Evaluation of Sr2MMoO6 (M = Mg, Mn) as anode materials in solid-oxide fuel cells: A neutron diffraction study

Sr2MMoO6 (M = Mg, Mn) double perovskites have recently been proposed as anode materials in solid-oxide fuel cells (SOFC). The evolution of their crystal structures has been followed by in situ temperature-dependent neutron powder diffraction from 25 degrees C room temperature (RT) to 930 degrees C by heating in ultrahigh vacuum (P-O2 approximate to 10(-6) Torr) in order to simulate the reducing atmosphere corresponding to the working conditions of an anode in a SOFC. At RT, the samples are described as tetragonal (I4/m space group) and monoclinic (P2(1)/n) for M = Mg, Mn, respectively. Sr2MMoO6 undergoes a structural phase transition from tetragonal to cubic (Fm-3m) below 300 degrees C; Sr2MMoO6 experiences two consecutive phase transitions to tetragonal (I4/m) and finally cubic (Fm-3m) at 600 degrees C and above. In the cubic phases, the absence of octahedral tilting accounts for a good overlap between the oxygen and transition-metal orbitals, resulting in a good electronic conductivity; a high mobility of the oxygen atoms is derived from the elevated displacement parameters, for instance 3.0 angstrom(2) and 4.6 angstrom(2) at 930 degrees C for M = Mg, Mn, respectively. Both factors contribute to the excellent performance described for these mixed ionic and electronic conductor oxides as anodes in single fuel cells. From dilatometric measurements, the thermal expansion coefficients (TEC) in the cubic region are 12.7 x 10(-6) K-1 and 13.0 x 10(-6) K-1 for M = Mg and Mn, respectively. These figures are comparable to those obtained from the mentioned structural analysis; moreover, the TECs for the cubic phases perfectly match those of the usual electrolytes in a SOFC. (C) 2013 American Institute of Physics. [http://dx.doi.org/10.1063/1.4774764]