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Structural and Dielectric Properties of Li0.5Bi0.5Ti0.8Zr0.2O3 Ceramics

DOI: 10.4236/oalib.1105563    88 Downloads   167 Views  

ABSTRACT

A polycrystalline orthorhombic compound of Li0.5Bi0.5Ti0.8Zr0.2O3 is synthesized by using a high temperature solid-state reaction technique at high temperature (i.e., at 900℃). The room temperature X-ray diffraction study assured the evolution of single-phase compound with orthorhombic structure. The dielectric analysis of Li0.5Bi0.5Ti0.95Zr0.05O3 explored over a broad frequency range (103 - 106 Hz) at various temperatures (33℃ - 500℃) displayed that the dielectric properties of the material are dependent on both frequency and temperature. Dielectric study reveals that the ferro to paraelectric phase transition of the studied compound is a temperature of 112℃. The nature of the variation of conductivity and value of activation energy in different regions, calculated from the temperature dependence of ac conductivity suggest that the conduction process is of mixed type (i.e., ionic–polaronic and space charge generated from the oxygen ion vacancies).

1. Introduction

The discovery of ferroelectricity in BaTiO3 is very crucial for the material researchers. Oxides of disparate structures like tungsten bronze, perovskite, layer structure etc., were analysed for application in numerous devices such as transducers, actuators, multi-layer capacitors, ferroelectric random-access memory and display, microwave dielectric resonators, etc. This galvanized the scientists to flourish thermally well-built modern electronic materials possessing high dielectric constant and low dielectric loss. Reckoning the above, the ferroelectric oxides of perovskite structural family have chiefly been probed for their convenient dielectric [1] , electro-optic [2] , nonlinear optic [3] pyroelectric [4] , piezoelectric [5] properties. A perfect perovskite structure can be represented by a general formula as ABO3 where A is a large cation (mono to trivalent) and B is a small cation (a transition metal ion). The A ions occupy the corners of the cube, which is 12 coordinated, while the B ions sit on the body center positions inside an oxygen octahedron, which are at the face center positions. Literature survey disclosed that an ample of research has been accomplished on perovskite type ferroelectric oxides niobates and tantalates [6] [7] [8] . We could not find any, but no work is executed on the structural, dielectric and electrical properties of current compound. Looking to the importance of the material, we have synthesised and analysed the structural and dielectric properties of a new compound having the chemical formula Li0.5Bi0.5Ti0.8Zr0.2O3 (LBTZ).

2. Experimental

2.1. Material Preparation

The polycrystalline sample Li0.5Bi0.5Ti0.8Zr0.2O3 (LBTZ) was prepared with apposite stoichiometric ratio of precursors; LiCO3, Bi2O3, TiO2, ZrO2 of high purity (>99.9%) were weighed and blended mechanically by an agate mortar for about 3 h. The powders were then calcined at optimized temperature and time (900˚C for 12 h) in air condition. The calcined powder thus procured was blended with PVA (poly vinyl alcohol) binder, grinded and were palletized ensuingly (about 10 mm in diameter and 1 - 2 mm thickness) under uniaxial pressure of 4 × 106 N/m2. Thereafter the pellets were sintered in air atmosphere at 950˚C for 12 h. Ultimately the pellets were laminated with high purity silver paint, and then heated at 150˚C for 2 h before executing the electrical measurements.

2.2. Material Charecterisation

X-ray diffraction (XRD) pattern of the material procured in a vast range of Bragg angle 2θ (200 ≤ 2θ ≤ 800) at a scanning speed of 30 min1 by an X-ray diffractometer (Rigaku, Miniflex) with CuKα radiation (λ = 1.5405 Å) at room temperature. The surface morphology of the pellet sample of the material was recorded with a high-resolution scanning electron microscope (SEM: JOEL-JSM model: 5800F). The dielectric characterisation of the sample was prosecuted in the temperature range of 32˚C - 5000˚C and frequency range of 1 kHz to 1 MHz, using a computer-controlled Hioki HiTester LCR meter.

3. Results and Discussion

3.1. Structural and Microstructural Analysis

Figure 1 shows the XRD pattern of the sample. Indexing of all the peaks of the

Figure 1. Room temperature XRD pattern (left), SEM (inset) and histogram (right) of Li0.5Bi0.5Ti0.8Zr0.2O3 ceramics.

pattern were done taking their 2θ values bya computer program package, “POWDMULT” [9] in distinct crystal system and cell configuration and are traced to be sharp and solo, which The peaks are discrete from those of ingredients ensuring the evolution of new single-phase compound. On the basis of the best agreement (based on least-squares refinement) between scrutinised (obs) and reckoned (cal) interplaner distance d (i.e., Σ(dobs − dcal) = minimum), an orthorhombic unit cell was selected with lattice parameters: a = 18.8065 (29) Ǻ, b = 8.3211 (29), c = 8.2920 (29) Ǻ (estimated standard deviation in parentheses) which are consistence with the reported ones [10] . The coherently average dispersed crystallite size (D) of the compound was computed to be ~ 15 nm using Scherrer’s equation; D = 0.89λ/(β1/2cosθhkl), where λ = 1.5405 Å and β1/2 = peak width of the reflection at half maxima [11] . The contributions of strain, instrumental error and other unknown effects in the peak broadening has not been taken into board during the crystallite size enumeration.

The room temperature SEM micrograph (Figure 1 (inset)) of the LBTZ compound, confirmed homogenously and non-uniform distribution of the grains over the entire surface of the sample. The grain size evaluated from the histogram Figure 1 (right) is traced to be of 4.9 μm. As expected, the grain size of the sample obtained here is gigantic in comparison to the crystallite size enumerated from Scherrer’s equation. Thus, a solo grain has large number of crystallites [12] .

3.2. Dielectric Analysis

The temperature variation of relative dielectric constant (εr) at some selected frequencies of compound is shown Figure 2. The plot firmly established the ferro to paraelectric phase transition at 112˚C. The value of εr is small at low temperatures which increases with rise in temperature. The dielectric constant (εr) at frequencies 10, 50, 100, 500 and 1000 kHz are found to be 225, 203, 159, 145 and 125 respectively at the transition temperature. The variation of tanδ follows the same pattern as that of εr. The increase in the value of tanδ may be due to (i) enhancement in the conductivity and (ii) reduction in the contribution of ferroelectric domain wall [13] .

3.3. Ac Conductivity Analysis

The ac electrical conductivity (σac) is calculated using the dielectric data and an empirical relation. σac = ωεrε0 tanδ, where ε0 = permittivity of free space and ω = angular frequency. Figure 3 shows the variation of σac as a function of temperature at frequencies 10 and 100 kHz.

The nature of the variation (Figure 3) is almost linear over a wide temperature region obeying the Arrhenius relation: σac = σ0 exp(−Ea/kBT) [14] , Each of graph is divided into two different regions independently of frequency. Every divided region is characterized by different slopes showing different activation energy. The solid line of figure shows the linear fit. The activation energy calculated from the slope of the curve at different temperatures has been compared in Table 1. Due to the dielectric phase transitions, abnormality in conductivity was

Figure 2. Variation of εr and tanδ (right) with temperature of Li0.5Bi0.5Ti0.8Zr0.2O3 at some selected frequencies.

Figure 3. Variation of σac with 1000/T of Li0.5Bi0.5Ti0.8Zr0.2O3 at two selected frequencies.

Table 1. Comparison of activation energy Ea (eV) of Li0.5Bi0.5Ti0.8Zr0.2O3 at two different frequencies in region I, II and III, calculated from σac vs. 1/T graphs.

observed for all the compound at temperatures nearly equal to its Curie temperature, which might be due to the dielectric phase transition. The value of σac increases with increase in temperature indicating negative temperature coefficient of resistance (NTCR) behavior. The increase in conductivity is due to the hopping action of the ions because of thermally activated electrons. At high temperature higher value of activation energy indicates that conductivity is for the movement of oxygen vacancies. Activation energy is low at high frequency as compared to that at the low frequency (Table 1). This is because at low frequencies the overall conductivity is the result of hopping of charge carriers over a large distance and at higher frequencies is restricted to only nearest neighboring defects sites [15] .

4. Conclusion

The polycrystalline sample of LBTZ was prepared by a solid-state-reaction route. Preliminary X-ray analysis confirms the single phase orthorhombic crystal structure at room temperature. The plot firmly established the ferro to paraelectric phase transition at 112˚C. The dielectric constant of the ceramics decreases with increasing frequency. The activation energy of the compound was found to be different in different regions indicating presence of different conduction mechanisms.

Acknowledgements

D. Panda acknowledges North Orissa University for the co-operation and help during his Ph.D research work. The authors are thankful to Prof. R.N.P. Choudhary, Professor, Department of Physics, ITER, Bhubaneswar who had helped us and permitted us to use his laboratary during synthesis of compound and some of analysis of its properties. D. Panda also acknowledges Department of Physics, Betnoti College Betnoti for allowing him to do some experimental work during his research.

Conflicts of Interest

The authors declare no conflicts of interest regarding the publication of this paper.

Cite this paper

Panda, D. , Mohanty, B. , Sahoo, P. and Choudhary, R. (2019) Structural and Dielectric Properties of Li0.5Bi0.5Ti0.8Zr0.2O3 Ceramics. Open Access Library Journal, 6, 1-6. doi: 10.4236/oalib.1105563.

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