Elsevier

Materials Chemistry and Physics

Volume 155, 1 April 2015, Pages 171-177
Materials Chemistry and Physics

High dielectric constant and low optical band gap studies of La-modified Ba(Fe0.5Nb0.5)O3 ceramics

https://doi.org/10.1016/j.matchemphys.2015.02.021Get rights and content

Highlights

  • BLFN ceramics were synthesized in single perovskite phase.

  • Defects and charge compensation mechanisms accounted the observed high εr.

  • BLFN samples exhibited wider absorption and narrow optical band gaps.

  • Oxygen vacancies accounted the dielectric relaxation and the narrow band gaps.

Abstract

Ba1-xLa2x/3(FeNb)0.5O3/BLFN (x = 0, 0.02, 0.04, 0.06, 0.08) ceramics in single perovskite phase were synthesized by solid state reaction route. Dielectric constant (εr) increased with the increase of La content in the BLFN ceramic samples. Defects and the charge compensation mechanisms were used to account the observed high εr and the relaxor like behavior of the BLFN ceramic samples. All the BLFN ceramic samples exhibited the wider absorption bands (200–900 nm) and the narrow optical band gaps (∼1.3 eV). Dielectric relaxation and the narrow band gaps suggested the formation of oxygen vacancies in the BLFN ceramic samples.

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

References (44)

  • M.M. Vijatovic et al.

    Properties of lanthanum doped BaTiO3 produced from nanopowders

    Ceram. Int.

    (2010)
  • A.R. West et al.

    Novel high capacitance materials:- BaTiO3:La and CaCu3Ti4O12

    J. Eur. Ceram. Soc.

    (2004)
  • N.G. Eror et al.

    Self-compensation in lanthanum-doped strontium titanate

    J. Solid State Chem.

    (1981)
  • M. Ganguly et al.

    Structural, dielectric and electrical properties of BaFe0.5Nb0.5O3 ceramic prepared by solid-state reaction technique

    Mater. Chem. Phys.

    (2011)
  • V. Shrivastava et al.

    Dielectric studies of La and Pb doped SrBi2Nb2O9 ferroelectric ceramic

    Mater. Lett.

    (2006)
  • F.D. Morrison et al.

    Doping mechanisms and electrical properties of La-doped BaTiO3 ceramics

    Int. J. Inorg. Mater.

    (2001)
  • C.-Y. Chung et al.

    Effects of lanthanum doping on the dielectric properties of Ba(Fe0.5Nb0.5)O3 ceramic

    J. Appl. Phys.

    (2004)
  • S.K. Kar et al.

    Effect of microwave sintering on structural, morphological and dielectric properties of Ba(FeNb)0.5O3 ceramics

    J. Electroceram.

    (2013)
  • I.P. Raevski et al.

    High dielectric permittivity in AFe1/2B1/2O3 nonferroelectric perovskite ceramics (A=Ba, Sr, Ca; B=Nb, Ta, Sb)

    J. Appl. Phys.

    (2003)
  • Y.Y. Liu et al.

    Giant dielectric response and relaxor behaviors induced by charge and defect ordering in SrFe1/2Nb1/2O3 ceramics

    Appl. Phys. Lett.

    (2007)
  • F.D. Morrison et al.

    Electrical and structural characteristics of lanthanum-doped barium titanate ceramics

    J. Appl. Phys.

    (1999)
  • E. Wu

    POWD, an interactive program for powder diffraction data interpretation and indexing

    J. Appl. Crystallogr.

    (1989)
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