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

Christian Berger, Edith Bucher, Andreas Egger, Anna Theresa Strasser, Nina Schrödl, Christian Gspan, Johannes Hofer, Werner Sitte

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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. 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 ≤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 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 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
Original languageEnglish
Pages (from-to)93-101
JournalSolid State Ionics
Publication statusPublished - 2018

ASJC Scopus subject areas

  • Materials Science(all)

Fields of Expertise

  • Advanced Materials Science

Treatment code (Nähere Zuordnung)

  • Basic - Fundamental (Grundlagenforschung)

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