Characterization and Rietveld Refinement of A-site deficient Lanthanum doped Barium Titanate
Introduction
Barium Titanate (BT) has been widely investigated and has experienced a renaissance in the past decade due to its long range of applications. It has its application in multilayer ceramic capacitors, electro mechanical system, electro optical system, pyroelectric detectors, piezoelectric actuators, MEMS, FeRAM devices, etc. [1].
Doped BT with rare earth elements has been an upcoming area of research for the past few decades and among them Lanthanum (La) has been a material of great importance. La behaves as a donor as it occupies the Ba site. Though undoped BT is electrically insulating, electrical resistance can be controlled through La doping. Even PTCR effect has been shown by donor doped BT [2], [3]. Rare earth elements especially La doped BT has been mostly popular as a dielectric material in Ni-MLCC as these are effective in Vo trapping and controlling the microstructure enhancing the reliability [4], [5], [6]. Solubility of La into the BT lattice has been quite high as compared to other rare earth elements resulting in a change in symmetry from tetragonal to cubic and a change in grain size which is again of importance [3], [7], [8]. Xinle et al. have also reported the same [9]. Beside this Aliouane et al. had been able to obtain relaxor behavior for more than 10% doping in charge compensated La doped BT. Structural distortion induced by La ions has been considered to be the reason behind [10].
Thus it has been well established that La doped BT is a perfect material to be used as a dielectric in capacitors with high dielectric constant, a stable capacitance value, long service life, low-loss factor, high insulation resistance, reduced Curie temperature, low voltage dependence and low temperature dependence of the dielectric constant over a wide temperature range [2], [3], [4], [5], [6], [7], [8], [9], [10], [11].
BT has a perovskite structure (ABO3) where Ba occupies the A site and Ti the B site. La is incorporated at the A site where it behaves as a donor according to the equation:
Creation of oxygen vacancies also takes place leading to semiconductive behavior, according to the equation:
The effect of Ln-substitution for Ba-ion can be expressed by Kröger–Vink notation as:
Since the substitution at the A site is off valent (Ba2+ being substituted by smaller La3+), so to obtain charge neutrality vacancy is created. Eq. (3) implies that for every two rare earth cation substitution at the A-site, three alkaline cations gets replaced creating one positively double charge vacancy, provided the charges are to be taken to the perfect lattice. This number of vacancy increases with increase in doping concentration [12], [13].
La doped BT ceramics were prepared according to the equation:where x = 0.00, 0.02, 0.04, 0.06, 0.08, and 0.10.
Excluding the composition x = 0.00, all the other compositions are non-stoichiometric, exhibiting A-site deficient perovskite type solid solutions according to the structural formula:
Ba1−xLa2x/3□x/3TiO3, where □ denotes A-site vacancy in the perovskite structure [10].
Though much work has been done in the past years in A-site and B-site doping of BT there is no clear picture about the various doping mechanism and defect chemistry associated with the various types of doping. In particular the doping of La in BT has been widely debated. This attracted us towards this zone of research with a core aim to justify charge compensated A-site deficient Lanthanum doped Barium Titanate in all possible respect.
Section snippets
Experimental procedure
Ba1−xLa2x/3TiO3 (0.00 ⩽ x ⩽ 0.10) ceramics were prepared through solid state reaction technique from reagents BaCO3 (99% Pure, Merck, India Ltd.), TiO2 (99% Pure, Merck, India Ltd.) and La2O3 (99.99% Pure, Sigma–Aldrich, USA). Powders were mixed in an appropriate amount and grinded with distilled water in an agate mortar. The homogeneous mixture was milled in a FRITSCH “Pulverisette 5” planetary mill for 10 h with Zirconium balls (5 mm diameter) and then heated at 1200 °C for 12 h. The process was
Results and discussion
Fig. 1 shows the XRD pattern of Ba1−xLa2x/3TiO3 ceramic powders calcined at 1400 °C for 4 h. All the compositions show the reflections of single phase perovskite structure. The diffraction pattern of all composition within the 2θ range of 44°–46° is shown in a magnified scale for clarity. The presence of (0 0 2), (2 0 0) peaks suggests tetragonal symmetry at room temperature for the compositions x ⩽ 0.04. A Gaussian fit showing the presence of both the peaks is shown in inset. However merging of both
Conclusions
Ba1−xLa2x/3TiO3 (x = 0.02, 0.04, 0.06, 0.08, and 0.10) were prepared by solid state reaction route. Rietveld Refinement confirm a compositionally induced phase transition from tetragonal to cubic symmetry at the composition x = 0.06. The same is supported through FTIR, Raman spectroscopy and temperature dependent dielectric study. FTIR study reveal distortion in the TiO6 octahedra and increase in Ti–O bond strength due to La substitution. Decrease in intensity, followed by a disappearance of the
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