Structural, optical, dielectric and thermoelectric properties on heavy rare earth dysprosium–erbium (Dy–Er)-doped strontium titanate ceramics

Abstract

Heavy rare earth dysprosium- and erbium-doped strontium titanate ceramics were prepared by using conventional solid-state reaction method. The composition, microstructure, spectroscopic, dielectric and thermoelectric properties are characterized to identify the influence of rare earth ions on the strontium titanate. The obtained result reveals that the structural changes take place from cubic to pyrochlore cubic phase with the increase of Dy–Er concentration in strontium titanate. The interaction of rare earth ions with strontium titanate reduces the band gap from 3.24 to 2.8 eV. The dielectric response as a function of frequency and temperature reveals detailed electrical properties of the material. AC conductivity increases with temperature due to enhanced mobility of hopping charge carriers and follows the universal power law with frequency. The activation energy decreases with increasing frequency. Frequency exponent (s) values below unity indicate translational motion with sudden hopping transitions. The real (M′) and imaginary (M″) parts of the electrical modulus confirm thermal activation and suggest a transition from long-range to short-range mobility with rising frequency. With the addition of rare earth ions, the Seebeck coefficient increases, whereas the thermal conductivity decreases, which leads to increase in the figure of merit with a maximum value of 0.20, 0.21, 0.23, 0.22 and 0.087 at 450 °C for ST, 25%, 50%, 75% and 100% doped compositions.