Acceptor Doping and Oxygen Vacancy Migration in Layered Perovskite NdBaInO₄‑Based Mixed Conductors

The Ca²⁺ and Ba²⁺ solubility on Nd³⁺ sites in new layered perovskite NdBaInO₄ mixed oxide ionic and hole conductor and their effect on the oxide ion conductivity of NdBaInO₄ were investigated. Among the alkaline earth metal cations Ca²⁺, Sr²⁺, and Ba²⁺, Ca²⁺ was shown to be the optimum acceptor–dopant for Nd³⁺ in NdBaInO₄ showing the largest substitution for Nd³⁺ up to 20% and leading to oxide ion conductivities ∼3 × 10^–4–1.3 × 10^–3 s/cm within 600–800 °C on Nd_0.8Ca_0.2BaInO_3.9 composition, exceeding the most-conducting Nd_0.9Sr_0.1BaInO_3.95 in the Sr-doped NdBaInO₄. Energetics of defect formation and oxygen vacancy migration in NdBaInO₄ were computed through the atomistic static-lattice simulation. The solution energies of Ca²⁺/Sr²⁺/Ba²⁺ on the Nd³⁺ site in NdBaInO₄ for creating the oxygen vacancies confirm the predominance of Ca²⁺ on the substitution for Nd³⁺ and enhancement of the oxygen vacancy conductivity over the larger Sr²⁺ and Ba²⁺. The electronic defect formation energies indicate that the p-type conduction in a high partial oxygen pressure range of the NdBaInO₄-based materials is from the oxidation reaction forming the holes centered on O atoms. Both the static lattice and molecular dynamic simulations indicate two-dimensional oxygen vacancy migration within the perovskite slab boundaries for the acceptor-doped NdBaInO₄. Molecular dynamic simulations on the Ca-doped NdBaInO₄ specify two major vacancy migration events, respectively, via one intraslab path along the <i>b</i> axis and one interslab path along the <i>c</i> axis. These paths are composed by two terminal oxygen sites within the perovskite slab boundaries

La_1+<i>x</i>Ba_1–<i>x</i>Ga₃O_{7+0.5<i>x</i>} Oxide Ion Conductor: Cationic Size Effect on the Interstitial Oxide Ion Conductivity in Gallate Melilites

Substitution of La³⁺ for Ba²⁺ in LaBaGa₃O₇ melilite yields a new interstitial-oxide-ion conducting La_1+<i>x</i>Ba_1–<i>x</i>­Ga₃O_{7+0.5<i>x</i>} solid solution, which only extends up to <i>x</i> = 0.35, giving a maximum interstitial oxygen content allowed in La_1+<i>x</i>Ba_1–<i>x</i>­Ga₃O_{7+0.5<i>x</i>} as about half of those allowed in La_1+<i>x</i>(Sr/Ca)_1–<i>x</i>­Ga₃O_{7+0.5<i>x</i>}. La_1.35Ba_0.65­Ga₃O_7.175 ceramic displays bulk conductivity ∼1.9 × 10^–3 S/cm at 600 °C, which is lower than those of La_1.35(Sr/Ca)_0.65­Ga₃O_7.175, showing the reduced mobility for the oxygen interstitials in La_1+<i>x</i>Ba_1–<i>x</i>­Ga₃O_{7+0.5<i>x</i>} than in La_1+<i>x</i>(Sr/Ca)_1–<i>x</i>­Ga₃O_{7+0.5<i>x</i>}. Rietveld analysis of neutron powder diffraction data reveals that the oxygen interstitials in La_1.35Ba_0.65Ga₃O_7.175 are located within the pentagonal tunnels at the Ga level between two La/Ba cations along the <i>c</i>-axis and stabilized via incorporating into the bonding environment of a three-linked GaO₄ among the five GaO₄ tetrahedra forming the pentagonal tunnels, similar to the Sr and Ca counterparts. Both static lattice atomistic simulation and density functional theory calculation show that LaBaGa₃O₇ has the largest formation energy for oxygen interstitial defects among La_1+<i>x</i>M_1–<i>x</i>­Ga₃O_{7+0.5<i>x</i>} (M = Ba, Sr, Ca), consistent with the large Ba²⁺ cations favoring interstitial oxygen defects in melilite less than the small cations Sr²⁺ and Ca²⁺. The cationic-size control of the ability to accommodate the oxygen interstitials and maintain high mobility for the oxygen interstitials in La_1+<i>x</i>M_1–<i>x</i>Ga₃O_7+0.5x (M = Ba, Sr, Ca) gallate melilites is understood in terms of local structural relaxation to accommodate and transport the oxygen interstitials. The accommodation and migration of the interstitials in the melilite structure require the tunnel-cations being able to adapt to the synergic size expansion for the interstitial-containing tunnel and contraction for the tunnels neighboring the interstitial-containing tunnel and continuous tunnel-size expansion and contraction. However, the large oxygen bonding separation requirement of the large Ba²⁺ along the tunnel not only suppresses the ability to accommodate the interstitials in the tunnels neighboring the Ba²⁺-containing tunnel but also reduces the mobility of the oxygen interstitials among the pentagonal tunnels

Localization of Oxygen Interstitials in CeSrGa₃O_7+δ Melilite

The solubility of Ce in the La_1–<i>x</i>Ce_<i>x</i>SrGa₃O_7+δ and La_{1.54–<i>x</i>}Ce_<i>x</i>Sr_0.46Ga₃O_7.27+δ melilites was investigated, along with the thermal redox stability in air of these melilites and the conductivity variation associated with oxidization of Ce³⁺ into Ce⁴⁺. Under CO reducing atmosphere, the La in LaSrGa₃O₇ may be completely substituted by Ce to form the La_1–<i>x</i>Ce_<i>x</i>SrGa₃O_7+δ solid solution, which is stable in air to ∼600 °C when <i>x</i> ≥ 0.6. On the other side, the La_{1.54–<i>x</i>}Ce_<i>x</i>Sr_0.46Ga₃O_7.27+δ compositions displayed much lower Ce solubility (<i>x</i> ≤ 0.1), irrespective of the synthesis atmosphere. In the as-made La_1–<i>x</i>Ce_<i>x</i>SrGa₃O_7+δ, the conductivity increased with the cerium content, due to the enhanced electronic conduction arising from the 4f electrons in Ce³⁺ cations. At 600 °C, CeSrGa₃O_7+δ showed a conductivity of ∼10^–4 S/cm in air, nearly 4 orders of magnitude higher than that of LaSrGa₃O₇. The oxidation of Ce³⁺ into Ce⁴⁺ in CeSrGa₃O_7+δ slightly reduced the conductivity, and the oxygen excess did not result in apparent increase of oxide ion conduction in CeSrGa₃O_7+δ. The Ce doping in air also reduced the interstitial oxide ion conductivity of La_1.54Sr_0.46Ga₃O_7.27. Neutron powder diffraction study on CeSrGa₃O_7.39 composition revealed that the extra oxygen is incorporated in the four-linked GaO₄ polyhedral environment, leading to distorted GaO₅ trigonal bipyramid. The stabilization and low mobility of interstitial oxygen atoms in CeSrGa₃O_7+δ, in contrast with those in La_1+<i>x</i>Sr_1–<i>x</i>Ga₃O_{7+0.5<i>x</i>}, may be correlated with the cationic size contraction from the oxidation of Ce³⁺ to Ce⁴⁺. These results provide a new comprehensive understanding of the accommodation and conduction mechanism of the oxygen interstitials in the melilite structure

Solid-State ²⁹Si NMR and Neutron-Diffraction Studies of Sr_0.7K_0.3SiO_2.85 Oxide Ion Conductors

K/Na-doped SrSiO₃-based oxide ion conductors were recently reported as promising candidates for low-temperature solid-oxide fuel cells. Sr_0.7K_0.3SiO_2.85, close to the solid-solution limit of Sr_1–<i>x</i>K_<i>x</i>SiO_{3–0.5<i>x</i>}, was characterized by solid-state ²⁹Si NMR spectroscopy and neutron powder diffraction (NPD). Differing with the average structure containing the vacancies stabilized within the isolated Si₃O₉ tetrahedral rings derived from the NPD study, the ²⁹Si NMR data provides new insight into the local defect structure in Sr_0.7K_0.3SiO_2.85. The Q¹-linked tetrahedral Si signal in the ²⁹Si NMR data suggests that the Si₃O₉ tetrahedral rings in the K-doped SrSiO₃ materials were broken, forming Si₃O₈ chains. The Si₃O₈ chains can be stabilized by either bonding with the oxygen atoms of the absorbed lattice water molecules, leading to the Q¹-linked tetrahedral Si, or sharing oxygen atoms with neighboring Si₃O₉ units, which is consistent with the Q³-linked tetrahedral Si signal detected in the ²⁹Si NMR spectra