Lithium-induced phase transitions in lead-free Bi0
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
3Nanoforce 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. 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 (BNT-BKT)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-BaTiO3-K0.5Na0.5NbO3 (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
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
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 thickness, the electroded area of the
sample, the vacuum permittivity, the measuring frequency, and the
real and imaginary part of the electrical impedance, respectively.
Current-polarization-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°C-150°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
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.
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 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 off-centering), 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
Cell
(weight %)
a (Å)
c (Å)
c/a
αr (deg.)
x = 0
R3c 93
5.491(0)
13.489(1)
2.46(1)
59.87
P4bm 7
5.534(1)
3.889(1)
0.70(1)
x = 0.05
R3c 86
5.497(0)
13.492(2)
2.45(1)
59.91
P4bm 14
5.504(2)
3.968(4)
0.72(2)
x = 0.15
R3c 43
5.494(0)
13.504(1)
2.46(1)
59.85
P4bm 57
5.502(0)
3.911(1)
0.71(1)
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.
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 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 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
(0.935-x)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
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.
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.
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 = 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-to-weakly 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.
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.
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