J Mater Sci: Mater Electron (2013) 24:305–316 DOI 10.1007/s10854-012-0744-3
Ferroelectric and pyroelectric properties of rare earth based tungsten–bronze compounds B. N. Parida • Piyush R. Das • R. Padhee R. N. P. Choudhary
•
Received: 8 February 2012 / Accepted: 28 April 2012 / Published online: 22 May 2012 Ó Springer Science+Business Media, LLC 2012
Abstract Complex polycrystalline materials [Li2Pb2R2 W2Ti4Nb4O30 (R = Dy, Sm)] of the tungsten bronze structural family have been synthesized using a high-temperature solid-state reaction (mixed-oxide) technique. The formation of the single phase compounds was checked using preliminary X-ray structural data/pattern. The nature and distribution of grains in the samples in the scanning electron micrographs confirm the good quality of the samples used for electrical characterization. Detailed studies of dielectric constant, tangent loss and electrical polarization as a function of temperature at different frequency confirmed the existence of ferroelectric properties in the materials at room temperature. Study of electrical properties (impedance, modulus, conductivity, etc.,) of the materials exhibits a strong correlation between their microstructures (i.e., bulk, grain boundary, etc.) and electrical parameters. The frequency dependence of ac conductivity suggests that the materials obey Jonscher’s universal power law. Pyroelectric study shows that the materials have good pyroelectric coefficient and figure of merit.
1 Introduction Since the discovery of ferroelectricity in BaTiO3 [1] in 1940s, a large number of oxides of similar and/or different structural families were studied in search of new ferroelectric oxide materials for device applications. Among all the ferroelectric oxides reported so far, some oxides of
B. N. Parida P. R. Das (&) R. Padhee R. N. P. Choudhary Department of Physics, Institute of Technical Education and Research, Siksha O Anusandhan University, Bhubaneswar 751030, India e-mail:
[email protected]
complex tungsten bronze (TB) structural family have been found very fascinating for piezoelectric, pyroelectric, microwave dielectric/resonators etc. devices at room temperature [1–12]. Particularly, lanthanides-doped compounds have received a considerable attention [13–16] of the researchers throughout the world because of their better structural stability and enhanced properties. The TB structure with a general formula [(A1)2(A2)4](C)4 [(B1)2(B2)8]O30, may be described as complex chains of distorted BO6 octahedral sharing corners which delimit, parallel to c-axis, three different types of interstices (A1),(A2) and (C) with 12-,15- and 9-fold coordination. (B1) and (B2) sites, arising out from two different types of BO6 octahedron, have C2v and C1 symmetry respectively [14–18]. Moreover, A-site is occupied by mono to trivalence cations, B-site is occupied by tetra to hexa-valent ions (W?6, Ti?4, Nb?5, Ta?5, V?5) and C-site is either unfilled or occupied by small ions. As the TB structure is a complex system, there is a large scope for modifications at different sites for device applications. Since the first report on ferroelectric properties in a tungsten bronze compound [4], ferroelectric and related properties in a large number of compounds of the TB structural family have been investigated in the past in the form of single crystal, ceramics and thin films. Structural, ferroelectric and electrical transport properties in some compounds related to this paper such as Pb3R3Ti5Nb5O30 (R = rare earth ion) have already been reported by several workers [5, 19–22]. Studies of ferroelectric and related properties in some multi-valence complex tungsten–bronze structure have been reported earlier in single crystal, ceramics and thin film samples [23], but not much work have been reported on compounds having all the six valences. In order to tailor the physical properties and ferroelectric phase transition temperature in some
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compounds with one to six valence, we have studied ferroelectric properties of various compounds including Na2Pb2Sm2W2Ti4Nb4O30 [6], Na2Pb2Nd2W2Ti4Nb4O30 [7], and Na2Pb2R2W2Ti4V4O30 (R = Gd, Eu) [8] which have provided many interesting properties useful for devices. Recently, pyro-electric properties of Ba5SmTi3Nb7O30 have been reported by Ganguly et al. [10]. More recently, ferroelectric phase transition and conduction mechanism in Li2Pb2Pr2W2Ti4O30 have been reported by Parida et al. [24]. Structural, dielectric and electrical properties of K2Pb2Dy2W2Ti4O30 have also been reported by Padhee et al. [9]. The above compounds have very good ferroelectric and electrical properties as obtained from their dielectric, polarization and impedance measurements. Though a lot of work has been done on TB structured compounds, no work has been reported on the ferroelectric and pyroelectric properties in the Li2Pb2R2W2Ti4Nb4O30 (R = Sm, Dy) complex system. Therefore, this work is an attempt to obtain new ferroelectric compounds with some interesting results useful for applications.
J Mater Sci: Mater Electron (2013) 24:305–316
(SEM) but that for LPDWTN(D = Dy) microstructure or texture of the pellet surface was recorded by HITACHI S3400N SEM. The dielectric (capacitance, dissipative factor), impedance and inductance parameters on a sintered pellet sample were measured as a function of frequency (1 kHz to 1 MHz) at different temperatures (25–500 °C) using a computer-controlled impedance meter [PSM LCR 4NL (Model: 1735, UK)] with a laboratory-designed and fabricated sample holder and furnace. A chromel–alumel thermo-couple and KUSAM MECO 108 digital millivoltmeter were used to record the temperatures. The polarization (hysteresis loop) of the material on the poled sample (electric field = 6 kV/cm, time = 8 h) was obtained at different temperatures using loop tracer (M/S Marine India, New Delhi). The pyroelectric current of the pellet sample was measured at different temperature (25–450 °C) by Keithley Instruments Inc., Model 6517B, 1347263, A10/700x at the heating rate of nearly 2 °C/min.
3 Results and discussion 2 Experimental
3.1 Structural analysis
The polycrystalline samples of Li2Pb2R2W2Ti4Nb4O30 (R = Sm, Dy) (LPRWTN) were synthesized by a hightemperature solid-state reaction technique using high-purity (AR grade) ingredients: Li2CO3, TiO2, Nb2O5 and WO3 (99 %, M/s LOBA Chemie Pvt. Ltd., India.), PbO (99.9 %, M/s E. Merck India Ltd.), R2O3 (R = Sm, Dy) (99.9 %, M/s Indian Rare Earth Ltd). These oxides and carbonate were mixed in dry (air) and wet (methanol) medium for several hours in agate mortar. The calcination temperature of the mixtures was decided and optimized (1,100 °C) on the basis of repeated firing/mixing for 4 h in alumina crucible. An X-ray diffraction (XRD) pattern and diffraction data of calcined powder were obtained at room temperature using X-ray powder diffractometer (Rigaku Miniflex). The CuKa radiation (k = 1.5405A0) was used to collect the XRD pattern/data of the above materials in a wide range of Bragg’s angles (h) (20° B 2h B 80°) at a scanning rate of 3°/min. The calcined fine powder of the materials was then cold pressed into cylindrical pellets (diameter 12 mm and 1–2 mm thickness) under a uni-axial pressure of 4 9 106 N m-2 using a hydraulic press. Polyvinyl alcohol (PVA) was used as a binder to prepare pellets. The pellets were then sintered at an optimized temperature (1,150 °C) and time (4 h) in air atmosphere. The sintered pellets were coated with high-quality silver paste, and dried at 160 °C for 8 h before taking dielectric and electrical measurements. For LPSWTN (S = Sm) the surface morphology of a gold-coated pellet samples was recorded by JEOL JSM-5800 scanning electron microscope
The XRD patterns of LPRWTN (R = Sm, Dy) recorded at room temperature (on powder sample) are compared in Fig. 1a. The diffraction pattern of both the compounds consists of a large number of sharp and single peaks. These peaks are different from those of the ingredients of the prepared compounds. Thus it clearly suggests the formation of single phase new compounds with better crystallization [25]. As the TB structural compounds have either tetragonal or orthorhombic structure, attempts were being made to indexed all the observed peaks of the XRD patterns in these crystal systems with different unit cell configurations using a standard computer program package ‘‘POWD’’ [26]. However, some very small peaks could not be indexed in the above crystal systems and selected cell configurations suggesting the presence of some unknown secondary phase [27]. At a glance inspection of indexed reflections indicates that the materials can have Pma2 space group (polar point group mm2) which is consistent with ferroelectric phase of the materials. However, with very limited powder diffraction data it is not possible to determine the space group uniquely. Further, the scattered crystallite or particle size (P) of the compounds was calculated using the broadening of some widely spread (over Bragg angles) strong and medium reflections in the Scherrer’s equation: [28]. The average value of P is found to be 11 and 12 nm for LPSWTN and LPDWTN respectively. The least-squares refined unit cell ˚ , b = 14.2515(09) A ˚, parameters are: a = 15.2279(09) A 3 ˚ ˚ c = 8.4449(09) A and volume V = 1,831.51 A for ˚, LPSWTN and that for LPDWTN are: a = 15.2238(18) A
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022 420 430 013 051 123 223 033 621 451 352 700 701 641 561 180 081 652 514 910 381 444 225 671 091 941 10 11 770 174
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LPSWTN LPDWTN
80
Bragg Angle (2 ϑ)
(b)
decrease with increasing atomic number, consequently the cell volume of the studied compositions decreases when going from Sm?3 to Dy?3. The orthorhombic distortion calculated using d = [b - a/b ? a] will be 0.0332 and 0.0331 for Sm and Dy containing compounds respectively. Furthermore, the TB structure compounds are built on five crystallographic sites. It is therefore difficult to precisely determine the R?3 ions coordination (12- or 15-fold coordination) based on the current results. However, the numerous structural studies show that the rare earth cations predominately prefer at the A1 site [29]. Figure 1b shows the surface microstructures/textures of LPRWTN (Sm, Dy). In spite of sintering at optimized high temperature some voids of irregular shape and dimension are seen. The small size grains are homogenously distributed throughout the surface of the sample. The rectangular dimension (length and breadth) of the micrograph is found to be in the range of 3–10 lm. 3.2 Dielectric properties
Fig. 1 a Indexed XRD pattern of LPSWTN and LPDWTN. b SEM micrograph of LPSWTN and LPDWTN
˚ , c = 8.4076(18) A ˚ and volume V = b = 14.2456(18) A 3 ˚ 1,823.51 A (the number indicated in parenthesis is estimated standard deviation of unit cell parameters). As the ionic radius of lanthanide ions of the compounds is known to
The temperature dependence of relative dielectric constant (er) and tangent loss (tand) at three different frequencies (10, 100 kHz and 1 MHz) is shown in Fig. 2. The value of er increases on increasing temperature up to a temperature [referred as transition temperature (Tc)], and then decreases. The nature of variation of tand with temperature follows the similar pattern as of er. The transition temperature of LPSWTN and LPDWTN was found to be 390 and 315 °C respectively. The decrease in transition temperature is due to decrease in ionic radii of rare earth ions. A large number of researches have shown that the value of transition temperature (Tc) decreases in TB structure of lead free lanthanide compounds with increasing ionic radius of rare earth cations. Josse et al. [30] reported that the accommodation of the R?3 in A1-tunnels induces the distortions in the anionic framework, depending on its ionic radius. The smaller size of R?3 provokes the greater distortion favoring an increase of Tc. In contrast, in lead based TB compositions the inverse effect can be observed [31]. It is observed that the value of emax (dielectric constant at Tc) at 10, 100, and 1,000 kHz is found to be 1,665, 662 and 405 for LPSWTN and 260, 233 and 219 for LPDWTN respectively. The Corresponding value of tandmax (tangent loss at Tc) at same frequency is found to be 3.51, 1.35 and 0.46 for LPSWTN and 0.28, 0.066 and 0.04 for LPDWTN respectively. The rate of increase of tand for both the samples is found to be small in the low temperature region, whereas at higher temperatures this rate increases significantly. There is a sharp increase in tand at higher temperatures in LPSWTN may be due to scattering of thermally activated charge carriers and some defects in the sample [10]. At higher temperatures the conductivity
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6
tan δ
1600
10kHz 100kHz 1MHz
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1MHz 1200
tan δ
εr
350
εr
10kHz LPSWTN
100kHz 800
4
2
1MHz
LPDWTN
0
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Fig. 2 a, b Variation of relative dielectric constant and dielectric loss with temperature of LPDWTN and LPSWTN
εr
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Temperature( C)
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Temperature( C)
begins to dominate, which in turn, is responsible for rise in tand. Also, at higher temperatures the contributions of ferroelectric domain walls to tand is less, which causes the rise in value of tand [32]. But in case of LPDWTN there is not much effect of scattering of thermally activated charge carriers, defects, dominance of conductivity and ferroelectric domain walls. The value of emax and tandmax of the titled compounds decreases with rise in frequency. Such characteristic is normally found in normal ferroelectric materials. Additionally, dispersion in tand at the higher temperatures can be seen in Fig. 2 in both the samples. This trend is also observed in some similar-type of compounds [6–8]. It is associated to ionic conductivity of the material, which may be related to loss of oxygen or PbO during sintering at high temperatures. Dielectric constant decreases with increase of frequency, which is a general characteristic of dielectric material. This is because of the absence of dipolar and ionic polarizations in the material at higher frequency. There is a dielectric anomaly (Fig. 2) in the titled compounds. The dielectric anomaly is assumed to be related to the ferroelectric–paraelectric phase transition. This assumption has been confirmed by appearance of hysteresis loops at different temperatures (below Tc). In the plot of er versus temperature, dielectric peaks were found to be broadened or diffused in the region of
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phase transition. In order to determine the degree of disorder in the compounds, a general expression [33] 1 1 aðT Tc Þc or er er max 1 1 ln ¼ c lnðT Tc Þ þ const: er er max was used, where er is relative dielectric constant at a temperature T and emax is its maximum value at Tc. The value of diffusivity (c = 1.33 and 1.44 at frequency 100 kHz and 1 MHz respectively for LPSWTN) and that for LPDWTN c = 1.06 and 1.2 at frequency 100 kHz and 1 MHz respectively, was calculated from the slope of log (1/er 1/emax) versus log (T - Tc) plots using the linear portion of the graph. The value of diffusivity was found to be between 1 (obeying Curie–Weiss law) and 2 (for completely disordered system) which confirms the presence of diffuse phase transition in the studied materials. The diffuse phase transition is usually observed in TB structure ferroelectrics, which can be explained by the presence of certain non-equivalent position in the unit cell at micro–nano-scopic levels [34]. It is known that the TB structured compounds lose oxygen during hightemperature sintering [10] which can be represented by using Kro¨ger and Vink notation[35]: Oo ! 1=2O2 " þV00o þ 2e where V00o denotes oxygen vacancies. The defects such as oxygen vacancies V00o induce disordering in the system [36]
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(a) LPSWTN
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data 100kHz data 1MHz fit linear 100kHz (γ =1.06) fit linear 1MHz (γ =1.2)
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ln(T-Tc ) Fig. 3 a, b ln
1 er
1 lnðT Tc Þ er max
as a result diffuse-type phase transition occurs. The low value of diffusivity in LPDWTN again confirms that there is less amount of defect due to less oxygen vacancy in comparison to that of LPSWTN (Fig. 3a, b). 3.3 Polarization study Figure 4a, b shows the hysteresis loop of LPSWTN and LPDWTN at different temperatures. Even with a small change (decreasing) in area of the loop on rising temperature in both the samples ferroelectric properties of the materials is confirmed. The small change in the remnant and spontaneous polarization is expected since the working temperature for recording the loop is much below the transition temperature. Because of the experimental limitation on further increase of temperature of the sample holder we could not record hysteresis loop above 200 °C to (a) determine actual phase transition temperature (Tc) and (b) confirmed it (as observed in our dielectric studies).
-6
-6 -0.20 -0.15 -0.10 -0.05
0.00
0.05
0.10
0.15
0.20
E(kV/cm) Fig. 4 a, b Temperature variation of hysteresis loop of LPSWTN and LPDWTN
3.4 Impedance and modulus spectroscopy Complex impedance spectroscopy (CIS) is a non-destructive and powerful technique to characterize temperature– frequency dependence of electrical properties of ferroelectrics materials. This technique is useful to separate the contributions of (1) bulk, (2) grain boundary and (3) electrode polarization effect in the material. An ac signal is applied across the pellet samples, and their output response was measured. The impedance measurements of the materials give us data having both real (resistive) and imaginary (reactive) components. For this purpose some basic equations of impedance and electrical modulus were used: Complex impedance ZðxÞ ¼ Z 0 jZ 00 ¼ Rs
j ; xCs
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Z'(k Ω)
where x = 2pf is the angular frequency; C0 is the geometrical capacitance, j = H-1. The subscripts p and s refer to the parallel and series circuit components respectively. The peak of the high frequency semicircular arcs in the complex impedance spectra enables us to calculate the relaxation frequency (xmax) of the bulk material using the equation:
40
R m0 n Z ¼ jx 1 þ x1 þ xjx2
where x1 = 2pf1 and x2 = 2pf2 are the first and second characteristic angular frequencies respectively. An excellent agreement between experimental and calculated values for both real and imaginary parts of impedance is
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300 C o 350 C o 375 C o 400 C fit data
16 14 12 10
30
8
(a) LPSWTN
20
6 4
10 2 0
0 1
10
100
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frequency(kHz)
xmax sb ¼ xmax Rb Cb ¼ 1 ) 2pfmax Rb Cb
35
(b) LPDWTN
30 25
Z''
80
60
20
Z'(k Ω)
where Rb = bulk resistance and Cb = bulk capacitance. Figure 5a, b shows the variation of Z 0 and Z 00 as a function of frequency at selected temperatures for LPSWT and LPDWTN. The value of Z 0 decreases with rise in frequency and temperature for both the samples. At high frequency the value of Z 0 of each temperature coincides implying the possible release of space charge [37]. Solid line represents theoretical fitting of experimental data. 00 The value of Z 00 attains a maxima (Zmax ) at higher 00 temperatures. The value of Zmax decreases with rise in temperature in both the sample. This explains the presence 00 of relaxation in the sample [38]. The broadening of Zmax peak with increase of temperature suggests the occurrence of temperature dependence of relaxation phenomenon in the materials. The relaxation process occurs due to the presence of immobile charges at low temperatures and defects and vacancies at higher temperatures [39, 40]. Again the imaginary part of the impedance plots indicating high frequency slopes is independent of temperature. On the other hand, low frequency slopes are strongly temperature dependent. These two temperature dependent slopes suggest that there are two distinct dispersion mechanism involved in the samples. The asymmetric behavior, which is similar to other ferroelectrics [40, 41], can be explained using the equivalent circuits: (CQR for low temperature and (CQR) (CR) for high temperature) are shown in Fig. 5a, b where C = A(jx)m-1 and Q = A(jx)n-1 are Jonscher’s universal capacitances [42]. The frequency dependence of the AC complex impedance can be expressed as: [41]
18
o
o
300 C o 350 C o 375 C o 400 C fit data
50
Complex admittance Y ¼ Y 0 þ jY 00 ¼ jxC0 e ¼ ðRP Þ1 þ jxCp Complex permittivity e ¼ e0 je00
Z'
Z''
Z''(k Ω)
eðxÞ
20
60
¼ M 0 þ jM 00 ¼ jxC0 Z;
Z'
40
o
400 C 425oC 450oC 475oC fit data
20
0
15
Z''(k Ω)
1
400 oC 425 oC 450 oC 475 oC fit data
Complex modulus MðxÞ ¼
J Mater Sci: Mater Electron (2013) 24:305–316
10 5 0
1
10
100
1000
10000
Frequency(kHz)
Cb
Cgb
CPE
Rb
Rgb
Fig. 5 a, b Variation of Z 0 and Z 00 with frequency of LPSWTN and LPDWTN
observed from non-linear curve fitting as shown in Fig. 7 (using the formula [41]. R0 Z 00 ¼ m n x þ xx2 x1 The variation of fitting parameters (m and n) with temperature is shown in Fig. 6. This plot (m vs. temperature) confirms in both the sample that it is close to unity and temperature independent. On the other hand the value of n is less than unity, and is temperature dependent. In the ferroelectric state the value of n decreases and it attains minimum near Tc, and subsequently increases with rise in temperatures in both
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Temperature( oC)
0.0022 0.005
Fig. 6 Variation of fitting parameter m and n with temperature for LPSWTN and LPDWTN
(b) LPDWTN
M'' 400 C 425oC 450oC 475oC fit linear
0.004
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M'
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the samples. The minimum value of n near (Tc) can be explained by restoring force between charge carriers and lattice [41]. The variation in the value of n can also be understood by the theory given by Dissado and Hill [43, 44]. According to them, the exponent n characterizes the magnitude of the correlation in a single dipole reorientation. The unity value corresponds to fully correlated transitions and zero value corresponds to fully uncorrelated transition. In our experiment n tends to minimum near Tc suggesting a strongly uncorrelated reorientation of the charge carrier polarization at transition point. The electrical modulus analysis is very useful to detect electrode polarization, grain boundary conduction effect, bulk properties electrical conductivity and relaxation time [45, 46]. Figure 7a, b shows the variation of M 0 and M 00 with frequency at selected temperatures. In both the samples values of M 0 approaches to zero at low frequency the monotonic dispersion with rise in frequency may be due to presence of conduction phenomenon and short range mobility of charge carriers. This implies the lack of restoring force for flow of charge under the influence of 00 steady electric field [47]. In both the samples the Mmax peak shifts to higher frequency side. This nature of dielectric relaxation suggests that the hopping mechanism of charge carriers dominates intrinsically in thermally activated process. Asymmetric broadening of the peak in the samples indicates spread of relaxation with different time constants, which suggests non- Debye type [48] of conduction mechanism in the materials. The solid line correspondence to theoretical fitting of experimental data using similar formula as
0.0020
o
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M''
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frequency(kHz)
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0.001
0.0002 0.0000
0.000 1
10
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frequency(kHz)
Fig. 7 a, b Variation of M 0 and M 00 with frequency at different temperature of LPSWTN and LPDWTN
C 1 M 00 ¼ m 0 n : x þ xx2 x1 The frequency dependence of imaginary component of impedance (Z 00 ) and modulus (M 00 ) is shown in Fig. 8 The combine plot of Z 00 and M 00 as a function of frequency is used to detect the presence of the smallest capacitance and the largest resistance [49].This plot also helps to distinguish whether relaxation process is due to short range or long range motion of charge carriers. For the short range process, peaks of Z 00 and M 00 will occur at different frequencies whereas for long range they will occur at same frequency [50, 51]. In the studied compounds there are mismatch of peaks of different temperatures which suggests short range motion of charge carrier and departure from ideal Debye-like behavior [50]. 3.5 Ac conductivity Studies of ac conductivity were carried out for better understanding of the frequency dependence of electrical properties of the materials. The frequency dependence of
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Fig. 8 Variation of Z 00 and M 00 with frequency at different temperatures of LPSWTN and LPDWTN
1000
400oC 425oC 450oC 475oC fit data
0.01
σac ( Ω-1m-1)
ac conductivity also provides information on the nature of charge carriers. The ac electrical conductivity (rac) was calculated using the dielectric data in an empirical relation, rac = xere0tand, where e0 is permittivity in free space and x is angular frequency. In order to have better understanding of conduction mechanism in the material, we have to follow Jonscher’s universal power law [39]:
100
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(b) LPDWTN
1E-3
rT ðxÞ ¼ rð0Þ þ r1 ðxÞ ¼ r0 þ Axn where r(0) is the frequency independent term giving dc conductivity and r1(x) is the purely dispersive component of ac conductivity. The exponent n can have a value between zero and one which represents the degree of interaction between mobile ions and lattices around them whereas A determines the strength of polarizability [52]. Figure 9a, b shows the variation of rac of the materials with frequency at different temperatures. At higher temperatures the conductivity curves show frequency independent plateau in the low frequency region whereas at higher frequencies rac a xn thus frequency dispersion is still maintained in both the samples. It is obvious that rac increases with rise in frequency but it is nearly independent at low frequency. Therefore, extrapolation of this part towards lower frequency side gives rdc. The increasing trend of rac with rise in frequency (in low frequency region) may be attributed to the disordering of cations between neighboring sites, and presence of space charges [53]. In the high frequency region the curves approach to each other. The nature of conductivity plots reveals that the curves exhibit low frequency dispersion phenomena obeying the Jonscher’s power law. According to Jonscher [39], the origin of the frequency dependence of conductivity lies in the relaxation phenomena arising due to mobile charge carriers. When a mobile charge carrier hops to a new site from its original position, it remains in a state
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1E-4 1
10
100
1000
frequency(kHz) Fig. 9 a, b Variation of ac conductivity as a function of frequency (conductivity spectrum) at different temperatures for LPSWTN and LPDWTN
of displacement between two potential energy minima. Also, the conduction behavior of the materials obeys the above power law (with a slope change governed by n in the low temperature region). The value of n \ 1 signifies that the hopping motion involves a translational motion with a sudden hopping whereas n [ 1 means that the motion involves localized hopping without the species leaving the neighborhood. The frequency at which change in slope takes place is known as hopping frequency of the polarons (xp), and is temperature dependent. The high frequency dispersion has been attributed to the ac conductivity whereas the frequency independent plateau region corresponds to the dc conductivity. The material obeys universal power law which is confirmed by a fit of above equation to the experimental data (Fig. 9a, b) where solid line correspondence to fitted curve and symbols are experimental data. From non-linear fitting it is found that the motion of
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3.6 Pyroelectric study
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(b) LPDWTN 5.0x10 -9
0.95 0.90
4.0x10 -9 0.85 3.0x10 -9
A
n
0.80
A 2.0x10 -9
0.75
1.0x10 -9
0.70 0.65
0.0 200
250
300
350
400
450
0.60 500
o
Temperature( C) Fig. 10 a, b Variation of fitting parameter A and n as the function of temperature for LPSWTN and LPDWTN
charge carriers in the samples is translational one because of small value of n (\1) [54]. Figure 10a, b shows the variation of A and n as function of temperature. It is seen that the value of n decreases with rise in temperature and becomes minimum near Tc in LPSWTN as well as LPDWTN. Again it increases with increase in temperature whereas preexponential factor A has reverse trend. The exponent n represents interaction between mobile ions with the lattice around them [52]. The observed minima at Tc suggest a strong interaction between the lattice and mobile ions. According to dynamic theory [55, 56], one of the transverse optical mode (soft mode) is weakened, and the restoring force tends to be zero at transition temperature Tc. Therefore, if charge carriers coupled with soft mode, they become very mobile at Tc, and thus conductivity will increase. The pre-exponential factor A determines the strength of polarizability. The maximum value of A at Tc suggests the presence of high polarizability (i.e., maximum dielectric constant [52].
A pyroelectric material exhibits a spontaneous electric polarization which is temperature-dependent. The pyroelectric effect arises as a result of the change of the spontaneous polarization with temperature [57]. Pyroelectric current (I) can be expressed as I ¼ CA
dT dt
where dT dt is the rate change of material’s temperature with time, A is the area of the sample/pellet electrodes and C is the pyroelectric coefficient. The pyroelectric coefficient passes through a peak at a temperature 387 °C for LPSWTN which is lower than ferroelectric transition temperature Tc and 367 °C for LPDWTN which is more than that of ferroelectric Tc (Fig. 11a). The maximum value of pyroelectric coefficient at these temperatures is found to be 5.99 9 106 lC/m-2 °C-1 and 3.6 9 105 lC/m-2 °C-1 respectively which is much higher than that of single crystal and ceramic illustrated in Table 1. It is known that in diffuse phase transition the rate of change of spontaneous polarization with temperature becomes maximum at temperature well below or above the transition temperature (Tc) [10] which confirms the materials have diffuse phase transition. Again with increase in ionic radius of R? cation, maximum value of pyroelectric coefficient increases. The piezoelectric coefficients (d33) are 0.21 and 0.24 pC/N respectively for LPSWTN and LPDWTN. Figure of merit (FOM) is an important parameter of pyroelectric materials for their heat sensing applications [58]. A pyroelectric device requires high values of FOM. To achieve maximum performance of the device, the pyroelectric material must have a high pyroelectric coefficient and a minimum dielectric constant as well as tangent loss. In literature [59] FOM is defined and calculated by using following formulas, FOM1 ¼
C er
where er is the relative permittivity of the material. In some devices the tan d, is the dominant noise source associated with the pyroelectric material. In this case a more useful FOM can be modified to C FOM2 ¼ pffiffiffiffiffiffiffiffiffiffiffiffiffiffi er tan d The FOM1 and FOM2 have been calculated from pyroelectric coefficient, dielectric constant and tan d at 100 kHz using above formula. The variation of FOM1 and FOM2 with temperature is shown in Fig. 11b and inset suggests both the materials have high figure of merit in comparison to other reported compound given in Table 1
123
314
J Mater Sci: Mater Electron (2013) 24:305–316
Fig. 11 a, b Variation of pyrocoefficient and figure of merit with temperature for LPSWTN and LPDWTN
0.40 6
Pyrocoefficient( Γ )(Cm-2o C-1)
5
Γ at Tc =5.99(Cm-2o C-1 ) (LPSWTN)
0.35
Γ at Tc =0.36(Cm-2o C-1) (LPDWTN)
0.30 0.25
4
0.20 3 0.15 2
0.10 0.05
1
Pyrocoefficient( Γ )(Cm-2o C -1)
(a)
0.00 0 0
100
200
300
-0.05 500
400
Temperature( oC) 0.010 -2 o
-1
-2 o
-1
(LPDWTN)FOM1=0.0015 (C m
(LPSWTN)FOM=0.0093 (C m
C )
-1
0.06 0.05
(LPDWTN)FOM 2
0.15
-2 o
at Tc=0.063)(C m
-1
C )
0.04
0.10
0.03 0.02
0.05
0.01 0.00
0.0012
0.0008
0.0004
0.00
0.002 0
-1
-2 o
-2 o
(LPSWTN)FOM2 at Tc=0.213)(C m C )
0.20
FOM1(C m C )(LPDWTN)
0.07
FOM2(C m-2 oC-1)(LPDWTN)
FOM2(C m-2 oC-1)(LPSWTN)
-2 o
-1
FOM1(C m C )(LPSWTN)
0.004
0.0016
0.25
0.008
0.006
(b)
C )
50 100 150 200 250 300 350 400 450
Temperature(°C)
0.0000
0.000
0
50
100
150
200
250
300
350
400
450
Temperature(°C)
and the materials are pyroelectric sensor with maximum efficiency at temperatures 387 and 367 °C which is lower and above the transition temperatures[10].
4 Conclusion Based on the above observation it is finally concluded that LPSWTN and LPDWTN have an orthorhombic TB crystal structure at room temperature. This compound shows diffuse-type of ferroelectric phase transition with transition temperature well above the room temperature. Detailed
123
studies of dielectric properties have shown a dielectric anomaly at 390 and 315 °C indicating the existence of ferroelectricity in the material. The experimental results on electrical properties indicate that the materials exhibit (1) conduction due to bulk material up to temperature 400 °C, (2) NTCR-type behavior and (3) temperature dependence of relaxation phenomena. The impedance spectrum was used to estimate the electrical conductivity. Modulus analysis indicates non-exponential type of conductivity relaxation in the material. The appearance of polarization– electric field hysteresis loop below transition temperature confirms the existence of ferroelectric properties in the
J Mater Sci: Mater Electron (2013) 24:305–316 Table 1 Comparison of pyroelectric coefficient (C), relative dielectric constant, loss tangent (tand) and figure of merit (FOM) of some standard ferroelectric materials [60] with those LPSWTN and LPDWTN
315
Material
C (lC/m2 C-1)
TGS
550
DTGS
550
LiTaO3
er
tand
FOM1 (lC/m2 K1)
FOM2 (lC/m2 K1)
55
0.025
10
464
43
0.02
12.5
593
230
47
\0.01
4.9
335
LiNbO3
83
28
NA
3.0
NA
Pb5Ge3O11
110
40
0.0005
2.8
246
SBN-50
550
400
0.003
1.4
502
PPZT
60–500
300
NA
0.02–1.67
NA
PZFNTU
380
290
0.0027
1.31
429.44
PbTiO3 PCWT4-24
180 380
190 220
0.01 0.01
0.95 1.73
130.59 256.20
LPSWTN
5.99 9 106
662
1.35
9,300
213,000
233
0.066
1,500
63,000
Sr0.5Ba0.5Nb2O6 -3,000
LPDWTN
3.6 9 10
5
materials. The pyroelectric FOM shows that the materials have good pyroelectric properties for device application.
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