A new defect chemistry model for Nb-doped SrCoO2.5+δ: The role of oxygen interstitials and delocalized-to-localized electron holes

Abstract

A new defect chemistry model featuring oxygen interstitials () and delocalized-to-localized delectron holes () as point defects is demonstrated under a perfect Brownmillerite SrCoO 2.5 reference framework to elucidate transport properties of Nb-doped SrCoO 2.5+ (SCN) system over a range of partial pressure of oxygen (Po 2) and temperature (T). With this new defect chemistry model, the electronic conductivity behaviors with T and Po 2 can be well interpreted by the change in concentration of d-electron holes migrating with a constant mobility. The important concentration contours of electron holes, excess electrons and oxygen interstitials are mapped out on the T-Po 2 domain, from which hole-concentration and conductivity under isostoichiometry are reconstructed. The partial/integral molar thermodynamic properties and thermodynamic factor of the SCN solid solution are also determined. A transitional delocalizedto-localized hole-transport mechanism with decreasing oxygen stoichiometry is discussed based on p=p(T) obtained under constant oxygen stoichiometry.

Structural and Electronic Features of Nb-Doped SrCoO3: Insight from First-Principles Calculations

“…For easy comparison, the 1 × 1 × 2 supercell structure for the cubic phase SrCoO 3 is shown in Figure b, where Co1 and Co2 are equivalent in the cubic structure (Co1 cubic = Co2 cubic ). For the tetragonal phase, the lattice parameters are a = b = 3.894 Å and c = 7.747 Å, which is in good agreement with the experimental findings ( a = b = 3.874 Å and c = 7.787 Å) . A further examination of the local structure of CoO6 octahedron in the tetragonal structure indicates that the flattened and elongated CoO6 octahedron alternate along the c axis.…”

“…The original structures of tetragonal and cubic phase were taken from the X-ray diffraction results . To model the 10% Nb-doped SCO (SrNb 0.1 Co 0.9 O 3‑δ or SCN10 in short), we first constructed a supercell of 3 × 3 × 1 unite cells with 90 atoms (O, 50; Sr, 18; Co, 18) based on the optimized tetragonal structure since it is more stable than the cubic structure.…”

“…Donor doping of higher oxidation-state cations into SCO, including on the Co site with Nb, Ta, Fe, Sc, and Sb − or on the Sr site with Y and La, − can stabilize the wanted cubic structure. Of these donor dopants, Nb has been shown the most effective structural stabilizer and performance retainer as an IT-SOFC cathode.…”

“…At T < ∼500 °C, a tetragonal phase containing ordered oxygen-vacancy superlattice appears. While it appears that extensive experimental studies on structural, thermal, and electrochemical properties of Nb-doped SrCoO 3−δ have been carried out in the literature, − an in-depth atomic-level understanding on the structural and electronic properties of Nb-doped SCO system is still lacking. However, atomic scale simulation is an important theoretical technique in the research of materials science.…”

Unusually Large Thermopower Change from +330 to −185 μV K–1 of Brownmillerite SrCoO2.5

“…The former is due to oxygen adsorption after vaporization of water molecules from the surface, and latter is most likely desorption of oxygen from the surface of the SrCoO2.5 film. [20][21][22] Next, we measured the optical conductivity of the SrCoO2.5 films by spectroscopic ellipsometry measurements to analyze the electronic structure. As shown in Fig.…”