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Near room temperature bismuth and lithium co-substituted BaTiO3 relaxor ferroelectrics family
Accepted Manuscript Near room temperature bismuth and lithium co-substituted BaTiO3 relaxor ferroelectrics family Hitesh Borkar, Vaibhav Rao, M. Tomar, Vinay Gupta, Ashok Kumar PII:
S0925-8388(17)34366-9
DOI:
10.1016/j.jallcom.2017.12.170
Reference:
JALCOM 44256
To appear in:
Journal of Alloys and Compounds
Received Date: 4 September 2017 Revised Date:
15 December 2017
Accepted Date: 16 December 2017
Please cite this article as: H. Borkar, V. Rao, M. Tomar, V. Gupta, A. Kumar, Near room temperature bismuth and lithium co-substituted BaTiO3 relaxor ferroelectrics family, Journal of Alloys and Compounds (2018), doi: 10.1016/j.jallcom.2017.12.170. 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.
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Near room temperature bismuth and lithium co-substituted BaTiO3 relaxor ferroelectrics family Hitesh Borkar1,2, Vaibhav Rao1,3, M Tomar4, Vinay Gupta5, Ashok Kumar1,2,*
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CSIR-National Physical Laboratory, Dr. K. S. Krishnan Marg, New Delhi 110012, India Academy of Scientific and Innovative Research (AcSIR), CSIR-National Physical Laboratory (CSIR-NPL) Campus, Dr. K. S. Krishnan Road, New Delhi 110012, India 3 Solar and Alternative Energy, Amity University, Jaipur, Rajasthan, 302006, India 4 Department of Physics, Miranda House, University of Delhi, Delhi 110007, India 5 Department of Physics and Astrophysics, University of Delhi, Delhi 110007, India
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Abstract
Eco-friendly lead-free relaxor ferroelectrics, Ba1-x(Bi0.5Li0.5)xTiO3 (BBLT) with x = 0.3, 0.4 and 0.5, have been synthesized by using the conventional solid-state reaction route. The crystal structure and phase purity were verified by x-ray diffraction (XRD) patterns using
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Rietveld refinement which revealed the presence of tetragonal (95 to 80%) and orthorhombic phases (5 to 20%) in the matrix. The temperature dependent dielectric constant and tangent loss dispersion spectra, slim ferroelectric-hysteresis, and nano-scale microstructures suggest the
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presence of short-range ordering (SRO)/polar nano-regions (PNRs) in the matrix. The dielectric spectra suggest that the dielectric dispersion increases with increase in Bi/Li concentrations
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which may be due to compositional fluctuations, ordering of cations at nano-scale and presence of metastable states of SRO and long-range ordering (LRO) regions. The nonlinear VogelFulcher relationship is fitted for maximum dielectric constant temperatures (Tm) which provides activation energy and freezing temperature of all compositions. The current density (J) versus electric field (E) (J-E) data obey bulk-limited current conduction mechanisms that may be due to a diffusive displacement of Ti4+ at octahedral center, the creation of random fields due to inhomogeneity at A-site, and trap-assisted hoping of charge carriers under high E-fields. 1
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Keywords: Relaxor-ferroelectric; Dielectric spectroscopy; Current conduction mechanism ; Polar nano-regions
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*Corresponding Author: Dr. Ashok Kumar (Email: [email protected])
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1. Introduction
Perovskite-based ferroelectric materials have attracted extensive interest to the
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researchers working on the development of transducers, sensors, energy harvesters, nonvolatile memory elements, optoelectronic devices, multilayer ceramic capacitors and micro-electromechanical systems, etc.,[1–4]. Among the various type of ferroelectrics, relaxor ferroelectrics have been found interesting due to large piezo-electric coefficients, electro-mechanical coupling, and interesting physics due to the presence of chemical in-homogeneities, ordering of cations and
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existence of PNRs in the matrix [5–9]. It is well proven that chemical and SRO ordering significantly improve the electro-mechanical and piezoelectric properties in relaxor ferroelectrics which make these systems suitable for industrial applications. Polar nano-regions (PNRs) are
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nano-scale local clusters of randomly oriented polarization which plays a key role in a large
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dielectric constant near the ferroelectric phase transition temperatures (Tm) and strong dielectric dispersion below Tm. In past, lead-based perovskite compounds, i.e. Pb0.85La0.1(Zr0.65Ti0.35)O3 (PLZT),
(1-x)[Pb(Mg1/3Nb2/3)O3]-x[PbTiO3]
(PMN–PT),
0.1Pb(Ni1/3Nb2/3)O3-
0.9Pb(Zr0.42Ti0.58)O3 (PNN-PZT), PbFe2/3W1/3O3 (PFW), PbFe1/2Nb1/2O3 (PFN), PbFe1/2Ta1/2O3 (PFT), PbSc0.5Nb0.5O3 (PSN), (PbxSr1-x)TiO3 (PST), Ba1−xPbx(Ti1−yZry)O3, etc., have shown above mentioned properties with strong dielectric dispersion near the phase transition temperatures [10–12].These systems showed large dielectric dispersion below Tm and merger of 2
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dielectric dispersion above Tm, slim ferroelectric hysteresis, and high piezoelectric and electromechanical properties. These compounds have drawbacks of toxicity and volatility associated with the lead compound. In this regards, environment-friendly and non-toxic
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compounds, i.e., ‘lead-free’ relaxor materials are the needs of modern-day investigation on relaxors and future lead-free sensors and actuators devices. It has also been reported that by the substitution of mono-valent and tri-valent cations on A-/B- sites of BaTiO3 leads to chemical in-
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homogeneities and cation ordering in the matrix which in turn develops PNRs and broad
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dielectric dispersion.
The lead-free perovskite materials have been investigated regarding their dielectric relaxation behavior, electrical and ferroelectric phase transition properties. These materials are as follows: KNbO3 (KN), doped-BaTiO3 (BT), (Bi1/2K1/2)TiO3(BKT), (Bi1/2Na1/2)TiO3 (BNT), Ba(Ti1-xYx)O3-x/2, etc., [13–16]. Among these systems, Bi0.5K0.5TiO3-based ceramics were widely
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investigated and used for various applications [17]. M. Naveed Ul-Haq et al. found the magnetodielectric properties in relaxor/ferrimagnetic composites. In these systems, the dielectric dispersion was control by an external applied magnetic field which may be useful for magnetic
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field sensors [18]. These materials are of primary interest of investigation due to various reasons;
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(i) BaTiO3-based materials find multiple uses in the commercial applications, (ii) A-/B- sites substituted-BaTiO3 often appeared as a ‘model' (non-lead based) ferroelectrics, (iii) quite different chemical mapping compared to classical relaxor like PMN, and (iv) even in relaxor states, these systems show well behaved remanent polarization and hysteresis (long-range ordering). Lead-free perovskite-type oxides show multiple polymorphic phase transitions, variation in dielectric permittivity and dielectric loss through chemical substitution, and complex microstructure-property relation. Relaxor behavior in the perovskites is mainly due to disorder 3
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in non-isovalent cations in the stoichiometric complex perovskite compounds. The systems such as Pb(Mg1/3Nb2/3)O3 (PMN) [19] or Pb(Sc1 /2Ta1/2)O3 (PST) [20] (in which Mg2+, Sc3+, Ta5+ and Nb5+ ions are entirely or moderately disordered at B-sub-lattice. Also for the non-stoichiometric
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solid solutions, like for Pb1−xLax(Zr1−yTiy)1−x/4O3(PLZT) [21,22] where the substitution of La3+ for Pb2+ ions lead to the vacancies. More degree of freedom can be obtained to add disorder on the Ba-site in BZT ceramic which is already a relaxor system with (Ti/Zr:65/35). The
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coexistence of ferroelectric (order) and relaxor (local and small region order) can be developed with the hetero-valent substitution of the Ba2+ cation by Bi3+ cations. Owing to its 6s2 lone pair
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(like Pb), the choice of Bi3+ (substituted for Ba2+) cation is favorable for the relaxor effect [23– 25].
In general, Random field theory is a well-accepted model for relaxors; this model suggests that the relaxors are a system with randomly oriented polarizations, random sites,
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defects, and lattice vacancies persist well above the Tm (i.e. para-electric region) which becomes the host lattice for these materials. Incorporation of Bi3+ cations in BaTiO3 lattice requires other mono-valent (Li/Na/K) cations to maintain the charge neutrality and vacancies. In this regard, we
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have chosen Bi+3/Li+1 cations to substitute the Ba2+ cations. These cations are attached to the host
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lattice which makes it almost impossible to move from one place to another. However, electrons and polarons can provide a large amount of electronic conduction under high E-fields and temperatures; sometimes it is possible that under large E-field and temperature, it is possible that Li may leave the lattice and support the formation of interstitial sites and ionic conduction. It has been reported that La-doped BaTiO3 generates a large number of electronic charge carriers at Asite which produces a substantial increase in conductivity [26]. In case of small amount of hetero-valent substitution at A-site, the sequence of ferroelectric phase transition was similar like 4
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to BaTiO3, however large amount of substitution may produce relaxor behavior, as can be seen in Ba1−yBi2y/3)TiO3, (0.09 < y < 0.15) system [27].
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Herein, we studied the relaxor behavior, temperature dependent dielectric properties, and effect of Bi3+/Li1+ substitution on BBLT systems. A suitable substitution of mono/dia/tri- valent cations at A-/B- site of perovskite structure provides complex microstructure with suitable
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cations ordering or compositionally inhomogeneous structure which in turn exhibits large dielectric dispersion in radio frequency range. Nonlinear behavior of frequency-dependent
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dielectric maximum temperatures is investigated, and possible freezing temperatures are obtained for various compositions. Leakage current behavior and possible current conduction mechanisms have been discussed. 2. Material and methods
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The Ba1-x(Bi0.5Li0.5)xTiO3 (BBLT) (x=0.3, 0.4, 0.5) polycrystalline ceramics prepared by the conventional solid-state reaction method. High purity (99.95 %) precursor powders of BaCO3, Li2CO3, Bi2O3, TiO2 were purchased from Sigma-Aldrich, latter their stoichiometric
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proportions are weighed according to desired materials. These green powders were mechanically mixed with IPA (isopropyl alcohol) for 2 hrs in an agate mortar for the homogeneous mixture.
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The dried powders were calcinated in a high-purity alumina crucible at 850 °C for 10 hrs. The calcined powders again re-grinded and mixed with PVA as a binder (10 wt%), then pressed at the pressure of 5-7 tons per square inch to form disk-shaped pellets with 10 mm in diameter and 1.5 mm in thickness. Finally, these pellets were sintered at an optimized temperature 1120-1200 °
C for 4 hrs depending on compositions. Phase purity of the compounds was confirmed by the X-
ray diffraction (XRD) technique using (Bruker AXS D8 Advance X-ray diffractometer Cu-kα;
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kα=1.54059A°) monochromatic radiation, in the 2ɵ range between 20° and 60°. Rietveld Refinement was carried out using Fullprof suit to understand the crystal structures. Grain growth, surface morphology, and elemental analysis on sintered pellets were carried out using a scanning
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electron microscope (SEM, Zeiss EVOMA-10) with measuring the uncertainty of ±7-10 %. The silver paint was used on both sides of pellets to prepare metal-insulator-metal capacitive structure. The capacitor structure was dried at 200 °C for 2-3 h to remove chemical residue,
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followed by the electrical measurements. The temperature dependent dielectric properties were studied using LCR meter (HIOKI-3532-50) at an oscillating amplitude of 500 mV in the 1kHz to
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1MHz frequency range, and temperature range from 300 K to 723 K. The Temperature dependent polarization versus electric field (P-E) hysteresis loops and room temperature leakage current were measured by using Radiant Ferroelectric Tester.
3.1
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3. Results and discussion
Structure and morphological study
In order to understand the crystal structure, the Rietveld refinement was carried out on
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XRD data of all three compositions (x= 0.3, 0.4, 0.5) and analyzed for Bi and Li substitution on Ba-site. Fig. 1 shows the x-ray diffraction patterns which fitted well for the mixed crystal
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structure model. These diffraction patterns were refined for tetragonal and orthorhombic crystal structure, the refined peaks matched well with Bragg’s peaks. The Rietveld refinement refined parameters were depicted in Table 1 which indicates that these systems are in the mixed crystal structure phase where the tetragonal structure is the dominating crystal structure. The tetragonal and orthorhombic structures relate the P4mm and Amm2 crystal symmetry, respectively. The lattice parameters were matched with their respective crystal structures. The fractional proportion
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of tetragonal to orthorhombic crystal structure was found to be 95:05, 80:20, and 88:12 for x=0.3, 0.4, 0.5, respectively. Fitting parameters was not too much convincing for x=0.5 due to more in-homogeneity of Bi/Li/Ba cations present at A-site of ABO3 structure and also the
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presence of pyrochlore phase. For x=0.5, the pyrochlore phases were (~0.5%) found at Bragg’s angle 29.06, mark as (*) which corresponds to the Bi2Ti2O7 [28,29]. When the content of Bi2O3 reached relatively high value, Bi2O3 reacts with dissociative TiO2 to produce Bi2Ti2O7. Bulk
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diffusion through grain boundary has been considered as main grain growth process during hightemperature sintering. The inhomogeneity in ordered and disordered lattice can reduce the
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activation energy of bulk diffusion; promote the mass transfer which enhances the sintering ability of ceramics [30]. An ideal cubic perovskite ABO3 structure, the coordination of the A-site is 12. The ionic radius of 12-coordinated Bi3+ and Li+ is comparable to the ionic radii for Ba2+ cation. Thus a limited amount of the substitution of Bi3+ and Li+ for Ba2+ will take place without
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pyrochlore phase but above than critical composition limit of bismuth and lithium substitution leads to the development of more impurity and secondary phases, it can be easily seen for the dielectric spectra of BBLT (x=0.5) [31]. The sintering temperature of BBLT based ceramics
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decreased effectively with an increase in Bi and Li substitution which may be due to the large
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activation energy of bulk diffusion during the high-temperature sintering process. The SEM micrographs for various BBLT compositions with x=0.3, 0.4, 0.5 were
illustrated in Fig. 2 (a), (b) & (c), respectively. It can be seen from micrographs that oblate spheroid grains were formed after the high-temperature sintering process. A comparison of the micrographs of Bi/Li substituted BBLT systems show that doping induces grains coalesces to form larger grains. It could also be noted that average grains and their distributions were nearly same with Li and Bi substitution. It is attributed to the development of an inter-granular 7
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interaction on the surface of grain that enhances merging of grains. An oblate spheroid type grains can be seen throughout matrix for BBLT ceramics and coalescence overlapping of grains reduced as the doping level increased. To confirm initial and final compositions of the product,
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an elemental analysis of the samples were carried out using EDX analysis as shown in Table 2. It is clear that BBLT systems are mostly free from any foreign impurity element. These systems exhibit negligible amount of impurity elements compared to the substituted elements and
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matched with initial and final ingredients. One cannot probe light element lithium using EDX
various elements. 3.2
Dielectric spectroscopy
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analysis which may result in high uncertainty in the calculation of proper compositions of
The temperature and frequency dependence of dielectric constant (ε) and tangent loss (tan
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δ) for the series of BBLT ceramics are shown in Fig. 3(a-f). Strong frequency dispersion is observed below the Tm (maximum dielectric constant temperature) for all the compositions with a tendency to merge above Tm. The magnitude of maximum dielectric constant decreased with
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increase in frequencies and shifted towards higher temperature side with an increase in probe frequencies. The dielectric dispersion of BBLT (x=0.4) is showing similar trends as with
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classical relaxor, however, for higher compositions (x=0.5) the dispersion is more chaotic. The second dielectric dispersion near the high temperature (~ 550 K) is due to thermally activated charge carriers. All three compositions show two dielectric maxima regions, one below to 450 K which is related to relaxor nature of the systems; however, at an elevated temperature nearly 550 K, a frequency dependent peak was observed, which may be due to ferroelectric to paraelectric or pseudo cubic to a cubic phase transition. A detail investigation is required to confirm the nature and category of high-temperature phase transitions. It may be achieved using a high8
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temperature XRD or neutron scattering, which is beyond the scope of the present investigation. Similar behavior was observed for NBT-BT systems near the morphotropic phase boundary. A similar trend of dielectric dispersion is observed for tangent loss data, however, in case of
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tangent loss below 450 K, the magnitude of tangent loss increased with increase in probe frequency and shifted towards higher temperature side. This signature of dielectric loss is considered as a fingerprint for relaxor systems. It can be easily observed from the high-
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temperature tangent loss data that their magnitude decreases with increase in probe frequency. The patterns of dielectric dispersion favor the presence of polar-nano regions and chemical
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inhomogeneity in the matrix which results in relaxor behavior in BBLT ceramics [32–34]. In the present system, it might be possible to form (Ba2+/ Bi3+/Li+) rich polar clusters on the analogy of A-site substituted relaxors, where inhomogeneity may introduce by substitution. The high degree of cation ordering-disordering, development of oxygen vacancies and deficiencies of volatile Li
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and Bi cations due to high sintering temperature modified the microstructure which results in large dielectric dispersion. The phase separation and compositional inhomogeneity are not observed from XRD data in these systems. Previously studied atomic-level disorder is an
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inherent property of the random-site structure. A relaxor behavior can be expected regardless of the overall size of chemically ordered domains [35]. It is sufficient to frustrate the long-range
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ordering of the A-site distributed cations to produce localized randomness. It is also evident from Fig. 3 (a,c,e) that diffuse dielectric peaks shift to higher temperature side with increase in the concentration of Bi and Li cations. It can be observed in Fig. 3(e) for the composition x=0.5 that the relaxor-ferroelectric phase transitions shift towards high temperature and reduces the magnitude of dielectric constant. The magnitude of tangent loss factor increases with increase in Bi/Li substitution due to increase in leakage current. Furthermore, peak broadening with
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increasing Bi and Li concentration is also evident. The random distribution of cations on A-site creates compositional variations of polar nano-regions which in turn lead to the locally varying ferroelectric transition temperature [36]. It is worth to be noted that temperature dependent
law (discussed in section 3.3) as reported previously [29,37]. Vogel–Fulcher analysis
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3.3
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dielectric maxima temperature (Tm) at various frequencies follows the nonlinear Vogel-Fulcher
To check the relaxor nature of materials, an empirical Vogel–Fulcher (VF) relation has
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been employed to account the dielectric relaxation phenomenon [38,39]. A thermally activated polarization reversal between two equivalent positions results in dielectric relaxation. Based on dielectric relaxation, the nonlinear VF model is used to study the activation energy Ea which requires crossing the barrier height between two equivalent polarization states and polarization
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flipping frequency (fo):
− E f = f 0 exp a k B (Tm − T f ) ………………………………………………………………………(1)
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where, f is the experimental frequency; f0 is polarization flipping frequency corresponding to
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asymptotically high temperatures; k B is Boltzmann’s constant; and Tf is the static freezing temperature. The dielectric constant maxima temperatures (Tm) with their corresponding frequency for all the compositions were fitted with equation (1) as shown in Fig. 4. The nonlinear VF fitting provides various parameters for the quantitative analysis of relaxor properties of all the compositions. These parameters suggest that the freezing temperature slightly shifted towards higher temperature side with an increase in Li/Bi compositions. The freezing temperature calculated from tangent loss spectra peaks for BBLT (x=0.5) is quite low 10
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compared to the dielectric maxima temperature, that is quite obvious since the tangent loss peaks always appear at least 50-100 K lower than the dielectric maximum temperature peaks. The activation energy was obtained in the range of 10 to 50 mV which is reasonably good for relaxor
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systems. Activation energy and polarization flipping frequency are found in the range of model relaxor systems due to cation ordering and compositional fluctuation in the matrix. These parameters are given in Table 3. Due to the high conductivity of BBLT sample with x = 0.5, we
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cannot precisely determine the Tm for low probe frequencies, so we have used high-frequency dielectric spectra and loss factor for VF fitting. The relaxor behavior can develop in any system
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due to many reasons such as microscopic composition fluctuation, the formation of nanoscale polar regions, coupling of order parameters, ordering of cations and local disorder through internal strain, or the randomly distributed electrical field [40–43]. In BBLT ceramic nanoscale polar order and their cooperative interaction leads to complex relaxation behavior. The
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substitution of Bi3+/Li+ cations for Ba2+ leads to the formation of compositional fluctuation that has a profound influence on the static and dynamic dielectric and optical properties [42,44,45].
Temperature dependent ferroelectric polarization
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3.4
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Fig.5 (a-d) shows the ferroelectric polarization (P) as functions of the electric field (E) measured at 2 Hz and in the temperature range from 293 K-423 K. Room temperature P-E loops of the BBLT ceramics are shown in Fig. 5(a), which indicates slim ferroelectric hysteresis for all the compositions that is one of the characteristics of classical relaxor-ferroelectrics. Fig. 5(a) suggests that the saturation polarization (Ps) decreases with increase in the concentration of Li and Bi ions. This is because of substituted cations reduce an alignment of dipoles concerning applied electric field. The leakage current also increases with increase in substitution due to 11
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development of free charge carriers in the matrix and their hopping to the nearest neighbor site. The spontaneous polarization (Ps) decreases (x=0.3, & 0.4) with an increase in temperature and persist till the dielectric maxima temperature. It becomes slimmer and almost negligible coercive
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field at elevated temperature. However, for x=0.5 the saturation polarization increases with temperature which may be due to high conductivity in the sample at elevated temperature. Slim hysteresis loops are due to the nano dipoles in the ceramics with weak cooperative interaction
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among themselves. These systems are the possible candidate for negative and positive
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Conduction mechanism
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electrocaloric effects [16,46].
Current density (J) versus electric field (E) and corresponding conduction mechanism observed for all the compositions of BBLT electroceramics are shown in Fig. 6(a-b).
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Symmetrical leakage current behavior is observed for these compositions BBLT x=0.3, 0.4, 0.5 until ± 60 kV applied an electric field. A significant enhancement in leakage current is observed with increase in the substitution of Bi and Li cations. The enhancement in leakage current may
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be due to development of mobile charge carriers under large applied electric field and active charge carriers at grain boundaries and interstitial site. The bound charge carriers accumulated at
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the surface and generated charge carriers drift away from the polarization in the material. To understand the mechanism responsible for conduction of BBLT ceramics, we considered three main mechanisms, conventional Schottky emission (SE) [47], bulk-limited Poole–Frenkel (PF) [48], and Space charge limited current (SCLC) conduction mechanisms [49]. These are models commonly used to describe carrier transport in oxide ceramics/ferroelectrics. We fit experimental data to these various models, but only SCLC fitting provides physical parameters which support the trap-assisted conduction mechanism.
For SCLC mechanism, experimental data should 12
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satisfy straight line fits depending on the type of charge transport. The magnitude of exponents should represent the various type of conduction process such as if n ~ 1 for ohmic, n ~ 2 for trap assisted and n ~ 3 for deep level charge trapping with free flow of carriers. When the internal
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field is dominated by space charge (either from free or trapped carriers), the current density (J) expected to have a power-law dependence on voltage, (J α En). In case of discrete traps, a
݆ௌ =
ଽఓ ఌೝ ఌ ɵா మ ଼ௗ
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quadratic voltage and cubic thickness (d) dependence are expected [50].
……………………………………………………………………………(2)
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Where E is applied field, εr is the dielectric constant of ceramics, εo is the dielectric constant of free space, µp is carrier mobility, and θ is the ratio of the total density of induced free carriers to trapped carriers. Fig. 6(b) shows SCLC fitting for x=0.3, 0.4, 0.5 and fitted parameters (exponents ~n) are found in the range of 1.34, 1.29, 1.04, respectively. It is possible to identify
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the data in all regions which look linear on the log-log plot and could, therefore, satisfy the power-law dependence. These findings suggest that substitution can relocate the anion (oxygen) vacancies and defects trapped across grains and grain boundaries and the density of charge
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carriers increases with increase in the magnitude of compositions. The SCLC mechanism is very common in polycrystalline ceramics and has been extensively studied. The concentration of
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trapping and de-trapping of the injected charge carriers decide the magnitude of leakage current. We believe that SCLC conduction is likely to be the dominant factor in leakage mechanism for all BBLT ceramics. 4. Conclusions We have successfully synthesized a new family of relaxor ferroelectrics near the room temperature. Interestingly, a mixed crystal structure with nearly 80-95% tetragonal phase and 13
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rest orthorhombic phase is identified using Rietveld refinement. The composition with x=0.4 shows near classical relaxor system which obey the nonlinear VF model and dielectric dispersion model. The characteristic parameters of these systems are also matched with other relaxor
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families. A well defined dielectric dispersion both in real and imaginary part was observed below Tm with further merge in Tm favors the relaxor properties. The formation of pure phase is difficult for higher compositions (x≥0.5), which leads to impurity phases with lots of free charge
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carriers responsible for high conductivity. A slim hysteresis loop is obtained for these systems support their relaxor nature. The conductivity is reasonably low at a high electric field which
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increases with increase in compositions and follows SCLC mechanism. An in-depth study of the microstructure is required to confirm the genuine reason for relaxor behavior and their microstructure-property relation. Acknowledgment
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AK acknowledges the CSIR-National Physical Laboratory five year project PSC-0111. Hitesh Borkar shows appreciation to UGC (SRF) to provide fellowship to carry out the Ph.D. program. Authors sincerely thank the Director, NPL New Delhi, Dr. Ranjana Mehrotra and Dr. Sanjay
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Yadav, for their constant encouragement.
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W. Mehl, R. H. Tredgold: Space Charge Conduction in Solids, Berichte Der
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Bunsengesellschaft Für Phys. Chemie. 70 (1966) 393–394.
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[50]
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Figure captions: Fig. 1. The experimental data points (red dots), simulated curve (continuous black line) and
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difference (bottom blue line), Braggs position (green line) profiles of the reflections obtained from the Rietveld analysis of the sintered pellets XRD data of Bi, Li modified BTO at room temperature using P4mm+Amm2 space group for a x=0.3(a), 0.4(b), and 0.5(c).
with x=0.3 (a), 0.4 (b), and 0.5 (c), respectively.
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Fig. 2. The SEM micrographs for surface topography and elemental analysis of BBLT ceramics
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Fig. 3. Temperature-dependent dielectric constant and tangent loss as function of frequency for Bi and Li modified BBLT ceramics with x=0.3(a, b), 0.4(c, d), 0.5(e, f), respectively. Fig. 4. 1/Tm as a function of the measured frequency of BBLT for x=0.3, 0.4 and 0.5. The symbols (dot) are the experimental data points, and the solid line is the corresponding fitting to
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the Vogel–Fulcher relationship as expressed in Eq. (1). V-F fitting using tangent loss data is also given in the figure where right-hand side y-axis is used for scaling.
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Fig. 5. (a) Room temperature ferroelectric measurement for different compositions at 2 Hz. Temperature dependent ferroelectric (P-E) data of BBLT measured for x=0.3 (b), 0.4 (c), and 0.5
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(d).
Fig. 6. (a) Leakage current as a function of applied electric field measured on poled BBLT for x=0.3, 0.4, and 0.5. (b) SCLC fitting for current density and electric field measurement as expressed in Eq. (2).
21
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Table 1 Refinement structural parameters and agreement factor for BBLT at room temperature with space group P4mm +Amm2. 70-30
60-40
3.998
4.136
4.001
b (Å)
3.998
6.275
4.001
c (Å) Crystal structure
4.014 Tetragonal
5.323 Orthorho mbic
Space Group Rbraggs Rf-factor
P4mm 2.17 1.57
Amm2 26.9 29.6
4.003
4.078
6.296
4.003
5.445
4.013 Tetragonal
5.302 Orthorhom bic
4.006 Tetragonal
7.027 Orthorhombic
P4mm 2.047 1.832
Amm2 6.98 7.79
P4mm 16.7 13.8
Amm2 50.8 47.7
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14.15 15.40 11.36 03.40
Rp Rw Rexp χ2 95
5
80
19.00 19.02 14.11 01.85
40.1 30.8 18.8 02.9 20
88
12
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Wt%
4.038
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a (Å)
50-50
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Refine Parameters
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Stoichiometric calculated
x=0.4
Ba
37.74
Obs. From EDX 45.35
Bi
24.61
14.2
Li
Stoichiometric calculated
31.46
31.39
x=0.5
Obs. From EDX 37.99
Stoichiometric calculated 25.52
Obs. From EDX 29.95
38.89
25.15
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x=0.3
15.62
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Elements
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Table 2 Comparison of elemental analysis of initial and final product from energy dispersive Xray element analysis (EDAX).
Beyond experimental limit 18.72
18.04
O
18.84
21.85
18.27
18.62
17.79
19.40
18.33
27.77
17.84
25.15
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Ti
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Table 3 Parameters obtained for Vogel–Fulcher (VF) relationship from temperature dependent dielectric study on the composition Ba1-x(Bi0.5Li0.5)xTiO3 (BBLT) solid solutions with x = 0.3,
Tf (K)
Ea (eV)
x=0.3
284
0.03
x=0.4 x=0.5
8.13x107
311
0.05
3.32 x109
322
0.02
6.7 x107
275
0.01
1.4 x107
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x=0.5 (from tangent loss)
f0 (Hz)
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Compositions
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0.4, 0.5 ceramic.
24
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Fig. 1
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(a) x=0.3
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(b) x=0.4
T
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Intensity (arb. units)
obs cal diff. braggs position
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(c) x=0.5
20
30
40
T/O T/O
T/O O
O
*
O
O
T
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T/O
T O
50
2θ (Degree)
60
25
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Fig. 2
26
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Fig. 3
800
1 kHz 5 kHz 10 kHz 50 kHz 100 kHz 500 kHz 1 MHz
0.1
1MHz
1MHz
1kHz
1MHz
400
500
600
1kHz
1000
0.01 700 300 1 kHz 5 kHz 10 kHz 50 kHz 100 kHz 500 kHz 1 MHz
1kHz
1MHz
800
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300
1kHz
1kHz
(b)
1MHz
400
500
1MHz
Mtan δ AN U
Dielectric Constant (ε)
600
(a)
1 kHz 5 kHz 10 kHz 50 kHz 100 kHz 500 kHz 1 MHz
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1kHz
1000
1kHz
0.1
600
700
1 kHz 5 kHz 10 kHz 50 kHz 100 kHz 500 kHz 1 MHz
1kHz
1MHz
(c)
600
300
600
1 kHz
1 kHz
400
500
T(K)
400 1 MHz
0.15
1MHz
500
1 kHz 5 kHz 10 kHz 50 kHz 100 kHz 500 kHz 1 MHz
(d)
600
700
1 kHz
0.1
1 MHz
(e)
1 MHz
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400
1 kHz 5 kHz 10 kHz 50 kHz 100 kHz 500 kHz 1 MHz
500
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800
400
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300
0.01 700 300 0.2
600
0.05
700
1 MHz
(f)
1 kHz
300
400
500
600
700
T(K)
27
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Fig. 4
exp. data points V-F fitted
3.2
3
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10
1.5 12
14
1.0
ln(f) Hz
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8
2.0
-1
2.8
2.5
3
x=0.4
6
3.0
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-1
x=0.5
3.0
2.6
x=0.5 (from loss)
10 /Tm (K )
10 /Tm (K )
x=0.3
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3.5
28
Fig. 5
70-30 60-40 50-50
-10
10 0
-10
(a)
-20 -60
-40
-20
0
20
E (kV/cm)
10 0
x=0.4
EP
-10
293 K 313 K 333 K 353 K 373 K 393 K 413 K 423 K
40
-20
-60 -40 -20 0 20 E (kV/cm)
60
20
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20
P (µC/cm2)
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P (µC/cm2)
0
x=0.3
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10
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-20 -60 -40 -20 0 20 E (kV/cm)
P (µC/cm2)
P (µC/cm2)
20
293 K 313 K 333 K 353 K 373 K 393 K 413 K 423 K
20
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(c) 40
60
10
293 K 313 K 333 K 353 K 373 K 393 K 413 K 423 K
(b) 40
60
x=0.5
0
-10 -20 -60 -40 -20 0 20 E (kV/cm)
(d) 40
60
29
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Fig. 6
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-5
10
x=0.3 x=0.4 x=0.5
-6
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-7
10
-8
10
(a) -60
-40
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J (A/cm2)
10
-20
0
20
40
60
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-5.6
(b)
-6.0
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log J (A/cm2)
-5.2
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E (kV/cm)
-6.4 -6.8 -7.2
x= 0.3 x= 0.4 x= 0.5 fitted data
0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 log E (kV/cm) 30
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Highlights A novel novel Relaxor Ferroelectrics Family is discovered.
•
Lead free A-site substituted relaxor with polar nano regions.
•
The maximum dielectric temperatures follow nonlinear Vogel-Fulcher relationship.
•
The microstructure-property relation is established based on electrical properties.
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S0925-8388(17)34366-9
DOI:
10.1016/j.jallcom.2017.12.170
Reference:
JALCOM 44256
To appear in:
Journal of Alloys and Compounds
Received Date: 4 September 2017 Revised Date:
15 December 2017
Accepted Date: 16 December 2017
Please cite this article as: H. Borkar, V. Rao, M. Tomar, V. Gupta, A. Kumar, Near room temperature bismuth and lithium co-substituted BaTiO3 relaxor ferroelectrics family, Journal of Alloys and Compounds (2018), doi: 10.1016/j.jallcom.2017.12.170. 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.
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Near room temperature bismuth and lithium co-substituted BaTiO3 relaxor ferroelectrics family Hitesh Borkar1,2, Vaibhav Rao1,3, M Tomar4, Vinay Gupta5, Ashok Kumar1,2,*
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1
CSIR-National Physical Laboratory, Dr. K. S. Krishnan Marg, New Delhi 110012, India Academy of Scientific and Innovative Research (AcSIR), CSIR-National Physical Laboratory (CSIR-NPL) Campus, Dr. K. S. Krishnan Road, New Delhi 110012, India 3 Solar and Alternative Energy, Amity University, Jaipur, Rajasthan, 302006, India 4 Department of Physics, Miranda House, University of Delhi, Delhi 110007, India 5 Department of Physics and Astrophysics, University of Delhi, Delhi 110007, India
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Abstract
Eco-friendly lead-free relaxor ferroelectrics, Ba1-x(Bi0.5Li0.5)xTiO3 (BBLT) with x = 0.3, 0.4 and 0.5, have been synthesized by using the conventional solid-state reaction route. The crystal structure and phase purity were verified by x-ray diffraction (XRD) patterns using
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Rietveld refinement which revealed the presence of tetragonal (95 to 80%) and orthorhombic phases (5 to 20%) in the matrix. The temperature dependent dielectric constant and tangent loss dispersion spectra, slim ferroelectric-hysteresis, and nano-scale microstructures suggest the
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presence of short-range ordering (SRO)/polar nano-regions (PNRs) in the matrix. The dielectric spectra suggest that the dielectric dispersion increases with increase in Bi/Li concentrations
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which may be due to compositional fluctuations, ordering of cations at nano-scale and presence of metastable states of SRO and long-range ordering (LRO) regions. The nonlinear VogelFulcher relationship is fitted for maximum dielectric constant temperatures (Tm) which provides activation energy and freezing temperature of all compositions. The current density (J) versus electric field (E) (J-E) data obey bulk-limited current conduction mechanisms that may be due to a diffusive displacement of Ti4+ at octahedral center, the creation of random fields due to inhomogeneity at A-site, and trap-assisted hoping of charge carriers under high E-fields. 1
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Keywords: Relaxor-ferroelectric; Dielectric spectroscopy; Current conduction mechanism ; Polar nano-regions
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*Corresponding Author: Dr. Ashok Kumar (Email: [email protected])
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1. Introduction
Perovskite-based ferroelectric materials have attracted extensive interest to the
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researchers working on the development of transducers, sensors, energy harvesters, nonvolatile memory elements, optoelectronic devices, multilayer ceramic capacitors and micro-electromechanical systems, etc.,[1–4]. Among the various type of ferroelectrics, relaxor ferroelectrics have been found interesting due to large piezo-electric coefficients, electro-mechanical coupling, and interesting physics due to the presence of chemical in-homogeneities, ordering of cations and
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existence of PNRs in the matrix [5–9]. It is well proven that chemical and SRO ordering significantly improve the electro-mechanical and piezoelectric properties in relaxor ferroelectrics which make these systems suitable for industrial applications. Polar nano-regions (PNRs) are
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nano-scale local clusters of randomly oriented polarization which plays a key role in a large
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dielectric constant near the ferroelectric phase transition temperatures (Tm) and strong dielectric dispersion below Tm. In past, lead-based perovskite compounds, i.e. Pb0.85La0.1(Zr0.65Ti0.35)O3 (PLZT),
(1-x)[Pb(Mg1/3Nb2/3)O3]-x[PbTiO3]
(PMN–PT),
0.1Pb(Ni1/3Nb2/3)O3-
0.9Pb(Zr0.42Ti0.58)O3 (PNN-PZT), PbFe2/3W1/3O3 (PFW), PbFe1/2Nb1/2O3 (PFN), PbFe1/2Ta1/2O3 (PFT), PbSc0.5Nb0.5O3 (PSN), (PbxSr1-x)TiO3 (PST), Ba1−xPbx(Ti1−yZry)O3, etc., have shown above mentioned properties with strong dielectric dispersion near the phase transition temperatures [10–12].These systems showed large dielectric dispersion below Tm and merger of 2
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dielectric dispersion above Tm, slim ferroelectric hysteresis, and high piezoelectric and electromechanical properties. These compounds have drawbacks of toxicity and volatility associated with the lead compound. In this regards, environment-friendly and non-toxic
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compounds, i.e., ‘lead-free’ relaxor materials are the needs of modern-day investigation on relaxors and future lead-free sensors and actuators devices. It has also been reported that by the substitution of mono-valent and tri-valent cations on A-/B- sites of BaTiO3 leads to chemical in-
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homogeneities and cation ordering in the matrix which in turn develops PNRs and broad
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dielectric dispersion.
The lead-free perovskite materials have been investigated regarding their dielectric relaxation behavior, electrical and ferroelectric phase transition properties. These materials are as follows: KNbO3 (KN), doped-BaTiO3 (BT), (Bi1/2K1/2)TiO3(BKT), (Bi1/2Na1/2)TiO3 (BNT), Ba(Ti1-xYx)O3-x/2, etc., [13–16]. Among these systems, Bi0.5K0.5TiO3-based ceramics were widely
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investigated and used for various applications [17]. M. Naveed Ul-Haq et al. found the magnetodielectric properties in relaxor/ferrimagnetic composites. In these systems, the dielectric dispersion was control by an external applied magnetic field which may be useful for magnetic
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field sensors [18]. These materials are of primary interest of investigation due to various reasons;
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(i) BaTiO3-based materials find multiple uses in the commercial applications, (ii) A-/B- sites substituted-BaTiO3 often appeared as a ‘model' (non-lead based) ferroelectrics, (iii) quite different chemical mapping compared to classical relaxor like PMN, and (iv) even in relaxor states, these systems show well behaved remanent polarization and hysteresis (long-range ordering). Lead-free perovskite-type oxides show multiple polymorphic phase transitions, variation in dielectric permittivity and dielectric loss through chemical substitution, and complex microstructure-property relation. Relaxor behavior in the perovskites is mainly due to disorder 3
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in non-isovalent cations in the stoichiometric complex perovskite compounds. The systems such as Pb(Mg1/3Nb2/3)O3 (PMN) [19] or Pb(Sc1 /2Ta1/2)O3 (PST) [20] (in which Mg2+, Sc3+, Ta5+ and Nb5+ ions are entirely or moderately disordered at B-sub-lattice. Also for the non-stoichiometric
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solid solutions, like for Pb1−xLax(Zr1−yTiy)1−x/4O3(PLZT) [21,22] where the substitution of La3+ for Pb2+ ions lead to the vacancies. More degree of freedom can be obtained to add disorder on the Ba-site in BZT ceramic which is already a relaxor system with (Ti/Zr:65/35). The
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coexistence of ferroelectric (order) and relaxor (local and small region order) can be developed with the hetero-valent substitution of the Ba2+ cation by Bi3+ cations. Owing to its 6s2 lone pair
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(like Pb), the choice of Bi3+ (substituted for Ba2+) cation is favorable for the relaxor effect [23– 25].
In general, Random field theory is a well-accepted model for relaxors; this model suggests that the relaxors are a system with randomly oriented polarizations, random sites,
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defects, and lattice vacancies persist well above the Tm (i.e. para-electric region) which becomes the host lattice for these materials. Incorporation of Bi3+ cations in BaTiO3 lattice requires other mono-valent (Li/Na/K) cations to maintain the charge neutrality and vacancies. In this regard, we
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have chosen Bi+3/Li+1 cations to substitute the Ba2+ cations. These cations are attached to the host
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lattice which makes it almost impossible to move from one place to another. However, electrons and polarons can provide a large amount of electronic conduction under high E-fields and temperatures; sometimes it is possible that under large E-field and temperature, it is possible that Li may leave the lattice and support the formation of interstitial sites and ionic conduction. It has been reported that La-doped BaTiO3 generates a large number of electronic charge carriers at Asite which produces a substantial increase in conductivity [26]. In case of small amount of hetero-valent substitution at A-site, the sequence of ferroelectric phase transition was similar like 4
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to BaTiO3, however large amount of substitution may produce relaxor behavior, as can be seen in Ba1−yBi2y/3)TiO3, (0.09 < y < 0.15) system [27].
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Herein, we studied the relaxor behavior, temperature dependent dielectric properties, and effect of Bi3+/Li1+ substitution on BBLT systems. A suitable substitution of mono/dia/tri- valent cations at A-/B- site of perovskite structure provides complex microstructure with suitable
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cations ordering or compositionally inhomogeneous structure which in turn exhibits large dielectric dispersion in radio frequency range. Nonlinear behavior of frequency-dependent
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dielectric maximum temperatures is investigated, and possible freezing temperatures are obtained for various compositions. Leakage current behavior and possible current conduction mechanisms have been discussed. 2. Material and methods
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The Ba1-x(Bi0.5Li0.5)xTiO3 (BBLT) (x=0.3, 0.4, 0.5) polycrystalline ceramics prepared by the conventional solid-state reaction method. High purity (99.95 %) precursor powders of BaCO3, Li2CO3, Bi2O3, TiO2 were purchased from Sigma-Aldrich, latter their stoichiometric
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proportions are weighed according to desired materials. These green powders were mechanically mixed with IPA (isopropyl alcohol) for 2 hrs in an agate mortar for the homogeneous mixture.
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The dried powders were calcinated in a high-purity alumina crucible at 850 °C for 10 hrs. The calcined powders again re-grinded and mixed with PVA as a binder (10 wt%), then pressed at the pressure of 5-7 tons per square inch to form disk-shaped pellets with 10 mm in diameter and 1.5 mm in thickness. Finally, these pellets were sintered at an optimized temperature 1120-1200 °
C for 4 hrs depending on compositions. Phase purity of the compounds was confirmed by the X-
ray diffraction (XRD) technique using (Bruker AXS D8 Advance X-ray diffractometer Cu-kα;
5
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kα=1.54059A°) monochromatic radiation, in the 2ɵ range between 20° and 60°. Rietveld Refinement was carried out using Fullprof suit to understand the crystal structures. Grain growth, surface morphology, and elemental analysis on sintered pellets were carried out using a scanning
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electron microscope (SEM, Zeiss EVOMA-10) with measuring the uncertainty of ±7-10 %. The silver paint was used on both sides of pellets to prepare metal-insulator-metal capacitive structure. The capacitor structure was dried at 200 °C for 2-3 h to remove chemical residue,
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followed by the electrical measurements. The temperature dependent dielectric properties were studied using LCR meter (HIOKI-3532-50) at an oscillating amplitude of 500 mV in the 1kHz to
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1MHz frequency range, and temperature range from 300 K to 723 K. The Temperature dependent polarization versus electric field (P-E) hysteresis loops and room temperature leakage current were measured by using Radiant Ferroelectric Tester.
3.1
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3. Results and discussion
Structure and morphological study
In order to understand the crystal structure, the Rietveld refinement was carried out on
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XRD data of all three compositions (x= 0.3, 0.4, 0.5) and analyzed for Bi and Li substitution on Ba-site. Fig. 1 shows the x-ray diffraction patterns which fitted well for the mixed crystal
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structure model. These diffraction patterns were refined for tetragonal and orthorhombic crystal structure, the refined peaks matched well with Bragg’s peaks. The Rietveld refinement refined parameters were depicted in Table 1 which indicates that these systems are in the mixed crystal structure phase where the tetragonal structure is the dominating crystal structure. The tetragonal and orthorhombic structures relate the P4mm and Amm2 crystal symmetry, respectively. The lattice parameters were matched with their respective crystal structures. The fractional proportion
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of tetragonal to orthorhombic crystal structure was found to be 95:05, 80:20, and 88:12 for x=0.3, 0.4, 0.5, respectively. Fitting parameters was not too much convincing for x=0.5 due to more in-homogeneity of Bi/Li/Ba cations present at A-site of ABO3 structure and also the
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presence of pyrochlore phase. For x=0.5, the pyrochlore phases were (~0.5%) found at Bragg’s angle 29.06, mark as (*) which corresponds to the Bi2Ti2O7 [28,29]. When the content of Bi2O3 reached relatively high value, Bi2O3 reacts with dissociative TiO2 to produce Bi2Ti2O7. Bulk
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diffusion through grain boundary has been considered as main grain growth process during hightemperature sintering. The inhomogeneity in ordered and disordered lattice can reduce the
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activation energy of bulk diffusion; promote the mass transfer which enhances the sintering ability of ceramics [30]. An ideal cubic perovskite ABO3 structure, the coordination of the A-site is 12. The ionic radius of 12-coordinated Bi3+ and Li+ is comparable to the ionic radii for Ba2+ cation. Thus a limited amount of the substitution of Bi3+ and Li+ for Ba2+ will take place without
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pyrochlore phase but above than critical composition limit of bismuth and lithium substitution leads to the development of more impurity and secondary phases, it can be easily seen for the dielectric spectra of BBLT (x=0.5) [31]. The sintering temperature of BBLT based ceramics
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decreased effectively with an increase in Bi and Li substitution which may be due to the large
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activation energy of bulk diffusion during the high-temperature sintering process. The SEM micrographs for various BBLT compositions with x=0.3, 0.4, 0.5 were
illustrated in Fig. 2 (a), (b) & (c), respectively. It can be seen from micrographs that oblate spheroid grains were formed after the high-temperature sintering process. A comparison of the micrographs of Bi/Li substituted BBLT systems show that doping induces grains coalesces to form larger grains. It could also be noted that average grains and their distributions were nearly same with Li and Bi substitution. It is attributed to the development of an inter-granular 7
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interaction on the surface of grain that enhances merging of grains. An oblate spheroid type grains can be seen throughout matrix for BBLT ceramics and coalescence overlapping of grains reduced as the doping level increased. To confirm initial and final compositions of the product,
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an elemental analysis of the samples were carried out using EDX analysis as shown in Table 2. It is clear that BBLT systems are mostly free from any foreign impurity element. These systems exhibit negligible amount of impurity elements compared to the substituted elements and
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matched with initial and final ingredients. One cannot probe light element lithium using EDX
various elements. 3.2
Dielectric spectroscopy
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analysis which may result in high uncertainty in the calculation of proper compositions of
The temperature and frequency dependence of dielectric constant (ε) and tangent loss (tan
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δ) for the series of BBLT ceramics are shown in Fig. 3(a-f). Strong frequency dispersion is observed below the Tm (maximum dielectric constant temperature) for all the compositions with a tendency to merge above Tm. The magnitude of maximum dielectric constant decreased with
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increase in frequencies and shifted towards higher temperature side with an increase in probe frequencies. The dielectric dispersion of BBLT (x=0.4) is showing similar trends as with
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classical relaxor, however, for higher compositions (x=0.5) the dispersion is more chaotic. The second dielectric dispersion near the high temperature (~ 550 K) is due to thermally activated charge carriers. All three compositions show two dielectric maxima regions, one below to 450 K which is related to relaxor nature of the systems; however, at an elevated temperature nearly 550 K, a frequency dependent peak was observed, which may be due to ferroelectric to paraelectric or pseudo cubic to a cubic phase transition. A detail investigation is required to confirm the nature and category of high-temperature phase transitions. It may be achieved using a high8
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temperature XRD or neutron scattering, which is beyond the scope of the present investigation. Similar behavior was observed for NBT-BT systems near the morphotropic phase boundary. A similar trend of dielectric dispersion is observed for tangent loss data, however, in case of
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tangent loss below 450 K, the magnitude of tangent loss increased with increase in probe frequency and shifted towards higher temperature side. This signature of dielectric loss is considered as a fingerprint for relaxor systems. It can be easily observed from the high-
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temperature tangent loss data that their magnitude decreases with increase in probe frequency. The patterns of dielectric dispersion favor the presence of polar-nano regions and chemical
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inhomogeneity in the matrix which results in relaxor behavior in BBLT ceramics [32–34]. In the present system, it might be possible to form (Ba2+/ Bi3+/Li+) rich polar clusters on the analogy of A-site substituted relaxors, where inhomogeneity may introduce by substitution. The high degree of cation ordering-disordering, development of oxygen vacancies and deficiencies of volatile Li
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and Bi cations due to high sintering temperature modified the microstructure which results in large dielectric dispersion. The phase separation and compositional inhomogeneity are not observed from XRD data in these systems. Previously studied atomic-level disorder is an
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inherent property of the random-site structure. A relaxor behavior can be expected regardless of the overall size of chemically ordered domains [35]. It is sufficient to frustrate the long-range
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ordering of the A-site distributed cations to produce localized randomness. It is also evident from Fig. 3 (a,c,e) that diffuse dielectric peaks shift to higher temperature side with increase in the concentration of Bi and Li cations. It can be observed in Fig. 3(e) for the composition x=0.5 that the relaxor-ferroelectric phase transitions shift towards high temperature and reduces the magnitude of dielectric constant. The magnitude of tangent loss factor increases with increase in Bi/Li substitution due to increase in leakage current. Furthermore, peak broadening with
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increasing Bi and Li concentration is also evident. The random distribution of cations on A-site creates compositional variations of polar nano-regions which in turn lead to the locally varying ferroelectric transition temperature [36]. It is worth to be noted that temperature dependent
law (discussed in section 3.3) as reported previously [29,37]. Vogel–Fulcher analysis
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3.3
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dielectric maxima temperature (Tm) at various frequencies follows the nonlinear Vogel-Fulcher
To check the relaxor nature of materials, an empirical Vogel–Fulcher (VF) relation has
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been employed to account the dielectric relaxation phenomenon [38,39]. A thermally activated polarization reversal between two equivalent positions results in dielectric relaxation. Based on dielectric relaxation, the nonlinear VF model is used to study the activation energy Ea which requires crossing the barrier height between two equivalent polarization states and polarization
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flipping frequency (fo):
− E f = f 0 exp a k B (Tm − T f ) ………………………………………………………………………(1)
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where, f is the experimental frequency; f0 is polarization flipping frequency corresponding to
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asymptotically high temperatures; k B is Boltzmann’s constant; and Tf is the static freezing temperature. The dielectric constant maxima temperatures (Tm) with their corresponding frequency for all the compositions were fitted with equation (1) as shown in Fig. 4. The nonlinear VF fitting provides various parameters for the quantitative analysis of relaxor properties of all the compositions. These parameters suggest that the freezing temperature slightly shifted towards higher temperature side with an increase in Li/Bi compositions. The freezing temperature calculated from tangent loss spectra peaks for BBLT (x=0.5) is quite low 10
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compared to the dielectric maxima temperature, that is quite obvious since the tangent loss peaks always appear at least 50-100 K lower than the dielectric maximum temperature peaks. The activation energy was obtained in the range of 10 to 50 mV which is reasonably good for relaxor
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systems. Activation energy and polarization flipping frequency are found in the range of model relaxor systems due to cation ordering and compositional fluctuation in the matrix. These parameters are given in Table 3. Due to the high conductivity of BBLT sample with x = 0.5, we
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cannot precisely determine the Tm for low probe frequencies, so we have used high-frequency dielectric spectra and loss factor for VF fitting. The relaxor behavior can develop in any system
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due to many reasons such as microscopic composition fluctuation, the formation of nanoscale polar regions, coupling of order parameters, ordering of cations and local disorder through internal strain, or the randomly distributed electrical field [40–43]. In BBLT ceramic nanoscale polar order and their cooperative interaction leads to complex relaxation behavior. The
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substitution of Bi3+/Li+ cations for Ba2+ leads to the formation of compositional fluctuation that has a profound influence on the static and dynamic dielectric and optical properties [42,44,45].
Temperature dependent ferroelectric polarization
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3.4
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Fig.5 (a-d) shows the ferroelectric polarization (P) as functions of the electric field (E) measured at 2 Hz and in the temperature range from 293 K-423 K. Room temperature P-E loops of the BBLT ceramics are shown in Fig. 5(a), which indicates slim ferroelectric hysteresis for all the compositions that is one of the characteristics of classical relaxor-ferroelectrics. Fig. 5(a) suggests that the saturation polarization (Ps) decreases with increase in the concentration of Li and Bi ions. This is because of substituted cations reduce an alignment of dipoles concerning applied electric field. The leakage current also increases with increase in substitution due to 11
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development of free charge carriers in the matrix and their hopping to the nearest neighbor site. The spontaneous polarization (Ps) decreases (x=0.3, & 0.4) with an increase in temperature and persist till the dielectric maxima temperature. It becomes slimmer and almost negligible coercive
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field at elevated temperature. However, for x=0.5 the saturation polarization increases with temperature which may be due to high conductivity in the sample at elevated temperature. Slim hysteresis loops are due to the nano dipoles in the ceramics with weak cooperative interaction
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among themselves. These systems are the possible candidate for negative and positive
3.5
Conduction mechanism
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electrocaloric effects [16,46].
Current density (J) versus electric field (E) and corresponding conduction mechanism observed for all the compositions of BBLT electroceramics are shown in Fig. 6(a-b).
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Symmetrical leakage current behavior is observed for these compositions BBLT x=0.3, 0.4, 0.5 until ± 60 kV applied an electric field. A significant enhancement in leakage current is observed with increase in the substitution of Bi and Li cations. The enhancement in leakage current may
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be due to development of mobile charge carriers under large applied electric field and active charge carriers at grain boundaries and interstitial site. The bound charge carriers accumulated at
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the surface and generated charge carriers drift away from the polarization in the material. To understand the mechanism responsible for conduction of BBLT ceramics, we considered three main mechanisms, conventional Schottky emission (SE) [47], bulk-limited Poole–Frenkel (PF) [48], and Space charge limited current (SCLC) conduction mechanisms [49]. These are models commonly used to describe carrier transport in oxide ceramics/ferroelectrics. We fit experimental data to these various models, but only SCLC fitting provides physical parameters which support the trap-assisted conduction mechanism.
For SCLC mechanism, experimental data should 12
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satisfy straight line fits depending on the type of charge transport. The magnitude of exponents should represent the various type of conduction process such as if n ~ 1 for ohmic, n ~ 2 for trap assisted and n ~ 3 for deep level charge trapping with free flow of carriers. When the internal
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field is dominated by space charge (either from free or trapped carriers), the current density (J) expected to have a power-law dependence on voltage, (J α En). In case of discrete traps, a
݆ௌ =
ଽఓ ఌೝ ఌ ɵா మ ଼ௗ
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quadratic voltage and cubic thickness (d) dependence are expected [50].
……………………………………………………………………………(2)
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Where E is applied field, εr is the dielectric constant of ceramics, εo is the dielectric constant of free space, µp is carrier mobility, and θ is the ratio of the total density of induced free carriers to trapped carriers. Fig. 6(b) shows SCLC fitting for x=0.3, 0.4, 0.5 and fitted parameters (exponents ~n) are found in the range of 1.34, 1.29, 1.04, respectively. It is possible to identify
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the data in all regions which look linear on the log-log plot and could, therefore, satisfy the power-law dependence. These findings suggest that substitution can relocate the anion (oxygen) vacancies and defects trapped across grains and grain boundaries and the density of charge
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carriers increases with increase in the magnitude of compositions. The SCLC mechanism is very common in polycrystalline ceramics and has been extensively studied. The concentration of
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trapping and de-trapping of the injected charge carriers decide the magnitude of leakage current. We believe that SCLC conduction is likely to be the dominant factor in leakage mechanism for all BBLT ceramics. 4. Conclusions We have successfully synthesized a new family of relaxor ferroelectrics near the room temperature. Interestingly, a mixed crystal structure with nearly 80-95% tetragonal phase and 13
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rest orthorhombic phase is identified using Rietveld refinement. The composition with x=0.4 shows near classical relaxor system which obey the nonlinear VF model and dielectric dispersion model. The characteristic parameters of these systems are also matched with other relaxor
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families. A well defined dielectric dispersion both in real and imaginary part was observed below Tm with further merge in Tm favors the relaxor properties. The formation of pure phase is difficult for higher compositions (x≥0.5), which leads to impurity phases with lots of free charge
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carriers responsible for high conductivity. A slim hysteresis loop is obtained for these systems support their relaxor nature. The conductivity is reasonably low at a high electric field which
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increases with increase in compositions and follows SCLC mechanism. An in-depth study of the microstructure is required to confirm the genuine reason for relaxor behavior and their microstructure-property relation. Acknowledgment
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AK acknowledges the CSIR-National Physical Laboratory five year project PSC-0111. Hitesh Borkar shows appreciation to UGC (SRF) to provide fellowship to carry out the Ph.D. program. Authors sincerely thank the Director, NPL New Delhi, Dr. Ranjana Mehrotra and Dr. Sanjay
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Yadav, for their constant encouragement.
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Figure captions: Fig. 1. The experimental data points (red dots), simulated curve (continuous black line) and
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difference (bottom blue line), Braggs position (green line) profiles of the reflections obtained from the Rietveld analysis of the sintered pellets XRD data of Bi, Li modified BTO at room temperature using P4mm+Amm2 space group for a x=0.3(a), 0.4(b), and 0.5(c).
with x=0.3 (a), 0.4 (b), and 0.5 (c), respectively.
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Fig. 2. The SEM micrographs for surface topography and elemental analysis of BBLT ceramics
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Fig. 3. Temperature-dependent dielectric constant and tangent loss as function of frequency for Bi and Li modified BBLT ceramics with x=0.3(a, b), 0.4(c, d), 0.5(e, f), respectively. Fig. 4. 1/Tm as a function of the measured frequency of BBLT for x=0.3, 0.4 and 0.5. The symbols (dot) are the experimental data points, and the solid line is the corresponding fitting to
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the Vogel–Fulcher relationship as expressed in Eq. (1). V-F fitting using tangent loss data is also given in the figure where right-hand side y-axis is used for scaling.
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Fig. 5. (a) Room temperature ferroelectric measurement for different compositions at 2 Hz. Temperature dependent ferroelectric (P-E) data of BBLT measured for x=0.3 (b), 0.4 (c), and 0.5
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(d).
Fig. 6. (a) Leakage current as a function of applied electric field measured on poled BBLT for x=0.3, 0.4, and 0.5. (b) SCLC fitting for current density and electric field measurement as expressed in Eq. (2).
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Table 1 Refinement structural parameters and agreement factor for BBLT at room temperature with space group P4mm +Amm2. 70-30
60-40
3.998
4.136
4.001
b (Å)
3.998
6.275
4.001
c (Å) Crystal structure
4.014 Tetragonal
5.323 Orthorho mbic
Space Group Rbraggs Rf-factor
P4mm 2.17 1.57
Amm2 26.9 29.6
4.003
4.078
6.296
4.003
5.445
4.013 Tetragonal
5.302 Orthorhom bic
4.006 Tetragonal
7.027 Orthorhombic
P4mm 2.047 1.832
Amm2 6.98 7.79
P4mm 16.7 13.8
Amm2 50.8 47.7
TE D
14.15 15.40 11.36 03.40
Rp Rw Rexp χ2 95
5
80
19.00 19.02 14.11 01.85
40.1 30.8 18.8 02.9 20
88
12
AC C
EP
Wt%
4.038
M AN U
a (Å)
50-50
SC
Refine Parameters
22
ACCEPTED MANUSCRIPT
Stoichiometric calculated
x=0.4
Ba
37.74
Obs. From EDX 45.35
Bi
24.61
14.2
Li
Stoichiometric calculated
31.46
31.39
x=0.5
Obs. From EDX 37.99
Stoichiometric calculated 25.52
Obs. From EDX 29.95
38.89
25.15
SC
x=0.3
15.62
M AN U
Elements
RI PT
Table 2 Comparison of elemental analysis of initial and final product from energy dispersive Xray element analysis (EDAX).
Beyond experimental limit 18.72
18.04
O
18.84
21.85
18.27
18.62
17.79
19.40
18.33
27.77
17.84
25.15
AC C
EP
TE D
Ti
23
ACCEPTED MANUSCRIPT
RI PT
Table 3 Parameters obtained for Vogel–Fulcher (VF) relationship from temperature dependent dielectric study on the composition Ba1-x(Bi0.5Li0.5)xTiO3 (BBLT) solid solutions with x = 0.3,
Tf (K)
Ea (eV)
x=0.3
284
0.03
x=0.4 x=0.5
8.13x107
311
0.05
3.32 x109
322
0.02
6.7 x107
275
0.01
1.4 x107
AC C
EP
TE D
x=0.5 (from tangent loss)
f0 (Hz)
SC
Compositions
M AN U
0.4, 0.5 ceramic.
24
ACCEPTED MANUSCRIPT
Fig. 1
RI PT
(a) x=0.3
SC M AN U
(b) x=0.4
T
TE D
Intensity (arb. units)
obs cal diff. braggs position
EP
(c) x=0.5
20
30
40
T/O T/O
T/O O
O
*
O
O
T
AC C
T/O
T O
50
2θ (Degree)
60
25
RI PT
ACCEPTED MANUSCRIPT
AC C
EP
TE D
M AN U
SC
Fig. 2
26
ACCEPTED MANUSCRIPT
Fig. 3
800
1 kHz 5 kHz 10 kHz 50 kHz 100 kHz 500 kHz 1 MHz
0.1
1MHz
1MHz
1kHz
1MHz
400
500
600
1kHz
1000
0.01 700 300 1 kHz 5 kHz 10 kHz 50 kHz 100 kHz 500 kHz 1 MHz
1kHz
1MHz
800
SC
300
1kHz
1kHz
(b)
1MHz
400
500
1MHz
Mtan δ AN U
Dielectric Constant (ε)
600
(a)
1 kHz 5 kHz 10 kHz 50 kHz 100 kHz 500 kHz 1 MHz
RI PT
1kHz
1000
1kHz
0.1
600
700
1 kHz 5 kHz 10 kHz 50 kHz 100 kHz 500 kHz 1 MHz
1kHz
1MHz
(c)
600
300
600
1 kHz
1 kHz
400
500
T(K)
400 1 MHz
0.15
1MHz
500
1 kHz 5 kHz 10 kHz 50 kHz 100 kHz 500 kHz 1 MHz
(d)
600
700
1 kHz
0.1
1 MHz
(e)
1 MHz
AC C
400
1 kHz 5 kHz 10 kHz 50 kHz 100 kHz 500 kHz 1 MHz
500
TE D
800
400
EP
300
0.01 700 300 0.2
600
0.05
700
1 MHz
(f)
1 kHz
300
400
500
600
700
T(K)
27
RI PT
ACCEPTED MANUSCRIPT
Fig. 4
exp. data points V-F fitted
3.2
3
TE D
10
1.5 12
14
1.0
ln(f) Hz
AC C
EP
8
2.0
-1
2.8
2.5
3
x=0.4
6
3.0
M AN U
-1
x=0.5
3.0
2.6
x=0.5 (from loss)
10 /Tm (K )
10 /Tm (K )
x=0.3
SC
3.5
28
Fig. 5
70-30 60-40 50-50
-10
10 0
-10
(a)
-20 -60
-40
-20
0
20
E (kV/cm)
10 0
x=0.4
EP
-10
293 K 313 K 333 K 353 K 373 K 393 K 413 K 423 K
40
-20
-60 -40 -20 0 20 E (kV/cm)
60
20
TE D
20
P (µC/cm2)
SC
P (µC/cm2)
0
x=0.3
M AN U
10
AC C
-20 -60 -40 -20 0 20 E (kV/cm)
P (µC/cm2)
P (µC/cm2)
20
293 K 313 K 333 K 353 K 373 K 393 K 413 K 423 K
20
RI PT
ACCEPTED MANUSCRIPT
(c) 40
60
10
293 K 313 K 333 K 353 K 373 K 393 K 413 K 423 K
(b) 40
60
x=0.5
0
-10 -20 -60 -40 -20 0 20 E (kV/cm)
(d) 40
60
29
ACCEPTED MANUSCRIPT
Fig. 6
RI PT
-5
10
x=0.3 x=0.4 x=0.5
-6
SC
-7
10
-8
10
(a) -60
-40
M AN U
J (A/cm2)
10
-20
0
20
40
60
EP
-5.6
(b)
-6.0
AC C
log J (A/cm2)
-5.2
TE D
E (kV/cm)
-6.4 -6.8 -7.2
x= 0.3 x= 0.4 x= 0.5 fitted data
0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 log E (kV/cm) 30
ACCEPTED MANUSCRIPT
Highlights A novel novel Relaxor Ferroelectrics Family is discovered.
•
Lead free A-site substituted relaxor with polar nano regions.
•
The maximum dielectric temperatures follow nonlinear Vogel-Fulcher relationship.
•
The microstructure-property relation is established based on electrical properties.
AC C
EP
TE D
M AN U
SC
RI PT
•