Cations size mismatch versus bonding characteristics: synthesis, structure and oxygen ion conducting properties of pyrochlore-type lanthanide hafnates

This work describes the synthesis, structural characterization and electrical properties of solid solutions with the general formula Gd₂Hf_2−xB_xO₇, where B = Ti⁴⁺, Sn⁴⁺ and Zr⁴⁺. All samples were successfully prepared in ~ 30 h, via a mechanochemical reaction in a planetary ball mill, using the corresponding elemental oxides as starting chemicals. The XRD and Raman spectroscopy analysis of the title samples revealed that on firing at 1500 °C Hf⁴⁺ substitution by Sn⁴⁺ and Ti⁴⁺ produces better ordered pyrochlore structures and decreases the electrical conductivity of Gd₂Hf₂O₇ by more than two orders of magnitude (from 2.7 × 10⁻⁴ at 700 °C to 8.71 × 10⁻⁷ and 1.12 × 10⁻⁶ Sm cm⁻¹, for Gd₂Sn₂O₇ and Gd₂Ti₂O₇, respectively). By contrast, the Gd₂Hf_2−xZr_xO₇ system remains disordered with conductivity increasing by almost an order of magnitude and reaching a value for Gd₂Zr₂O₇ of 1.55 × 10⁻³ Sm cm⁻¹ at 700 °C, whereas the activation energy for oxygen migration decreases in both, the Sn- and Ti-containing systems, and increases slightly in the Zr-containing solid solution. These changes cannot be only explained when taking into account the cations size ratio criteria; the covalency of the <B–O> metal bond plays also a key role in determining the structural characteristics and electrical properties of the title three systems.