Phase Transition in Perovskite Oxide La_0.75Sr_0.25Cr_0.5Mn_0.5O_3-δ Observed by in Situ High-Temperature Neutron Powder Diffraction

(La 0.75 Sr 0.25 )Cr 0.5 Mn 0.5 O 3- δ (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 La 0.75 Sr 0.25 Cr 0.5 Mn 0.5 O 3 (LSCM) exhibits a rhombohedral structure with space group R 3̄ c (167), a = 5.4479(1) Å, β = 60.477(1)°, and V = 115.563 Å 3 at room temperature. Conductivity and thermal expansion data in air exhibit an anomalous change starting at ∼400 °C which we here demonstrate as being correlated to a phase transition. The perovskite phase undergoes a R 3̄ c → Pm 3̄ m, rhombohedral to cubic phase transition over the temperature range from 500 to over 1100 °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 °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 La 0.75 Sr 0.25 Cr 0.5 Mn 0.5 O 3- δ exhibits a primitive cubic structure with space group Pm 3̄ m, which is the same as the major phase of La 0.75 Sr 0.25 Cr 0.5 Mn 0.5 O 3 in air at high temperatures; therefore, the stresses due to phase changes on redox cycling may be minimized.