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Structural and electrical studies in La-substituted SrBi2Nb2O9 ferroelectric ceramics
ARTICLE IN PRESS
Physica B 371 (2006) 337–342 www.elsevier.com/locate/physb
Structural and electrical studies in La-substituted SrBi2Nb2O9 ferroelectric ceramics Vaibhav Shrivastavaa, A.K. Jhaa,, R.G. Mendirattab a
Department of Applied Physics, Delhi College of Engineering, Delhi 110042, India School of Applied Sciences, Netaji Subhas Institute of Technology, Delhi 110075, India
b
Received 26 January 2005; received in revised form 20 October 2005; accepted 23 October 2005
Abstract In this paper, structural and electrical studies of A-site lanthanum-substituted Sr1xLaxBi2Nb2O9; x ¼ 0:020:5 ceramics are reported. The introduction of lanthanum produces pore-free microstructure upto x ¼ 0:3. A diffuse ferroelectric to paraelectric phase transition is observed in the doped samples and positive temperature coefficient of resistance (PTCR) is observed in x ¼ 0:0, 0.4 and 0.5 samples whereas x ¼ 0:120:3 samples show negative temperature coefficient of resistance (NTCR). Curie temperature decreases continuously with increase in lanthanum content. r 2005 Elsevier B.V. All rights reserved. Keywords: Donor doping; Space-charge polarisation; DC conductivity; Dielectric loss
1. Introduction Bismuth-layered SrBi2Nb2O9 (SBN) and SrBi2Ta2O9 (SBT) ceramics have drawn attention of researchers due to their excellent fatigue characteristics as compared to those of Pb(Zr,Ti)O3 (PZT) ceramics [1,2]. Therefore, doping in SBN- and SBT-type layered ceramics to improve the electrical and structural properties have been a matter of interest [3,4]. These ceramics show high values of Curie temperature and fatigue resistance, but suffer from high dielectric loss and low dielectric constant for which presence of space-charge in the structure is believed to be the cause [5]. In SBN and SBT ceramics role of donor type dopants can play a significant role due to their nature to deliver free charge in the structure while occupying the desired sites. Forbess et al. [6] have reported an increase in Curie temperature on doping 0.1 off-valent lanthanum. The unavailability of sufficient literature on these studies and their significance led us to carryout this research. In this paper, the effects of A-site lanthanum substitution on the structural and electrical properties of Sr1xLaxBi2 Nb2O9 ceramics are reported for x ¼ 0:020:5. The work Corresponding author. Tel.: +91 11 30972376; fax: +91 11 27871023.
E-mail address: [email protected] (A.K. Jha). 0921-4526/$ - see front matter r 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2005.10.131
is also aimed at providing an idea of optimum composition for better NvRAM, capacitor and other applications.
2. Experimental Samples were prepared using solid-state reaction method taking SrCO3, La2O3, Bi2O3 and Nb2O5 (all from Aldrich) in stoichiometric proportions. The powders were thoroughly mixed and passed through sieve of appropriate size. Mixtures were calcined at 900 1C in air. Polyvinylalcohol (Aldrich) 2 wt.% solution was mixed in all powdered samples in proportions and then molded into disc shape pellets by applying pressure of 270 MPa. Pellets were sintered in air at 1150 1C for 2 h. X-ray diffractograms were taken for all the calcined and sintered samples on Philips X-ray diffractometer PW 1710 using CuKa radiation of wavelength 1.54439 A˚ in the range 101p2yp701 at a scanning rate of 0.051/s. The micrographs of fractured surfaces of a few sintered pellets were recorded on a Cambridge Stereo Scan 360 scanning electron microscope (SEM). All pellets were coated with silver paste and cured at 600 1C for half an hour. Dielectric measurements were taken on an HP 4192A Impedance Analyser operating at oscillation level of 1 V in the frequency range 50 Hz–1 MHz
ARTICLE IN PRESS V. Shrivastava et al. / Physica B 371 (2006) 337–342
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Fig. 1. X-ray diffractograms of a few sintered Sr1xLaxBi2Nb2O9 samples.
and a frequency of 100 kHz was chosen for measuring Curie temperatures of all samples. DC conductivity of all the samples were measured using conventional two-probe laboratory-made setup with titanium base plate and tip. All temperature measurements were taken at a heating rate of 3 1C/min. X-ray diffractograms of a few sintered samples are shown in Fig. 1. The position of peaks indicates the formation of perovskite structure. The samples with x ¼ 0:020:3 show high crystallinity. The x ¼ 0:4 and 0.5 were less crystalline as evident from the disappearance of a few peaks of low intensity. The intensity of a few peaks is observed to decrease regularly with increase in lanthanum content. An increase in the width of a few peaks in diffractograms of x ¼ 0:4 and 0.5 samples is observed. From the observed reduction in the intensities of major peaks upto x ¼ 0:3, suppression in the perovskite phase is estimated, as peak intensity is an indication of phase concentration. In addition, background noise, which indicates the amorphous content in the structure, is observed to increase in the samples with x ¼ 0:4 and 0.5 for higher 2y values. The added lanthanum is likely to go to the strontium site because of similar ionic radius and coordination number. The lanthanum is not likely to occupy the sites of bismuth and niobium due to similar reasons. The details of lanthanum occupancy and a few structural distortion parameters have been reported elsewhere [7]. Fig. 2 shows the SEM images of microstructure of fractured surfaces of x ¼ 0:0, 0.1, 0.3 and 0.5 samples. The undoped SBN shows poor microstructure with porosity and maximum grains of average size below 1 mm as in
Fig. 2a. Lanthanum as dopant in small quantities is known [8] to reduce porosity by promoting uniform grain growth; the same is observed in doped samples up to x ¼ 0:3 samples with average grain size 5 mm. The microstructure of x ¼ 0:5 sample shows grains of average size below 1 mm and having irregular shape with no ordered boundary. High concentrations of lanthanum, i.e., more than 30 atomic% may induce formation of defects [9] in the structure because of high solid solubility nature and occupancy onto other sites. Fig. 3 shows the variation of dielectric constant (e) with frequency in the range 50 Hz–1 MHz at room temperature for all the samples. A slow decrease in dielectric constant is observed over the studied frequency range for x ¼ 0:1, 0.2 and 0.3 samples. Samples with x ¼ 0:0, 0.4 and 0.5 show a rapid decrease up to 1 kHz followed by a slow decrease at higher frequencies. This rapid decrease in dielectric constant is attributed to space-charge polarisation saturation. Normally the space-charge saturates up to 1 kHz, however, in a few donor-doped ceramics it may extend up to 100 kHz [10]. For frequencies higher than 1 kHz the dielectric constant increases with increase in lanthanum content upto x ¼ 0:3 and decreases for x ¼ 0:4 and 0.5. All samples show a decrease in e-values at frequencies above 100 kHz, which is an indication of nearby relaxation peak of saturation. The introduction of lanthanum is known to increase dielectric constant [11,12] because of its high ionic polarisibility. Also, it contributes to the space-charge polarisation by the creation of vacancies in the structure to maintain charge neutrality. The dielectric constant of undoped sample is affected, besides other factors, by the evaporation of bismuth oxide during sample preparation.
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Fig. 2. SEM fracture photographs of x ¼ 0:0, 0.1, 0.3 and 0.5 samples.
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Fig. 3. Dielectric constant versus frequency behaviour of Sr1xLaxBi2 Nb2O9 ceramics.
This results in space-charge generation the effect of which is felt more at lower frequencies [5]. Low e-values at higher frequencies are also expected because of saturation of space-charge polarisation near 1–100 kHz. This is indicated in Fig. 3 where large saturation of space charge occurs near 1 kHz for x ¼ 0:0, 0.4 and 0.5 samples and complete saturation of the same has been shown near 100 kHz. The x ¼ 0:2 sample does not show such a loss of space-charge polarisation; further investigations are needed to explain this observation.
Fig. 4. Dielectric constant versus temperature behaviour of Sr1xLax Bi2Nb2O9 ceramics.
In Fig. 4, dielectric constant versus temperature behavior of the studied compositions is shown. The observations have been plotted using Microcal Origin data plot. These measurements were taken at frequency 100 kHz near and above which space-charge and orientational polarisations cease off and only ionic and electronic contributions persists [5]. These ceramics are known to have single phase transitions corresponding to Curie temperature where dielectric constant is a maximum. Curie temperature and maximum dielectric constant (emax.) both show a decrease with increase in lanthanum content up to x ¼ 0:5. Dielectric peak is broadened with increase in x which is similar to diffuse phase transition type behavior of
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Fig. 5. Tetragonal strain and Curie temperature versus composition x of Sr1xLaxBi2Nb2O9 ceramics.
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100 kHz, similar to undoped sample. Sample with x ¼ 0:2 shows greater loss for frequencies less than 1 kHz compared to x ¼ 0:1 and 0.3 samples but it shows a continuous decrease in loss value up to 1 MHz. The increase in loss values beyond 100 kHz is attributed to relaxation losses due to ion jump or electron hopping [17] in samples with x ¼ 0:0, 0.1, 0.3, 0.4 and 0.5. The frequency 80 kHz at which all the samples show minimum loss is significant, and after this loss is higher for x ¼ 0:4 and 0.5 samples compared to x ¼ 0:0 sample. The x ¼ 0:4 and 0.5 samples are possibly defect structures as indicated by disappeared peaks in X-ray diffractograms. At higher frequencies more energy should be dissipated during the rotation of dipoles in a defect structure therefore high dielectric loss is expected in x ¼ 0:4 and 0.5 samples. The higher loss values of x ¼ 0:0 sample are attributed to the presence of space-charge due to the evaporation of bismuth oxide during sintering which is also possibly the cause for relatively higher dielectric constant values up to 1 kHz. Dielectric loss versus temperature behaviour of the studied samples is shown in Fig. 7. The samples are observed to possess dielectric loss peaks at different temperatures. The peaks in samples with x ¼ 0:1, 0.2 and 0.3 are observed in a wide temperature range whereas x ¼ 0:0, 0.4 and 0.5 samples show peaks at small intervals of temperature. The dielectric loss values show peaks with respect to temperature due to the following reasons: (i) if a dielectric is a mixture of two or more polar substances and (ii) due to different character of losses in chemically individual dielectric like dipolar and structural, etc. The maximum dielectric loss is expected at a temperature near which phase transition occurs, i.e., in the neighbourhood of Curie temperature. In the present work, observations of dielectric loss are in accordance with that of Curie
106
Fig. 6. Dielectric loss versus frequency behaviour of Sr1xLaxBi2Nb2O9 ceramics.
relaxors. Such transition behavior is explained on the basis of the formation of micro-polar regions [13,14], which have their respective transition temperatures. A decrease in Curie temperature with increase in lanthanum content is attributed to the overall decrease in tetragonal lattice strain [15]. The lead-doped samples have also shown similar behavior. The observed values of tetragonal strain with Curie temperature as a function of composition are shown in Fig. 5. Fig. 6 shows the dielectric loss as a function of frequency at room temperature. The undoped SBN sample shows huge loss, which decreases with frequency up to 80 kHz and thereafter increases again; a known dielectric dispersion behaviour [16]. Among the doped samples, x ¼ 0:3 sample shows minimum loss at low frequencies which increases appreciably after about 100 kHz. The x ¼ 0:4 and 0.5 samples show large loss values before and after about
Fig. 7. Dielectric loss versus temperature behaviour of Sr1xLaxBi2Nb2O9 ceramics.
ARTICLE IN PRESS V. Shrivastava et al. / Physica B 371 (2006) 337–342
temperature measurements. From Fig. 7a, where measured dielectric loss values for x ¼ 0:0, 0.4 and 0.5 are plotted, it is clear that the loss increases with temperature only for x ¼ 0:0 and 0.4 samples but for x ¼ 0:5 insignificant changes are observed. The peaks observed in x ¼ 0:1 to 0.3 samples are possibly relaxational peaks [7,18] at different temperatures, the reasons for such behaviour have been described earlier. The x ¼ 0:2 sample shows almost similar behaviour but its first peak is of less height compared to x ¼ 0:1 and 0.3 the reason for which is possibly less space-charge in this sample compared to x ¼ 0:1 and 0.3 samples. This observation is in accordance with Fig. 3 where a steep decrease in dielectric constant value is observed for x ¼ 0:1 and 0.3 samples at frequencies after 100 kHz with an exception for x ¼ 0:2 sample, which shows less steeper curve. In x ¼ 0:4 and 0.5 samples, possibly number of defects is increased due to lanthanum occupancy onto the sites of bismuth and niobium ions such that at low temperatures ions may participate for conduction. Therefore, insignificant loss peaks are observed which should be due to space-charge relaxational losses (structural losses) merged with dipolar relaxational losses [19]. Such significant relaxational peaks are not expected to be observed in defect structures where phase transitions are observed in a wide range of temperature [20,21]. This seems to be the cause for observed loss behaviour of x ¼ 0:4 and 0.5 samples. Bulk ionic DC conductivities of all the samples are plotted in Fig. 8. It is observed that room temperature conductivity shows an increasing trend up to x ¼ 0:3 followed by a decrease at higher values of x, (Table 1). To ensure that this was indeed the case the observations were taken several times and the same trend was noticed each time. In Table 1, DC conductivity values at different temperatures are listed. The x ¼ 0:4 and 0.5 samples in their paraelectric phase show positive temperature coeffi-
Fig. 8. DC conductivity behaviour of Sr1xLaxBi2Nb2O9 ceramics.
341
Table 1 Activation energy and high temperature conductivity of samples Composition (x) in Sr1xLaxBi2Nb2O9
0.0 0.1 0.2 0.3 0.4 0.5
DC (eV) PE phase
1.05 1.29 1.15 0.85 0.25 0.47
s ( 108 S/cm) DC (at RT)
DC (600 1C)
0.83 0.84 0.87 0.97 0.82 0.79
425 464 487 515 577 1672
cient of resistance (PTCR) in the neighbourhood of phase transition temperature; the x ¼ 0:0 sample shows an identical behaviour but to a much smaller extent. Such behaviour has been reported in BaTiO3 insulators. The PTCR behaviour is the result of negative charge carrier conduction under the external electric field and is known as a characteristic of metals [22]. The samples with x ¼ 0:1, 0.2 and 0.3 show negative temperature coefficient of resistance NTCR throughout the temperature range, which is a characteristic of insulators [23]. It indicates that lanthanum addition either increases positive charge in the structure or eases the evaporation of negative charge carriers, i.e., oxygens from the structure. The second possibility is ruled out because of very high bond strength of lanthanum with oxygen and the first possibility is reasonable due to donor nature of lanthanum. The observed PTCR behaviour in samples with x ¼ 0:4 and 0.5 could be due to the lanthanum occupying other cationic sites available in the structure. It is not unreasonable to assume that as lanthanum concentration increases, it not only occupies strontium sites but also other cationic sites available in the structure, resulting in the formation of unwanted heterogeneous phases [15,24]. If such multiple site-occupancy occurs, marked changes in conductivity and microstructure are expected. The same is observed in present work for x ¼ 0:4 and 0.5 samples. This is also supported by the observed increased peak width in X-ray diffractograms (Fig. 1) and by the presence of micro-cracks in SEM fracture photographs (Fig. 2) for x ¼ 0:4 and 0.5 samples. As lanthanum exists in stable oxidation state of +3 and strontium in +2, substitution of two lanthanum ions onto two strontium sites produces one strontium vacancy in the structure to preserve charge neutrality. These bismuth layer ceramics have been reported to have oxygen vacancies produced during sample preparation [20]. It is also known that donor doped ceramics do not favour oxygen vacancy formation. The occupancy of lanthanum onto bismuth sites should not affect the conductivity of compositions because of same valence state of +3, but if it occupies niobium site of valence state +5, a decrease in conductivity is expected, which is observed (Table 1, RT column). Higher lanthanum content in studied compositions like x ¼ 0:4 and 0.5 might induce bismuth and niobium ions to
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become free from their respective sites and to participate in ionic conduction. Additionally, it will provide two free electrons to the structure, which possibly results in PTCR. DC activation energy for all the samples is calculated using slope of obtained Arrhenius plots in the temperature range of paraelectric phase; obtained values are listed in Table 1. The x ¼ 0:1 sample is found to possess maximum activation energy of 1.29 eV and the sample with x ¼ 0:4 shows a minimum of 0.25 eV, both in their paraelectric phase. 3. Conclusions Lanthanum doping in bismuth layer SBN ceramics onto A-site yields pore-free microstructure up to x ¼ 0:3. The x ¼ 0:3 sample shows minimum dielectric loss values up to 80 kHz; the values increase thereafter at higher frequencies up to 1 MHz. The sample with x ¼ 0:2 shows minimum dielectric loss at 1 MHz and does not show dielectric frequency relaxation up to this frequency. The x ¼ 0:2 sample is therefore best suited for high frequency applications. Additionally, this sample offers high dielectric constant of 700 at Curie temperature of 336 1C. The PTCR feature of x ¼ 0:0, 0.4 and 0.5 samples can also be utilised for sensor applications although these sample show high dielectric loss and low dielectric constant. Flattened dielectric response of these ceramics provides wide range of applications. High ionic conductivity of x ¼ 0:4 and 0.5 samples may yield high fatigue resistance for memory applications. References [1] C.A.P. de Arauzo, J.F. Scott, Science 246 (1989) 1400. [2] C.A.P. Arauzo, J.D. Cuchlaro, L.D. Mcmillan, M.C. Scott, J.F. Scott, Nature 374 (1995) 627.
[3] Y. Wu, M.J. Forbess, S. Seraji, S.J. Limmer, T.P. Chou, C. Nguyen, G.Z. Cao, J. Appl. Phys. 90 (10) (2001) 5296. [4] Y. Shimakawa, Y. Kubo, Y. Nakagawa, S. Goto, T. Kamiyama, H. Asano, F. Izumi, Phys. Rev. B 61 (10) (2000) 6559. [5] D.B. Dove, L.L. Hench (Eds.), Principles of Electronic Ceramics Part A, London, 1971, p. 125, 495. [6] M.J. Forbess, S. Seraji, Y. Wu, C.P. Nguyen, G.Z. Cao, Appl. Phys. Lett. 76 (20) (2000). [7] V. Shrivastava, A.K. Jha, R.G. Mendiratta, Solid State Commun. September 2004, in review. [8] R.C. Buchnan (Ed.), Principles of Electronic Ceramics, Marcel Dekkar, New York, 1991, p. 154. [9] G.H. Haertling, C.E. Land, J. Am. Ceramic Soc. 54 (1971) 1. [10] R.C. Buchnan (Ed.), Principles of Electronic Ceramics, Marcel Dekkar, New York, 1991, pp. 38–39. [11] G.H. Haertling, J. Am. Ceramic Soc. 82 (4) (1999) 797–818. [12] I. Bunget, M. Popescu (Eds.), Physics of Solid Dielectrics, Elsevier, Amsterdam, Oxford, New York, 1984, p. 209. [13] L.L. Hench, J.K. West (Eds.), Physics of Electronic Ceramics, Wiley, New York, 1995, p. 277. [14] V. Shrivastava, A.K. Jha, R.G. Mendiratta, Solid State Commun. 133 (2) (2005) 125–129. [15] Y.H. Xu (Ed.), Ferroelectric Materials, Elsevier Science Publishers B.V., 1991, p. 165. [16] I.S. Zheludev (Ed.), Physics of Crystalline Dielectrics, vol. 2, Plenum Press, New York, 1971, p. 521. [17] G.A. Smolenski, V.A. Isupov, A.I. Agranoskaya, S.N. Popov, Sov. Phys. Solid State 2 (1961) 2584. [18] B. Tareev (Ed.), Physics of Dielectric Materials, Mir Publishers, Moscow, 1971, p. 148. [19] C. Ang, Z. Yu, P.M. Vilarinho, J.L. Baptista, Phys. Rev. B 57 (1998) 7403. [20] Y. Wu, M.J. Forbess, S. Seraji, S.J. Limmer, T.P. Chou, G. Cao, J. Appl. Phys. 89 (10) (2001) 5647. [21] A. Chen, Y. Zhi, L.E. Cross, Phys. Rev. B 62 (2000) 228. [22] Y.H. Xu (Ed.), Ferroelectric Materials, Elsevier Science Publishers B.V., 1991, p. 193. [23] R.C. Buchnan (Ed.), Principles of Electronic Ceramics, Marcel Dekkar, New York, 1991, pp. 250–251. [24] G.H. Haertling, Ferroelectrics 75 (1987) 25.
Physica B 371 (2006) 337–342 www.elsevier.com/locate/physb
Structural and electrical studies in La-substituted SrBi2Nb2O9 ferroelectric ceramics Vaibhav Shrivastavaa, A.K. Jhaa,, R.G. Mendirattab a
Department of Applied Physics, Delhi College of Engineering, Delhi 110042, India School of Applied Sciences, Netaji Subhas Institute of Technology, Delhi 110075, India
b
Received 26 January 2005; received in revised form 20 October 2005; accepted 23 October 2005
Abstract In this paper, structural and electrical studies of A-site lanthanum-substituted Sr1xLaxBi2Nb2O9; x ¼ 0:020:5 ceramics are reported. The introduction of lanthanum produces pore-free microstructure upto x ¼ 0:3. A diffuse ferroelectric to paraelectric phase transition is observed in the doped samples and positive temperature coefficient of resistance (PTCR) is observed in x ¼ 0:0, 0.4 and 0.5 samples whereas x ¼ 0:120:3 samples show negative temperature coefficient of resistance (NTCR). Curie temperature decreases continuously with increase in lanthanum content. r 2005 Elsevier B.V. All rights reserved. Keywords: Donor doping; Space-charge polarisation; DC conductivity; Dielectric loss
1. Introduction Bismuth-layered SrBi2Nb2O9 (SBN) and SrBi2Ta2O9 (SBT) ceramics have drawn attention of researchers due to their excellent fatigue characteristics as compared to those of Pb(Zr,Ti)O3 (PZT) ceramics [1,2]. Therefore, doping in SBN- and SBT-type layered ceramics to improve the electrical and structural properties have been a matter of interest [3,4]. These ceramics show high values of Curie temperature and fatigue resistance, but suffer from high dielectric loss and low dielectric constant for which presence of space-charge in the structure is believed to be the cause [5]. In SBN and SBT ceramics role of donor type dopants can play a significant role due to their nature to deliver free charge in the structure while occupying the desired sites. Forbess et al. [6] have reported an increase in Curie temperature on doping 0.1 off-valent lanthanum. The unavailability of sufficient literature on these studies and their significance led us to carryout this research. In this paper, the effects of A-site lanthanum substitution on the structural and electrical properties of Sr1xLaxBi2 Nb2O9 ceramics are reported for x ¼ 0:020:5. The work Corresponding author. Tel.: +91 11 30972376; fax: +91 11 27871023.
E-mail address: [email protected] (A.K. Jha). 0921-4526/$ - see front matter r 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2005.10.131
is also aimed at providing an idea of optimum composition for better NvRAM, capacitor and other applications.
2. Experimental Samples were prepared using solid-state reaction method taking SrCO3, La2O3, Bi2O3 and Nb2O5 (all from Aldrich) in stoichiometric proportions. The powders were thoroughly mixed and passed through sieve of appropriate size. Mixtures were calcined at 900 1C in air. Polyvinylalcohol (Aldrich) 2 wt.% solution was mixed in all powdered samples in proportions and then molded into disc shape pellets by applying pressure of 270 MPa. Pellets were sintered in air at 1150 1C for 2 h. X-ray diffractograms were taken for all the calcined and sintered samples on Philips X-ray diffractometer PW 1710 using CuKa radiation of wavelength 1.54439 A˚ in the range 101p2yp701 at a scanning rate of 0.051/s. The micrographs of fractured surfaces of a few sintered pellets were recorded on a Cambridge Stereo Scan 360 scanning electron microscope (SEM). All pellets were coated with silver paste and cured at 600 1C for half an hour. Dielectric measurements were taken on an HP 4192A Impedance Analyser operating at oscillation level of 1 V in the frequency range 50 Hz–1 MHz
ARTICLE IN PRESS V. Shrivastava et al. / Physica B 371 (2006) 337–342
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Fig. 1. X-ray diffractograms of a few sintered Sr1xLaxBi2Nb2O9 samples.
and a frequency of 100 kHz was chosen for measuring Curie temperatures of all samples. DC conductivity of all the samples were measured using conventional two-probe laboratory-made setup with titanium base plate and tip. All temperature measurements were taken at a heating rate of 3 1C/min. X-ray diffractograms of a few sintered samples are shown in Fig. 1. The position of peaks indicates the formation of perovskite structure. The samples with x ¼ 0:020:3 show high crystallinity. The x ¼ 0:4 and 0.5 were less crystalline as evident from the disappearance of a few peaks of low intensity. The intensity of a few peaks is observed to decrease regularly with increase in lanthanum content. An increase in the width of a few peaks in diffractograms of x ¼ 0:4 and 0.5 samples is observed. From the observed reduction in the intensities of major peaks upto x ¼ 0:3, suppression in the perovskite phase is estimated, as peak intensity is an indication of phase concentration. In addition, background noise, which indicates the amorphous content in the structure, is observed to increase in the samples with x ¼ 0:4 and 0.5 for higher 2y values. The added lanthanum is likely to go to the strontium site because of similar ionic radius and coordination number. The lanthanum is not likely to occupy the sites of bismuth and niobium due to similar reasons. The details of lanthanum occupancy and a few structural distortion parameters have been reported elsewhere [7]. Fig. 2 shows the SEM images of microstructure of fractured surfaces of x ¼ 0:0, 0.1, 0.3 and 0.5 samples. The undoped SBN shows poor microstructure with porosity and maximum grains of average size below 1 mm as in
Fig. 2a. Lanthanum as dopant in small quantities is known [8] to reduce porosity by promoting uniform grain growth; the same is observed in doped samples up to x ¼ 0:3 samples with average grain size 5 mm. The microstructure of x ¼ 0:5 sample shows grains of average size below 1 mm and having irregular shape with no ordered boundary. High concentrations of lanthanum, i.e., more than 30 atomic% may induce formation of defects [9] in the structure because of high solid solubility nature and occupancy onto other sites. Fig. 3 shows the variation of dielectric constant (e) with frequency in the range 50 Hz–1 MHz at room temperature for all the samples. A slow decrease in dielectric constant is observed over the studied frequency range for x ¼ 0:1, 0.2 and 0.3 samples. Samples with x ¼ 0:0, 0.4 and 0.5 show a rapid decrease up to 1 kHz followed by a slow decrease at higher frequencies. This rapid decrease in dielectric constant is attributed to space-charge polarisation saturation. Normally the space-charge saturates up to 1 kHz, however, in a few donor-doped ceramics it may extend up to 100 kHz [10]. For frequencies higher than 1 kHz the dielectric constant increases with increase in lanthanum content upto x ¼ 0:3 and decreases for x ¼ 0:4 and 0.5. All samples show a decrease in e-values at frequencies above 100 kHz, which is an indication of nearby relaxation peak of saturation. The introduction of lanthanum is known to increase dielectric constant [11,12] because of its high ionic polarisibility. Also, it contributes to the space-charge polarisation by the creation of vacancies in the structure to maintain charge neutrality. The dielectric constant of undoped sample is affected, besides other factors, by the evaporation of bismuth oxide during sample preparation.
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Fig. 2. SEM fracture photographs of x ¼ 0:0, 0.1, 0.3 and 0.5 samples.
400 x=0.0 x=0.1 x=0.2 x=0.3 x=0.4 x=0.5
350
Dielectric constant
300
250
200
150
100
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102
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104 Frequency (Hz)
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Fig. 3. Dielectric constant versus frequency behaviour of Sr1xLaxBi2 Nb2O9 ceramics.
This results in space-charge generation the effect of which is felt more at lower frequencies [5]. Low e-values at higher frequencies are also expected because of saturation of space-charge polarisation near 1–100 kHz. This is indicated in Fig. 3 where large saturation of space charge occurs near 1 kHz for x ¼ 0:0, 0.4 and 0.5 samples and complete saturation of the same has been shown near 100 kHz. The x ¼ 0:2 sample does not show such a loss of space-charge polarisation; further investigations are needed to explain this observation.
Fig. 4. Dielectric constant versus temperature behaviour of Sr1xLax Bi2Nb2O9 ceramics.
In Fig. 4, dielectric constant versus temperature behavior of the studied compositions is shown. The observations have been plotted using Microcal Origin data plot. These measurements were taken at frequency 100 kHz near and above which space-charge and orientational polarisations cease off and only ionic and electronic contributions persists [5]. These ceramics are known to have single phase transitions corresponding to Curie temperature where dielectric constant is a maximum. Curie temperature and maximum dielectric constant (emax.) both show a decrease with increase in lanthanum content up to x ¼ 0:5. Dielectric peak is broadened with increase in x which is similar to diffuse phase transition type behavior of
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340
6.47
450
6.44 350
300 6.41
Curie temperature (°C)
Tetragonal strain
400
250
6.38 0
0.1
0.2 0.3 x-values
0.4
200 0.5
Fig. 5. Tetragonal strain and Curie temperature versus composition x of Sr1xLaxBi2Nb2O9 ceramics.
0.7 x=0.0 x=0.1 x=0.2 x=0.3 x=0.4 x=0.5
0.6
Dielectric loss
0.5 0.4 0.3 0.2 0.1 0
102
103
104 Frequency (Hz)
105
100 kHz, similar to undoped sample. Sample with x ¼ 0:2 shows greater loss for frequencies less than 1 kHz compared to x ¼ 0:1 and 0.3 samples but it shows a continuous decrease in loss value up to 1 MHz. The increase in loss values beyond 100 kHz is attributed to relaxation losses due to ion jump or electron hopping [17] in samples with x ¼ 0:0, 0.1, 0.3, 0.4 and 0.5. The frequency 80 kHz at which all the samples show minimum loss is significant, and after this loss is higher for x ¼ 0:4 and 0.5 samples compared to x ¼ 0:0 sample. The x ¼ 0:4 and 0.5 samples are possibly defect structures as indicated by disappeared peaks in X-ray diffractograms. At higher frequencies more energy should be dissipated during the rotation of dipoles in a defect structure therefore high dielectric loss is expected in x ¼ 0:4 and 0.5 samples. The higher loss values of x ¼ 0:0 sample are attributed to the presence of space-charge due to the evaporation of bismuth oxide during sintering which is also possibly the cause for relatively higher dielectric constant values up to 1 kHz. Dielectric loss versus temperature behaviour of the studied samples is shown in Fig. 7. The samples are observed to possess dielectric loss peaks at different temperatures. The peaks in samples with x ¼ 0:1, 0.2 and 0.3 are observed in a wide temperature range whereas x ¼ 0:0, 0.4 and 0.5 samples show peaks at small intervals of temperature. The dielectric loss values show peaks with respect to temperature due to the following reasons: (i) if a dielectric is a mixture of two or more polar substances and (ii) due to different character of losses in chemically individual dielectric like dipolar and structural, etc. The maximum dielectric loss is expected at a temperature near which phase transition occurs, i.e., in the neighbourhood of Curie temperature. In the present work, observations of dielectric loss are in accordance with that of Curie
106
Fig. 6. Dielectric loss versus frequency behaviour of Sr1xLaxBi2Nb2O9 ceramics.
relaxors. Such transition behavior is explained on the basis of the formation of micro-polar regions [13,14], which have their respective transition temperatures. A decrease in Curie temperature with increase in lanthanum content is attributed to the overall decrease in tetragonal lattice strain [15]. The lead-doped samples have also shown similar behavior. The observed values of tetragonal strain with Curie temperature as a function of composition are shown in Fig. 5. Fig. 6 shows the dielectric loss as a function of frequency at room temperature. The undoped SBN sample shows huge loss, which decreases with frequency up to 80 kHz and thereafter increases again; a known dielectric dispersion behaviour [16]. Among the doped samples, x ¼ 0:3 sample shows minimum loss at low frequencies which increases appreciably after about 100 kHz. The x ¼ 0:4 and 0.5 samples show large loss values before and after about
Fig. 7. Dielectric loss versus temperature behaviour of Sr1xLaxBi2Nb2O9 ceramics.
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temperature measurements. From Fig. 7a, where measured dielectric loss values for x ¼ 0:0, 0.4 and 0.5 are plotted, it is clear that the loss increases with temperature only for x ¼ 0:0 and 0.4 samples but for x ¼ 0:5 insignificant changes are observed. The peaks observed in x ¼ 0:1 to 0.3 samples are possibly relaxational peaks [7,18] at different temperatures, the reasons for such behaviour have been described earlier. The x ¼ 0:2 sample shows almost similar behaviour but its first peak is of less height compared to x ¼ 0:1 and 0.3 the reason for which is possibly less space-charge in this sample compared to x ¼ 0:1 and 0.3 samples. This observation is in accordance with Fig. 3 where a steep decrease in dielectric constant value is observed for x ¼ 0:1 and 0.3 samples at frequencies after 100 kHz with an exception for x ¼ 0:2 sample, which shows less steeper curve. In x ¼ 0:4 and 0.5 samples, possibly number of defects is increased due to lanthanum occupancy onto the sites of bismuth and niobium ions such that at low temperatures ions may participate for conduction. Therefore, insignificant loss peaks are observed which should be due to space-charge relaxational losses (structural losses) merged with dipolar relaxational losses [19]. Such significant relaxational peaks are not expected to be observed in defect structures where phase transitions are observed in a wide range of temperature [20,21]. This seems to be the cause for observed loss behaviour of x ¼ 0:4 and 0.5 samples. Bulk ionic DC conductivities of all the samples are plotted in Fig. 8. It is observed that room temperature conductivity shows an increasing trend up to x ¼ 0:3 followed by a decrease at higher values of x, (Table 1). To ensure that this was indeed the case the observations were taken several times and the same trend was noticed each time. In Table 1, DC conductivity values at different temperatures are listed. The x ¼ 0:4 and 0.5 samples in their paraelectric phase show positive temperature coeffi-
Fig. 8. DC conductivity behaviour of Sr1xLaxBi2Nb2O9 ceramics.
341
Table 1 Activation energy and high temperature conductivity of samples Composition (x) in Sr1xLaxBi2Nb2O9
0.0 0.1 0.2 0.3 0.4 0.5
DC (eV) PE phase
1.05 1.29 1.15 0.85 0.25 0.47
s ( 108 S/cm) DC (at RT)
DC (600 1C)
0.83 0.84 0.87 0.97 0.82 0.79
425 464 487 515 577 1672
cient of resistance (PTCR) in the neighbourhood of phase transition temperature; the x ¼ 0:0 sample shows an identical behaviour but to a much smaller extent. Such behaviour has been reported in BaTiO3 insulators. The PTCR behaviour is the result of negative charge carrier conduction under the external electric field and is known as a characteristic of metals [22]. The samples with x ¼ 0:1, 0.2 and 0.3 show negative temperature coefficient of resistance NTCR throughout the temperature range, which is a characteristic of insulators [23]. It indicates that lanthanum addition either increases positive charge in the structure or eases the evaporation of negative charge carriers, i.e., oxygens from the structure. The second possibility is ruled out because of very high bond strength of lanthanum with oxygen and the first possibility is reasonable due to donor nature of lanthanum. The observed PTCR behaviour in samples with x ¼ 0:4 and 0.5 could be due to the lanthanum occupying other cationic sites available in the structure. It is not unreasonable to assume that as lanthanum concentration increases, it not only occupies strontium sites but also other cationic sites available in the structure, resulting in the formation of unwanted heterogeneous phases [15,24]. If such multiple site-occupancy occurs, marked changes in conductivity and microstructure are expected. The same is observed in present work for x ¼ 0:4 and 0.5 samples. This is also supported by the observed increased peak width in X-ray diffractograms (Fig. 1) and by the presence of micro-cracks in SEM fracture photographs (Fig. 2) for x ¼ 0:4 and 0.5 samples. As lanthanum exists in stable oxidation state of +3 and strontium in +2, substitution of two lanthanum ions onto two strontium sites produces one strontium vacancy in the structure to preserve charge neutrality. These bismuth layer ceramics have been reported to have oxygen vacancies produced during sample preparation [20]. It is also known that donor doped ceramics do not favour oxygen vacancy formation. The occupancy of lanthanum onto bismuth sites should not affect the conductivity of compositions because of same valence state of +3, but if it occupies niobium site of valence state +5, a decrease in conductivity is expected, which is observed (Table 1, RT column). Higher lanthanum content in studied compositions like x ¼ 0:4 and 0.5 might induce bismuth and niobium ions to
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become free from their respective sites and to participate in ionic conduction. Additionally, it will provide two free electrons to the structure, which possibly results in PTCR. DC activation energy for all the samples is calculated using slope of obtained Arrhenius plots in the temperature range of paraelectric phase; obtained values are listed in Table 1. The x ¼ 0:1 sample is found to possess maximum activation energy of 1.29 eV and the sample with x ¼ 0:4 shows a minimum of 0.25 eV, both in their paraelectric phase. 3. Conclusions Lanthanum doping in bismuth layer SBN ceramics onto A-site yields pore-free microstructure up to x ¼ 0:3. The x ¼ 0:3 sample shows minimum dielectric loss values up to 80 kHz; the values increase thereafter at higher frequencies up to 1 MHz. The sample with x ¼ 0:2 shows minimum dielectric loss at 1 MHz and does not show dielectric frequency relaxation up to this frequency. The x ¼ 0:2 sample is therefore best suited for high frequency applications. Additionally, this sample offers high dielectric constant of 700 at Curie temperature of 336 1C. The PTCR feature of x ¼ 0:0, 0.4 and 0.5 samples can also be utilised for sensor applications although these sample show high dielectric loss and low dielectric constant. Flattened dielectric response of these ceramics provides wide range of applications. High ionic conductivity of x ¼ 0:4 and 0.5 samples may yield high fatigue resistance for memory applications. References [1] C.A.P. de Arauzo, J.F. Scott, Science 246 (1989) 1400. [2] C.A.P. Arauzo, J.D. Cuchlaro, L.D. Mcmillan, M.C. Scott, J.F. Scott, Nature 374 (1995) 627.
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