Dielectric behavior and impedance analysis of lead-free CuO doped (Na0.50K0.50)0.95(Li0.05Sb0.05Nb0.95)O3 ceramics

Accepted Manuscript Dielectric behavior and Impedance analysis of Lead-free CuO doped (Na0.50K0.50)0.95(Li0.05Sb0.05Nb0.95)O3 ceramics Rashmi Rani, Seema Sharma, Radheshyam Rai, Andrei L. Kholkin PII:

S1293-2558(12)00348-2

DOI:

10.1016/j.solidstatesciences.2012.10.025

Reference:

SSSCIE 4614

To appear in:

Solid State Sciences

Received Date: 2 December 2011 Revised Date:

12 September 2012

Accepted Date: 24 October 2012

Please cite this article as: R. Rani, S. Sharma, R. Rai, A.L. Kholkin, Dielectric behavior and Impedance analysis of Lead-free CuO doped (Na0.50K0.50)0.95(Li0.05Sb0.05Nb0.95)O3 ceramics, Solid State Sciences (2012), doi: 10.1016/j.solidstatesciences.2012.10.025. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

ACCEPTED MANUSCRIPT

AC C

EP

TE D

M AN U

SC

RI PT

Title of the Manuscript: Dielectric behavior and Impedance analysis of Lead-free CuO doped (Na0.50K0.50)0.95(Li0.05Sb0.05Nb0.95)O3 ceramics Authors name: Rashmi Rani1, Seema Sharma1, Radheshyam Rai2* and Andrei L. Kholkin2 1 Ferroelectric Research Laboratory, Department of Physics, A. N. College, Patna 800013, India 2 Department of Ceramics and Glass Engineering and CICECO, University of Aveiro, 3810-193, Aveiro, Portugal appropriate graphical abstract Variation of real and imaginary part of impedance of (Na0.50K0.50)0.95(Li0.05Sb0.05Nb0.95)O3 and 0.8wt% CuO doped (Na0.50K0.50)0.95(Li0.05Sb0.05Nb0.95)O3 samples at 5000C. a b

ACCEPTED MANUSCRIPT

Dielectric behavior and Impedance analysis of Lead-free CuO doped (Na0.50K0.50)0.95(Li0.05Sb0.05Nb0.95)O3 ceramics Rashmi Rani1, Seema Sharma1, Radheshyam Rai2* and Andrei L. Kholkin2 Ferroelectric Research Laboratory, Department of Physics, A. N. College, Patna 800013, India 2 Department of Ceramics and Glass Engineering and CICECO, University of Aveiro, 3810-193, Aveiro, Portugal

RI PT

1

SC

ABSTRACT

Pure (Na0.50K0.50)0.95(Li0.05Sb0.05Nb0.95)O3 (NKNLS) and CuO doped NKNLS perovskite

M AN U

structured ferroelectric ceramics were prepared by the solid-state reaction method. x wt% of CuO (x = 0.2 - 0.8wt%) was added in the NKNLS ceramics. X-ray diffraction patterns indicate that single phase was formed for pure NKNLS while a small amount of second phase (K6Li4Nb10O30~3%) was present in Cu2+ doped NKNLS ceramics. Dielectric anomalies around the temperatures of 1200C and 3500C have been identified as the ferroelectric-paraelectric

TE D

transition (orthorhombic to tetragonal and tetragonal to cubic) temperatures for pure NKNLS compound. The electrical behavior of the ceramics was studied by impedance study in the high temperature range. Impedance analysis has shown the grain and grain boundary contribution

EP

using an equivalent circuit model. The impedance response in pure and Cu2+ doped NKNLS

AC C

ceramics could be resolved into two contributions, associated with the bulk (~grains) and the grain boundaries. From the conductivity studies, it is found that activation energies are strongly frequency dependent. The activation energy obtained from dielectric relaxation data may be attributed to oxygen ion vacancies. Keywords: Lead-free; NKN; Impedance spectroscopy; MPB; NTCR. *Corresponding author Email address: [email protected], [email protected]

1

ACCEPTED MANUSCRIPT

1.

Introduction Lead containing materials, such as Pb(ZrTi)O3, Pb(Mg1/3Nb2/3)O3–PbTiO3, and

Pb(Zn1/3Nb2/3)O3–PbTiO3 are currently applied in industries as actuator, transducer, and sensor

RI PT

materials. The traditional piezoelectric ceramics or single crystals are mostly Pb-based perovskite materials where lead oxide takes at least 70wt% of the total [1]. A common knowledge is that lead will have brought a great threat to the environment due to the toxicity of

SC

lead oxide and its high vapor pressure during processing. Lead-free piezoelectric ceramics such as Tungsten Bronze type materials, Bilayer structured materials, and Perovskite type materials

M AN U

have been intensively studied. An increasing attention has been paid to lead free or lead content reduced piezoelectric ceramics in recent years.

In the last few years, Na0.5K0.5NbO3 (NKN)-based compositions with a morphotropic phase boundary (MPB) have shown greater advantages over another typical lead-free

TE D

piezoelectric candidate material system, (Bi0.5Na0.5)TiO3 (BNT)-based MPB materials, such as higher Curie temperatures (Tc) up to nearly 4000C, better piezoelectric and electromechanical properties, and so on. NKN ceramics have two-phase transition temperatures above room

EP

temperature, one from orthorhombic to tetragonal at ~200 0C and another from tetragonal to cubic at Tc (~4200C). However, pure NKN ceramics are known to be difficult to densify fully by

AC C

ordinary sintering method. Two reasons for this phenomenon are as follows: (1) The phase stability of pure NKN is 1140 0C, which makes high sintering temperature impossible. (2) Oxygen deficiency is another problem in the preparation, which results from high temperature processing and gives rise to electronic conductivity. This is because Na2O evaporates during the sintering which decreases the poling efficiency and piezoelectric properties by reducing the resistivity of the specimens. Therefore, it is necessary to develop a method of decreasing the sintering temperature of NKN-based ceramics to below 1000 0C. Furthermore, the sintering of 2

ACCEPTED MANUSCRIPT

NKN ceramics at low temperatures would enable them to be applied to multilayer devices. Other than pressure-assisted sintering of NKN compounds [2], many other routes such as sintering aids [3-6], control of stoichiometry [7,8], alkaline or rare earth elements doping [7,9-11], solid

RI PT

solutions [12-14] with ATiO3 (A: Ca, Sr, or Ba), and so on, have been utilized to improve their densification. Sintering aids such as ZnO [4], CuO [15-17], V2O5 [18], MnO2 [19] have also been employed to improve the sinter-ability of NKN ceramics. CuO is now established as an

SC

effective sintering aid for NKN-based ceramics, enabling liquid phase sintering at temperatures around 9600C and able to modify the microstructure and piezoelectric properties. In this study,

M AN U

Li, and Sb were added to NKN ceramics by forming a solid solution to promote sintering and enhance the electrical properties. The effects of CuO additive on the densification, phase transition behavior and electrical properties have been studied. 2.

Impedance Theory

TE D

As it is well established that impedance measurements using impedance data is an effective method to study (a) properties of the intragranular and interfacial regions and their interrelations, (b) their temperature and frequency dependent phenomena in order to separate the

EP

individual contributions from the total impedance and (c) their interfaces with electronically conducting electrodes [20–24]. Each impedance parameter can be used to highlight a particular

AC C

aspect of the response of a sample. Also, it enables us to evaluate the relaxation frequency, which is an intrinsic property of the sample, independent of its geometrical factors. More interestingly, the impedance measurements enable us to eliminate the error, if any, due to stray frequency effects. The frequency dependence of various impedance parameters of a material can be described via the permittivity (ε*), impedance (Z*), admittance (Y*), electric modulus (M*) and dielectric loss or dissipation factor (tan δ) [18]. A simultaneous analysis of the permittivity

3

ACCEPTED MANUSCRIPT

(ε), impedance (Z), admittance (Y) and modulus (M) can provide a complete description of the frequency dependent properties of materials. Impedance data of materials that have capacitive (reactive) and resistive components,

RI PT

when represented in the Impedance plot [i.e., the negative of the imaginary part (-Im Z) in the y axis and the real part (Re Z) in the x axis—each point corresponding to a different frequency], lead to a succession of semicircles. For example, a polycrystalline material usually presents grain

SC

and grain boundary properties with different time constants leading to two successive semicircles. In this case, the intercept of the high frequency semicircle with the real axis is the

M AN U

bulk resistance (Rb) of the sample. Hence, the bulk electrical conductivity (σb) is written σb = 1/Rb * l/A, where l is the thickness and A the area of the electrode deposited on the sample. Electrical conductivity of ceramic materials is thermally activated. The relaxation frequency (f0) of the material, independently of the geometrical parameter of the sample, is found at the apex of

TE D

the semicircle and fulfils the condition 2π f0RbCb=1. From this relation, the bulk capacitance of the material (Cb), also called the geometric capacitance can be calculated. Experimental procedures

3.1.

Materials and Methods

EP

3.

Pure (Na0.50K0.50)0.95(Li0.05Sb0.05Nb0.95)O3 (NKNLS) and x wt% CuO doped NKNLS

AC C

(where x = 0.2, 0.4, 0.6 and 0.8) ceramics were prepared by the conventional mixed oxide route. The starting materials were high purity metal oxide or carbonate powders, Nb2O5 (>99.9%), K2CO3 (>99.8%), Na2CO3 (>99.8%), Sb2O3, Li2CO3 (>99.99%), and CuO (>99.9%). The carbonates were dried at 2500C for 6h prior to use, then all the powders were weighed according to the required compositions and mixed for 48h using propan-2-ol and zirconia media. The powders were calcined at 850–930 0C for 4h then CuO (0.2–0.8 wt%) was added, wet milled for 24–48h and dried. Pellets were prepared by pressing powders in a 10mm diameter die at 4

ACCEPTED MANUSCRIPT

100MPa, then sintering at 850 0C - 11000C for 2–6h and finally cooling to room temperature at 1800C/h. 3.2. Structural Characterization

RI PT

Product densities were determined from weight and dimension measurements. The density of the sintered samples increased with CuO content in NKNLS ceramics.

The crystal structures were examined by X-ray diffraction. This was undertaken using a Philips

SC

Analytical, X’pert-MPD, employing CuKα radiation under the conditions 50kV and 40mA. The samples were scanned at an interval of 0.030 /min for 2θ in the range 10–800. The identification

M AN U

of the peaks was carried out using the Topas23 refinement programme. The initial atomic coordinates for the NKN crystal structure were taken from Kumada et al. [25]. Microstructural examination of the fractured surface of the ceramics was carried out by scanning electron microscopy (SEM) (JEOL 6300 and Philips XL30, equipped with energy

3.3.

TE D

dispersive spectrometers (EDS) for chemical analysis). Electrical Characterization

For dielectric measurements, the disc shaped samples were ground on SiC paper to

EP

reduce the thickness to less than 1mm and coated with silver paste. Dielectric constant, loss tangent and impedance were determined by use of PSM1735 Impedance Analyzer at frequencies

AC C

1kHz to 10 MHz; samples were heated from room temperature to 500 0C in a Carbolite (MTF9/15/130) tube furnace. 4.

Results and discussion

4.1. Structure Analysis

Fig. 1 a-e shows the XRD patterns of the NKNLS ceramics sintered at 10800C and CuO added NKNLS ceramics sintered at 970 0C for 2 h. A homogeneous orthorhombic phase was developed for the specimens with x = 0. However, this orthorhombic phase gradually changed to 5

ACCEPTED MANUSCRIPT

the tetragonal phase when the CuO content exceeded 0.4, and tetragonal phase was formed for the specimens with x > 0.6. Therefore, both orthorhombic and tetragonal phases coexisted for the specimens with 0.4 < x < 0.6. In addition, the homogeneous NKNLS phase was developed for

RI PT

the specimens with x = 0, but peaks for K6Li4Nb10O30 (KLN) second phase indicated by the asterisks were found when CuO is introduced and their intensity increased [26]. This KLN second phase was also observed in the (1-x)NKN-xLiNbO3 ceramics and its presence was related

SC

to the formation of the liquid phase due to the Na2O evaporation, which occurred at temperatures above 950 0C [26]. The melting temperature of the KLN second phase was approximately 960 0C

M AN U

[27], indicating that it existed as a liquid phase during the sintering of the NKNLS ceramics at 970 0C and assisted their densification. Therefore, it is considered that liquid phase assisted sintering occurred in the NKNLS ceramics. Refinement of the spectrum for undoped NKNLS showed 65% could be attributed to orthorhombic and the rest to the tetragonal phase. The CuO

TE D

doped NKLN samples showed very similar XRD spectra, but there was broadening of the (200) peak position [28]. Accurate determinations of orthorhombic to tetragonal phase ratios and lattice parameters must await appropriate high resolution synchrotron radiation XRD

EP

investigations. The use of dopants can usually be expected to expand possible application of the materials by introducing special properties, for example, soft or hard properties in a

AC C

piezoelectric material for various applications. In this work, NKNLS ceramics are modified by introducing a small amount of copper oxide (CuO) in ABO3 perovskite structure. Cu ions are anticipated to enter into the B site, generating oxygen vacancies. The CuO addition did not generate any evident lattice distortion as can be seen from the XRD. On one hand, the formation of oxygen vacancies as a charge equilibrium mechanism tends to cause the lattice shrinkage and on the other hand, considering that Cu2+ has a relatively large ionic radius compared with Nb5+ (ionic radii: 0.73 and 0.64 Å for Cu2+ and Nb5+, respectively, 6

ACCEPTED MANUSCRIPT

CN = 6), lattice expansion can be expected by increasing the content of CuO. These two effects compensate with each other, leading to non shift in the position of diffraction peaks of the XRD pattern. Microstructure Analysis

RI PT

4.2.

Figure 2 a-e shows the SEM images of the fractured surface of the NKLNS ceramics with 0.2
SC

depicted in Fig. 2. Average grain size in undoped ceramics was 2-3 µm and CuO added ceramics was 1-2 µm respectively. Densities of ceramics were also slightly increased and reached ~96.5%

M AN U

by adding small amounts of CuO. In addition, grain morphology changed from sharp-cornered cubical grains with smooth surfaces to cut cornered grains with rough surfaces. It appeared that introducing the Cu2+ altered the growth behavior of the grains by decreasing the surface energy. Based on the Jackson solid–liquid interface model, high entropy of fusion (∆Sf >2R; R:1.9872

TE D

cal/mol K) thermodynamically favors the layer by layer growth and, therefore, creates atomically smooth growth surface [27]. However, at lower entropy of fusion (∆Sf <2R), there is no preferential growth and hence random growth with rough surfaces occurs. It was hypothesized

EP

that the addition of Cu2+ decreased the entropy of fusion in NKNLS system by lowering its surface free energy, thus giving rise to the observed rough surfaces. Figure 2f shows the EDS

AC C

spectra of NKNLS composition (x = 0.5) sintered at 10800C for 2h. All the peaks of the elements present in the NKNLS are observed and assigned. Peaks for Cu and C are from the grid used in TEM measurements. Li could not be detected by the EDS, as was also reported by others [28]. 4.3. Dielectric Analysis

Dielectric-temperature dependence of relative permittivity (εr´) and imaginary permittivity (εr´´) dielectric loss were measured upon cooling at the frequency of 10 kHz in the temperature range of Room Temperature (RT)–500 0C. 7

It showed (Figure 3a) similar

ACCEPTED MANUSCRIPT

temperature–permittivity behaviors with three distinctive regions separated by two phase transitions. For undoped ceramics, the orthorhombic–tetragonal (To–t) and tetragonal–cubic (Curie temperature; Tc) phase transitions occurred at 110 0C and 3300C, respectively [29-31].

RI PT

The low temperature phase transition temperature (orthorhombic–tetragonal; To–t) and the ferroelectric-paraelectric (FE-PE) transition temperature shifted towards higher temperature upon addition of Cu2+. The dielectric value increased with CuO doping in the system indicating

SC

that the improved dielectric values may have been due to the increased density. Temperature dependence of εr´´ (Figure 3b) showed the similar trend of changes. At

M AN U

temperatures >3000C and particularly above Tc, the addition of Cu2+ resulted in a sharp increase in εr´´. This was mainly due to a decrease in bulk resistivity of NKNLS ceramics at higher temperature. In low frequency region = 1 kHz, at ~ 4000C a dielectric relaxation peak in pure NKNLS is observed. The authors suggest that the peak results from the contribution of the

TE D

reorientation of the Na1+ and Nb5+ ions coupling with the thermally activated conduction electrons which appear due to ionization of the oxygen vacancies; these electrons interact with the dipoles of the Na1+ and Nb5+ ions and contribute to the dielectric relaxation peak [32].

EP

Whereas, in the high frequency region the mobility increases for the charge carriers and the concentration of intrinsic oxygen vacancies and electrons become smaller. It is well known that

AC C

in perovskite ferroelectrics the oxygen vacancies could be formed in the process of sintering due to the escape of oxygen from the lattice [33, 34]. Thus, this anomaly could be correlated with a low-frequency relaxation process due to oxygen vacancies. The semiconductive grains in ceramic is believed to lose traces of oxygen during sintering at high temperature as per [35] Kroger Vink notation of defects. O x o = ½ O2 (g) + V x o V x o = V · o + e’ 8

ACCEPTED MANUSCRIPT

V · o = V”· o + e” Thus excess electrons and oxygen vacancies are formed in the reduction reaction O x o = ½ O2 (g) + V”· o + 2e

RI PT

These electrons trapped by oxygen vacancies can be thermally activated, thus enhancing the conduction process. Doubly charged oxygen vacancies are considered to be the most mobile charges in perovskite ferroelectrics and play an important role in conduction [35]. The broadness

SC

in the ε’ versus temperature may be attributed to the structural disorder and compositional fluctuations in the ceramics. The magnitude of εr´' decreases with increase in frequency; this may

M AN U

be due to the presence of interfacial polarization which arises due to difference in conductivity in the material. The motion of the charge carriers gets interrupted at the grain boundary due to lower conductivity. In case of polycrystalline ceramics, this is commonly observed if the grains are semi-conducting and the grain boundaries are insulating [34]. Complex Impedance Analysis

TE D

4.4.

Figure 4 a-e shows temperature dependent spectra of NKNLS and CuO doped (x=0.8) samples. The impedance spectrum is featured by semicircular arcs. The nature of variation of the

EP

arcs with temperature and frequency provides various clues of the materials. The plots typically consist of single semicircular arc, and as the temperature increases it resolved further. For

AC C

NKNLS and CuO doped NKNLS compounds the Impedance plot comprise of two semicircular arcs whose center lies below the real axis. The most acceptable approach to interpret the depression of semicircles is static distribution of relaxation times. All the impedance plots in NKNLS exhibits the phenomena of decentralization, in which the centers of semi-circles that compose the total electric response centered below the real axis making an angle (φ) with x-axis. It has been observed that the value of φ is found to increase with increase in temperature and at the transition temperature the value of φ is maximum, further increase in temperature results 9

ACCEPTED MANUSCRIPT

decrease in the value of φ. In addition, it has been observed that the shape of the diagram (Fig. 4) suggests that the electrical response is composed of overlapping of two semi-circles. Also, the high overlapping degree suggests that each contribution presents very similar relaxation

RI PT

frequency. The small semi circles definition is assigned to very similar values of most relaxation frequency, with relation each relaxation phenomenon detected, one to grain and other to grain boundary. Therefore, the overlapping increases with decreasing of magnitude of the difference

SC

between the most relaxation frequencies. The width of the overlapping decreases with increase in temperature. The electric and dielectric properties of NKLNS system are well represented by two

M AN U

parallel QR elements in series for pure NKNLS and two parallel CQR elements in series for doped NKNLS, where C=Capicitance, Q=Leaky Capicitance and R=Resistance. The first relaxation phenomenon was observed at lower frequencies which represents the grain boundary contribution to the electrical response. The second one, in the high frequency range was observed

TE D

which corresponds to the specific properties of grains or bulk.

According to Debye’s model, a material having single relaxation time gives rise to an ideal semicircle centered on the real axis. The first semicircle (at higher frequencies) attributed to

EP

transport phenomena in the bulk and another at lower frequencies, is related to transport phenomena at grain boundary [33]. An equivalent circuit is being used to provide a complete

AC C

picture of the system and establish the structural property relationship of the materials. Comparison of complex impedance plots (symbols) with fitted data (lines) using commercially available software ZSimp WIN Version 2 has been given in Figure 5(a,b). To Model the nonDebye response, constant phase element (CPE) are used extensively in equivalent electrical circuits for fitting of experimental impedance data in addition to resistors and capacitors. Figure 6 (a,b) exhibits the temperature dependency of real part of impedance (Z΄) against frequency. All the plots show a low frequency dispersion followed by a plateau region, and finally merging of 10

ACCEPTED MANUSCRIPT

all the curves at and above 10 kHz irrespective of temperature. The dispersion region spreads in a high frequency regime with increase in temperature. The initial decrease in Z' value with frequency may be due to a slow dynamics relaxation process in the material probably due to

RI PT

space charge. At high temperatures, low frequency plateau region is observed. This may be related to the dc conductivity property of the material, and the finally merger of the pattern at higher frequency may be attributed to the release of space charge [34]. The value of Z' also

SC

shows a decreasing trend with rise in the temperature showing the phenomenon typical to the negative temperature coefficient of resistance (i.e. NTCR type) behavior in semiconductors.

M AN U

Further, the impedance value is observed to increase first in the low frequency region with rise in temperature followed by a relative decrease at subsequently higher temperatures. The result may be related to a change in the charge-ordering pattern with temperature; the effect showing clearly perceptible change in its behavior in the low frequency region. Figure 7 (a,b) shows the

TE D

variation of imaginary part of the impedance with frequency at different temperatures. The curves are asymmetric and deviating from ideal Debye like curve and supporting the behaviour of Impedance plots. The appearance of peaks at a characteristic frequency ωmax (=2πfmax)

EP

dependent on temperature can be related to the type and strength of the electrical relaxation phenomena in the material possible due to increasing relaxation in the sample. The magnitude of

AC C

Z΄΄ maxima decreases with temperature indicating increasing loss in the resistive property of the sample. This behavior of impedance pattern arises possibly due to the presence of space charge in materials [35]. 4.5.

AC conductivity Analysis The ac conductivity relates to the dielectric constant and dielectric loss through the

relation σac=ωεεotanδ. Figure 8 (a,b) shows the variation of ac conductivity for x = 0 and x = 0.8 ceramics as a function of frequency at different temperatures for all the compounds. As 11

ACCEPTED MANUSCRIPT

temperature increases, the frequency at which the dispersion becomes prominent shifts to higher frequency region and the conductivity dispersion region decreases. The nature of variation of curves indicates characteristic dispersion phenomena in low frequency region for all the

RI PT

temperatures. This frequency dependence of conductivity obeys Jonscher’s power law equation [36] σω= σdc + Aωn(0
SC

and A is the frequency independent but weakly temperature dependent term. It has been observed that low frequency dispersion obeying the power law feature σωn changes its slope

M AN U

governed by n. The frequency at which change in slope takes place is known as hopping frequency (op). At higher frequency, the hopping takes place by charge carriers through trap sites separated by energy barriers of varied heights. The number of charge carriers with high relaxation time due to higher energy barriers respond in low frequency region might be less in

TE D

numbers and so the conductivity is lower at low frequencies. While the more charge carriers are with low barrier heights and these charge carriers easily respond with the frequency and at higher frequency they showed higher conductivity. The thermal energy of the charge carriers is

EP

increased at higher temperatures and so the potential height is also reduced. The slopes of the curves, which is designated as n, were calculated at different temperatures. The value of n less

AC C

than unity is associated with charge carriers or with extrinsic dipoles arising from the presence of defects and impurities in the sample [37]. In CuO doped samples, conductivity spectrum shows a low frequency dispersion region followed by a high frequency plateau region. The low frequency region has been attributed to the ac-conductivity whereas the frequency independent plateau region corresponds to the dc conductivity. The dispersion behaviour may be due to the presence of space charge while in the plateau region space charge vanishes. This is reasonable since the space charge vanishes at higher frequency domain. At lower temperatures σac linearly varies with 12

ACCEPTED MANUSCRIPT

frequency and nonlinearity occurs in the high frequency region. This behaviour suggests that the hopping mechanism might be playing an important role in conduction process in the low temperature region [41]. Thus the trend of ac-conductivity characteristics with frequency

RI PT

represents universal power law.

Fig. 9 shows the Arrehenius plot of the σac vs. 103/T for x=0 and 0.8 respectively. A small change in slope is observed around Curie temperature. But this change is prominent only at

SC

high frequencies. This is because at low frequencies oxygen vacancies become active with other defects and prevents the growth of ferroelectric phase, and hence there is no clear peak appears

M AN U

in the slope. At low temperature region the ac conductivity becomes less temperature dependent. The increasing trend of σac in the low frequency range may be due to the disordering cations between the neighboring sites and the presence of space charges that vanishes at higher temperatures and frequencies. The high conductivity in NKNLS is believed to be due to the

TE D

oxygen vacancies generated because of the doping of Cu2+. A frequency independent relation between ac-conductivity and inverse of absolute temperature in the high-temperature region suggests the validity of relation σ = σo exp(-EA/kβT) where σ0, Ea and kβ represent the pre-

EP

exponential factor, activation energy of the mobile charge carriers and Boltzmann constant, respectively. The activation energy values obtained for NKNLS and Cuo doped (x=0.8) ceramics

AC C

were 0.92 and 0.90 respectively. On the basis of high values of activation energy it can be concluded that oxygen vacancies (doubly ionized) are the most likely charge carriers operating in these ceramics [42]. At room temperature the oxygen vacancies exhibit a low mobility, whereby the ceramic samples exhibit an enhanced resistance. However with rising temperature they are activated and contributed to the observed electrical behavior. 5. Conclusions

13

ACCEPTED MANUSCRIPT

The present work reports the results of our investigation on the dielectric and electrical properties of NKNLS prepared by high temperature solid state reaction method using the CIS technique. The powders were calcined at 850–930 0C and sintered at 850–1100 0C. Small

RI PT

additions of CuO reduced the sintering temperature and increased the density to 96.5% theoretical. A change of crystalline structure from orthorhombic to tetragonal was found with increase in CuO content. Average grain size as determined from SEM study was 2-3 µm for

SC

undoped ceramics and 1-2 µm for CuO added ceramics. The electrical properties indicate that the material exhibits (a) bulk conduction up to a temperature of 325 °C, (b) grain boundary

M AN U

conduction at higher temperature (up to 500 °C), (c) negative temperature coefficient of resistance (NTCR) behavior, and (d) temperature-dependent relaxation phenomena. The electrical conduction in the ceramics may be due to the mobility of the oxide ion (O2−) vacancies

Acknowledgement

TE D

at higher temperatures.

EP

Seema Sharma is grateful to Department of Science and Technology, Govt of India for providing financial support for this project. Radheshyam Rai is grateful to the Foundation for

AC C

Science and Technology of Portugal (FCT) for financial support within the projects PTDC/CTMCER/115085/2009 and PTDC/FIS/108025/2008.

14

ACCEPTED MANUSCRIPT

References

AC C

EP

TE D

M AN U

SC

RI PT

[1] B. Jaffe, W.R. Cook, H. Jaffe, Piezoelectric Ceramics, Academic Press, New York, 1971. [2] J.F. Li, K. Wang, B.P. Zhang, L.M. Zhang, J. Am. Ceram. Soc. 89 (2006) 706-709. [3] R.Z. Zuo, J. Rodel, R.Z. Chen, L.T. Li, J. Am. Ceram. Soc. 89 (2006) 2010-2015. [4] S.H. Park, C.W. Ahn, S. Nahm, J.S. Song, Jpn. J. Appl. Phys. 43 (2004) L1072-L1074. [5] C.W. Ahn, H.C. Song, S. Nahm, S.H. Park, K. Uchino, S. Priya, H.G. Lee, N.K. Kang, Jpn. J. Appl. Phys. 44 (2005) L1361- -L1364. [6] M. Matsubara, T. Yamaguchi, K. Kikuta, S. Hirano, Jpn. J. Appl. Phys. 43 (2004) 71597163. [7] Z.S. Ahn, W.A. Schulze, J. Am. Ceram. Soc. 70 (1987) C–18-C-21. [8] J. Yoo, K. Lee, K. Chung, S. Lee, K. Kim, J. Hong, S. Ryu, C. Lhee, Jpn. J. Appl. Phys. 45 (2006) 7444-7448. [9] B. Malic, J. Bernard, J. Holc, D. Jenko, M. Kosec, J. Eur. Ceram. Soc. 25 (2005) 2707-2711. [10] S.N. Murty, K. Umarantham, A. Bhanamathi, Ferroelectrics 82 (1988) 141- 147. [11] M. Kosec, D. Kolar, Mater. Res. Bull. 10 (1975) 335-339. [12] Y. Chang, Z. Yang, L. Wei, B. Liu, Mater. Sci. Eng. A 437 (2006) 301-305. [13] M. Kosec, V. Bobnar, M. Hrovat, J. Bernard, B. Malic, J. Holc, J. Mater. Res. 19 (2004) 1849-1854. [14] Y. Guo, K. Kakimoto, H. Ohsato, Jpn. J. Appl. Phys. 43 (2004) 6662-6666. [15] N.M. Hagh, K. Kerman, B. Jadidian, A. Safari, J. Eur. Ceram. Soc. 29 (2009) 2325–2332. [16] H. Y. Park, C.W. Ahn, K.H. Cho, S. Nahm, H.G. Lee, H.W. Kang, D.H. Kim, K.S. Park. J. Am. Ceram. Soc. 90 (2007) 4066–4069. [17] H.Y. Park, J.Y. Choi, M.K. Choi, K.H. Cho, S. Nahm, H.G. Lee, J Appl. Phys. 104 (2008) 034103-034110. [18] I.T Seo, H.Y. Park, N.D. Van, M.K. Choi, S. Nahm, H.G. Lee. IEEE Trans. Ultranson. Ferroelectric Freq. Contr. 56 ( 2009) 2337–2342. [19] D.K. Lin, K.W. Wok, H.L.W. Chan. J Alloys Compd. 461 (2008) 273–278. [20] A. R. West, D. C. Sinclair, N. Hirose, J. Electroceram. 1 (1997) 65-71. [21] J. R. Macdonald, Impedance Spectroscopy, Emphasizing Solid Materials and Systems, Wiley Interscience Publ., New York, 1987. [22] J. T. S. Irvine, D. C. Sinclair, A. R. West, Adv. Mater. 2 (1990) 132-138. [23] D. C. Sinclair, A. R. West, J. Appl. Phys. 66 (1989) 3850-3857. [24] R. Gerhardt, J. Phys. Chem. Solids 55 (1994) 1491-1506. [25] N. Kumada, T. Kyoda, Y. Yonesaki, T. Takei, N. Kinomura Mater Res Bull 42 (2007) 1856–1862. [26] H.C. Song, K.H. Cho, H.Y. Park, C.W. Ahn, S. Nahm, K. Uchino, S.H. Park, J. Am. Ceram. Soc. 90 (2007) 1812-1816. [27] F. Azough, M. Wegrzyn, R. Freer, S. Sharma, D. Hall, J. Europ. Ceram. Soc. 31 (2011) 569–576. [28] Martin, E. and Glicksman, Diffusion in Solids: Field Theory, Solid-State Principles and Applications, John Wiley Inter-science Publishers, New York, NY, 1999. [29] K.S. Lee, J.H. Yoo, Current Applied Physics. 12 (2012) 798-802. [30] Rashmi Rani, Seema Sharma, Radheshyam Rai, Andrei L. Kholkin, Materials Research Bulletin. 47 (2012) 381-386. [31] Laijun Liu, Yanmin Huang, Yunhua Li, Liang Fang, Hichem Dammak, Huiqing Fan, Mai Pham Thi, Materials Letters. 68 (2012) 300-302. 15

ACCEPTED MANUSCRIPT

AC C

EP

TE D

M AN U

SC

RI PT

[32] C. Ang, Z. Yu, L.E. Cross, Phys. Rev. B 62 (2000) 228–236. [33] R.R. Neurgoanker, L.E. Cross, Mater. Res. Bull. 21 (1986) 893-899. [34] M.A.L. Nobre, S. Lanfredi, Mater. Lett. 47 (2001) 362-366. [35] C. Ang, Z. Yu, Z. Jing, P. Lunkenheimer, A. Loidl, Phys. Rev. B 61(2000) 3922-3926. [36] M. N. Rahaman, Ceramic Processing and Sintering, Dekker, New York, 1995. [37] S. Sen, R.N.P. Choudhary, A. Tarafdar, P. Pramanik. J. Appl. Phys. 99 (2006) 124114124122. [38] J. R. Macdonald, Impedance Spectroscopy, Wiley, New York, 1987. [39] D.K. Pradhan, R.N.P. Choudhary, C. Rinaldi, R.S. Katiyar, J. Appl. Phys. 106 (2009) 024102-024112. [40] A.K. Jonscher, Nature 267 (1977) 673- 679. [41] A. Mansingh, A. Dhar, J. Phys. D: Appl. Phys. 18 (1985) 2059-2072. [42] S.S.N. Bharadwaja, P. Victor, P. Venkateswarlu, S.B. Krupanidhi, Phys. Rev. B 65 (2002) 174106.

16

ACCEPTED MANUSCRIPT

Figure captions

RI PT

Figure 1. Room temperature XRD patterns of (Na0.50K0.50)0.95(Li0.05Sb0.05Nb0.95)O3 and (x)wt% CuO doped (Na0.50K0.50)0.95(Li0.05Sb0.05Nb0.95)O3 samples with different concentration x = 0.2, 0.4, 0.6 and 0.8.

SC

Figure 2. SEM micrographs of (Na0.50K0.50)0.95(Li0.05Sb0.05Nb0.95)O3 and (x)wt% CuO doped (Na0.50K0.50)0.95(Li0.05Sb0.05Nb0.95)O3 samples with different concentration x = 0.2, 0.4, 0.6 and 0.8.

M AN U

Figure 3. (a) Variation of εr´, (b) Variation of εr´´ of (Na0.50K0.50)0.95(Li0.05Sb0.05Nb0.95)O3 and 0.8wt% CuO doped (Na0.50K0.50)0.95(Li0.05Sb0.05Nb0.95)O3 as a function of temperature at 10 kHz respectively. Figure 4. Variation of real and imaginary part of impedance with temperature of (Na0.50K0.50)0.95(Li0.05Sb0.05Nb0.95)O3 and 0.8wt% CuO doped (Na0.50K0.50)0.95(Li0.05Sb0.05Nb0.95)O3 samples. Figure 5. (a,b) Variation of real and imaginary part of impedance of (Na0.50K0.50)0.95(Li0.05Sb0.05Nb0.95)O3 and 0.8wt% CuO doped (Na0.50K0.50)0.95(Li0.05Sb0.05Nb0.95)O3 samples at 5000C.

TE D

Figure 6. (a,b) Variation of real part (Z´) of (Na0.50K0.50)0.95(Li0.05Sb0.05Nb0.95)O3 and 0.8wt% CuO doped (Na0.50K0.50)0.95(Li0.05Sb0.05Nb0.95)O3 samples with frequency at different temperature.

EP

Figure 7. (a,b) Variation of imaginary part impedance (Z″) of (Na0.50K0.50)0.95(Li0.05Sb0.05Nb0.95)O3 and 0.8wt% CuO doped (Na0.50K0.50)0.95(Li0.05Sb0.05Nb0.95)O3 samples with frequency at different temperature.

AC C

Figure 8 (a,b). Variation of polarization conductivity σac with frequency at different temperatures for (Na0.50K0.50)0.95(Li0.05Sb0.05Nb0.95)O3 and 0.8wt% CuO doped (Na0.50K0.50)0.95(Li0.05Sb0.05Nb0.95)O3 samples. Figure 9. Temperature dependence of the σac of (Na0.50K0.50)0.95(Li0.05Sb0.05Nb0.95)O3 and 0.8wt% CuO doped (Na0.50K0.50)0.95(Li0.05Sb0.05Nb0.95)O3 samples.

17

211

0.8

*

202 220

*

32000

201 102

002 200

*K6Li4Nb10O30

111

001 100

110

40000

SC

RI PT

ACCEPTED MANUSCRIPT

M AN U

Intensity (a.u)

0.6

24000

0.4

16000

0.2

8000

0.0

0 30

40

TE D

20

50

60

70

2θ

AC C

EP

Figure 1.Room temperature XRD patterns of (Na0.50K0.50)0.95(Li0.05Sb0.05Nb0.95)O3 and (x)wt% CuO doped (Na0.50K0.50)0.95(Li0.05Sb0.05Nb0.95)O3 samples with different concentration x = 0.2, 0.4, 0.6 and 0.8.

18

ACCEPTED MANUSCRIPT

0.2

SC

RI PT

0.0

0.6

0.8

AC C

EP

0.8

TE D

M AN U

0.4

Figure 2. SEM micrographs of (Na0.50K0.50)0.95(Li0.05Sb0.05Nb0.95)O3 and (x)wt% CuO doped (Na0.50K0.50)0.95(Li0.05Sb0.05Nb0.95)O3 samples with different concentration x = 0.2, 0.4, 0.6 and 0.8.

19

RI PT

ACCEPTED MANUSCRIPT

8000

At 10 kHz NKNLS+0.8%CuO 4000

NKNLS

SC

6000

3000

2000

NKNLS

M AN U

εr´´

εr´

4000

NKNLS+0.8%CuO

2000

1000

0

0

100

200

300

400

0

500

0

100

200

300

400

500

0

Temperature( C)

TE D

Temperature ( C)

AC C

EP

Figure 3. (a) Variation of εr´, (b) Variation of εr´´of (Na0.50K0.50)0.95(Li0.05Sb0.05Nb0.95)O3 and 0.8wt% CuO doped (Na0.50K0.50)0.95(Li0.05Sb0.05Nb0.95)O3 as a function of temperature at 10 kHz respectively.

20

ACCEPTED MANUSCRIPT

1200 0

300

NKNLS NKNLS+0.08CuO

250

600

150

100

400

50

200 0 0

0 0

200

400

600

800

1000

50

100

150

1200

200

SC

20

30 0

425 C

18

NKNLS NKNLS+0.08CuO

M AN U

16 14

20

250

300

Z'(k Ω)

Z'(kΩ)

25

400 C

200

Z''(kΩ)

Z''(kΩ)

800

0

NKNLS NKNLS+0.8CuO

RI PT

1000

350 C

0

450 C NKNLS NKNLS+0.08CuO

Z''(kΩ)

Z''(kΩ)

12

15

10 8 6

10

4

5

2 0

0

0 0

5

10

15

20

25

2

4

6

8

10

12

14

16

18

20

Z'(kΩ)

30

TE D

Z'(kΩ)

50

0

475 C NKNLS NKNLS+0.08CuO

EP

40

AC C

Z´´(kΩ)

30

20

10

0 0

10

20

30

40

50

Z´(k Ω )

Figure 4 (a-e). Variation of real and imaginary part of impedance with temperature of (Na0.50K0.50)0.95(Li0.05Sb0.05Nb0.95)O3 and 0.8wt% CuO doped (Na0.50K0.50)0.95(Li0.05Sb0.05Nb0.95)O3 samples.

21

RI PT

ACCEPTED MANUSCRIPT

b

TE D

M AN U

SC

a

AC C

EP

Figure 5 (a,b) Variation of real and imaginary part of impedance of (Na0.50K0.50)0.95(Li0.05Sb0.05Nb0.95)O3 and 0.8wt% CuO doped (Na0.50K0.50)0.95(Li0.05Sb0.05Nb0.95)O3 samples at 5000C.

22

RI PT

ACCEPTED MANUSCRIPT

b

a 600 0

1200

Z'(kΩ)

1000 800 600

SC

1400

NKNLS+0.8%CuO 500

400

Z'(kΩ)

300 C 0 325 C 0 350 C 0 375 C 0 400 C 0 425 C 0 450 C 0 475 C 0 500 C

M AN U

NKNLS

1600

300

0

300 C 0 325 C 0 350 C 0 375 C 0 400 C 0 425 C 0 450 C 0 475 C 0 500 C

200

400

100

200 0

0

1

10

Frequency (kHz)

100

1000

0.1

TE D

0.1

1

10

100

1000

Frequency(kHz)

AC C

EP

Figure 6. (a,b) Variation of real part (Z´) of (Na0.50K0.50)0.95(Li0.05Sb0.05Nb0.95)O3 and 0.8wt% CuO doped (Na0.50K0.50)0.95(Li0.05Sb0.05Nb0.95)O3 samples with frequency at different temperature.

23

RI PT

ACCEPTED MANUSCRIPT

b

a 3500

1200

Z''(k Ω)

2500

100

50

2000

z"(kΩ)

0 100

1500

1000

10000

Frequency (kHz)

1000

SC

150

300

200

800

600

0 10

400

1000

0

400C 0 425C 0 450C 0 475C 0 500C

100

Z''(kΩ)

400 C 0 425 C 0 450 C 0 475 C 0 500 C

3000

300 C 0 325 C 0 350 C 0 375 C 0 400 C 0 425 C 0 450 C 0 475 C 0 500 C

M AN U

0

0

NKNLS+0.8%CuO

0

200

z''(k Ω)

NKNLS

100

1000

300 C 0 325 C 0 350 C 0 375 C 0 400 C 0 425 C 0 450 C 0 475 C 0 500 C

Frequency(kHz)

200

500

0

0

0.1

1

10

100

1000

0.1

TE D

Frequency(kHz)

1

10

100

1000

Frequency(kHz)

AC C

EP

Figure 7. (a,b) Variation of imaginary part impedance (Z″) of (Na0.50K0.50)0.95(Li0.05Sb0.05Nb0.95)O3 and 0.8wt% CuO doped (Na0.50K0.50)0.95(Li0.05Sb0.05Nb0.95)O3 samples with frequency at different temperature.

24

RI PT

ACCEPTED MANUSCRIPT

b

a

NKNLS+0.8%CuO

0.01

σac (Ω-m)-1

M AN U

0.01

σac(Ω-m)-1

1E-3

0

300 C 0 350 C 0 400 C 0 450 C 0 500 C

1E-4

1E-5 1

SC

0.1

NKNLS

10

100

0

300 C 0 350 C 0 400 C 0 450 C 0 500 C

1E-4

1E-5

1

TE D

Frequency (kHz)

1000

1E-3

10

100

1000

Frequency (kHz)

AC C

EP

Figure 8 (a,b). Variation of polarization conductivity σac with frequency at different temperatures for (Na0.50K0.50)0.95(Li0.05Sb0.05Nb0.95)O3 and 0.8wt% CuO doped (Na0.50K0.50)0.95(Li0.05Sb0.05Nb0.95)O3 samples.

25

RI PT

ACCEPTED MANUSCRIPT

-7

NKNLS NKNLS+0.8%CuO

At 10kHz -8

SC

-10 -11 -12 -13 -14 -15 1.5

2.0

M AN U

σac (Ω-m)-1

-9

2.5

3.0

3.5

3

10 /T (K)

AC C

EP

TE D

Figure 9. Temperature dependence of the σac of (Na0.50K0.50)0.95(Li0.05Sb0.05Nb0.95)O3 and 0.8wt% CuO doped (Na0.50K0.50)0.95(Li0.05Sb0.05Nb0.95)O3 samples.

26