Pyrochlore Rare-Earth Hafnate RE2Hf2O7 (RE = La and Pr) Nanoparticles Stabilized by Molten-Salt Synthesis at Low Temperature.

Complex oxides of the RE2Hf2O7 series are functional materials that exist in the fluorite or pyrochlore phase depending on synthesis method and calcination temperature. In this study, we investigate the process of synthesis, crystal structure stabilization, and phase transition in a series of RE hafnate compounds, synthesized by the coprecipitation process of a single-source complex hydroxide precursor followed with direct calcination or molten-salt synthesis (MSS) method. Phase pure RE2Hf2O7 (RE = Y, La, Pr, Gd, Er, and Lu) ultrafine nanostructured powders were obtained after calcinating the precursor in a molten salt at 650 °C for 6 h. Moreover, we demonstrate that the MSS method can successfully stabilize ideal pyrochlore structures for La2Hf2O7 and Pr2Hf2O7 in the nanodomain, which is not possible to achieve by direct calcination of the coprecipitated precursor at 650 °C. We propose mechanisms to elucidate the differences in these two synthesis methods and highlight the superiority of the MSS method for the production of RE hafnate nanoparticles.