High dielectric constant and low optical band gap studies of La-modified Ba(Fe0.5Nb0.5)O3 ceramics
Introduction
Oxide based perovskite materials, possessing high εr (∼104) over a wide temperature range (100–600 K), are widely used in capacitors, memory, sensors etc. applications [1], [2], [3], [4], [5], [6]. Ba(FeNb)0.5O3/BFN system belongs to this category of materials [7], [8]. The origin of high εr and dielectric relaxation process in the BFN system is attributed to both the intrinsic and extrinsic phenomena [9], [10]. Extrinsic phenomena such as grain–grain boundary effect, space charge effect, Maxwell–Wagner type interface polarization etc. are used to account the room temperature high εr values in the BFN ceramics [11], [12]. These extrinsic phenomena in a material can arise due to presence of point defects, vacancies, free electrons, inhomogeneities etc. To achieve the enhanced electrical properties, the ABO3 based perovskite materials can be easily modified with a suitable substitution. The dopants, to be substituted at A or B site in the ABO3 unit cell, should be suitably chosen for a specific enhancement in the electrical properties. Lanthanum (La3+) dopant is mostly used for stabilizing the structure as well as shifting the Curie temperature in ferroelectric materials towards low temperature side [13]. Substitution of aliovalent dopant La3+ ions in place of Ba+2 ions (A-site) in the BFN system can cause the A site vacancies, which can further lead to the improved and stabilized temperature dependent dielectric properties.
In the present study BLFN (x = 0, 0.02, 0.04, 0.06, 0.08)/BFN, BLFN2, BLFN4, BLFN6, BLFN8 ceramics are synthesized by solid state reaction route. The structural, microstructural, dielectric and optical properties of the BLFN ceramic samples are studied and discussed in detail.
Section snippets
Experimental
Ba(1-x)La2x/3(FeNb)0.5O3 (x = 0.0,0.02,0.04,0.06 and 0.08) ceramics were prepared by solid state reaction route. The stoichiometric amounts of BaCO3, La2O3, Fe2O3 and Nb2O5 (of purity ≥ 99%) powders were taken as the starting raw chemicals. These powders are ball milled with zirconia balls using acetone as the grinding media. The optimum calcination temperature for the single perovskite phase formation was optimized at 1250 °C for 4 h. The calcined powders were mixed with 3 wt% polyvinyl
Structural study
XRD patterns, shown in Fig. 1, confirm the formation of single perovskite phase in all the BLFN ceramic samples, calcined at 1250 °C for 4 h. The lattice parameters and the crystal structure of the calcined BLFN ceramic samples are determined from the XRD data by using the least-squares fit method of a standard computer program (POWD) [14]. The best agreement between the observed (obs) and the calculated (cal) interplanar spacing ‘d’ (i.e. minimum ΣΔd = Σ (dobs − dcal)) is considered for
UV–Vis optical properties study
Fig. 5(a) shows the optical diffuse reflectance spectra (DRS) in the UV–Vis region of all the La doped BLFN ceramic samples. A broad diffuse absorption band within the 200–910 nm wavelength range is observed in all the BLFN ceramic samples. The spectra of all the BLFN ceramic samples exhibit three broad bands, centered at ∼264, 480 and 890 nm wavelengths, respectively. The absorption band decreases with the increase of La doping concentration in the BLFN ceramic samples.
The band gap absorption
Conclusions
BLFN ceramic samples were synthesized in single perovskite phase by solid state reaction route. Density increased and the average grain size decreased with the increase in La content in the BLFN ceramic samples. Defect mechanisms such as: space charge polarization, oxygen vacancies etc. were used to explain the origin of high εr in the BLFN ceramic samples. Narrow optical band gaps (∼1.3 eV) with a wide absorption spectrum were observed in all the BLFN ceramic samples. Dielectric relaxation and
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