Synthesis and characterization of the novel K₂NiF₄-type oxide Pr₂Ni_0.9Co_0.1O_4+δ

Pr₂Ni_0.9Co_0.1O_4+δ (PNCO) powder was synthesized via a freeze drying process by mixing and shock freezing of aqueous metal acetate solutions, vacuum freeze drying of the resulting precursor and thermal treatment to obtain the complex oxide. X-ray powder diffraction and Rietveld refinement confirmed that the material was mainly single phase (< 1 wt% Pr₆O₁₁ as secondary phase) with an orthorhombic K2NiF4-type unit cell at room temperature. Precision thermogravimetry between 30 °C and 900 °C showed an irreversible mass increase at T ≥ 750 °C and pO₂ = 0.2 bar which indicated the transition to a higher order Ruddlesden-Popper phase Pr₄(Ni,Co)₃O₁₀ − x and PrOy. Di fferential scanning calorimetry in pure Ar and 20% O₂/Ar showed a structural phase transition from the orthorhombic to a tetragonal modification at approximately 440 °C. Thermal expansion measurements between 30 °C and 1000 °C at different oxygen partial pressures (1 × 10⁻³ ≤ pO₂/bar ≤ 1) indicated two different regions, corresponding to the orthorhombic low-temperature phase up to 400 °C and the tetragonalhigh-temperaturephasefrom400 °Cto1000 °C.TheelectronicconductivityofPNCOwasintherange of 65 ≤σ_e/S cm⁻¹ ≤ 90 (600–800 °C). The chemical surface exchange coefficient for oxygen (kchem) was obtained from in-situ dc-conductivity relaxation experiments between 600 °C and 800 °C and 10⁻³ bar oxygen partial pressure. At temperatures close to 600 °C PNCO exhibited significantly faster oxygen exchange kinetics than the Co-free material Pr₂NiO_4+δ (PNO). For example, the surface exchange coefficient of PNCO at 600 °C wasaround2 × 10−5 cm s−1,whilekchem ofPNOwasapproximatelyoneorderofmagnitudesmaller.However, at 800 °C both compounds showed similar oxygen exchange rates due to a lower activation energy of k_chem for PNCO (~80 kJ mol⁻¹) as compared to PNO (~160 kJ mol⁻¹). Post-test analyses of the specimens used for conductivity relaxation measurements showed the formation of small Pr₆O₁₁ particles on the surface