Study of Structural and Dielectrical Properties of Lead Free Polycrystalline Electro Ceramics Ba 5 CaTi 2 Nb 8 O 30 ( BCTN ) for Microwave Tunable Device Applications

Tungsten bronze structure ceramics have found vital potential in many applications such as actuators, transducer, electro-optic, ferroelectric random access memory and microwave devices. These type ceramics are extensively used in many industrial applications due to their spontaneous polarization and wellknown for its high dielectric constant, low dielectric loss, low leakage current density, good thermal stability and high piezoelectric coefficient. In present work, Ba5CaTi2Nb8O30 (BCTN) has been synthesized first time through solid state reaction method. The microstructures, dielectric, ferroelectric, ferromagnetic and Raman spectra have been investigated by means of X-ray diffraction, Scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDAX), LCR meter, P-E loop tracer, VSM and Raman spectrometer respectively. X-ray diffraction study revealed the formation of single phase tetragonal structure with space group P4bm. The crystallite size was observed to be in the range 14.4 nm. Detailed dielectric properties of BCTN compound as function of temperature at different frequencies show that sample exhibits diffuse type transition at curie temperature 316 ̊C. The P-E and M-H studies confirmed the coexistent ferroelectricity and magnetism at room temperature.


Introduction
Tungsten bronze (TB) ferroelectric materials belong to the most important family of dielectric material.Due to their flexible and open nature structure, this compound received extraordinary attention and has been found to be beneficial for various device applications such as transducers, actuators, electro-optic device, piezoelectric sensors, and FERM (ferroelectric random access memories) etc. [1] [2] [3] [4] [5].Tungsten bronze (TB) structure compound consisting of a framework of distorted BO 6 octahedral corner sharing by three different types of interstices (A, B, C) which are available for many cations occupation having general formula (A 1 ) 2 (A 2 ) 4 (C) 4 (B 1 ) 2 (B 2 ) 8 O 3 [6] where A 1 and A 2 sites are commonly filled by divalent or trivalent cations, B 1 and B 2 sites by tetravalent or pentavalent cations and C site being small, regularly remains vacant giving the general formula A 6 B 10 O 30 .So in comparison to provskite structure, there is a scope for substitution of several cations at different interstitial sites (i.e., A 1 , A 2 , B 1 and B 2 ) that may alter the physical properties of the compound for various device applications [7].Ganguly et al. [8] have been reported the structural, dielectric and electrical studies of Ba 5 SmTi 3 Nb 7 O 30 ferroelectric ceramics.Wang Chen et al. [9] have been reported the structural, dielectric and magnetic proper-

Experimental
Polycrystalline sample of composition Ba 5 CaTi 2 Nb 8 O 30 (BCTN) was synthesized by solid state reaction method.Reagents grade BaCO 3 (99.9%),CaCO 3 (99.9%),TiO 2 (99%) and Nb 2 O 5 (99.9%) (All from M/S Aldrich, USA) used as starting precursor.All starting precursor material was taken into their stoichiometric proportions.The material was thoroughly grind in an agate mortar for 7 -8 hours and then passed through sieve of appropriate size (75 micron).The grind powder mixture was calcined at 1100˚C for 20 hrs in an alumina crucible to form desired phase.Then calcined yellowish color obtained solid solution ground and subsequently admixed with 5 wt% polyvinyl alcohol (M/S Aldrich, S. Jindal  of sintered pellets were analyzed using scanning electron microscope (Hitachi S-3700 N).The sintered pellets were polished to 10 mm thickness and applied silver paste on both sides of pellets to form electrodes, the pellets were then cured at 350˚C for 30 minutes.The dielectric measurement was carried out using LCR meter (Agilent 4284A) operating at oscillation amplitude 1 V. P-E hysteresis loops were recorded at room temperature using an automatic hysteresis loop tracer based on Sawyer-Tower circuit.The magnetic properties were determined by using Lake-Shore's fully integrated sample magnetometer Raman measurement was performed using Horiba Jobin-Yvon LabRAM HR model equipped with a laser of 514.5 nm wavelengths in the scan range of 100 -1600 cm −1 .

Structural Study
The observed X-ray diffraction pattern for the synthesized BCTN ceramic is shown in Figure 1.The peak position and relative intensity of all diffraction peak are observed to be matching well with the standard powder diffraction file of JCPDS card No. 00-039-0258 and thereby confirming the tetragonal tungsten bronze structure with space group P4bm.Ganguly, P. et al. [16] have reported the similar diffraction pattern for Ba 5−x Ca x SmTi 3 Nb 7 O 30 (x = 0) synthesized by solid state reaction method.The crystallite size of synthesized BCTN ceramic was calculated by using the Debye Scherer's equation [17] [18]: 0.9 D cos where D gives the crystallite size, λ is incident wavelength, β is FWHM, and θ is the glancing angle.The observed crystallite size of BCTN was found to be 14.43 nm.The lattice parameters were calculated by using the relation reported by Ganguly et al. [8].The values of lattice parameters a, b, & c were observed to be 8.3311 Å, 8.3311 Å and 12.1289 Å respectively.

Surface Morphology
Room temperature SEM micrographs of gold coated BCTN ceramics sintered at 1300˚C shown in Figure 2. It is observed that specimen has fine grains and is well known that with high sintering temperature well developed grain with uniform grain distribution is observed [19].The grain size of BCTN ceramic is 6 µm.The grain size of BCTN ceramic can be explained on the basis of driving force.This driving force is relative to difference between free energy of strained matrix and strain free crystal.Its result decrease grain boundary area and total boundary energy and increase the grain size of sample.With increasing grain size porosity of specimen decreases [20] [21].Thus increase in grain and decrease in porosity may be attributed due to small interfacial energy.This interfacial energy is proportional to grain diameter.

The Stability of the Crystal Structure
The stability of the tetragonal tungsten bronze (TTB) structure compound can be determined by Tolerance factor (t) discussed by Wakiya et al. [22] According to the general formula of TTB, two types of A sites are present, one is A1 site with 12 fold coordination which is identical as perovskite structure and A2 site with 15 fold coordination which occupies pentagonal site.Thus value of tolerance factor (t) can be calculated by following relation [23] ( ) ( ) ( ) 23 12 3 where t A1 and t A2 represent the tolerance factor of A 1 and A 2 sites, R A , R B and R O are ionic radius of A sites, B sites and oxygen respectively.
In TTB structure tolerance factor and stability relationship analyzed by the combined relationship of above two equations which can be expressed as: ( ) The crystal structure is stable when value of t is closer to 1. Since the calculated value of tolerance factor (t) for BCTN ceramic is 0.63640 which is slightly small than 1 hence showing a slight instability of the synthesized TB ceramics.

EDAX Studies
In order to confirm the chemical composition and stoichiometric proportions of the typical samples of Ba 5 CaTi 2 Nb 8 O 30 (BCTN) ceramics, energy dispersive analysis of X-ray (EDAX) study is carried out and the typical EDAX spectra for ceramic is presented in Figure 3.All the ions present in Ba 5 CaTi 2 Nb 8 O 30 ceramics are observed to be in good stoichiometric proportions as desired.Table 1 shows the elemental percentage and calculated atomic formula with nominal composition.

Dielectric Studies
It is well known that the dielectric constant and dielectric loss of the ferroelectric Materials Sciences and Applications    The increase in value of dielectric constant can be understood as: The sintering of compound is carried out in muffle furnace of very high temperature, so there is possibility of re-oxidation around the grain during the cooling process and lattice pick up oxygen vacancies from the ambient.This re-oxidation creates a strong insulating layer around the grain with bulk resistance.Its result can developed very large conductive difference between bulk and grain boundary.And grain boundary is responsible for large capacitance and resistance compared to grain interior subsequent to surface charge accumulation and interfacial polarization therefore leads to increase dielectric constant [25] [26] [27].Secondly, the value of maximum dielectric constant can be attributed due to well defined or well developed grain and shown in Figure 2. Well defined grain makes the domain wall motion easier and due to this dielectric constant value increase.

Diffuse phase transition
The dielectric peak is broadened indicating the existence of diffuse phase transition.The quantitative evaluation of the diffusivity or disorderness in a sample can be calculated by modified Curie-Weiss law [28]: where, ε r is relative permittivity at temperature T and ε max is maximum relative permittivity at temperature T c , C' is Curie constant.The parameter γ is regarded as a measure of the diffuseness or disorderness of the ferroelectric to paraelectric phase transition.γ = 1 corresponds to the normal ferroelectric behavior while γ lies between 1 (normal Curie-Weiss behavior) and 2 (for completely disordered system) confirming the diffuse type phase transition in the sample [29].In present work value of γ observed in between 1 < γ < 2, that is correspond to diffuse phase transition.This observation is in conformity with observed phase transition in Figure 4(a).
The plots of ln(1/ε r − 1/ε max ) as a function of ln(T − T c ) at 1 kHz, 10 kHz and 100 kHz are shown in Figure 4(c).The calculated value of diffusivity was found to be γ = 1.54 in the material which show the existence of diffuse phase transition in the material.This anomaly in tanδ (of diffuse type material) may be accepted due to dielectric relaxation in the material.At above mentioned frequencies, the variation of dielectric loss with temperature show that dielectric loss has smaller value at lower temperature but its value increase sharply at higher temperature [30].This sharp increase in tanδ at higher temperature may be attributed due to ferroelectric domain wall contribution and space charge polarization.It is observed that Ba 5 CaTi 2 Nb 8 O 30 (BCTN) ceramic has large dielectric constant and low dielectric loss which make it suitable material for tunable device applications.

Raman Spectroscopy
Raman spectroscopy is powerful tool to study the local distortion, strain and any Materials Sciences and Applications disorder present in crystalline solids because vibrational spectrum having shorter length scale characteristic.In Figure 5 Raman spectra plotted intensity versus the difference in wave number between incident beam and scattered beam and peak observed in correspondence to phonon frequency [31].In present work Raman scattering used to study the change in degree of ordering and any distortion present in the sample [32] [33].Recently this spectroscopy used for the study of multiferroic materials.Room temperature Raman spectra of BCTN compound studied in frequency range 50 -1600 cm −1 which depicts various mode of vibrations i.e.A 1 (TO 1 ), A 1 (TO 2 ), A 1 (TO 3 ) and A 1 (LO

ties of Ba 3
SrLn 2 Fe 2 Nb 8 O 30 (Ln = La, Nd, Sm) filled tungsten bronze ceramics.From extensive literature study, it has been analyzed that lots of works have been done on rare earth substituted compound of these families such as Ba 5 RTi 3 Nb 7 O 30 , Ba 4 R 2 Ti 4 Nb 6 O 30 (R = Nd, Sm, Eu, Gd, Dy), Ba 5 HoTi 3 V 7 O 30 which shows diffused phase transition above room temperature [10] [11] [12] [13] [14] and Ba 6 MNb 9 O 30 (M = Ga, Sc, In), Sr 4 CaLaTi 3 Nb 7 O 30 showing relax or behaviour with tetragonal tungsten bronze structure [15].It is well known fact that the properties of BCTN are dependent on the particle size, type of substituent and the synthesizing technique.Since there is no report available on the synthesizing of BCTN tungsten bronze ceramic by solid state reaction method, therefore, we aimed to synthesize Ba 5 CaTi 2 Nb 8 O 30 (BCTN) polycrystalline compound by solid state reaction technique and motivated to investigate the effect of synthesizing technique on the phase, microstructure, ferroelectric, ferromagnetic and Raman properties of BCTN ceramic.

Figure 4 (
a) and Figure4(b) show the temperature dependence of dielectric constant ( ε ′ ) at frequencies of 1 kHz, 10 kHz, 100 kHz.In Ba 5 CaTi 2 Nb 8 O 30 compound, shows the sharp transition in dielectric constant at Curie temperature T c , where dielectric constant is maximum.It is observed that compound has dielectric anomaly at 316˚C (the Curie temperature T c ) depicts the occurrence of ferroelectric-paraelectric phase transition[24].It is also analyzed that compound has the same transition temperature (T c ) at above revealed frequencies, indicating that compound does not have relax or behavior.

Figure 4 (
Figure 4(b) shows the temperature variation of dielectric loss (tanδ) from room temperature to 500˚C at the frequencies of 1 kHz, 10 kHz and 100 kHz.

Figure 6
Figure 6 shows the P-E hysteresis loop of sintered sample recorded at room temperature.The observed value of remenant polarization (P r ) is 0.742 μC•cm −2 and coercive field (E c ) is 11.805 kV•cm −1 .The variation of magnetization as a function of applied field is shown in Figure 7.The values of magnetic parameters such as saturation magnetization, remanence magnetization, coercivity H c (Oe), and squareness ratio M r /M s are observed to be 7.65 × 10 −4 emu/g, 7.22 × 10 −5 emu/g, 65.51 Oe and 0.09 respectively.

Ba 5
CaTi 2 Nb 8 O 30 ferroelectric ceramic is successfully synthesized by solid state reaction technique with tetragonal phase structure.The SEM micrographs depicted the presence of well-developed grain with uniform grain distribution and the grain size of BCTN ceramic is 6 µm.BCTN ceramic shows a diffuse phase of ferroelectric-paraelectric phase transition with T c = 316˚C.Dielectric loss is observed to very small loss and it is independent of temperature around 250˚C then decreases with increasing frequency.Ferroelectric and Ferromagnetism measured at room temperature which predicted a narrow hysteresis loop of et al.

Table 1 .
Ba 5 CaTi 2 Nb 8 O 30 ceramics elemental percentage and calculated atomic formula with nominal composition.
[34]at 265.958 cm −1 , 628.33 cm −1 , 1010.94 cm −1 and 1124.84 cm −1 .Peak at low wave number < 200 cm −1 in Raman spectra rise due to external lattice vibration involving motion of cations relative to oxygen octahedron frame work of NbO 6 .The sharp A 1 (TO 2 ) band at 628.33 cm −1 which correspond to vibration of A site against NbO6 octahedral ion at B site.A 1 (TO 3 ) mode is due to vibration of Nb and O ions which are located in plane perpendicular to it.A 1 (LO 3 ) mode is due to phonon propagating along c axis which is characteristic feature of tetragonal tungsten bronze structure in BCTN compound.The modes at 265.958 cm −1 , 628.33 cm −1 and 1010.94cm−1signifies long range ferroelectric ordering[34].