Materials Chemistry and Physics 130 (2011) 95– 103
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Materials Chemistry and Physics
j ourna l ho me pag e: www.elsev ier .com/ locate /matchemphys
-site substitution effect of strontium on bismuth layered CaBi4Ti4O15 ceramicsn electrical and piezoelectric properties
mit Tanwar ∗, Maya Verma, Vinay Gupta, K. Sreenivasepartment of Physics and Astrophysics, University of Delhi, Delhi 110007, India
r t i c l e i n f o
rticle history:eceived 22 October 2010eceived in revised form 24 May 2011ccepted 31 May 2011
eywords:aman spectroscopy and scatteringourier transform infrared spectroscopyFTIR)hase transitions
a b s t r a c t
Strontium substituted CaBi4Ti4O15 ceramics with the chemical formula Ca1−xSrxBi4Ti4O15 (CSBT)(x = 0.0–1.0) have been prepared through conventional solid state route. The formation of single phasematerial with orthorhombic structure was verified from X-ray diffraction with incorporation of Sr sub-stitution. Decrease in a-axis displacement of Bi ion in the perovskite structure in the CSBT ceramics wereobserved from the relative changes in soft mode (20 cm−1) in the Raman spectra, and increase in Srincorporation shows the shift in ferroelectric to paraelectric phase transition temperature. The dielectricproperties for all the CSBT ceramic compositions are studied as a function of temperature over the fre-quency range of 100 Hz–1 MHz. Curie’s temperature was found to be function of Sr substitution and withincrease in the Sr concentration the phase transition becomes sharper and phase transition temperature
◦
ielectric properties gets shifted towards lower temperature (790–545 C). The behavior of ac conductivity as a function offrequency (100 Hz–1 MHz) at low temperature (<500 ◦C) follows the power law and attributed to hoppingconduction mechanism. Sr substitution results in the increase in piezoelectric coefficients (d33) whereaspiezoelectric charge coefficient values were found comparable to that of PZT at room temperature. Rel-ative changes in soft modes due to Sr incorporation results in high piezoelectricity in the CSBT ceramics.
. Introduction
Bismuth layered structured material are of great technologicalnterest because of their applications as thermally stable capacitors,ositive temperature coefficient devices, microwave applications,on-volatile ferroelectric memories, high temperature piezoelec-ric transducers, sensors, actuators and photonics devices [1–3].igh temperature sensing devices are required for detecting vibra-
ion in aerospace, automotive, power generating industry and oilell drilling under harsh thermal condition [4,5]. Bismuth lay-
red structured ferroelectric (BLSF) ceramics with high ferroelectrichase transitions are the potential candidates for high temperatureensing applications [4,5].
The general formula of BLSF is (Bi2O2)2+(Am−1BmO3m+1)2−
here A is mono, di, or trivalent ion or a mixture of them to form dodecahedron, and B is a combination of cations well suitedo form an octahedron and m is an integer usually lying in theange 1–4. The CaBi4Ti4O15 with layer m = 4 are subgroup of BLSF
amily and is potential candidate for high temperature piezoelec-ric applications due to its high Curie temperature (Tc = 790 ◦C)nd piezoelectric voltage coefficient g33 (8 × 10−3 V m N−1), which
is comparable to lead based piezoelectric ceramics, i.e. for PZTg33 = 26 × 10−3 V m N−1 [6].
It has been shown that the properties of the BLSF compoundsare strongly dependent on the number of perovskite unit and thechemical elements at the A or B site and properties of a mate-rial can be tailored by doping with a suitable dopant at either ofthese sites. There are many reports available in the literature forA and B site substitution in the perovskite layers of CBT ceram-ics and its influence on the structural, dielectric, piezoelectric andferroelectric properties has been studied [7–9]. Wu and cowork-ers studied the influence of various A-site dopants (Ca2+, Ba2+) andB-site dopants (W6+, V5+) on dielectric and piezoelectric proper-ties of BLSF material and correlated with the difference in ionicradii [7]. Substitution of host cations by dopant having smallerionic radius will result in an increased rattling space leading to ahigher transition temperature and large dielectric constant at tran-sition point, while dopant having larger radius will result in lowertransition temperature with lower dielectric constant at transi-tion temperature. Enhanced piezoelectricity due to the changesobserved in soft modes in Raman spectra have been reported byvarious authors [10,11]. However no report are available in the lit-
erature regarding the changes observed in soft modes due to Srsubstitution corresponds to a-axis displacement of Bi ion in per-ovskite of CSBT ceramics which yield enhanced piezoelectric andferroelectric properties. Furthermore the detailed studies on ac
onductivity and dielectric properties of Sr substituted CBT ceram-cs have not been reported as a function of temperature. In theresent study single phase Sr substituted CBT ceramics were pre-ared from solid state route and effect of Sr substitution werenalyzed on structural and electrical properties of Calcium bismuthitanate ceramics and attempts have been made to correlate theoftening of Raman modes with the ferroelectric and piezoelectricroperties.
. Experimental
Ceramic samples of Ca1−xSrxBi4Ti4O15 (CSBT) were prepared by conventionalolid-state reaction technique. Reagent-grade oxide or carbonate powders of Bi2O3,rCO3, TiO2, and CaCO3 with 99% purity were weighed and mixed in stoichiometricatio. The materials were thoroughly mixed and ball milled for 24 h with ZrO2 ballsing distilled water as mixing media. After calcining at 1000 ◦C the ground andall milled powders were mixed with binder (5%PVA). The dry powder was pressed
nto circular disks of 10 mm diameter using hydraulic press. The pellets were reac-ively sintered at 1200 ◦C for 1 h, using a conventional muffle furnace. Densitiesf pellets were measured from Archimedes’s buoyancy principle. Crystallographictructure was analyzed by X-ray diffractometer (XRD) (Philips PW1830) using CuK�
adiation. The Raman scattering measurements were performed in the backscat-ering geometry using a Jobin-Yvon T64000 triple monochromator equipped with
charge coupled detector. A JEOL scanning electron microscope (model JSM-840)as used to study the microstructure of the prepared ceramics. FTIR studies wereone (KBr used as base material) from Perkin Elmer RX-1 FTIR spectrometer. Theoth faces of pellets were polished with fine emery paper and coated with plat-
num film by rf sputtering. The dielectric properties of CSBT ceramic were studiedn metal-ferroelectric-metal (MFM) configuration using Agilent 4284A LCR meter as
function of frequency in the range 100 Hz–1 MHz. dc conductivity measurementas done with Keithley 256 source measure unit at 50 V. Temperature dependent
tudies were made using specially designed sample cell over the wide range of tem-erature from room temperature to 900 ◦C. Piezoelectric properties were studiedsing d33 Piezotest (piezometer).
Fig. 2. SEM of CSBT ceramic samples
Fig. 1. XRD pattern for CSBT ceramic samples for different Sr concentration.
3. Results and discussion
3.1. Structural properties
Fig. 1 shows the X-ray diffraction pattern for Ca Sr Bi Ti O
1−x x 4 4 15(CSBT) ceramics samples where x varying from 0.0 to 1.0 in step of0.25, sintered at 1200 ◦C for 1 h. All the peaks obtained in the XRDpattern were well indexed with the reported data on CBT ceramic
for different Sr concentration.
A. Tanwar et al. / Materials Chemistry and Physics 130 (2011) 95– 103 97
Table 1Comparison of Curie’s temperature, critical exponent and activation energy of stron-tium substituted CaBi4Ti4O15 ceramics.
S. no. Ceramic samples Tc (◦C) Curie’s criticalexponent �
Fig. 3. Raman spectra for CSBT ceramic samples for different Sr concentration.
the Raman peak observed at low frequency ∼20 cm for highstrontium concentration is the soft mode [17] and is attributedto the relative a-axis displacement in the ferroelectric phase ofBi ions in the perovskite with respect to the chains of TiO6 octa-
ndicating the formation of single phase of the bismuth oxide layerype structure with m = 4 [12–15]. The lattice parameters were esti-
ated and results indicates that the crystal symmetry of both CBT
a = 5.44 ´A, b = 5.47 ´A and c = 40.53 ´A) and SBT (a = 5.49 ´A, b = 5.42 ´A
nd c = 40.84 ´A) [16] is orthorhombic. A shift in the XRD peak of1 1 9) and (0 2 1 0) towards lower angle was seen indicating thencorporation of Sr content in CBT ceramics. The perovskite layerf CBT is composed of four TiO6 octahedra and size of the octahedraepends upon the Ti–O bond length. Ti–O bond in the perovskitetructure is partly covalent and both the ionic radii and charge ofhe A site cation affects the percentage of ionic character of Ti–Oond [15]. Large ionic radii of Sr (1.20 A) as compared to Ca (1.04 A)
on leads to large Ti–O bond length which affects the size of theerovskite structure and increases the lattice parameters ‘a’ and ‘c’.
Fig. 2 shows the SEM observed on CSBT ceramics surfaces withifferent Sr concentration. SEM micrographs reveals that the grainize increases for low substitution of strontium x < 0.5 and wasound to be maximum for x = 0.5 strontium substitution and thenecreases for higher Sr substitution (x > 0.5) and the CSBT ceramicample becomes less dense and more porous (for x > 0.5) as com-ared to lower Sr substitution (x < 0.5). The observed variations inhe measured values of density shows a slight increase from 94% to5% of the theoretical density with increase in Sr content from 0.0o 0.5. It can be observed that with the increase in grain size (x < 0.5)ensity increases and found to be maximum for x = 0.5 and there-fter decrease for higher Sr substitution (x > 0.5) with the relatedecrease in density from 95% to 89%.
Fig. 3 shows the Raman spectra recorded at room tempera-ure under the back scattering configuration of the prepared CSBTeramics as a function of x. The well defined phonon modes around5, 81,160, 266, 558, 697 and 855 cm−1 were observed for the CBTeramic and found to be well agreement with the reported data17]. The Raman mode observed at 855 cm−1 is reported due to B1g
ode corresponds to symmetric Ti–O stretching vibration [17]. Thisncrease towards higher wave number is attributed to the heavy
ass and larger size of the Sr ion which increases the force con-tant of Sr–Ti and Sr–O bond and thus require high frequency fori–O vibration [18]. The mode at 720 cm−1 peak is due to the bandtretching vibration of TiO6 octahedra and the band at 275 cm−1 isor B2g, B3g mode and this mode corresponds to the O–Ti–O bend-ng vibration. A shift towards the higher wave number for these
odes were observed with increase in Sr substitution in CBT andpproaches the frequency reported for bulk SBT ceramic samples17]. This shift is attributed to the heavy mass and large size of Sron as compared to Ca ion which affects the motion of the Ti–Otretching and bending vibration [18].
Influence of strontium doping on the low frequency Ramanpectra of CBT is shown in Fig. 4. In BLSFs, three displacive modesxist: (1) the relative a-axis displacement in the ferroelectric phasef Bi ions in the perovskite with respect to the chains of TiO6 octahe-ra; (2) the rotational mode of the TiO6 octahedra along the a-axis;
nd (3) another rotational mode of the TiO6 octahedra along the c-xis [19,20]. In the present study, mode at 60 cm−1 appears and isue to the displacement of Bi3+ ions in the Bi2O2 layer, whereas
Fig. 4. Raman spectra at low wavenumber for CSBT ceramic samples for differentSr concentration.
−1
Fig. 5. FTIR spectra for CSBT ceramic samples for different Sr concentration.
98 A. Tanwar et al. / Materials Chemistry and Physics 130 (2011) 95– 103
frequ
htspliciW(
Fig. 6. Temperature variation of dielectric constant at different
edra [21]. Softening of this mode increases with the decrease inhe Ca concentration as full width half maxima decreases for hightrontium concentration. Effect of a-axis displacement of Bi ion inerovskite structure can also be observed in the increase in the
attice parameter ‘a’ (Fig. 1). Zhang et al. observed the same behav-or of change in lattice parameter ‘a’ due to Sr substitution in CBT
eramics [12]. Fig. 4 also shows that the mode at 20 cm−1 for CBTs highly underdamped as compared to SBT ceramic samples [17].
ith the incorporation of strontium in the perovskite structurereplacing calcium) the peak at 95 cm−1 and 156 cm−1 corresponds
encies for CSBT ceramic samples for different Sr concentration.
to the vibrational mode of A site ions and displacement of A-site Biion in the pseudo perovskite respectively and peaks of both thesemodes become more prominent which is attributed to the largersize and heavy mass of the strontium ion as compared to the cal-cium ion [22]. The substitution of heavier Sr2+ for lighter Ca2+ ionsin CBT ceramics is expected to shift phonon mode at 20 cm−1 and
95 cm−1 towards lower frequency [23,24]. However, no significantshift in the position of these modes were observed with increaseSr concentration (Fig. 4), indicating that other factors are playing acrucial role. One possible reason may be in the force constant due to
A. Tanwar et al. / Materials Chemistry
Fp
srofS
s4tctbttibembcfdc
3
tatamwttoractCZwi
ig. 7. Temperature variation of dielectric constant at 1 MHz for CSBT ceramic sam-les as a function of strontium dopant.
ubstitution of Sr having large size in comparison to that of Ca, andesults in the increase in wavenumber because the binding energyf Sr is less than that of Ca [25]. The counterbalancing of decrease inorce constant with the substitution of lighter Ca by heavier massr gives no significant shift in the modes at 56 cm−1 and 158 cm−1.
Fourier transform infrared spectra (FTIR) of CSBT samples ishown in Fig. 5. Two well defined absorption bands at around30 cm−1 and 840 cm−1, and one broad band in the range 580 cm−1
o 680 cm−1 were observed in the spectra of all prepared CSBTeramic samples. The broad absorption band at around 580 cm−1
o 680 cm−1 is assigned to Ti–O stretching vibration and the lowerand at 430 cm−1 to the Ti–O bending vibration and the absorp-ion band at 840 cm−1 is assigned to the vibrations arising fromhe strongly covalently bonded (Bi2O3)2+ layers [26]. The stretch-ng vibration is expected to occur at frequencies higher than theending vibration, from the comparison of the change in potentialnergy due to repulsive forces between the ions in the two nor-al vibrations [27]. The peak for bending vibrations was found to
e shifted towards the lower wavenumber with increase in Sr con-entration for composition x ≥ 0.5 and indicate that the interatomicorce constant between the Ti–O bond decreases which might beue to the larger size of the strontium ion as compared to thealcium ion (Fig. 5) [28].
.2. Dielectric properties
The variation of dielectric constant (ε′) as a function of tempera-ure for all prepared CSBT samples measured at different frequencyre shown in Fig. 6. A well defined phase transition from ferroelec-ric to paraelectric is observed at around 790 ◦C and 550 ◦C for CBTnd SBT ceramic samples respectively (Table 1), and is in agree-ent to the reported results on CBT and SBT ceramics by otherorkers [16,27,29,30]. A significant shift in the Curie’s tempera-
ure (Tc) was observed with Sr concentration which is attributed tohe structural distortion due to strontium substitution. As the sizef Sr ion is greater than the Ca which leads to the decrease in theattling space for TiO6 octahedra (as discussed in Raman spectra)nd results in shift to lower phase transition temperature [31]. Fig. 7ompare the change in transition temperature with Sr concentra-ion and it can be observed that sharpness of the phase transition at
urie’s temperature increases for high strontium concentration. L.heng et al. observed the same behavior of transition temperatureith Sr substitution (inset of Fig. 7). The loss (tan ı) increases with
ncrease in temperature which can be attributed to the increase in
and Physics 130 (2011) 95– 103 99
conductivity with temperature of the CSBT ceramic samples [16](as discussed later).
The measured dielectric data of CSBT ceramics has also beenanalyzed according to the Curie Weiss law given by
1ε
− 1εm
= 1C
(T − Tm)� (1)
where � and C are material constant depending on the composi-tion of the samples. For normal ferroelectrics, � equals to 1 andfor relaxor it is equal to 2 [32]. The value of � is determined bythe slopes of the fitting curve between log((1/ε) − (1/εm)) versuslog(T − Tm) and the plot was found linear and value of � was aboutone (Table 1), indicating the ferroelectric nature of all preparedCSBT ceramics samples.
3.3. Electrical measurements
ac conductivity �ac(ω) and dc conductivity �dc of CSBT ceramicsamples for different Sr concentration were measured as a functionof temperature. Fig. 8 shows the variation of ac and dc conductivity,�ac(ω), of CSBT ceramic samples sintered at 1200 ◦C at five fixedfrequencies as a function of reciprocal of temperature (1000/T)(K−1). The behavior of �ac(ω) with temperature is similar to thatobserved in many other low mobility oxide materials reported byvarious workers [33–35]. The variation of �dc, of CSBT ceramicswas found to be in accordance with the Arrhenius behavior givenby
� = �o exp(
− Ea
KT
)(2)
where Ea is the activation energy, K is the Boltzmann constantand T is the temperature. The estimated value of activation energyfor all CSBT ceramics samples listed in Table 1, and were foundto be less than half the band gap value of CBT (∼3.36 eV) andSBT (∼3.5 eV) ceramics, indicating extrinsic conduction. The acti-vation energy is generally associated with the acceptor or donorlevels. However frequency dependent conductivity (as discussedlater) shows that the dominant conduction mechanism may be dueto hopping of charge carriers, and the activation energy may beassociated with activated mobility rather than charge carriers. Theobserved variation of �ac(ω) with temperature could be dividedinto two distinct regions, low temperature region (300–500 K)and high temperature region (500–1150 K). In the lower tempera-ture region (300–500 K) �ac(ω) shows strong frequency dispersionand weak temperature dependence (Fig. 10). In this region �ac(ω)increases with an increase in frequency. In high temperature region(500–1150 K) �ac(ω), shows a strong temperature dependent andweak frequency dependence. It may be noted that the measuredvalue of �ac(ω) is comparable to �dc, in the higher temperatureregion.
The plot of �ac(ω) for CSBT ceramics samples as a function of fre-quency at different temperature are shown in Fig. 9. The variationof �ac(ω) with frequency was found in accordance with the powerlaw,
�AC = Aωs (3)
where A is a temperature dependent parameter and s is the expo-nent having values between 0 and 1 [36]. The value of s for all CSBTceramic samples varies from 0.98 to 0.21 with increase in temper-ature from room temperature to 850 K. The observed variation in�ac(ω) indicates that the conduction mechanism in CSBT ceramicsamples is due to hopping of charge carriers between the sites
having variable barrier heights and random separation. A devia-tion of �ac(ω) from power law was observed at higher temperature(>500 K). The dispersion in the ac conductivity at low frequenciesdecreases for higher Sr concentration.
100 A. Tanwar et al. / Materials Chemistry and Physics 130 (2011) 95– 103
uencie
itta
Fig. 8. Variation of ac conductivity with 1000/T measured at different freq
The frequency dependence of dielectric loss of CSBT ceram-
cs sintered at 1200 ◦C as can be seen in Fig. 10 as a function ofemperature. The observed behavior can be divided in two dis-inct region. In the low temperature region (<500 K), ε′(ω) shows
weak frequency dispersion and small temperature dependence
s for the CSBT ceramic samples having different strontium concentration.
whereas, in higher temperature region (>500 K) a strong fre-
quency dispersion and temperature dependence was observed.The contribution of dc conductivity is clearly reflected in theobserved strong frequency dispersion of ε′′(ω) at higher tem-perature. Electrical equivalent circuit for ceramic sample can
A. Tanwar et al. / Materials Chemistry and Physics 130 (2011) 95– 103 101
th a s
bftol
Fig. 9. Variation of ac conductivity with frequency at different temperatures wi
e represented by a parallel combination of capacitance (Cp), a
requency dependence resistance due to short range polariza-ion Rac(ω), and a dc resistance Rdc due to long range transportf charge carriers [37,38]. Cp and Rp are the value of equiva-ent capacitance and resistance measured for the prepared CSBT
tep of 50 K for CSBT ceramic samples having different strontium concentration.
ceramic and the measured dielectric loss, ε′′meas can be written
as
ε′′ac +
(�dc
ωεo
)= ε′′
meas (4)
102 A. Tanwar et al. / Materials Chemistry and Physics 130 (2011) 95– 103
re ran
wωdop
Fig. 10. Variation of dielectric loss with frequency in a wide temperatu
here ε′′meas is the measured dielectric loss at a given frequency
and εo is the permittivity of free space and �dc is the measuredc conductivity and ε′′
ac = (�ac/ωεo). For CSBT ceramic, the valuef �dc is much lower then �ac, therefore, ε′′
meas ≈ ε′′ac at low tem-
erature (<500 K) which corresponds to the dispersion behavior of
ge for the CSBT ceramic samples for different strontium concentration.
ε′′meas at low frequency. However at higher temperature (>500 K),
�dc is comparable to �ac or even higher, ε′′meas is dominated by the
term having �dc. Therefore an increase in the measured value ofε′′
meas at low frequency and higher temperature was observed forall CSBT ceramic samples (Fig. 10).
A. Tanwar et al. / Materials Chemistry
Fi
3
fsfetIhsaiAiclAtolwtt2ca
4
CstSdsttw
[[
[[[
[[[
[[[
[
[[[[
[[[[[[[[
[[
[
[[[
ig. 11. Variation of piezoelectric coefficients (d33 and g33) as a function of Sr contentn CSBT ceramic samples.
.4. Piezoelectric measurements
Piezoelectric property of CSBT ceramics were measured as aunction of Sr dopant. The samples were electrically poled in ailicon oil bath at 150 ◦C under the electric field of ∼80 kV cm−1
or 30 min to induce piezoelectricity. The measured value of piezo-lectric coefficient d33 for all CSBT ceramics are measured at roomemperature, and its variation with Sr content is shown in Fig. 11.t may be noted that with increase in Sr content d33 increasesowever a relatively higher value of piezoelectric coefficients foramples having high concentration of strontium (x ≥ 0.5) may bettributed to the relative change in the soft modes of CBT ceram-cs (as discussed in Raman analysis) due to Sr incorporation at-site in the perovskite structure. A similar finding for an increase
n d33 has been observed by J. Zheng et al. with strontium dopedalcium bismuth titanate ceramics and attributed this increase toattice distortion due to increase in lattice parameter ‘a’ [12,39].
similar finding of an increase in room temperature d33 withhis increase in softening effect in the perovskite structure werebserved by other coworkers with Nb, Sm and La doped bismuthayered ceramics [40–42]. Piezoelectric voltage coefficient increase
ith increase in strontium concentration from 11.2 × 10−3 V m N−1
o 19.9 × 10−3 V m N−1 which is only a few times smaller thanhose of lead based piezoelectric ceramics (g33 for PZT is6 × 10−3 V m N−1). Therefore, the prepared ceramic compositionsan be considered as a good candidate for piezoelectric sensorspplications [6].
. Conclusions
To summarize, strontium substituted CBT ceramicsa1−xSrxBi4Ti4O15 (x = 0.0–1.0 in step of 0.25) were synthe-ized by solid state reaction method. Structural distortion withhe Sr substitution has been observed by XRD and Raman analysis.ubstitution of Sr concentration at Ca site increase the a-axisisplacement of the Bi ion in the perovskite structure and thus
oft mode at 20 cm−1 was found to increase with Sr incorpora-ion. Temperature dependent dielectric studies indicate phaseransition temperature is dependent on the Sr substitution andas found to decrease with increase in Sr concentration. All CSBT
[[[
and Physics 130 (2011) 95– 103 103
ceramics follow the Curie Weiss law with critical exponent “�”equals to one indicating the ferroelectric nature of all the preparedCSBT ceramic. Piezoelectricity was found to increase with increasein Sr substitution which is attributed to the relative change in softmode in the perovskite structure of CBT ceramics.
Acknowledgements
Authors are thankful to Prof. R.S. Katiyar, University of PuertoRico, San Juan, USA for Raman studies and fruitful discussion. Thesupport from the DRDO and DST, Government of India are highlyacknowledged. One of the authors (A.T.) would like to thank theCouncil of Scientific & Industrial Research (CSIR), India for theresearch fellowship.
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