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Lithium-Induced Phase Transitions in Lead-Free Bi0.5Na0.5TiO3-Based Ceramics
Giuseppe Viola1,3, Ruth McKinnon1, Vladimir Koval2, Arturas Adomkevicius1, Steve Dunn1
and Haixue Yan*1,3
1
School of Engineering and Materials Science, Queen Mary University of London, 380 Mile
End Road, London E1 4NS, UK
2
Institute of Materials Research, Slovak Academy of Sciences, Watsonova 47043 53 Kosice,
Slovak Republic
3
Nanoforce Technology Ltd, 380 Mile End Road, London E1 4NS, UK
ABSTRACT: Lithium substituted 0.95[0.94(Bi0.5Na(0.5-x)Lix)TiO3-0.06BaTiO3]-0.05CaTiO3
materials include the polar rhombohedral R3c and weakly polar tetragonal P4bm phases. On
increasing lithium content, the (R3c/P4bm) phase ratio decreased, while the rhombohedral
and tetragonal lattice distortions remained the same. The temperature corresponding to the
shoulder in the dielectric permittivity shows no clear shift with respect to lithium substitution,
due to the rhombohedral distortion remaining constant. Electrical poling produced an increase
of the rhombohedral phase fraction, together with a rise of the rhombohedral and tetragonal
distortion. This confirmed the occurrence of a phase transition from the weakly polar to the
polar phase during electrical poling. Four peaks found in the current-electric field (I-E) loops
are related to reversible electric field-induced transitions. By studying the temperature
dependence of the current peaks in the I-E loops, it was found that the minimum temperature
where these electric field-induced transitions take place decreases with increasing lithium
substitution.
KEYWORDS: ferroelectrics, polarization, dielectrics, bismuth sodium titanate.
1
1. INTRODUCTION
Bismuth-based perovskites have attracted worldwide research interest over recent years for
the development of lead-free ferroelectric materials1-3 in a range of technological
applications, including piezoelectric actuators and capacitors because of their large electric
field-induced strain4 and antiferroelectric-like polarization-electric field (P-E) loops.5,6 Many
different compositional modifications of the basic Bi0.5Na0.5TiO3 (BNT) have been studied
and significant progress on how to obtain desired properties by compositional variation have
been made. For example several solid solutions based on two end members have been
investigated including Bi0.5Na0.5TiO3-BaTiO3 (BNT-BT)7, Bi0.5Na0.5TiO3-Bi0.5K0.5TiO3 (BNTBKT)8 and Bi0.5Na0.5TiO3-K0.5Na0.5NbO3 (BNT-KNN)9, where the presence of morphotropic
phase boundaries have been found, which generally determine an enhancement of dielectric
and piezoelectric properties at the corresponding compositions. Further elemental additions
led to the development of more complex solid solutions, with Bi0.5Na0.5TiO3-BaTiO3K0.5Na0.5NbO3 (BNT-BT-KNN),10-12 Bi0.5Na0.5TiO3-BaTiO3-Bi0.5K0.5TiO3 (BNT-BT-BKT)13,14
and Bi0.5Na0.5TiO3-BaTiO3-CaTiO3 (BNT-BT-CT)3 as indicative examples of three-end
member systems. The latter are becoming increasingly interesting because they offer more
degrees of freedom to obtain desired properties that cannot be achieved from two-end
member systems.
Bismuth sodium titanate was considered to be rhombohedral at room temperature,15-17
although more recent structural refinements suggest monoclinic symmetry Cc.18,19 However,
the local and average structures are still under debate.20-22 In BNT-based systems,
temperature variations and/or the application of an electric field induce modification of
crystal structures and properties.
Characteristic temperatures can be identified from the temperature dependence of dielectric
permittivity and loss. The temperature Tm indicates the temperature corresponding to the
2
maximum dielectric constant, and Ts corresponds to a visible shoulder in the dielectric
permittivity, above which the frequency dispersion significantly reduces. Based on early
TEM investigations Ts does not correspond to any structural transition,17 although more
recent studies suggested that Ts could be related to the appearance of a non-ferroelectric
phase with antiphase tilting.20
As described earlier, the structure of BNT-based materials is sensitive to the application of
an electric field. According to in-situ TEM studies on 0.91BNT-0.06BT-0.03KNN the
reversible appearance of ferroelectric domains can be observed by applying an alternating
electric field,23 which is linked to the observed antiferroelectric-like P-E loops. The
temperature range within which these reversible electric field-induced transitions take place
is dependent on composition. The widening of this temperature range could be beneficial for
energy storage applications.5 For this purpose, in this paper, lithium (Li) was used to
substitute sodium (Na) in Bi0.5Na0.5TiO3-BaTiO3-CaTiO3. Lithium is known to partially
substitute Na in BNT to form a solid solution,24 with a promotion of a weakly polar
tetragonal phase,25,26 which could result in a reduction of the temperature where the electric
field-induced transitions from weakly polar- to-polar phase start to take place, with the
presence of antiferroelectric-like loops. In BNT-based materials, the antiferroelectric-like
loops were initially attributed to phase transitions between antiferroelectric and ferroelectric
structures. However, there is no direct evidence to support antiferroelectric order using
diffractions methods including XRD and TEM.15-18 The main objective of this work is to
clarify the relationship between current peaks, antiferroelectric-like P-E loops and electric
field-induced phase transitions in BNT-based materials.
2. EXPERIMENTAL SECTION
3
Ceramic powders were prepared by a solid state reaction process. Oxides and carbonates of
the raw materials Bi2O3 (99.9% Sigma-Aldrich), TiO2 (99.8% Sigma-Aldrich), Na2CO3
(99.5% Alfa Aesar), BaCO3 (99.8% Alfa Aesar), CaCO3 (99.5% Alfa Aesar) and Li2CO3
(99.0
%
Alfa
Aesar)
were
weighed
according to
the
stoichiometric
formula
0.95[0.94(Bi0.5Na(0.5-x)Lix)TiO3-0.06BaTiO3]-0.05CaTiO3 (x = 0, 0.05 and 0.15). The mixture
was ball milled for 4h in nylon pots using ethanol and zirconia balls. The slurry was then
dried in air, calcined at 850°C for 4h and ball milled again for 24 h to homogenise the particle
size. After drying, the obtained powder was sieved through a 250 μm mesh. A cold-press was
used to compact pellets which were then sintered at the optimised temperature of 1150˚C for
4h in a conventional furnace.
The density of the pellets was measured using the Archimedes’ method in de-ionised water.
Room temperature X-ray diffraction (XRD) was performed on calcined powders and
ceramics, using a Siemens D5000 diffractometer (Siemens AG, Karlsruhe, Germany)
operating at 40 kV and 30 mA with Cu Kα radiation. Data from the X-ray diffractometer of
powders were analyzed by the Rietveld method27 using the FullProf program28. Silver paste
(Gwent Electronic Materials Ltd., C2011004D5, Pontypool, U.K.) was homogenously
brushed and fired at 600°C for 10min to obtained smooth electrodes for electrical
characterisation. Impedance spectroscopy (frequency range 100Hz-1MHz) was performed at
room temperature using an impedance analyser (Agilent 4294A, Hyogo, Japan). The
temperature dependence of the dielectric constant and loss was measured from room
temperature up to 600°C, by applying an alternating voltage of 1V at five different
frequencies in the range 1kHz-1MHz, using an LCR meter (Agilent, 4284A, Hyogo, Japan)
connected to a tube furnace with controlled temperature. The relative dielectric permittivity r
was calculated as r = Cd/(A0); the dielectric loss tan was obtained as tan = R/Xc and the
capacitance C was calculated as C = 1/(2fXc); where d, A, 0, f, R and Xc is the sample
4
thickness, the electroded area of the sample, the vacuum permittivity, the measuring
frequency, and the real and imaginary part of the electrical impedance, respectively. Currentpolarization-electric field (I-P-E) hysteresis loops were measured using a hysteresis tester
(NPL, Teddington, U.K.) in a silicone oil bath, at different temperatures in the range 25°C150°C using triangular voltage waveforms29 at a frequency of 10Hz.
3. RESULTS AND DISCUSSION
The relative density of the ceramics were estimated as ρ = 97.2, 97.7 and 96.8% for x = 0,
0.05 and 0.15, respectively. The room-temperature XRD patterns of powders (Fig.1) and
ceramics (Fig.2) show perovskite single-phase structure, within the limit of XRD accuracy,
for all the investigated compositions. The effect of poling on the crystal structure was
investigated by comparing the XRD patterns of the as-sintered and poled ceramics. A
thorough analysis of the diffraction spectra revealed that the observed XRD profiles
corresponded to a superposition of two main structural components. For x = 0 (Fig. 1a), the
dominant spectral contribution (93%) is attributed to the presence of a rhombohedral phase
(space group R3c) with unit cell parameters ah  5.491 Å and ch  13.489 Å in the hexagonal
representation. The reflections of the minor phase ( 7%) were indexed in the tetragonal
P4bm system with the lattice constants at  5.534 Å and ct  3.889 Å. It is well known that
the R3c structure is polar, with parallel displacements of the A- and B-site cations along the
[111] direction, while the tetragonal P4bm is weakly polar.15 The term “weakly polar”, used
to describe the tetragonal phase, represents a small, non-zero dipole moment, typical of a
weakly polar ferrielectric order, which is locally produced by the near-equal displacement of
the A- and B-site cations in opposite direction along the polar axis.15 The Rietveld refinement
of the XRD patterns for the compounds (x = 0, 0.05, 0.15) revealed that the P4bm structure
5
Figure 1. Room-temperature XRD patterns for 0.95[0.94(Bi0.5Na(0.5-x)Lix)TiO3-0.06BaTiO3]0.05CaTiO3 powders: (a) x = 0, (b) x = 0.05, and (c) x = 0.15. The black circles are
experimental data, the red line is the calculated profile from Rietveld refinement and the blue
line is the difference profile between the observed and calculated diffraction patterns. The
allowed Bragg reflections for the corresponding space groups in a proposed structural model
(R3c+P4bm) are indicated by green ticks. The insets illustrate the enlarged views of
diffractograms in selected 2θ ranges to demonstrate the deconvolution of Bragg peaks.
6
transforms gradually from a minor phase into a dominating one with increasing Li amount,
suggesting a composition driven polar-to-weakly polar phase
Figure 2. XRD of unpoled and poled ceramics with x = 0, 0.05 and 0.15.
transition in the Li-substituted BNT-BT-CT compounds. Similar substitution-driven
structural transitions from the polar R3c to antipolar and/or nonpolar phases have recently
been reported, for example, in rare earth modified BiFeO3 multiferroics.30,31 The coexistence
of the rhombohedral (43%) and tetragonal (57%) phases was observed for the x = 0.15
compounds (Fig. 1c). The chemical substitution driven tilting of Ti – O octahedron associated
with the rhombohedral-to-tetragonal phase transition reduces the number of equivalent
7
positions of the Bi3+/Na+/Ba2+/Ca2+/Li+ cations in the A-site and Ti4+ cations at the B-site of
the perovskite from 6 to 2 owing to a remarkable difference in ionic radius of substituting Li+
ion ( 1.24 Å)32 and substituted Bi3+/Na+/Ba2+/Ca2+ ions ( 1.34 Å – 1.61 Å).32,33 The
tetragonal P4bm structure stabilizes Ti4+ ions at the centre of its octahedron (reduced offcentering), resulting in a gradual transformation of the polar R3c phase into a weakly polar
state upon lithium addition. The refined lattice parameters are listed in Table 1.
Table 1 Calculated lattice parameters of 0.95[0.94(Bi0.5Na(0.5-x)Lix)TiO3-0.06BaTiO3]-0.05CaTiO3
compounds (x = 0, 0.05 and 0.15) by refinement of room-temperature XRD data.
Sample
Phase fraction
a (Å)
c (Å)
c/a
αr (deg.)
93
5.491(0)
13.489(1)
2.46(1)
59.87
P4bm 7
5.534(1)
3.889(1)
0.70(1)
R3c
86
5.497(0)
13.492(2)
2.45(1)
P4bm 14
5.504(2)
3.968(4)
0.72(2)
R3c
43
5.494(0)
13.504(1)
2.46(1)
P4bm 57
5.502(0)
3.911(1)
0.71(1)
(weight %)
x=0
x = 0.05
x = 0.15
Cell
R3c
59.91
59.85
In general, the substitution of Na with Li in BNT-BT-CT solid solutions leads to shrinkage of
the tetragonal unit cell. However, lattice distortions remained almost constant upon
substitution: ct/at  0.71 for the tetragonal phase and ch/ah  2.46 and αr59.87º for the
rhombohedral phase. The XRD peaks of BNT-based ceramics near 2θ=40° (Fig. 2) show
significant variations after poling. Both rhombohedral and tetragonal distortions increased, as
evidenced by the XRD peaks shift to the lower angle side. In addition, according to the
fitting, the fraction of rhombohedral phase increased after poling.
8
Figure 3 shows the frequency dependence of the dielectric permittivity and loss of three
unpoled ceramics in the range 100Hz-1MHz. The dielectric constants of the three materials
Figure 3. Frequency dependence of the dielectric permittivity and loss highlighting the
composition-induced structural transitions between the rhombohedral phase and tetragonal
phase.
decreased with increasing frequency, which can be attributed to the fact that at higher
frequencies fewer dipoles can follow the applied alternating electric field. The dielectric
permittivity first increases and then decreases with increasing Li concentration for the entire
range of frequency studied, while the loss shows similar values (Fig.3). The fraction of
rhombohedral phase was estimated as 93%, 86% and 43% in ceramics x = 0, 0.05, and 0.15,
respectively. The maximum permittivity value observed in x = 0.05 is attributed to the most
favorable coexistence of polar and weakly polar phases (among the studied compositions),
with the polar phase being dominant. The ferroelectric domains in this ceramic easily vibrate
9
under the AC field as the connections between the domains are weak and these are thought to
be surrounded by a weakly polar tetragonal phase. The lower permittivity of ceramic x = 0.15
compared to x = 0.05 can be related to the fact that the main phase is tetragonal P4bm which
is weakly polar.
Figure 4 shows the temperature dependence of permittivity and loss of the three
compositions from room temperature to 600°C, at five different frequencies in the range
Figure 4. Temperature dependence of dielectric permittivity and loss for (a) x = 0, (b) x = 0.05
and (c) x = 0.15 and highlights composition-induced phase transitions influence on the
temperature dependence for the materials investigated.
1kHz-1MHz. There is no clear shift of the broad permittivity peak at Ts for all three ceramics
which is probably due to the fact that all three materials investigated have a similar distortion
in the rhombohedral polar phase (Table 1). It was previously proposed that the temperature
dependence of the dielectric permittivity in BNT-based materials reflects a cross-over
between two dielectric relaxation processes with increasing temperature, which involve polar
nano-regions of different phase and having different temperature evolution.34 The frequency
dispersive anomaly at Ts can be associated with the relaxation of the polar nanoregion of the
10
rhombohedral phase.34 The anomaly at Tm instead was deconvoluted in two different
processes taking place with increasing temperature, which include two events: a) the
disappearance of the rhombohedral phase (frequency independent anomaly in permittivity);34
and b) the relaxation process of the polar nanoregions with tetragonal symmetry.34,35 The
present dielectric data can be interpreted within this framework. Interestingly, the permittivity
maximum becomes broader with increasing lithium content, so much so that the difference
between the temperatures Ts and Tm can no longer be defined (Fig.4). This is related to the
increase of tetragonal phase fraction with increasing lithium content in a way that the
relaxation process of the rhombohedral polar nanoregion is progressively swamped in that of
the tetragonal polar nanoregions, making the distinction of Ts and Tm more difficult from the
dielectric data. Similar features have previously been reported for the system (0.935x)Bi0.5Na0.5TiO3–0.065BaTiO3–xSrTiO3 which exhibits a structural modification from
rhombohedral to pseudocubic with increasing x.36
Figure 5 shows the I-P-E loops of the three ceramics, generated at different temperatures in
the range 25-150˚C with the application of an alternating electric field of 70 kV/cm and a
frequency of 10 Hz. Ceramics with x = 0 and x = 0.05 behave as ferroelectrics up to 75°C,
as evidenced by the presence of domain switching current peaks at the coercive field Ec.29
The coercive field decreases with increasing temperature as typically occurs in ferroelectrics.
In the temperature range T<75°C, the I-E loops of x = 0.15 ceramics shows four current peaks
at ±EF and ±EB. The amplitude dependence of the P-E and I-E loops of ceramic x = 0.15 at
25˚C shows a clear step change above a certain electric field amplitude (E>50kV/cm see
Fig.6), above which the four current peaks appear (Fig.6b). The P-E loops generated when
E≤50kV/cm do not lie within those generated at higher fields (see for instance the light blue
curve in Fig.6a). This is thought to be due to the occurrence of an additional polarization
mechanism at E>50kV/cm, as previously reported.3 The multiple current peaks can be
11
interpreted as follows; the subscripts F and B stand for “forward” and “backward”. In x =
0.15 ceramic at low temperature the current peaks corresponding to ±EF and ±EB both appear
during electrical loading. At cycling regime conditions, the polarization effects produced in
correspondence of +EF and -EF are recovered during electric field reversal at -EB and +EB.
Figure 5. I-P-E loops of ceramics at different temperatures at applied field E = 70 kV/cm and
10 Hz frequency.
12
Figure 6. (a) P-E and (b) I-E loops of ceramic x = 0.15 at room temperature at different
electric field amplitudes and 10Hz frequency, (c) illustrates the absence of EB in the first
electrical cycle (interval a-b) at E = 60kV/cm.
13
This is suggested by the fact that the condition P = 0 is re-established in the current valley
between ±EB and ±EF.3 In order to further prove that the current peaks at ±EB only appear
after the polar phase has been induced, two electric field bursts of 60 kV/cm and 10 Hz were
applied on a virgin sample and the current was monitored for the entire test. The
correspondent I-E plot in Fig.6c, shows that the peak at +EB is absent in the very first
electrical cycle (interval a-b), while the current peaks at ±EB appear in the cycling regime
conditions after the polar phase was induced at +EF in the very first cycle (interval a-b). The
presence of multiple current peaks during electrical loading at ±EF and ±EB is also visible in
ceramic x = 0 and x = 0.05 at 100°C (Fig.4). With further temperature increase, the current
peak corresponding to ±EB appear during electric field unloading indicating that the
polarization effects produced by ±EF can be recovered during unloading. The threshold fields
–EB (+EB) and +EF (-EF) move further away from each other with increasing temperature and
the electric field-induced transition becomes less hysteretic (the threshold fields ±EF and ±EB
become closer). In the plots associated with ceramics x = 0.05 it can be clearly seen that ±EF
and ±EB both increase with increasing temperature. In particular, the increase of ±EF with
increasing temperature suggests that the polarization mechanisms taking place at ±EF become
increasingly hindered with increasing temperature. These effects are all related to increasing
stability of the weakly polar phase with increasing temperature. The present results support
the existence of electric field induced transitions in Li-doped BNT-based systems linked with
electrical current peaks. The weakly polar tetragonal phase reversibly transforms into a polar
order during the application of the electric field. The temperature at which such transitions
take place is related to the relative amount of polar and weakly polar phase initially present in
the material, and it reduces with increasing amount of tetragonal phase. In fact, according to
our XRD analysis, the fraction of the weakly polar tetragonal phase P4bm increases with
increasing lithium content in the compositions studied. This is the reason why in ceramic x =
14
0.15 the temperature at which the weakly polar-to-polar transitions take place is lower than in
the case of x = 0 and x = 0.05.
4. CONCLUSIONS
In summary, the coexistence of polar rhombohedral R3c and weakly polar tetragonal P4bm
phases was found in 0.95[0.94(Bi0.5Na(0.5-x)Lix)TiO3-0.06BaTiO3]-0.05CaTiO3 lead-free
systems. The substitution of sodium for lithium increased the relative fraction of the weakly
polar phase over the polar one. Both rhombohedral and tetragonal lattice distortions kept at
the same level with increasing lithium concentration. Four current peaks in I-E loops can be
attributed to field-induced transitions from weakly polar-to-polar phases and from polar-toweakly polar phases. The onset temperature of the electric field-induced transitions decreased
with increased lithium content. This decrease is a result of an increased fraction of weakly
polar phase.
15
AUTHOR INFORMATION
Corresponding Author *E-mail: [email protected]
Author Contributions
The manuscript was written through contributions of all authors. All authors have given
approval to the final version of the manuscript.
ACKNOWLEDGMENT
Vladimir Koval would like to acknowledge The Grant Agency of the Slovak Academy of
Sciences for financial support on Projects: No. 2/0053/11, and No. 2/0057/14.
16
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