In situ X-ray and neutron diffraction of the Ruddlesden-Popper compounds (RE2-xSrx)0.98(Fe0.8Co0.2)1-yMgyO4-δ (RE=La, Pr): Structure and CO2 stability

The crystal structure of the Ruddlesden-Popper compounds (La1.0Sr1.0)(0.98)Fe0.8Co0.2O4-delta and (La1.2Sr0.8)(0.98)(Fe0.8Co0.2)(0.8)Mg0.2O4-delta was investigated at 1000 degrees C in N-2 (a(O2)=1 x 10(-4)) by in-situ powder neutron diffraction. In-situ powder X-ray diffraction (PXD) was also employed to investigate the temperature dependence of the lattice parameters of the compounds in air and the oxygen activity dependence of the lattice parameters at 800 degrees C and 1000 degrees C. The thermal and chemical expansion coefficients, determined along the two crystallographic directions of the tetragonal unit cell, are highly anisotropic. The equivalent pseudo-cubic thermal and chemical expansion coefficients are in agreement with values determined by dilatometry. The chemical stability in CO2 containing environments of various Ruddlesden-Popper compounds with chemical formula (RE2-xSrx)(0.98)(Fe0.8Co0.2)(1-y)MgyO4-delta (RE=La, Pr), as well as their stability limit in H-2/H2O =4.5 were also determined by in-situ PXD for x=0.9, 1.0 and y=0, 0.2.