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

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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 (< 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.

Details

Titel in ÜbersetzungSynthese und Charakterisierung des neuartigen K2NiF4 Oxides Pr2Ni0.9Co0.1O4+δ
OriginalspracheEnglisch
Seiten (von - bis)93-101
ZeitschriftSolid state ionics
StatusVeröffentlicht - 1 Mär 2018