Structural, Electrical and Ferroelectric Properties of Lead-free Bi(Fe0.85dy0.15)o3 Electroceramic Compound

This paper mainly reports detailed studies of structural, dielectric, impedance and ferroelectric properties of Bi(Fe 0.85 Dy 0.15 )O 3 (termed as BFDO15), fabricated via solid state reaction (SSR) method. Analysis of X-ray diffraction (XRD) data confirm the crystal symmetry changes from rhombohedral to orthorhombic symmetry. The scanning electron micrograph collected through field emission scanning electron microscopy (FE-SEM) shows a regular distribution of grains over the sample surface. The elemental composition of the sample was examined by using energy dispersive X-ray micro-analysis (EDXMA) and this confirms the existence of constituent elements of the sample. The electrical measurement was carried out using a computer-controlled phase sensitive multimeter (PSM) in a frequency range of 1 kHz - 1000 kHz and temperature range of 25 ºC – 500 ºC. Study of dielectric properties shows high dielectric permittivity and small value of dielectric loss in the sample. The frequency dependent impedance and electrical modulus analysis reveals the presence of a semiconducting nature and non-Debye type relaxation process. Analysis of ac-conductivity with respect to frequency obeys the universal Jonscher’s power law. The electric p olarization study shows enhancement in ferroelectric property of the material. Hence, based on the significant enhancement found in the structural, electrical and ferroelectric properties of BFDO15 material, it could be a promising candidate for modern device applications.


Introduction
A tremendous research interest in multiferroic materials, which are having simultaneously both the ferroelectricity as well as ferromagnetic order in a single phase at higher temperatures, have been experienced since the past few decades. Development of lead-free multiferroic materials added some advantages in our environment concerned and health issues of human beings as they are considered as toxic free materials for electronic device applications including information storage media, actuators, spintronics, transducers, sensors and optoelectronic devices [1][2][3]. Despite the tenacious efforts had been paid for the development of novel single phase multiferroics, bismuth ferrite (BFO) is perhaps the most aware material which have both magnetic and a strong ferroelectric (multiferroic) behavior at/above room temperature.
Bismuth ferrite possesses an ABO3 type distorted perovskite rhombohedral symmetry with a space group of R3c below the ferroelectric Curie temperature (Tc = 830 °C), which have the unit cell value of a = b = 5.558 Å and c = 13.87 Å and its rhombohedral angle is 89.3º-89.48º [4,5]. BFO becomes antiferromagnetic with G-type structure below its magnetic Neel temperature (TN = 620 °C). The main reason of the ferroelectricity of BFO is originated from 6s lone pair electrons of Bi 3+ ions while the ferromagnetic/antiferromagnetic property was attributed to partly filled d orbital of Fe 3+ ions [6]. Despite being multiferroicity at/above room temperature, BFO possesses some inherent drawbacks such as structural distortion, high current leakage density, high coercive field, low remnant polarization due to irregular magnetic spin structure which hindered its potential device applications [7,8]. However, researchers had identified the ways to strengthening and improving the multiferroic properties of BFO by substituting other certain components at AB or A/B site. It has learned from the literature review that doping at the A-site using alkaline earth ions is an effective way for enhancing the multiferroic property of BFO [9][10][11][12]. Several groups of researchers had attempted through SSR method, sol-gel technique and solution combustion method at A-site substitution using some rare earth ions including Dy 3+ , Gd 3+ , Sm 3+ , La 3+ and found that enhancement in magnetic properties as well as electrical features of bismuth ferrite [13][14][15][16]. Some investigations reported that substitution by several divalent ions such as Ca 2+ [17], Mn 2+ [18], Ba 2+ [19] and Sr 2+ , Pb 2+ [20] at both sites (AB-site) of BFO had also improved the structural and ferroelectric properties of BFO.
Based on the literature survey, most of the Dy doped BFO reported are at A-site doping or co-doping with some other elements while B-site Dy substitution has not yet been widely explored. In this work, we mainly focussed on the impact of B-site Dy substitution on structural, dielectric, electrical (i.e. impedance, modulus, conductivity) and ferroelectric features of BFO having a composition of Bi(Fe0.85Dy0. 15 Toledo digital balance (ML204/A01) was used for weighing the primary ingredients. All the ingredients were mixed homogeneously using mortar and pestle in a dry medium (air) for 3 h and wet (methanol) medium for 3 h in order to obtain a regular mixture sample. The mixed powder has been heated at an optimized temperature and time (750 ºC for 8 h). In order to make a sample pellet, 2% of polyvinyl alcohol (binder) was added to the heated powder. The cylindrical shaped pellets having a diameter of 11.75 mm and thickness 1.8 mm were made by applying iso-static pressure of 5x10 4 Nm -2 . Sintering process of the sample pellets was carried out at an optimized temperature 800 ºC in an electric furnace for 6 h. The high quality alumina crucibles of a cylindrical, boat and tray were utilized during the calcination and sintering process of the material.

Characterization of material
The basic crystal structure and phase formation of BFDO15 was measured by XRD (D8 Advance, Bruker) by applying CuKα (λ=1.5405 Å) radiation at 2º/min scan speed over Bragg's angle θ (20º ≤ 2θ ≤ 80º). POWDMULT software was utilized for measuring the unit cell parameters, the crystal system and Miller indices of BFDO15. The sample surface morphology has been measured using the FE-SEM (Carl Zeiss) for studying the distribution of grains, grain size, microstructure and density of grains. The elemental study of a sample was carried out by using energy dispersive X-ray spectroscopy (EDXS). The cylindrical shaped pellets were flattened with the help of a fine emery paper in order to get parallel and smooth surface of pellet, and then coated with a high quality silver paste (Alfa Aesar) and dried at 150 ºC for 1 h in order to eliminate the moisture. The dielectric constant, tangent loss and other electrical parameters were measured using a high precision computer-interfaced PSM (N4L, 1735) over a frequency range of 1 kHz -1000 kHz and temperature (25 ºC -500 ºC). A room temperature electric field polarization of the sample was measured using P-E loop tracer (Marine, India).

Results and discussion
3.1. Structural study Fig. 1 represents room temperature XRD pattern of sintered Bi(Fe0.85Dy0.15)O3 material. The sharp and narrow X-ray diffraction peaks indicate the proper crystalline nature as well as the development of a pure phase material. However, few peaks which have a very small intensities corresponds to the Bi25FeO40 and Bi2Fe4O9 crystal system [21] (shown as * symbol in the figure) and such a small impurity is a common problem in bismuth-based materials [22].
Measurement of the unit cell dimension was carried out using X-ray data refinement and indexing software known as "POWDMULT" [23]. The unit cell dimensions and crystal system of the sample were estimated according to the best agreement between (a) the observed and  Table 1. Sizes of each sample particles (P) were estimated by Scherrer's formula [24]: where λ=0.15407 nm, k is constant = 0.89, θ is maximum peak position and β1/2 is peak width at half maximum. The approximate crystallite size of BFDO15 is 38 nm. the help of a standard software called "ImageJ" and approximated to be 1.039 μm. The microstructural study was carried out using the EDXS as shown in Fig. 2b. This analysis confirmed the presence of bismuth, iron, dysprosium and oxygen (its constituent elements) with their respective atomic ratios and the purity of the sample. Moreover, EDXS analysis also reveals that BFDO15 material was effectively synthesized without impurities. The weight and atomic percentage of the constituent elements of the sample are tabulated in Table 2. capacitance (C|| ) were recorded with the help of high accuracy phase sensitive multimeter connected with a computer-controlled temperature furnace. Therefore, in order to know the potential applications and better understanding, it is an essential to study the dielectric properties of the sample. Dielectric constant of the sample is calculated by using the general capacitance formula given below:

Microstructural analysis
herein, p C = capacitance in parallel mode, t = thickness of the sample pellet, A = area of the sample pellet and 0  = permittivity of free space. for a sintered BFDO15 material. Initially, a temperature independent nature of dielectric permittivity has been detected at low temperature range, however, ɛr slowly increases with an increasing temperature for all the operating frequencies and the first hump was observed at 164 °C, which may be originated due to the magnetic transition in the as-prepared material [25].
Generally, due to the existence of different types of polarization (i.e. ionic, atomic, orientation, space charge) at low frequency region, higher value of ɛr was found at low frequency curves  Table 3. then results small value of dielectric permittivity [28]. Moreover, most of the dipoles are not strong enough to follow the increasing frequency at higher frequency region which results decrease of dielectric permittivity at higher frequency region. Therefore, a significant contribution only from electronic polarization is received at higher frequency among the different types of polarizations, which subsequently results small value of dielectric constant [29]. The dispersive spectrums found at lower frequency region suggests the effect of grain boundary which spreads throughout the sample surface [30].

Frequency dependent dielectric property
The graphical representation of dielectric loss (tanδ) as function of frequency is shown in Fig. 4b. The nature of the plot may be explained by a two-layer model which was proposed based on Koop's theory by Maxwell and Wagner [31]. This theory states that at low frequency, electrons are very effective as compared with other factors whereas grains are more active as compared with electrons at high frequencies. Due to the high resistive nature at grain boundary, more energy is required for moving charge carriers at low frequency region, which subsequently results high value of dielectric loss. On the other hand, a small amount of energy is used for moving charge carriers at higher frequency region due to the high conductivity and then consequences small value of dielectric loss at higher frequencies.

Electrical impedance spectroscopy 3.4.1 Real (Z') and imaginary (Z'') component of impedance study
Electrical impedance spectroscopy is one of the most powerful technique for investigating the dielectric and electrical characteristics of advanced ceramic materials. It has been commonly used for studying the grain and grain boundary effects and interface effect in a polycrystalline dielectric and ionic conductors [32,33]. The nature of electrical response as well as characteristics of real and imaginary components of the sample also can be examined using the experimental data. In addition, this technique is utilized for examining dynamics of the movement of ions in solid materials. Generally, an ac-signal is applied to silver coated pellet sample in order to measure the electrical response of the sample specimen, then analyzed and presented in different formats for studying its electrical properties. The real (Z') and imaginary ) can be determined by using the given formula [29]: herein, τ = RC = relaxation time, ω = angular frequency.  1-40 kHz). Then, the impedance spectrums amalgamate regardless to temperature at higher frequency division (40-1000 kHz), which was associated to the release of space charge and electrode effect [34]. The presence of decrement in barrier features with increasing temperature may also be associated with the increase of conductivity at higher frequencies [35]. The dispersive spectrum at low frequency division has also been noticed from the plot, that shows the decreasing nature of Z' with an increasing temperature suggesting the negative temperature coefficient of resistance (NTCR) behavior of the as-prepared material, it is the common feature of a semiconducting materials. The decreasing trend of Z' with a growing temperature as well as frequency is good in line with our other impedance result [29]. process/strength exists in a specific frequency [36]. The electrical relaxation processes of the sample are generally originated due to existence of immobile charge carriers (electron) at lower temperature and vacancies/defects created at higher temperature [37]. We can imagine the spread of relaxation time from the width of peaks as shown in Fig. 4b. According to the ideal Debye type relaxation, the centre of a semicircles should lie on/above the real axis. Nevertheless, it is obvious from the plot that the centre of a semi-circular arcs were falls below the real axis (x-axis), which confirms the presence of non-Debye type of electrical relaxation process in BFDO15 material [38]. Moreover, the graph clearly shows that the grain and grain boundary contribution of relaxation process at lower temperature (˂250 °C) and higher temperature (>275 °C) spectrums, respectively. The behavior of semi-circular arcs reveals the existence of distribution of relaxation time which were originated by irregular distribution of grains in our sample [39].

Electrical modulus spectroscopy
The electrical modulus spectroscopy is one of the most useful technique for examining the nature of electrode polarization, electrical conductivity, conduction mechanism, relaxation time and the effect of bulk and grain boundary properties of the as-prepared material [40]. In addition, this method can also be utilized for studying several types of electrical processes which developed in the present studied compound. The real (M') and imaginary part (M'') of modulus can be determined by using the mathematical relation [41] given below: herein, , all the symbols have their usual meanings.  peak of M'' shifted towards higher frequency region when the temperature increases and faster movement of charge carriers was observed and these results decreases in a relaxation time [44,45]. The trend behavior of M'' clearly indicates the existence of temperature dependent relaxation process and hopping charge mechanism in the material [46,47]. Moreover, the continuous increasing curve of the M'' spectrum for all the selected temperatures signifies that the ions can successfully jump from one site to the neighbouring site [48]. In addition, the asymmetric broadening of peaks/curves found in the plot recommends non-Debye type of relaxation process in the material.

Electrical conductivity study
Electrical conductivity analysis of a compound is an important method for determining electrical conduction mechanism which is required for examining transport properties of material. Conduction mechanism of materials is mainly influenced by microstructure of the sample and defects. In perovskite materials, the synthesis process such as calcination and sintering temperature, atmosphere, time, etc. can generate oxygen as well as cation vacancies and other defects [49]. Fig. 7a shows the change of ac-conductivity versus frequency at some temperature sets for Bi(Fe0.85Dy0.15)O3 material. It is noted that the value of ac-conductivity (σac) linearly increases as the frequency increase and tends to merge all the spectra at high frequency site. The frequency dependent electrical conductivity may be divided into two regions: (a) A plateau found at low frequency region and (b) a dispersion at high frequency region. At lower frequency, ac-conductivity seems not depends on frequency by forming a plateau. This frequency independent region is considered as zero-frequency conductivity (σo or σdc), which occurs due to thermally influenced space charge carriers. On the other hand, the frequency dependent conductivity was found at higher frequency region, so this site is considered as acconductivity (σac). Therefore, the σac may be calculated from the dispersive region (higher frequency site) using Jonscher's power law:  is the total conductivity, () dc  is the direct current conductivity of a sample, A is constant depending on temperature and n is the temperature dependent exponential. According to this, when a mobile charge carrier hops from one site to its neighbouring site, it persists in an oscillation state between two possible minimum energy [50]. Generally, at low frequency, the ions jump from their original site to the neighbouring vacant sites, whereas at higher frequencies, some ions move back to their original site through hopping process, which may results increase in conductivity at low frequencies [51]. The exponent n is related to the hopping rate and relaxation time. The hopping process becomes slower as compared with site relaxation, if n ˂ 1, whereas if n > 1, back-hopping conduction process is faster as compared with site relaxation [52]. The plot clearly shows that σac slowly increases as the temperature increase upto certain temperature and then suddenly falls at high temperature. However, it sharply increases as the temperature rises at further higher temperature region. The behavior of σac trend follows Arrhenius conductivity relation:

 
, where symbols have their usual meanings. In addition, the continuous increases of σac with an increasing temperature confirmed the NTCR behavior of a sample. It has been observed that all the different frequency spectrum were merged at high temperatures which attributed to the recombination of released space charge at higher temperature [53]. Since, the hopping of electron and polaron plays an important role in a conduction mechanism, the value of activation energy, Ea is high at lower frequency. It is also noticed from the plot that the activation energy rapidly increases with an increasing temperature while it decreases with an increasing frequency. As shown in Fig Table 4. Therefore, the density and distributing nature of oxygen vacancy effects the width and height of Schottky barrier and then influenced the resistance of a material which subsequently affected the polarization switching behavior of the sample [54]. Finally, instead of being observed an unsaturated loop, a significant enhancement in the value of remnant polarization has been observed in the present studied sample as compared with pure BFO [55].

Conclusion
The electro-ceramic material, Bi(Fe0.90Dy0.10)O3 was fruitfully synthesized via a conventional SSR route. The crystal structure was studied using powder XRD method and found that the structure changes from rhombohedral to orthorhombic symmetry and the average crystallite                      Figure 1 The XRD pattern of sintered Bi(Fe0.85Dy0.15)O3 material.