(Nd/Pr)NiO: Reaction Intermediates and Redox Behavior Explored by in Situ Neutron Powder Diffraction during Electrochemical Oxygen Intercalation.

Oxygen intercalation/deintercalation in PrNiO and NdNiO 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 PrNiO and NdNiO, 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 P4/ ncm space group, which on further reduction transform, again in a two-phase reaction step, toward the respective stoichiometric (Pr/Nd)NiO phases, with Bmab space group. Electrochemical oxidation does, however, not proceed fully reversibly for both cases: while the reoxidation of NdNiO is limited to the tetragonal intermediate phase with δ = 0.10, the homologous PrNiO can be reoxidized up to δ = 0.17, showing orthorhombic symmetry. For the intermediate tetragonal phase, we were able to establish for PrNiO 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.