Tunability of structure from ordered to disordered and its impact on ionic conductivity behavior in the Nd2−yHoyZr2O7 (0.0 ≤ y ≤ 2.0) system

Abstract

The present paper describes a detailed investigation of the nature of the order–disorder transition in the Nd_2−yHo_yZr₂O₇ system, over the entire composition range (0.0 ≤ y ≤ 2.0), using a combination of diffraction and spectroscopic analysis techniques. The complete series of solid solutions has been prepared by the gel combustion technique followed by high temperature sintering. Detailed structural characterization of the prepared samples using X-ray diffraction studies confirmed their single-phase solid solution formation throughout the series, as well as identifying the transformation of the system from an ordered pyrochlore structure (0.0 ≤ y ≤ 0.8) to a disordered fluorite-type structure (0.8 < y ≤ 2.0). Micro-Raman studies further supported the structural information. Micro-structural studies by SEM revealed dense products with uniform grain distribution in these materials. Comprehensive studies were performed to investigate the ionic conductivity behavior of these solid solutions in correlation with the structural transformations using AC impedance spectroscopy. A detailed analysis of ionic conductivity and related parameters, such as activation energies, etc. corresponding to individual conduction mechanisms such as bulk and grain boundary conduction, clearly correlated the effect of structural transformation on these charge transport properties. The composition Nd_1.2Ho_0.8Zr₂O₇ with the pyrochlore structure has been found to show the highest ionic conductivity in the entire series, which has been attributed to a combined effect of increasing carrier concentration and their preferred movement through an ordered array of vacant anion sites.