Synthesis and characterization of the novel K^sub 2^NiF^sub 4^-type oxide Pr^sub 2^Ni^sub 0.9^Co^sub 0.1^O^sub 4 + d

Pr2Ni0.9Co0.1O4 + 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. Differential 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 tetragonal high-temperature phase from 400 °C to 1000 °C. The electronic conductivity of PNCO was in the range of 65 ≤ σe/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 Pr2NiO4 + δ (PNO). For example, the surface exchange coefficient of PNCO at 600 °C was around 2 × 10− 5 cm s− 1, while kchem of PNO was approximately one order of magnitude smaller. However, at 800 °C both compounds showed similar oxygen exchange rates due to a lower activation energy of kchem 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.