Mechanochemical synthesis, crystal structure and ion conduction in the Gd2Hf2−x Ti x O7 system

This work describes the mechanochemical synthesis, structural characterization and electrical properties of an interesting group of novel ionic conductors, with general formula Gd2(Hf2−x Ti x )2O7. Different compositions in this system (x = 0, 0.4, 0.8, 1.2, 1.6 and 2) were obtained at room temperature, via a mechanochemical reaction between the corresponding elemental oxides, and characterized by using XRD, Raman spectroscopy and SEM. The XRD structural analysis by the Rietveld method revealed that all the Hf-containing compositions show a disordered fluorite-like structure instead of the expected pyrochlore-like atomic ordering, and the cation size mismatch criteria for pyrochlore stability. Increasing Ti content promotes a phase transformation to the pyrochlore structure with post-milling thermal treatments, which takes place in all samples on annealing at 1200 °C, except for Gd2Hf2O7. These results were confirmed by Raman spectroscopy, which also suggests that the x = 0.4 sample has the highest degree of oxygen disorder in the system and that this disorder decreases with increasing Ti4+ content. Finally, all samples show the pyrochlore structure on firing at 1500 °C. Activation energies E dc for oxygen migration were determined by using impedance spectroscopy and found to be within the ~0.9–1.2 eV range, whereas conductivity σ dc values at 700 °C vary from 1.12 × 10−6 to 2.75 × 10−4 S cm−1, with decreasing conductivity as Ti4+ content increases.