A Mixed / Superstructure in NASICON Ionic Conductors: Neutron Diffraction Study of Li2FeTi(PO4)3 and Li2FeZr (PO4)3

The evolution of crystal and magnetic structures of Sr2Fe2O5 with temperature was studied using neutron powder diffraction. The measurements were made in the temperature range from room temperature up to 1223.15 K and the patterns were analyzed using the Rietveld method. The oxide undergoes two phase transitions: (i) from the antiferromagnetic to the paramagnetic state and (ii) from the orthorhombic to the cubic structure. We discuss the changes of structural parameters with temperature and propose a new structural model for the cubic phase. The diffraction patterns were refined using Ibʹmʹ2, Ibm2, and Fm3c symmetry groups in the antiferromagnetic, the paramagnetic and the cubic state, respectively, with Z=4.