Ba0.9Co0.7Fe0.2Nb0.1O3-delta outperforms as a cathode in solid-oxide fuel cells (SOFC), at temperatures as low as 700-750 degrees C. The microscopical reason for this performance was investigated by temperature-dependent neutron powder diffraction (NPD) experiments. In the temperature range of 25-800 degrees C, Ba0.9Co0.7Fe0.2Nb0.1O3-delta shows a perfectly cubic structure (a = a(0)), with a significant oxygen deficiency in a single oxygen site, that substantially increases at the working temperatures of a SOFC. The anisotropic thermal motion of oxygen atoms considerably rises with T, reaching B-eq approximate to? 5 angstrom(2) at 800 degrees C, with prolate cigar-shaped, anisotropic vibration ellipsoids that suggest a dynamic breathing of the octahedra as oxygen ions diffuse across the structure by a vacancies mechanism, thus implying a significant ionic mobility that could be described as a molten oxygen sublattice. The test cell with a La0.8Sr0.2Ga0.83Mg0.17O3-delta electrolyte (similar to 300 mu m in thickness)-supported configuration yields a peak power density of 0.20 and 0.40 W cm(-2) at temperatures of 700 and 750 degrees C, respectively, with pure H-2 as fuel and ambient air as oxidant. The electrochemical impedance spectra (EIS) evolution with time of the symmetric cathode fuel cell measured at 750 degrees C shows that the Ba0.9Co0.7Fe0.2Nb0.1O3-delta cathode possesses a superior ORR catalytic activity and long-term stability. The mixed electronicionic conduction properties of Ba0.9Co0.7Fe0.2Nb0.1O3-delta account for its good performance as an oxygen-reduction catalyst.