetragonal phases to a pseudocubic phase occurred for unpoled
specimens at x around 0.03 while for poled samples with x
higher than 0.02, rhombohedral phase was observed. It is
suggested that during the poling process as well as under
applying electric field, the pseudocubic phase of Li100x
ceramics with x higher than 0.02 was not stable and transformed to rhombohedral phase. This behavior is in good
agreement with the lack of negative strain in bipolar electric
field induced strain curves as well as negligible remnant
polarization in the P–E hysteresis loops, which are
discussed below.
Polarization versus electric field hysteresis loops of Li- and
La-doped BNKT were measured at a frequency of 0.3 Hz at
room temperature and the results are shown in Fig. 3. A
saturated square hysteresis loop was observed for ceramics
with low dopant levels, indicating a relatively large remnant
polarization (Pr) as well as coercive field (Ec). In case of Li
doping, the saturated P–E loop was maintained up to x ¼ 0.05
while La doping brought about gradual slimming in the P–E
loop with increasing doping level.
The effect of dopant level on coercive field, remnant
polarization, piezoelectric constant, and mechanical quality
factor of BNKT-based ceramics are compared in Fig. 4 for Li
and La doped specimens. Li doping seems to induce hardening
of BNKT by generation of O-vacancies to maintain charge
neutrality in the lattice [29]. It has been known that oxygen
vacancies is one of the main reasons of reducing the domainwall motion [22,26,30,31], resulting in higher Ec (Fig. 4(a)) of
Li100x ceramics. On the other hand, the drastic decreases in Pr
(Fig. 4(b)) and Ec (Fig. 4(a)) of La-doped BNKT ceramics
could be attributed to ferroelectric-relaxor phase transition,
which was olso observed in BNKT doped ceramics doped with
Zr, Hf, Nb, Sn, and Ta [7,8,9,23,24]. The highest d33 value
(172 pC/N) was observed in La2 and an improved Qm (253)
was measured for Li2, as presented in Fig. 4(c) and (d). These
Fig. 2. XRD patterns as a function of doping level: (a) Li100x, (b) La100x, and (c) Poled-Li100x; x¼ 0–0.05.
T.H. Dinh et al. / Ceramics International 41 (2015) S458–S463
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Fig. 3. Comparison of Li- and La-doping effects on the P–E hysteresis loop of BNKT ceramics.
Fig. 4. Effect of doping level on (a) the coercive field (Ec), (b) remnant polarization (Pr), (c) piezoelectric constant (d33), and (d) mechanical quality factor (Qm) of
Li- and La-doped BNKT ceramics.
Fig. 5. Effect of dopant content on the bipolar S–E hysteresis loops of BNKT ceramics.
observations can be explained based on the hardening and
softening effect of acceptor and donor doping on BNKT-based
ceramics, respectively [22,29].
Fig. 5 shows bipolar electric-field-induced strain loops of Liand La-doped BNKT ceramics. An unmodified BNKT exhibited a
butterfly-shaped strain curve, which has been typically observed in
ferroelectric materials. With increasing the dopant content, the
negative strain, Sneg of Li100x ceramics maintained almost
constant, however, that of La100x specimens decreased. From
the S–E loops, both Sneg and maximum bipolar strain/electric field
(d n33 ) as a function of dopant content were determined and plotted
in Fig. 6.
A significant difference between two different dopants can be
seen in composition dependence of negative strain, Sneg (Fig. 6(a))
and converse piezoelectric constant dn33 (Fig. 6(b)). The existence
of considerable Sneg at both þ Ec and –Ec implies that there are
ferroelectric domains whose orientations reversed when the
external field is reversed. It is seen in Fig. 6 that La-doping leads
to an abrupt drop in the Sneg at x¼ 0.03, while Sneg is insensitive to
doping level for Li-doped specimens. This observation is also
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T.H. Dinh et al. / Ceramics International 41 (2015) S458–S463
Table 1
Ferroelectric and electromechanical properties of Li100x and La100x ceramics;
x¼ 0–0.05.
Composition
Pr
(μC/cm2)
Ec
(kV/cm)
d33
(pC/N)
Qm
kp
(%)
Smax/Emax
(pm/V)
Li1
Li2
Li3
Li4
Li5
La0
La1
La2
La3
La4
La5
22
20
21
19
19
19
20
17
2
1
0.5
33
34
35
36
36
36
33
21
7
3
0.5
114
112
110
103
105
130
156
172
5
2
2
214
253
208
172
166
165
110
51
–
–
–
31
38
34
33
33
31
31
29
–
–
–
216
233
250
216
233
250
250
300
650
400
216
Fig. 6. Variation of (a) negative strains (Sneg) and (b) converse piezoelectric
constant (d33) as a function of A-site doping levels.
Acknowledgments
consistent with the results observed in the P–E loops in Fig. 3, and
also supports the fact that La-doping induces the ferroelectricrelaxor phases transition in BNKT. It should be pointed out that a
large strain enhancement is observed when Sneg abruptly decreases
when 3 mol% La is doped, where the highest Smax/Emax of
650 pm/V is also attained as can be seen in Fig. 6(b) [32]. Similar
phenomena were also observed in BNKT ceramics doped/modified
with Zr [23], Hf [24], Nb [7], Sn [9], Ta [8], LiTaO3 [11], SrTiO3
[33], (K0.5Na0.5)NbO3 [34], and Sr(K1/4Nb3/4)O3 [35].
The various ferroelectric and electromechanical properties of
Li100x and La100x ceramics with x¼ 0.00–0.05 are summarized
in Table 1. It should be noted that the highest Qm and kp values
were measured for Li2, which were 253 and 38%, respectively.
The bipolar Smax/Emax (dn33 ) reached 650 pm/V at 60 kV/cm in La3
and the highest d33 was measured as 172 pC/N for La2.
4. Conclusions
This work systematically compared the effects of Li- and
La-doping on the crystal structure, microstructure and electromechanical properties of lead-free BNKT ceramics. Two
dopants showed definite differences in terms of ferroelectric
and strain properties. Li doping resulted in hardening of BNKT
that was evidenced by the increased ferroelectricity as well as
mechanical quality factor. On the contrary, La doping induced
a ferroelectric-relaxor phase transition when 3 mol% La was
doped, where an abnormal increase in the strain was observed
like previous reports on B-site donor doped BNKT. The
present work is believed to be meaningful in view of
experimental demonstration of definite differences in the
effects between A-site acceptor and donor doping on the
material properties of Bi-based perovskite ceramics.
Conflict of interest
We declare that we do not have any commercial or
associative interest that represents a conflict of interest in
connection with the work submitted.
This work was supported by the National Research Foundation of Korea (NRF) Grant (2013R1A1A2058917). M.R.
Bafandeh thanks the financial support of the University of
Kashan.
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