(Nd/Pr)₂NiO_4+δ: Reaction Intermediates and Redox Behavior Explored by in Situ Neutron Powder Diffraction during...

Oxygen intercalation/deintercalation in Pr₂NiO_4+δ and Nd₂NiO_4+δ was followed by in situ neutron powder diffraction during electrochemical oxidation/reduction, in a dedicated reaction cell at room temperature. For both systems three phases, all showing the same line width, were identified. The starting phases Pr₂NiO_4.23 and Nd₂NiO_4.24, considered with an average orthorhombic Fmmm symmetry, although both show a slight monoclinic distortion, get reduced in a two-phase reaction step to tetragonal intermediate phases with 0.07 ≤ δ ≤ 0.10 and P4₂/ ncm space group, which on further reduction transform, again in a two-phase reaction step, toward the respective stoichiometric (Pr/Nd)₂NiO_4.0 phases, with Bmab space group. Electrochemical oxidation does, however, not proceed fully reversibly for both cases: while the reoxidation of Nd₂NiO_4+δ is limited to the tetragonal intermediate phase with δ = 0.10, the homologous Pr₂NiO_4+δ can be reoxidized up to δ = 0.17, showing orthorhombic symmetry. For the intermediate tetragonal phase, we were able to establish for Pr₂NiO_4.09 a complex anharmonic displacement behavior of the apical oxygen atoms, as analyzed by single-crystal neutron diffraction and maximum entropy analysis, in agreement with a low- T diffusion pathway for oxygen ions, activated by lattice dynamics.