Exploring the ionic conductivity of A2Ti2O7 (A = Y and Gd) pyrochlore: Experimental and DFT approach

Abstract

This work investigates the structural and electrical properties of titanate pyrochlore materials, GT (Gd₂Ti₂O₇) and YT (Y₂Ti₂O₇), synthesized using the sol-gel method that can be used as solid electrolytes in intermediate temperature solid oxide fuel cells (IT-SOFCs). Both materials were found to have single-phase pyrochlore structures with cubic lattices and Fd-3m space groups after X-ray diffraction analysis. Rietveld refinement revealed that substituting Gd³⁺ with lower ionic radii cation of Y³⁺ reduced lattice constants, lattice volume, and crystallite size, attributed to differences in ionic radius and potential hybridization effects. The oxygen x parameter that decides the disorder in the structure obtained through Rietveld refinement is higher for GT (0.431) than the recommended range of 0.3125–0.375, along with the presence of superstructure peaks confirms the disordered pyrochlore structure in GT sample. Raman spectroscopy has consistent vibrational modes across both samples, while SEM indicated larger particle sizes for YT. X-ray photoelectron spectroscopy (XPS) analysis clarifies higher surface oxygen ratios in YT (67.6 %), which is crucial for oxygen ion transport. The O1s spectra of both samples show O_48f facilitating conductivity and O_8b corresponding to defect sites. Despite YT having higher dislocation density (0.313) and lattice strain (0.0058), GT exhibited lower defect formation energy (−5.82 eV) based on DFT results, favouring oxygen vacancy formation and enhanced ionic conductivity (2.07 × 10⁻³) at 700 °C. The highest ionic conductivity for YT is obtained at 600 °C of 2.33 × 10⁻³ with defect formation energy of −4.96 eV. The findings emphasize the critical role of structural disorder and defect sites in optimizing ionic conductivity.