Dielectric and microstructural properties of BaTiO3 and Ba0.9925Er0.0075TiO3 ceramics

BaTiO3 and Ba0.9925Er0.0075TiO3 ceramics were investigated regarding their dielectric and microstructure properties via conventional solid state reaction method. The phase pure samples were obtained when heated at 1400 ̊C for overnight. The effect of Er3+ doped into BaTiO3 on dielectric properties and microstructural properties was investigated for composition of BaTiO3 and Ba0.9925Er0.0075TiO3. The analysis was made by X-ray Diffraction (XRD), Scanning Electron Microscopy (SEM) and Impedance Analyzer. The XRD patterns of BaTiO3 and Ba0.9925Er0.0075TiO3 are phase pure and identical with tetragonal perovskite structure with space group of P4mm. The lattice parameters and unit cell volume of BaTiO3 increased by doping with Erbium as the crystallite size decreased. Measurements of dielectric properties were carried out as a function of temperature up to 200 ̊C at different frequencies. Ba0.9925Er0.0075TiO3 exhibit the high value of dielectric constant (ε=6179) at Curie temperature (TC) of 120 ̊C. SEM analysis of BaTiO3 and Ba0.9925Er0.0075TiO3 ceramics showed that the grain sizes of BaTiO3 and Ba0.9925Er0.0075TiO3 were ranged from 3.3m-7.8m and 2.2m-4.7m respectively.


Introduction
The Barium titanate (BaTiO 3 ) -based ferroelectric ceramics have gained importance in electronic industry as they have high dielectric constant and offer wide range of applications such as capacitors, sensors, actuators, power transmission devices, memory devices and high energy storage devices [1][2]. BaTiO 3 has a perovskite structure and due to this perovskite structure, BaTiO 3 has intrinsic capability to host different dopant ions with different sizes. The influence of dopant ion depends on the substitutional site (Ba 2+ or Ti 4+ ) as well as on the defects itself. The ionic radius of the dopant ion is essential for the determination of the substitutional site of host material. The ions with small ionic radii such as La 3+ and Ce 3+ will substitute into A-site whereas the ions with bigger ionic radii such as Lu 3+ and Yb 3+ possibly substitute at B site of BaTiO 3 structure. The ions that have amphoteric behaviour such as Y 3+ , Dy 3+ , Ho 3+ and Er 3+ can occupy both cation lattice site in the BaTiO 3 structure [3][4]. The influence of the dopant incorporation into BaTiO 3 as well as the influence of some transition metal and large rare earth ions in the structural and electrical properties of BaTiO 3 ceramics has been extensively investigated. Among them, Erbium (Er 3+ ) has been widely used due to its great technological interest. It has been previously reported by Leyet et. al. [5] that with the Erbium cation incorporation into BaTiO 3 crystalline lattice according to the formula Ba 1-x Er x TiO 3 , increase in dielectric constant is observed. This suggests that the incorporation of Er 3+ into BaTiO 3 crystalline lattice can be greatly improve the dielectric properties of BaTiO 3 .The aim of this work is to investigate the structural and electrical properties of Er doped BaTiO 3 as a function of the Er 3+ concentration as well as to correlate the structure properties of pure BaTiO 3 and Ba 0.9925 Er 0.0075 TiO 3 .

Experimental
BaTiO 3 and Ba 0.9925 Er 0.0075 TiO 3 compositions were prepared via conventional solid state reaction method. The chemicals used were high purity of BaCO 3 (99.99% Pure, Sigma-Aldrich), USA), TiO 2 (99.99%, Sigma-Aldrich) and Er 2 O 3 (99%, Sigma-Aldrich) powders. BaCO 3, TiO 2 and Er 2 O 3 were weighed using A&D Weighing HR250AZ pharma balance. Powder samples were mixed and ground using pestle and mortar. Samples were ground for half an hour by hand using acetone as the medium for mixing. Samples were then pressed into pellets shape by using uniaxial press with 1 tonne pressured. Pellets were then heated in air at high temperature between 1000 o C and 1400 o C at 5 o C/min for overnight. The crystal structures of the samples were confirmed by using X-ray Diffraction (Bruker,D2-Phaser) with Cu Kα radiation (λ = 1.5406). Samples were measured in range of 2θ = 20 o -80 o using step size of 0.02. The sintering temperature for electrical measurement was 1300 o C for 3 hours with heating and cooling rate of about 5 o C/min in air. For electrical measurements, the surfaces of sintered pellets were painted with silver conductive paint. The electrical characterization is measured using Impedance Analyser (Hioki IM3570) from room temperature to 200˚C at 1kHz, 10 kHz and 100 kHz. The microstructure was studied by using Scanning Electron Microscopy (SEM). The SEM micrographs were obtained using a JEOL JSM-6460LA SEM. The grains size distribution was measured using SEM image analysis software, Smile View.

XRD Analysis
Fig . 1 shows the X-ray Diffraction (XRD) patterns of BaTiO 3 and Ba 0.9925 Er 0.0075 TiO 3 samples heated at 1400˚C for overnight. The XRD patterns show similar XRD pattern with an early reported BaTiO 3 tetragonal perovskite structure [6,7]. The XRD patterns are the same and show the single phase tetragonal perovskite structure with space group of P4mm. The lattice parameters and unit cell volume for the sample BaTiO 3 composition sintered at 1400 o C were a = 3.9918 Å and c = 4.0338 Å and unit cell volume, V = 64.28 Å 3 . The lattice parameters and unit cell volume for the sample Ba 0.9925 Er 0.0075 TiO 3 composition sintered at 1400 o C were a = 3.9940 Å and c = 4.0362 Å and unit cell volume, V = 64.38 Å 3 . Table 1 shows the structure parameters of BaTiO 3 and Ba 0.9925 Er 0.0075 TiO 3 obtained. An increase in lattice parameters of BaTiO 3 as doped with Er can be observed. The average crystallite size values of pure BaTiO 3 and Erbium doped BaTiO 3 ceramics, revealing there is a decrease in the size suggesting the effect of dopant on crystallite size of BaTiO 3 . The crystallite sizes were evaluated through the Scherrer's equation. It is commonly determined by measuring the Bragg's peak width at the half maximum intensity and by using the Scherrer's formula (Eq. 1), where d is the crystalline size, λ is the wavelength of the x-ray, ß is the peak width at the half maximum intensity and theta is the Bragg's angle [7]. (1)

Electrical Analysis
The dielectric behavior and dielectric loss as a function of temperature for the sintered pellets of BaTiO 3 and Ba 0.9925 Er 0.0075 TiO 3 at 1 kHz is shown in Fig. 2 [8]. Some of rare earth doping that increase the T C are Yb doped BaTiO 3 and Lu doped BaTiO 3 which is quite similar to the current Er doped BaTiO 3 system. Based on Fig. 2, an increase in dielectric constant from 5194 to 6502 for BaTiO 3 and Ba 0.9925 Er 0.0075 TiO 3 can be observed at 1 kHz. The increase in dielectric constant can be due to the modifications in the grain size, resulting from the incorporation of the erbium in the BaTiO 3 structure [5]. As for the dielectric loss, it can be seen that the dielectric loss of BaTiO 3 and Ba 0.9925 Er 0.0075 TiO 3 were less than 0.1 at temperature below 120˚C. Much higher dielectric loss which is more than 0.1 observed at higher temperature above 120˚C up to 200˚C. Ba 0.9925 Er 0.0075 TiO 3 shows the highest dielectric constant at 1 kHz which increased by almost 1000 as compared to the BaTiO 3 . The T C of Ba 0.9925 Er 0.0075 TiO 3 increased by 10˚C from T C of BaTiO 3 . However, as the frequency increased, the dielectric constant values decreased. The decreasing dielectric constant values across the frequencies may be due to space charge polarization as reported by Ganguly et al. [9]. Fig. 3(a) shows the variation of capacitance value for undoped BaTiO 3 and Er doped BaTiO 3 at x=0.0075. The capacitance values increases from about ~3x10 -9 Fcm -1 to ~4x10 -9 Fcm -1 by doping Er into BaTiO 3 . These results are consistent with the results of dielectric constant. Fig. 3(b) Fig. 4 shows that the surface morphology of sintered BaTiO 3 and Ba 0.9925 Er 0.0075 TiO 3 by using SEM. The relation between dielectric constant and the grain size of sintered BaTiO 3 and Ba 0.9925 Er 0.0075 TiO 3 were observed. These micrographs were captured by 1000x magnification using Secondary Electron (SE) mode.

Microstructural Analysis
From Fig. 4(a) and 4(b), the irregular shape of grains can be seen for both samples. However, there is difference in the grain size as shown in grain size distribution graph in Fig. 5(a) and 5(b). The grain size of BaTiO 3 was in between 3.3m-7.8m and the grain size for Ba 0.9925 Er 0.0075 TiO 3 was ranged from 2.2m-4.7m. The grain size of Ba 0.9925 Er 0.0075 TiO 3 is much smaller as compared to BaTiO 3 . Therefore, the dielectric constant of Ba 0.9925 Er 0.0075 TiO 3 sample is much higher than BaTiO 3 samples.

Conclusions
Doping study using Erbium doped BaTiO 3 was studied with the effect of small amount of Er to the A-site of BaTiO 3 . The dielectric behaviour of BaTiO 3 improved by doping. The highest dielectric constant observed at Er doped BaTiO 3 with dielectric constant maximum at T C about 120˚C which is 6502. As for dielectric loss, the value is lower than 0.1. Besides, the capacitance and conductivity also improved significantly. The capacitance increased from ~3x10 -9 Fcm -1 to ~4 x 10 -9 Fcm -1 . The conductivity was between ~10 -7 Scm -1 and ~10 -6 Scm -1 . The effects of Erbium doping on the microstructural properties were verified as the grain size decreased as Er doped into BaTiO 3 . The grain size of BaTiO 3 was in between 3.3 m -7.8 m which is larger as compared to the grain size for Ba 0.9925 Er 0.0075 TiO 3 ranged from 2.2 m -4.7 m. Finally, it can be concluded that by doping Er into BaTiO 3 , the dielectric properties were improved and can be correlated with microstructure properties.