Exploring local disorder in fast oxygen ion conductors by atomic pair distribution function analysis

Our approach is to explore the real structure by Pair Distribution Function (PDF) analysis on a highly oxygen doped K₂NiF₄-type oxide and to investigate especially any kind of oxygen displacements. La₂NiO₄ is one special candidate as it combines two important points: a sufficiently high maximum extra-oxygen stoichiometry being La₂NiO_4.18 combined with a tetragonal lattice symmetry. We performed X-Rays (XRPD) and Neutron Powder Diffraction (NPD) experiments since both techniques can be considered complementary probes. In fact, X-rays scattering lengths are proportional to the atomic number, while neutrons scattering lengths b just depend on the single isotopes. In particular, considering a natural isotopic compositions, the neutrons b values for La, Ni and O are 8.24, 10.3 and 5.803 Angstroms respectively. For that reason, neutrons are more sensitive to oxygen related parameters than X-rays. X-ray diffraction data were recorded in-situ on warming from room temperature up to 450 Celsius degrees. The results show the in-plane disordering of the apical O site, supporting the split of the apical site of the NiO octahedra