Synthesis and characterization of the novel K2NiF4-type oxide Pr2Ni0.9Co0.1O4 (+) (delta)

Pr2Ni0.9Co0.1O4 (+ delta) (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 degrees C and 900 degrees C showed an irreversible mass increase at T >= 750 degrees C and pO(2) = 0.2 bar which indicated the transition to a higher order Ruddlesden-Popper phase Pr-4(Ni,Co)(3)O-10 (-) (x) and PrOy. Differential scanning calorimetry in pure Ar and 20% O-2/Ar showed a structural phase transition from the orthorhombic to a tetragonal modification at approximately 440 degrees C. Thermal expansion measurements between 30 degrees C and 1000 degrees C at different oxygen partial pressures (1 x 10(-3) <= pO(2)/bar <= 1) indicated two different regions, corresponding to the orthorhombic low-temperature phase up to 400 degrees C and the tetragonal high-temperature phase from 400 degrees C to 1000 degrees C. The electronic conductivity of PNCO was in the range of 65 <= sigma(e)/S cm(-1) <= 90 (600-800 degrees C). The chemical surface exchange coefficient for oxygen (k(chem)) was obtained from in-situ dc-conductivity relaxation experiments between 600 degrees C and 800 degrees C and 10 3 bar oxygen partial pressure. At temperatures close to 600 degrees C PNCO exhibited significantly faster oxygen exchange kinetics than the Co-free material Pr2NiO4 + delta (PNO). For example, the surface exchange coefficient of PNCO at 600 degrees C was around 2 x 10(-5) cm s(-1), while k(chem) of PNO was approximately one order of magnitude smaller. However, at 800 degrees C both compounds showed similar oxygen exchange rates due to a lower activation energy of k(chem) for PNCO (similar to 80 kJ mol(-1)) as compared to PNO (similar to 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.