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

Abstract

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 (<1wt% 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=0.2bar 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/Scm−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 10−3bar 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 2×10−5cms−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 (~80kJmol−1) as compared to PNO (~160kJmol−1). Post-test analyses of the specimens used for conductivity relaxation measurements showed the formation of small Pr6O11 particles on the surface.