Proton conductivity in the Gd2O3–HfO2 system

Abstract

In the present work, extensive studies of ionic conductors with pyrochlore and fluorite structures in the Gd₂O₃–HfO₂ system are carried out. Gd₂Hf₂O₇ and Gd_2.1Hf_1.9O_6.95, Gd_2.16Hf_1.84O_6.92, and Gd₂HfO₅ solid solutions known as “stuffed” pyrochlores were synthesized by mechanical activation of oxides followed by high temperature annealing at 1500–1600 °C. The main investigation methods were X-ray diffraction with Rietveld refinement, SEM microscopy and impedance spectroscopy in dry and wet air and N₂. Proton conductivity was first found in Gd₂HfO₅ (1 × 10⁻⁶ at 600 °C), and solid solutions based on it, doped with lanthanum and calcium with pure fluorite structure. Proton conduction of the best proton-conducting ceramics Gd_1.9La_0.1HfO₅ (3 × 10⁻⁶ at 600 °C) was confirmed by measurements in O₂/D₂O, Ar/D₂O atmospheres. At all temperatures, the conductivity σD₂O was lower than that of σH₂O. Gd_1.9Ca_0.1HfO_4.95 has a higher oxygen ion component of conductivity in dry air than Gd₂HfO₅, but the proton conductivity is the same. An attempt was made to synthesize the HEO composition (Lu_0.2Y_0.2Ho_0.2Nd_0.2La_0.2)₂HfO₅, which turned out to be two-phase, containing ∼70% fluorite and ∼30% pyrochlore. The two-phase HEO (Lu_0.2Y_0.2Ho_0.2Nd_0.2La_0.2)₂HfO₅ shows a total conductivity of ∼3 × 10⁻⁶ at 600 °C and ∼3 × 10⁻⁵ at 750 °C, respectively.