Oxygen‐Deficient R 2 MoO 6– δ (R = Tb, Dy, Y, Ho, Er, Tm, Yb) with Fluorite Structure as Potential Anodes in Solid Oxide Fuel Cells

The members of the family of oxygen‐defective fluorites R 2 MoO 6– δ (R = Tb, Dy, Y, Ho, Er, Tm, Yb) have been synthesized under reducing conditions and characterized in order to evaluate their potential use as anodes in intermediate‐temperature solid oxide fuel cells (IT‐SOFCs). A neutron powder diffraction (NPD) investigation demonstrates that they are all cubic, in the Fm $\bar {3}$ m space group, exhibiting a random distribution of R and Mo cations over the metal sites of the fluorite structure with crystallographic formulaMO 2– δ /3 (M = R 2/3 Mo 1/3 ), Z = 4. These materials are highly oxygen‐deficient fluorites with oxygen stoichiometries ranging from 5.2(1) (R = Y, Tm) to 4.8(1) (R = Tb, Dy, Ho, and Er) oxygen atoms per formula unit, accompanied by high values of the displacement parameters at room temperature (ca. 5 Å 2 ), which suggests a high mobility of the oxygen atoms. An in situ NPD study of the thermal evolution of the crystal structure for Ho 2 MoO 6– δ confirms the absence of phase transitions and a high oxygen‐vacancy concentration and mobility [ B (O) = 6.7(1) Å 2 ] in the network under the usual working conditions of an anode in a SOFC ( P O2 ≈ 10 –6 Torr, 500–700 °C). The thermal expansion coefficients of the sintered samples exhibit values around 9 × 10 –6 K –1 from 300 to 900 °C under 5 % H 2 /95 %N 2 , which perfectly match the thermal expansions of the other components of a SOFC. The maximum conductivity values are displayed for the fluorite containing Tb, ranging from 0.03 (200 °C) to 1.1 (900 °C) S cm –1 with an activation energy of 0.31 eV.