Structural, dielectric and electrical studies in tungsten doped SrBi₂Ta₂O₉ ferroelectric ceramics

Abstract

The effect of tungsten doping on the structural, dielectric and impedance properties of SrBi₂Ta₂O₉ (SBT) ferroelectric ceramics is reported. Tungsten doped SrBi₂(W_xTa_1−x)₂O₉ (0.0 ≤ x ≤ 0.20) ceramics were synthesized by the solid state reaction method. X-ray diffractograms of the samples reveal the single phase layered perovskite structure formation with tungsten content x ≤ 0.05. Variation in the lattice parameters has been explained in terms of the limited structural constraint and relaxation imposed by the Bi–O interlayer. Dielectric constant (ɛ) and dielectric loss (tan δ) measurements as a function of temperature reveal a decrease in the Curie temperature (T_c) from 320 °C (for x = 0.0) to 291 °C (for x = 0.025) and an increasing trend over the doping range of 0.05 ≤ x ≤ 0.20. Significant reduction in the dielectric loss with tungsten addition have been observed. The observed changes in ɛ and tan δ with frequency and temperature have been considered in the light of oxygen and cationic vacancies in tungsten doped samples. Dielectric constant of the samples increases with tungsten doping at their respective Curie temperatures. These have been viewed in terms of relative dominance of the ionic and electronic polarizations. Bulk conductivity of the samples as deduced through ac impedance investigations indicate an increased electronic conduction beyond the observed solubility limit of 0.05 of tungsten doping.

Introduction

Ceramic materials having ferroelectric behaviour have found many applications in electronics and optics. A large number of ferroelectric ceramic devices exploit properties that are indirect consequence of ferroelectricity, viz. dielectric, piezoelectric, pyroelectric and electro-optic properties. Ferroelectric thin films have received considerable attention because of their use in non-volatile memory applications [ferroelectric random-access memories (FRAMs)] [1], [2], [3]. Extensive work has been reported on lead zirconium titanate (PZT), but one of the problems with PZT is its fatigue resistance. PZT tends to degrade most of the initial amount of switching charge after 10⁶–10⁸ cycles of full polarization switching [4]. More recently, bismuth oxide layered ferroelectric materials based on SrBi₂Ta₂O₉ (SBT), SrBi₂Nb₂O₉ (SBN) and their solid solutions have created the interest due to their excellent ferroelectric properties (high remnant polarization, P_r, and low coercive field, E_c) characterized by limited polarization fatigue up to 10¹² cycles and a low leakage current [5], [6], [7], [8], [9].

Layered perovskite strontium bismuth tantalate, SrBi₂Ta₂O₉, is a member of the Aurivillius family having the general formula (Bi₂O₂)²⁺(A_n−1B_nO_3n+1)²⁻, where A = Ca²⁺, Ba²⁺, Sr²⁺, Bi²⁺, etc.; B = Fe³⁺, Ti⁴⁺, Nb⁵⁺, Ta⁵⁺, W⁶⁺, Mo⁶⁺, etc., and n indicates the number of corner sharing octahedral forming the perovskite like slabs [10]. Most of the works reported on the improvement of the dielectric and ferroelectric properties are based on A-site substitutions [11], [12], [13]. For example, the replacement of Sr²⁺ ions by a smaller cation Ca²⁺ results in an increase in its dielectric constant and the Curie temperature, T_c [14]. However, there has not been any extensive study on the dielectric properties of the layered perovskite, SBT, through B-site (Ta⁵⁺) substitutions with other cations of higher oxidation state. We report here the effect of tungsten substitution for tantalum on the structural, dielectric and impedance properties of SrBi₂Ta₂O₉ ferroelectric ceramics.