Structural and electrical properties of Barium Titanate (BaTiO3) and Neodymium doped BaTiO3 (Ba0.995Nd0.005TiO3)

Barium titanate (BaTiO3) and Neodymium (Nd) doped BaTiO3 with composition Ba0.995Nd0.005TiO3 were prepared using conventional solid state reaction method to study the dielectric properties of materials. Pure phase samples were found at final heating temperature of 1400 oC for overnight. X-ray diffraction analysis reveals the changes in the lattice parameter and unit cell volume of the pure perovskite tetragonal structure with space group (P4mm). Electrical analysis is carried out to investigate the dielectric properties, conductivity behaviour and dielectric loss of BaTiO3 and Ba0.995Nd0.005TiO3. Ba0.995Nd0.005TiO3 have a broaden dielectric peaks with high permittivity of 8000 and reasonably low loss tan δ which is about 0.004 (1 kHz).


Introduction
Dielectric ceramics are widely used in advance microelectronics technologies such as capacitor and microwave communication [1]. A high dielectric constant ceramic had received a lot of attention in reducing the size of the microelectronics circuits where high permittivity and low dielectric loss are required for many electronics application. BaTiO3 composition modified by doping have been extensively studied for high dielectric materials applications.
In previous work reported by Shaikh & Vest [2], BaTiO3 structure are very sensitive to the doping level and will change the electrical behaviour of pure BaTiO3. In order to improve the properties of BaTiO3, some modifications on BaTiO3 structure was made and rare earth materials were of interest due to a huge improvement in the permittivity as reported by Abdul Hamid et. al. [3] in Lanthanum (La) doped BaTiO3 and Samarium (Sm) doped BaTiO3 by Ganguly et al. [4].
Neodymium (Nd) doped BaTiO3 was previously studied by other researcher, however, in this study, we present a simple study about the physical behaviour and electrical behaviour change by small amount of Nd doping to the A-site perovskite of BaTiO3. The trivalent Nd is very interesting to study because it previously reported that small amount of Nd will exhibit a PTCR effect and change the electrical behaviour from insulating to semiconducting [5]. Instead of that, in term of structure, the radius of Nd 3+ (1.27Å) is very close to the radius of Ba 2+ (1.61Å) and small size of Ti 4+ (0.605.Å) at the center of the octahedral site towards one of the other corner oxygens. In view of the radius, it is possible for Nd 3+ to enter into the A-sites of BaTiO3 perovskite and effect the properties of BaTiO3 ceramics. In this work, the phase analysis, structural analysis and electrical behaviour of BaTiO3 and Ba 0.995 Nd 0.005 TiO3 was presented. The incorporation location of small amount of doping Nd doped to BaTiO3 was identified using Rietveld Refinement analysis.

Experimental Procedure
High purity of raw materials BaCO3 (99%), TiO2 (99%) and Nd2O3 (99%) from Sigma-Aldrich were used as starting materials. Stoichiometric amount of materials were mixed and grounds using pestle and mortar. Samples were pressed uniaxially using handpress into pellets and heated at final temperature of 1400 ᵒC for 12 hours with heating rate 5ᵒC/min. The phase purity of the samples were analyzed using the X-ray Diffraction (XRD: D2-ADVANCE, Bruker, Germany) from 2θ, 20 o -130 o . Structural studied obtained were refined using the General Structure Analysis Software (GSAS). For impedance measurements, pressed pellets were sintered at 1400ᵒC for 6 hours. Silver paste was used as electrode. Samples was painted with silver paste on both pellets' surfaces. Pellets were connected to the Platinum (Pt) leads of a conductivity jig which was mounted inside a horizontal tube furnace. Impedance data were recorded on a stepwise heating cycle using HIOKI IM 3522 Impedance Analyzer instrumentation in frequency range 10 Hz to 100 kHz.  Figure 1 shows the XRD patterns of BaTiO3 and Ba0.995Nd0.005TiO3 respectively. The reflection patterns of Ba0.995Nd0.005TiO3 remain similar as BaTiO3 sample, except with the presence of broad peak at 2: 22ᵒ and 51ᵒ respectively. All samples of the BaTiO3 and Nddoped BaTiO3 were indexed with tetragonal phase with the unit cell a = 3.9937 Å, c= 4.0342 Å and a= 3.9964 Å, c = 4.0288 Å, volume = 64.38 Å and volume = 64.34 Å respectively. The space group belong to P4mm. Figure 2 shows the variation of lattice parameters and unit cell volume of BaTiO3 and Ba0.995Nd0.005TiO3.

Result and Discussion
Introducing the small amount of Nd content into pure BaTiO3 at A-sites perovskite structure, resulting the changes as shown in Figure 2(a) and 2(b). A decrement in the lattice parameter and unit cell volume was observed. The decrement in lattice parameter and unit cell volume means that the lattice is shrinkage and the size of the unit cell become smaller. This effect is might be due to the size incompability of Nd with ionic radii of Nd 3+ and Ba 2+ , thus causes an overall shrinkage of unit cell and volume.
XRD data for Ba1-xNdxTiO3 (0 ≤ x ≤ 0.015) were collected and the structure was refined using the atomic coordinates of tetragonal perovskite (ICSD No: 245945) as astarting parameters. The cation site occupancies were varied with Ba and Nd on A-sites and Ti on B-sites. The optimized parameters for refinement were detailed in [6,7]. The refinement result,  mechanism, suggest by the refinement result are mostly reliable. Fitting parameters (Rp and Rwp) indicate acceptable agreement between refined and observed XRD patterns for BaTiO3 and Ba0.995Nd0.005TiO3 ceramics with both tetragonal structure. Some variations in occupancy values were showed due to the replacement of Ba atoms by Nd atoms which indicates that (Ba or Nd) atoms located in the same A-site occupy the same site in a fractional percentage. Note that there are small variations in atomic positions related to the titanium and oxygen atoms while the barium and neodymium atoms keep their fixed positions within the structure. Figure  3(a-d) shown the difference profile of (a) BaTiO3 and (b) Ba0.995Nd0.005TiO3 after refinement. Results obtained by using the Rietveld refinement method indicate acceptable agreement between observed XRD patterns and fitted theoretical results. This indicate the success of the Rietveld refinement method which displays small differences near zero in the intensity scale.
Permittvity at fixed frequency of 1 kHz, 10 kHz and 100 kHz for BaTiO3 and Ba0.995Nd0.005TiO3 sintered at1400 ᵒC for 6 hours are shown in Figure 4 respectively. Data show sharp maximum permittivity for all samples in accordance with literature. Permittivity at Tc was remarkedly decreased for Nd-doped BaTiO3 as compared to undoped BaTiO3. Maximum permittivity for pure BaTiO3 about 8500, at curie temperature (Tc) of 120 ᵒC, while for Nd-doped BaTiO3, permittivity at Tc are 8000, 5000 and 2000 at different frequency. At room temperature, the permittivity of Nd-doped BaTiO3 show remarkable value as compared to permittivity of BaTiO3. Small amount of Nd shows a very interesting result by shifting the Tc towards lower temperature, and slight decreament in permittivity change at Tc. This might be related to the high dipole polarisation of rare-earth based-Nd, microstructure and grain size of the sample. The permittivity reaches a peak at the Curie point and falls of at higher temperatures in accordance with the Curie-Weiss law [8]. This phenomena were strongly related with the shifting of maximum permittivity at Tc of Nd-Doped BaTiO3 at tetragonal phase to cubic phase above Tc. Generally, in heavily doped BaTiO3 ceramics grain size is usually suppressed and the permittivity of BaTiO3 ceramics attains the maximum when the grain size is 1μm or less [9]. Factors that influence permittivity include type of dopant, doping mechanism, processing conditions and grain size [10]. On the other hand, high permittivity may be associated with sample geometry and, in particular, with thin layer effects associated with grain boundaries, surface layers or sample-electrode contacts [11].
Doping Concentration   Figure 5 shows the temperature dependence in dielectric loss of BaTiO3 and Ba0.995Nd0.005TiO3 at a frequency of 1 kHz. The dielectric loss of the Ba0.995Nd0.005TiO3 is very low at much lower temperature which is below 0.04 loss. The dielectric loss of BaTiO3 and Ba0.995Nd0.005TiO3 at frequency of 1 kHz is very low below Tc and increase a little bit to 0.06 about Tc. However the loss was very minimal as frequency increased in Figure 5(b) and 5(c). Minimum dielectric loss as low as 0.2 was mantained at lower temperature, however, rapidly increased at high temperature. By doping Nd, below Tc, the dielectric loss was bound from 0.04 to 0.004. Figure 6 (a) and (b) shows the frequency dependence conductivity of BaTiO3 and Ba0.995Nd0.005TiO3 at frequency of 1 kHz, 10 kHz and 100 kHz sintered at 1400ᵒC for 6 hours. The conductivity of BaTiO3 changes with the frequencies of 1 kHz to 100 kHz. At 1 kHz, the conductivity is about 5.4x10 -8 Scm -1 . However a small increase of 10 -7 Scm -1 at 10 kHz and increased again at 100 kHz to 10 -6 Scm -1 . The conductivity at Tc for BaTiO3 is higher compared to other temperatures and much higher at higher frequency. The conductivity of Ba0.995Nd0.005TiO tend to increase from 10-8 to 10-6 Scm -1 approaching Tc at 110ᵒC.  The range of conductivity at low frequency of 1 kHz and 100 kHz were about 9.0 x 10 -7 Scm -1 and 1x10 -5 Scm -1 respectively. The conductivity difference considered high, with two orders of magnitude. This is in agreement with the exact conductivity of a semiconductor material, correlated with the observed properties and related phenomenon responsible [10]. Therefore, conductivity was greatly affected by temperature and frequency of range 1 kHz to 100 kHz. Data collection was limited to frequencies < 1 MHz, due to increases in inductive effects that influenced the impedance data [11].

Conclusion
The effect of Nd incorporation to the A-sites of perovskite BaTiO3 was studied. This effect is seen by observing the lattice and volume change in the structure of BaTiO3 with percentage 0.68% of shrinkage. However, the structure was still mantain with tetragonal unit cell and the structural site of Nd was confirmed by rietveld analysis with Nd at A-sites. The permittivity maximum was at Tc with 8500 and reduced to 8000 by doping at low frequency. However at room temperature, the substitution of Nd show a very high permittivity compared to pure BaTiO3 at three different frequencies. The conductivities of BaTiO3 and Ba0.995Nd0.005TiO3 increased by increasing the frequency and temperature.
The authors acknowledge the financial support from the Ministry of Education, Malaysia for funding this project (FRGS Grant No: 9003-00496) and Universiti Malaysia Perlis (UniMAP) for the opportunities of performing this research.