Electrical and thermophysical properties of mechanochemically obtained lanthanide hafnates

This contribution presents the synthesis and thermophysical characterization of seven lanthanide hafnates Ln(2)Hf(2)O(7) (Ln=Sm3+, Eu3+, Gd3+, Dy3+, Y3+, Ho3+, Yb3+); 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 (Sm3+, Eu3+, Gd3+). 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 sigma(dc) and lower E-dc than their fluorite analogues (sigma(dc) at 750 degrees C>10(-3) Scm(-1) vs <510(-4) Scm(-1), respectively). The same trend also holds for the thermal resistivity at high temperatures; the highest thermal resistivity and thus, lowest was obtained for Eu2Hf2O7 (similar to 1.3Wm(-1)K(-1) at 800 degrees C). Therefore, Ln(2)Hf(2)O(7) phases might be attractive component materials for electrochemical devices and thermal insulating coatings.