Influence of aliovalent cation substitution on structural and electrical properties of Gd2(Zr1−xMx)2O7−δ (M = Sc, Y) systems

Abstract

Fluorite-type zirconate compositions of the form Gd₂(Zr_1−xM_x)₂O_7−δ (M = Sc and Y; x = 0, 0.1, 0.2, 0.3, 0.4) were prepared by a solid state reaction route and the influence of aliovalent cation substitution on the structural and electrical properties were investigated using powder X-ray diffraction, Raman spectroscopy, transmission and scanning electron microscopy and impedance spectroscopy techniques. Despite being in the same group of elements, the two substituent cations introduced oxygen vacancies with contrasting influence on the parent lattice and ionic conductivity. The larger ionic radius of Y³⁺ forced the lattice to expand which along with the increased anion defects lowered the energy barrier for charge transport enhancing the ionic conductivity. Whereas, in Sc-substituted compositions, oxygen vacancies dominated over ionic size causing the lattice to contract and their cooperative behavior turned out to be disadvantageous for long range conduction processed with increased activation energies resulting in lowering the ionic conductivity. The changes in rigidity of the inter-ionic bonds and the lattice volume introduced by the substituent cations also influenced the thermal expansion behavior of the materials. This work demonstrates that the influence of oxygen vacancies on the ionic conductivity is also dependent on the size of the aliovalent substituent ion.