This contribution presents the synthesis and thermophysical characterization of seven lanthanide hafnates Ln2Hf2O7 (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 σdc and lower Edc than their fluorite analogues (σdc 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 Eu2Hf2O7 (κ~1.3·W·m−1·K−1 at 800°C). Therefore, Ln2Hf2O7 phases might be attractive component materials for electrochemical devices and thermal insulating coatings.

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