Journal of Ceramic Processing Research. Vol. 16, No. 5, pp.

565~571 (2015) J O U R N A L O F

Ceramic
Processing Research

Morphotropic Phase boundary in BNT-BZT solid solution: A study by Raman

spectroscopy and electromechanical parameters
B. Parijaa, T.Badapandab* and S. Panigrahic
a
Govt. (Auto) college, Rourkela. Odisha -769004 India
b
Department of Physics, C V Raman College Engineering, Bhubaneswar, Odisha, India
c
Department of Physics, National Institute of Tehnology, Rourkela, Orissa, India

Lead-free perovskite ferroelectric ceramics of (1-x)(Bi0.5Na0.5)TiO3-x Ba(Zr0.25Ti0.75)O3 [abbreviated as (1-x)BNT-xBZT] have

been synthesized via conventional solid-state reaction route. Structural changes of the solid-solutions were investigated by using
rietveld refinement of X-ray diffraction (XRD) data and Raman spectroscopy over the composition range 0 ≤ x ≤ 0.08. The
unique phase transition behaviours are discussed in relation to the growth of Ba2+TiO3 and Zr4+TiO3 clusters upon the
substitution of Ba2+ for (Bi0.5Na0.5)2+cations and Zr4+ for Ti4+ in the (1-x)BNT-xBZT solid-solutions. The splitting of (111) and
(200) plane represents the presence of morphotropic phase boundary within the solid solution at x = 0.05. Raman spectroscopy
exhibited a splitting of the (TO3) mode at x = 0.05 and confirmed the presence of MPB region. The piezoelectric properties
of the solid solution increase with rise in Ba(Zr0.25Ti0.75)O3 content and shows optimum value at x = 0.05 owing to the co-
existence of two ferroelectric phases. Based on these results, it is suggested that the morphotropic phase boundary in the
studied system lies in the composition x = 0.05.

Key words: (Bi0.5Na0.5)TiO3; Ba(Zr0.25Ti0.75)O3, Solid-solutions, Morphotropic Phase boundary, Reitveld refinement, Raman
spectroscopy, electromechanical properties.

Introduction RT. The crystal structure changes to the tetragonal

(P4bm) and then to the cubic (Pm3m) structure at
Structure-property relationships are an important approximately 300 and 540 oC, respectively, upon
aspect in material physics as the knowledge of these heating [2-6].
can be utilized to develop novel materials with In recent years, lead-free perovskite ceramics have
improved qualities. Perovskite-based ferroelectric and attracted considerable attentions as one of the important
piezoelectric materials have been at the center of materials because of its outstanding advantages in free
research because their simple structure facilitates the control atmosphere and no lead pollution. The intriguing
understanding of the structure property relationships. phase transitions of both BNT and BNT-based solid
Among the perovskites, Bi0.5Na0.5TiO3 ceramic with a solutions as a function of composition and temperature
rhombohedral structure has been considered to be a make them an excellent model for studies on the phase
good candidate for lead-free piezoelectric ceramics transition behaviours of relaxor ferroelectrics [7-10].
because of its strong ferroelectric nature at room BNT ceramic is difficult due to its high conductivity.
temperature, high Curie temperature (TC = 320 oC), Furthermore, the piezoelectric properties of BNT
relatively larger remnant polarization (Pr = 38 μC/ ceramic are too low to be for practical application. In
cm2), and coercive field of Ec = 73 kV/cm at room order to improve the poling process and then obtain
temperature [1]. In BNT, the bismuth and sodium good piezoelectric properties, some modification or
cations occupy the corners of a cubic unit cell, oxygen doping of the BNT system has been carried out. It has
cations occupying the face centers forming an been reported that BNT-based ceramics with their
octahedral and a titanium cation in the center of the compositions modified with the addition of various
oxygen octahedra. BNT is one among the few A-site ceramic oxides make it easier to be poled compared
disorder perovskite material, having a mixture of Bi3+ with pure BNT ceramics and improved piezoelectric
and Na1+ ions. BNT is a perovskite-structured properties [11-17]. The other feature of BNT is that it
ferroelectric with a rhombohedral symmetry (R3c) at forms a morphotropic phase boundary (MPB) with
other perovskites having tetragonal symmetry such as
BaTiO3 and Bi1/2K1/2TiO3, and very strong piezoelectric
*Corresponding author: properties are obtained for the MPB compositions [17-
Tel : +91-9437306100
Fax: +91-674 2462022 31]. Thus, BNT-based solid solutions have attracted
E-mail: badapanda.tanmaya@gmail.com much attention as lead-free piezoelectric materials.

565
566 B. Parija, T.Badapanda and S. Panigrahi

Most studies on BNT-based solid solutions have by investigating the linear shrinkage and bulk density
focused on the MPB composition suitable for actuator of the sintered pellets. The bulk density and apparent
applications. Among, the solid solutions, the solid porosity were investigated using Archimedes principle
solution with BT has been shown to have enhanced (water immersion technique) and all the compositions
physical properties corresponding to a rhombohedral- have well sintered at 1150 oC. The structural modifications
tetragonal morphotropic phase boundary (MPB) [32]. of the sintered samples have investigated using XRD and
Ba(Zr1_xTix)O3 (BZT) ceramics also have received its Rietveld refinement. The prepared ceramic samples
much attention for its excellent dielectric properties were polarized at room temperature under 35 kV/cm in
[33-36]. In the present work, ZrO2 has been added into silicon oil for 20 minutes. The piezoelectric coefficients
the B site of the pervoskie BNBT ceramics in order to d33 of the samples were measured using a quasi-
enhance the piezoelectric properties. There are very static piezoelectric d33-meter (YE2730A, China). The
few reports available on the BNT-BZT solid solutions impedance (Z) and phase (θ) versus frequency with
[37-40]. The present composition is chosen because steps in the range 100 Hz to 1 MHz were measured
Ba(Zr0.25Ti0.75)O3 (BZT) is an important B-site complex using Hioki 3532 impedance-phase gain analyser. The
perovskite-type cubic structure with space group resonant and anti-resonant frequencies for the planar
(Pm3m) and relaxor ferroelectric in nature. The detailed vibration modes used to calculate the planar coupling
structural and dielectric study was already reported by coefficient (Kp) mechanical quality factor (Qm) and the
the authors [41]. In recent years, Raman spectroscopy frequency constant (Np) by the following equations:
1
has been widely used for studying structural phase –---
2
0.395fr
transitions in piezoelectric systems exhibiting MPBs. Kp = -------------------------------
- (1)
The present study reports the X-Ray diffraction, Raman (fa – fr) + 0.574
2 2 2 –1
scattering studies and electromechanical properties of Qm = fa [2πRfCfr(fa – fr )] (2)
BNT-BZT solid solution with a special emphasis on
πd
searching for the MPB. Np = ⎛-------⎞ fr (3)
⎝3.4⎠
Experiment where fr and fa are resonant and anti-resonant
frequencies, Rf, C and d are resonant impedance,
The samples were prepared by a conventional mixed electrical capacitance and diameter of the pellet
oxide process. In the first step (Bi0.5Na0.5)TiO3 (BNT) respectively.
and Ba(Zr0.25Ti0.75)O3 (BZT) master batches were made
from Barium Carbonate-BaCO3 (reagent grade Meark, Result and Discussion
India), Zirconium Oxide-ZrO2 (reagent grade, Loba
chem.), Titanium Oxide-TiO2 (reagent grade, Merck), X-Ray diffraction study
Bismuth Oxide-Bi2O3 (reagent grade Meark, India) and The X-ray diffraction patterns of the (1-x)BNT-
Sodium Carbonate-Na2CO3 (reagent grade, Merck). All (x)BZT ceramics for all compositions are shown in the
the precursors are having purity of 99.9%. Appropriate Fig 1, indicating the samples have pure single phase
amount of reagents were mixed in a zirconia media for perovskite structure with no trace of non-stoichiometry
12 hrs using a laboratory designed ball milling unit. induced second phase and BZT has diffused into the
The BNT sample was calcined at 700 oC for 4 hrs and lattice to form a solid-solution. As the BZT fraction in
at 850 oC for 4 hrs with intermediate grinding and
mixing. The BZT sample was calcined at 1000 oC for 4
hrs and at 1250 oC for 4 hrs with intermediate grinding and
mixing. The phase purity of both the master samples was
investigated using X-ray diffraction (XRD), (Xpert MPD,
Philips, UK). Appropriate amount of BNT and BZT
were mixed thoroughly to obtained (1-x)(Bi0.5Na0.5)TiO3-
xBa(Zr0.25Ti0.75)O3 (abbreviated as BNT-BZT hereafter)
solid-solution. The phase stability of the compositions was
investigated using XRD. The phase solubility of the
compositions was investigated using XRD and Raman
spectroscopy (ENWAVE OPTRONICS-EZRAMAN). The
granules were made by adding 3% polyvinyl alcohol as a
binder. The granules were sieved and uniaxially cold
pressed with a load of 6 tonnes to obtain discs with a
diameter of 11 mm. The discs were decarbonised at
550 oC and then sintered between 1050 and 1175 oC for Fig. 1. X-Ray diffraction pattern of (1-x)(Bi0.5Na0.5)TiO3-x
4 hrs. The optimum sintering temperature was determined Ba(Zr0.25Ti0.75)O3 ceramics (a) 20-70 o (b) 46-48 o.
Morphotropic Phase boundary in BNT-BZT solid solution: A study by Raman spectroscopy and... 567

Table 1. Rietveld refinement results and atomic coordinates of 0.95(Bi0.5Na0.5)TiO3-0.05Ba(Zr0.25Ti0.75)O3 ceramics (MPB region).
Phase Atom x y z Biso
Bi 0 0 0.2903(13) 0.3840(18)
Ba 0 0 0.2903(13) 0.3840(18)
Rhomb. (R3c) Na 0 0 0.2903(13) 0.3840(18)
Ti 0 0 0.0177(14) 0.2840(19)
O1 0.1596(15) 0.3297(16) 0.8866(17) 0.9734(23)
Bi 0 0 0 0.3201(16)
Ba 0 0 0 0.3201(16)
Na 0 0 0 0.3201(16)
Tetra. (P4mm)
Ti 0.5 0.5 0.5356 0.2485(17)
O1 0.5 0.5 −0.10931 0.5447(18)
O2 0.5 0 0.41957 0.0374(19)

Table 2. Refinement results (phase percentage, cell parameter and cell volume) of the crystal structure of 0.95(Bi0.5Na0.5)TiO3-
0.05Ba(Zr0.25Ti0.75)O3 ceramics.
Lattice parameters Cell volume
x Crystal system Space group Phase
a = b (Å) c (Å) (Å3)
Rhomb R3c 55.3% 5.5202(3) 13.5229(5) 357.2(8)
0.07
Tetra. P4 mm 44.7% 3.9053(4) 3.9015(2) 59.5 (7)

the solid-solution increases, the (1 1 0) peaks of these

BNT-BZT ceramics shift monotonically to lower 2è
angel, suggesting a consecutive increase in lattice
parameter and cell volume as a function of the BZT
fraction due to large radius of Ba2+ (1.61 Å) as
compared with (Bi0.5Na0.5)2+ (~ 1.40 Å). The X-ray
diffraction patterns of these ceramics also demonstrate
the co-existence of two phases, with increasing BZT
fraction. Only the (2 0 0) peak is observed in the
diffraction patterns for BNT-BZT (x ≤ 0.04) ceramics,
indicating that there is only a rhombohedral structure.
However, (2 0 0) and (0 0 2) peaks observed in the
diffraction patterns for BNT-BZT (x = 0.08) ceramics
reveal the existence of its tetragonal structure. At x =
2
0.05 the peak at around 47 o is slightly asymmetrical, (The reliability factors are Rp = 16.3%, Rwp = 15.8%, χ =2.22,
featured with slightly splitting of the (2 0 0) and (0 0 2) Rexp = 10.6% and RBragg = 9.8%)
peaks. The more BZT content leads to the wider Fig. 2. Rietveld refinement plots of 0.95(Bi0.5Na0.5)TiO3-0.05Ba
(Zr0.25Ti0.75)O3 ceramic (MPB region).
separation between (2 0 0) and (0 0 2) peaks, indicating
the increasing tetragonality of the lattice. Therefore, it
can be suggested that the morphotropic phase boundary value are 16.3%, 15.8%, and 2.22, respectively. The
(MPB) of (1-x)BNT-xBZT lies in the composition x = result indicates that 0.95BNT-0.05BZT is composed of
0.05 at room temperature, where rhombohedral (R3c) 55.3% rhombohedral phase and 44.7% tetragonal
and tetragonal (P4 mm) phases co-exists. Table 1 and phase, so that 0.95BNT-0.05BZT is confirmed to be an
Table 2 summarize the Rietveld refinement result MPB composition. Also, the results obtained from the
carried out in the MPB composition of (1-x)BNT-xBZT Rietveld refinement method, shown a good agreement
solid-solutions, where the setting parameters of R3c between to the XRD patterns experimentally measured
were referred to Corker et al. [42] and those of P4 mm and theoretical line profile of (1-x)BNT-xBZT ceramic
were set according to the crystallographic limitation of solid-solutions with BNT and BZT Rietveld refinement
its space group. pattern.
Replacing the refined phase by the coexisting
rhombohedral and tetragonal phases resulted in a Raman scattering study
good fitting between the observed intensities and the Raman spectroscopy represents another local probe
calculated intensities of 0.95(Bi0.5Na0.5)TiO3-0.05Ba to study NBT-x%BT [43,44]. Previously reported,
(Zr0.25Ti0.75)O3 ceramic, as shown in Fig. 2 where the BNT has rhombohedral symmetry with space group
2
final R factor (Rp) and weighted R-factor (Rwp) and χ R3c (C63v) and BZT has cubic symmetry with space
568 B. Parija, T.Badapanda and S. Panigrahi

forces in the cubic ferroelectric phase split each of the

A1 and E modes into longitudinal and transverse modes.
.However, in the present case most of the analysis has
been done with respect to the rhombohedral symmetry
of BNT. Prior experimental investigations have shown
that these peaks are broad and cannot all be resolved
from each other but can be categorized into those
pertaining to one of three frequency ranges [43, 44].
These are: (1) a low wave number range of 100-200
cm_1 which is believed to be related to Na-O vibrations;
(2) a mid wave number range of 200-400 cm_1 which is
believed to be related to Ti-O vibrations; and (3) a high
wave number range of 400-800 cm_1 which is believed
to be related to oxygen octahedral vibrations and
Fig. 3. Room temperature Raman spectra of (1-x)(Bi0.5Na0.5)TiO3- rotations.
xBa(Zr0.25Ti0.75)O3 ceramic. Fig. 3 represents the Raman spectroscopy study of
(1-x)(Bi0.5Na0.5)TiO3-xBa(Zr0.25Ti0.75)O3 ceramics with 0 ≤
x ≤ 0.08. In BNT, rhombohedral structure with space group
R3c (Z = 2) and two clusters units it is possible to observed
13 Raman-active mode, which can be represented as
(ΓRaman = 7A1 + 6E) [45]. However, it is possible to
detect only five Raman-active modes observed in the
range from 100 to 1000 cm−1 in agreement with the
works reported by the Rout et al. [46] and Eerd et al.
[47]. Due to the disorder into-A site related to distorted
octahedral [BiO6] and [NaO6] clusters, these BNT
ceramics with rhombohedral structure presents 13
Raman-active modes by the following representation:
(ΓRaman = 4A1 + 9E) that have been seen analyzed and
reported by the Petzelt et al. [48]. In Fig. 3, the Raman
peaks of BNT and BNT-BZT solid-solutions are
relatively broad, which can be caused by the distorted
octahedral [BiO6] and [NaO6] clusters or disorder in A-
site of rhombohedral structure. Also this behaviour can
be provoked possible due to presence of the disorder
structural or distorted octahedral [TiO6] clusters at short-
range in both (rhombohedral and cubic) lattices. The first
Raman-active A1(TO1) mode at around (146 cm−1) is
related to network modifiers or distorted octahedral
[BiO6] and [NaO6] clusters. The second Raman-active
E(TO2) mode can be deconvoluted in three Raman peaks
in the regions of 279 cm−1. This mode is assigned at
stretching arising from the bonds due the presence of
octahedral [TiO6] clusters at short-range. The mode
shows anomaly at x = 0.05 and starts splitting into two
bands that shift apart from each other with further
Fig. 4. (a) Fitting of the Raman spectra of 0.95(Bi0.5Na0.5)TiO3- increase in BZT content. The splitting of the B peak(s)
0.05Ba(Zr0.25Ti0.75)O3 ceramic with Lorentzian line shape (b) at room temperature with increasing x for NBT-x%BT
Variation of intensity of different modes in the Raman spectra reveals a change in symmetry (relative to NBT), to a
versus BZT concentration. structure whose irreducible representation has a higher
number of Raman active modes.The third Raman-
group Pm3m (O1h) at room temperature. As far as BZT active (LO2) mode with low intensity is related to short-
is concerned, in the cubic relaxor ferroelectric phase of range electrostatic forces associated with the lattice
the O1h point group, each of the T1u modes splits into a ionicity [49]. According to Dobal et al. [50], the (TO3)
double degenerate E mode and non-degenerate A1 modes situated at around 542 cm−1 is ascribed to the
mode while the T2u silent mode splits into B1+E modes. (← O ← Ti → O →) stretching symmetric vibrations of
Furthermore the existence of long-range electrostatic the octahedral [TiO6] clusters. This mode is common in
Morphotropic Phase boundary in BNT-BZT solid solution: A study by Raman spectroscopy and... 569

materials with type-perovskite structure. However the

spectral signature of bands shows a change at x = 0.05
and separate in to two distinct bands at x = 0.07. At
higher composition (0.08) the band appears to split into
three weak Raman bands, similar observations are
found in BaTiO3 and PbTiO3 compounds in earlier
reports [51, 52]. Finally, the (LO3) mode found at
812 cm−1 is due to presence of the sites within the
rhombohedral lattice pre containing octahedral distorted
[TiO6] clusters [53]. These modes are classified into
longitudinal (LO) and transverse (TO) components
because of the electronic structure with polar character
of lattice.
Fig. 4 (a) Room temperature Raman spectrum of
0.95BNT-0.05BZT (MPB, composition) ceramics &
(b)Variation of the maximum intensity of different modes
in the Raman spectra with % of BZT concentration. In
Fig.4(a) fitted by Lorentzian area function for MPB
composition of BNT-BZT solid-solutions, which clearly
shows the overlapping of Raman bands due to
anharmonicity in lattice during molecular vibrations. In
Fig. 4 (b) shows the minimum intensity occurs at x =
0.05 due to the maximum strain in the lattice and
existence of MPB composition, which is well aliened
with the studies of XRD phase analysis. The dramatically
enhanced intensity of the Raman bands when the x value
is increased further, suggests the occurrence of a long
range ordering of the corresponding phases involved
[54-56].

Microstructural Study:
Fig. 5 shows the SEM images of natural surface for
Fig. 5. (a-h) SEM micrographs of (1-x)(Bi0.5Na0.5)TiO3-x Ba
(1-x) BNT-(x) BZT ceramics sintered at 1150 oC for (Zr0.25Ti0.75)O3 ceramic.
4 hours. The pure BNT sample presents rectangular
grain morphology while the BZT addition changes the
grain shape to spherical shape. All sample surface and 5.83 g/cm3, about more than 96% of the theoretical
grains present regular geometry with compact structure. density.
BNT disk appears to be poly-dispersed in both size and
shape due to inhomogeneous grain growth. On the Piezoelectric and electromechanical study:
contrary, the addition of BZT results in the inhibition Fig. 6 shows the frequency impedance/spectra of the
of grain growth, so the crystals of the BNT-BZT appear BNT-BZT solid solution at its MPB composition. Fig. 7
to be more uniform in both size and shape. Several (a) presents the piezoelectric and electromechanical
possible mechanisms were reported to show how the properties of (1-x) BNT-xBZT ceramics. The piezoelectric
liquid-phase could homogenize the microstructure [57]. constant d33 and electromechanical coupling factor kp
The first possibility is that the liquid-phase assists display a similar variation, enhancing with the increasing
rearrangement of the matrix particles at the inclusion/matrix of x through a maximum value in a composition near the
interface into a more efficient packing configuration. The MPB and then tending to decrease. A strong compositional
second possibility is that the liquid-phase goes into the dependence of d33 and Kp was observed. The sample with
polycrystalline inclusions by capillary action to break them x = 0.05, which is close to the MPB as demonstrated by the
up and homogenize the microstructure. It can also be seen XRD patterns in Fig. 1, displays the best value of 131 pC/
that the grain size reduces with increase in BZT content in N and the electromechanical coupling factor kp reaches to
the compositions up to x = 0.04. The reason may be the maximum value of 23%. It is evident that the MPB
that Ba2+ abounds in crystal boundary, which prevents between the orthorhombic and tetragonal perovskite
the ion from migrating and restrains the growing of phases plays an important role in enhancing the
grains. The SEM observation confirms that the (1-x) piezoelectric properties. The instability in the domain
BNT-(x) BZT ceramics are densely sintered and all structure might facilitate the polarization switching and
compositions have high density density between 5.67 thus improve the piezoelectricity. In the MPB region, the
570 B. Parija, T.Badapanda and S. Panigrahi

it can be seen that the mechanical quality factor Qm of

the specimens decreases with increasing x, reaches the
minimum value at x = 0.05 and then shows a slight
increase with more values of x. A tendency of
frequency constant Np of the specimens similar to the
Qm can be seen, which confirms the MPB at x = 0.05.
The changes of Qm and tan d with structures are
believed to be associated with the domain wall motion.
Increasing tetragonal distortion induces strain in the
crystal lattice, which would hinder the domain wall
mobility and stabilize the domain configuration during the
application of an external field, and therefore, Qm
Fig. 6. The frequency vs. impedance/phase spectrum of 0.95BNT- increases and tan d decreases with increasing BZT content.
0.05BZT ceramic (MPB composition). From the above electromechanical properties of the solid
solution BNT-BZT systems, it can be concluded that the
good piezoelectric and electromechanical properties lie in
near MPB composition range similar to other systems. It is
attributed to an increase in the number of possible
spontaneous polarization direction for the compositions
near the MPB due to the coexistence of rhombohedral
and cubic phases. This is also explained by having
equivalent energy for the coexistence of rhombohedral
and cubic phases can be transformed each other in
poling process, which enhance the piezoelectric and
electromechanical activities.

Conclusions
(Bi0.5Na0.5)TiO3-Ba(Zr0.25Ti0.75)O3 solid-solution ceramics
were successfully synthesized by a conventional solid state
reaction rout. X-ray diffraction analysis has shown that all
the compositions are characterised by a pure perovskite
structure without any parasite phase. It is observed that the
addition of BZT allows a phase transformation from the
rhombohedral phase to the tetragonal phase. By the
Rietveld refinement analysis, the MPB composition is
found at x = 0.05 which presents 55.3% of the
rhombohedral phase and 44.7% of the tetragonal phase.
Also, Raman spectroscopy study has been confirmed the
existence of morphotropic phase boundary (MPB) at
Fig. 7. (a) piezoelectric co-efficient (d33) and electromechanical x = 0.05 composition. The piezoelectric measurement
coupling factor (kp) and (b) Mechanical quality factor (Qm) and shows piezoelectric coefficient increases with an
frequency constant (Np) of BNT-BZT ceramics with % of BZT increase in BZT content and optimum value at x = 0.05.
concentration. Also electromechanical study shows optimum results at
x = 0.05 confirming the MPB composition. Both the
polarization vector can easily switch (upon application structural and electrical properties show that the solid
of a small field) between all the allowed polarization solution has a MPB around x = 0.05 which is expected to
orientations, and hence enhances the piezoelectric be a new candidate for lead-free piezoelectric material.
coefficient [58].
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