Dielectric properties of alkaline earth (D = Ba,Sr,Ca) and rare earth (T = La, Nd) substituted polycrystalline D1-xTx(Ti0.5Fe0.5) O3 perovskite

Abstract

Various polycrystalline D1-xTx(Ti0.5Fe0.5)O3, D=Ba, Sr, Ca; T=La, Nd, were prepared by the standard solid state reaction technique. Powders of the ingredients were mixed thoroughly in stoichiometric amount and were calcined at 950°C for 12 h. Pellet shaped samples prepared from each composition were sintered at 1200 and 1300°C for 5 h. X-ray diffraction (XRD) and optical microscopy are used to carry out the structural and surface morphology analyses. The XRD data confirm that all compositions are single phase perovskite structures. The samples of Ba1-xTx(Ti0.5Fe0.5)O3 are of tetragonal structure and are transformed to cubic structure with increasing sintering temperature. The samples of Sr1-xTx(Ti0.5Fe0.5)O3 and Ca1-xTx(Ti0.5Fe0.5)O3 are of cubic and orthorhombic structures respectively. The lattice parameters of Ba1-xTx(Ti0.5Fe0.5)O3 and Sr1-xTx(Ti0.5Fe0.5)O3 decrease with increasing La or Nd content. However, the lattice parameters of Ca1-xTx(Ti0.5Fe0.5)O3 increase with increasing La content but decrease with increasing Nd content. The theoretical density and the bulk density increase but the porosity decreases with increasing La or Nd content of all the compositions of Ba1-xTx(Ti0.5Fe0.5)O3, Sr1-xTx(Ti0.5Fe0.5)O3 and Ca1-xTx(Ti0.5Fe0.5)O3. The average grain size increases in Ba1-xTx(Ti0.5Fe0.5)O3 but decreases in Sr1-xTx(Ti0.5Fe0.5)O3 and Ca1-xTx(Ti0.5Fe0.5)O3 with increasing La or Nd content. Also, the average grain size of samples sintered at 1300°C are larger than that of the samples sintered at 1200°C. The dielectric measurements as a function of frequency and compositions were carried out at room temperature. The dielectric constant (ε/) remains fairly constant up to a few kHz and becomes independent of frequency at higher frequencies for all of the compositions of Ba1-xTx(Ti0.5Fe0.5)O3, Sr1-xTx(Ti0.5Fe0.5)O3 and Ca1-xTx(Ti0.5Fe0.5)O3. The values of ε/ are found to increase in Ba1-xTx(Ti0.5Fe0.5)O3 but decrease in Sr1-xTx(Ti0.5Fe0.5)O3 and Ca1-xTx(Ti0.5Fe0.5)O3 samples with increasing La or Nd content. The dielectric loss tangent (tan δ) is low at lower frequencies but it increases with the increase of frequency of all the compositions of Ba1-xTx(Ti0.5Fe0.5)O3 and Ca1-xTx(Ti0.5Fe0.5)O3. But tan δ is high at lower as well as higher frequencies for all the compositions of Sr1-xTx(Ti0.5Fe0.5)O3. This phenomenon can be explained by the Maxwell–Wagner model. The ac conductivity (σac) is derived from the dielectric measurements and it increases with increasing of frequency for all the compositions of Ba1-xTx(Ti0.5Fe0.5)O3, Sr1-xTx(Ti0.5Fe0.5)O3 and Ca1-xTx(Ti0.5Fe0.5)O3. The σac is found to increase in Ba1-xTx(Ti0.5Fe0.5)O3 but decrease in Sr1-xTx(Ti0.5Fe0.5)O3 and Ca1-xTx(Ti0.5Fe0.5)O3 with increasing La or Nd content. Frequency dependence of real (Z/) and imaginary (Z//) parts of the complex impedance for different composition were measured at room temperature. The complex impedance plot exhibits a tendency of formation of a single semicircular arc for all compositions. Different parameters are determined by fitting the experimental data with Cole-Cole empirical formula. A dominance of grain boundary resistance (Rgb) is observed. Rgb decreases in Ba1-xTx(Ti0.5Fe0.5)O3 but increases in Sr1-xTx(Ti0.5Fe0.5)O3 and Ca1-xTx(Ti0.5Fe0.5)O3 with increasing La or Nd content. The parameter α is greater than zero. This confirms the presence of non-Debye type of relaxation in the materials of all the compositions of Ba1-xTx(Ti0.5Fe0.5)O3, Sr1-xTx(Ti0.5Fe0.5)O3 and Ca1-xTx(Ti0.5Fe0.5)O3.