Phase transition in perovskite oxide La_0.75Sr_0.25C_r0.5Mn_0.5O_3-d observed by in situ high temperature neutron powder diffraction

(La0.75Sr0.25)Cr0.5Mn0.5O3-delta (LSCM) has recently been shown to be an efficient redox stable anode for solid oxide fuel cells (SOFCs). The structure of LSCM has been investigated by X-ray diffraction and neutron diffraction to further understand its properties under SOFC operating conditions. Samples were prepared with nominal A-site deficiency; however, neutron diffraction demonstrates that the A-site deficiency is actually minimal or even null, with spinel impurity compensating for low content of A-site species. This was not apparent from XRD. The perovskite oxide La0.75Sr0.25Cr0.5Mn0.5O3 ( LSCM) exhibits a rhombohedral structure with space group R3c (167), a = 5.4479(1)A, beta = 60.477(1)degrees, and V = 115.563 A(3) at room temperature. Conductivity and thermal expansion data in air exhibit an anomalous change starting at similar to 400 degrees C which we here demonstrate as being correlated to a phase transition. The perovskite phase undergoes a R3c -> Pm3m, rhombohedral to cubic phase transition over the temperature range from 500 to over 1100 degrees C as observed using in situ high-temperature neutron powder diffraction. The fraction of the cubic phase increases with increasing temperature and reaches 85% at 1000 degrees C. The phase transition is gradual; therefore, any sudden volume change due to a phase transition would be minimized, allowing a good electrolyte/anode interface on thermal cycling. The reduced form of La0.75Sr0.25Cr0.5Mn0.5O3-delta exhibits a primitive cubic structure with space group Pm3m, which is the same as the major phase of La0.75Sr0.25Cr0.5Mn0.5O3 in air at high temperatures; therefore, the stresses due to phase changes on redox cycling may be minimized.