Effect of B-site cationic substitution on the structural, spectroscopic, and conductivity behaviour of Ho₂(Hf₁₋ₓZrₓ)₂O₇ (x=0 and 1)

Ideally, a solid electrolyte which is a central component of SOFC should exhibit high anionic or cationic ionic conductivity at the proposed operating temperatures. In most cases, the performance is compromised when operating above 1000 °C due to poor mechanical, thermal and chemical stability of selected functional and non-functional materials. In this context, pyrochlores are one of the potential candidates due to their high conductivity, flexibility to accommodate large cations, and high mechanical, thermal and chemical stability. In this study, we report the synthesis of nano-powders of Ho₂Hf₂O₇ (HH) and Ho₂Zr₂O₇ (HZ) pyrochlore ceramics by eco-friendly alginate mediated ion-exchange process also known as Leeds Alginate Process (LAP) and provide further insight into the structure - conductivity relationship of HH and HZ compounds by EXAFS studies. Both the compositions were sintered at temperatures, ranging from 1100 °C-1500 °C at 2 h dwell time to achieve the desired high-density ceramic with stable pyrochlore structure. X-ray diffraction and Extended X-ray absorption fine structural (EXAFS) analysis showed superlattice reflections and complex coordination geometry of these oxides. The coordination number and disorder factor in the case of HZ were found to be more stable than the HH sample, as evident from the EXAFS. Impedance spectroscopy and dc-conductivity analysis showed a better charge transport behavior in HZ ceramics than in HH making HZ as a preferred solid electrolyte for SOFC. The conductivity of Ho₂Zr₂O₇ is comparable with the best-known fluorite oxide-ion conductor such as Sc₂O₃-stabilized Zro₂ at 500 °C.