Electrical and thermophysical properties of mechanochemically obtained lanthanide hafnates

Abstract

Abstract This contribution presents the synthesis and thermophysical characterization of seven lanthanide hafnates Ln 2 Hf 2 O 7 (Ln=Sm 3+ , Eu 3+ , Gd 3+ , Dy 3+ , Y 3+ , Ho 3+ , Yb 3+ ); the title samples were prepared at room temperature by mechanically milling stoichiometric mixtures of the corresponding elemental oxides. Irrespective of the lanthanide ion involved, milling promotes the formation of highly disordered fluoritelike materials. Postmilling thermal treatments facilitate the formation of the fluorite ordered derivative, the pyrochlore structure, but only for the larger lanthanides (Sm 3+ , Eu 3+ , Gd 3+ ). Impedance spectroscopy measurements revealed that these materials show a moderate‐to‐good oxygen ion conductivity at high temperatures; furthermore, those adopting the pyrochlore structure give higher σ dc and lower E dc than their fluorite analogues (σ dc at 750°C>10 −3 S·cm −1 vs <5·10 −4 S·cm −1 , respectively). The same trend also holds for the thermal resistivity at high temperatures; the highest thermal resistivity and thus, lowest κ was obtained for Eu 2 Hf 2 O 7 (κ~1.3·W·m −1 ·K −1 at 800°C). Therefore, Ln 2 Hf 2 O 7 phases might be attractive component materials for electrochemical devices and thermal insulating coatings.