Role of Ga³⁺ and Cu²⁺ in the High Interstitial Oxide-Ion Diffusivity of Pr₂NiO₄‑Based Oxides: Design Concept of Interstitial Ion Conductors through the Higher-Valence d¹⁰ Dopant and Jahn–Teller Effect

We have investigated the crystal structure, nuclear- and electron-density distributions, electronic structure, and oxygen permeation rate of three K2NiF4-type oxides of Pr2(Ni0.75Cu0.25)0.95Ga0.05O4+δ, Pr2Ni0.75Cu0.25O4+δ, and Sr2Ti0.9Co0.1O4–ε, in order to study the role of d10 Ga3+, Jahn–Teller Cu2+, and interstitial oxygen O3 in the high oxygen diffusivity of Pr2(Ni0.75Cu0.25)0.95Ga0.05O4+δ. The composition Pr2(Ni0.75Cu0.25)0.95Ga0.05O4+δ has a larger amount of interstitial oxygen O3 atoms (δ = 0.31 at room temperature (RT)) compared with Pr2Ni0.75Cu0.25O4+δ (δ = 0.19 at RT) and the oxygen deficient Sr2Ti0.9Co0.1O4–ε (ε = 0.02 at RT). The interstitial O3 atom is stabilized by (1) the substitution of (Ni,Cu)2+ by higher valence Ga3+, (2) static atomic displacements of the apical O2 oxygen, and (3) local relaxation near d10 Ga3+. Nuclear-density distributions of Pr2(Ni0.75Cu0.25)0.95Ga0.05O4+δ and Pr2Ni0.75Cu0.25O4+δ at high temperatures have visualized the −O2–O3–O2– diffusional pathway of oxide ions, which indicates an interstitialcy diffusion mechanism. Doping of the Jahn–Teller Cu2+ in Pr2NiO4+δ stabilizes the high-temperature disordered tetragonal I4/mmm phase and makes the apical O2 atoms more mobile. The apical O2 is more mobile compared to the equatorial O1, because the longer covalent (Ni,Cu,Ga)–O2 bond is weaker than the shorter (Ni,Cu,Ga)–(equatorial O1) one, as evidenced by the experimental and theoretical electron-density analysis. The interstitial O3 is more mobile due to the lower coordination number (CN = 4) compared with the lattice O1 and O2 atoms (CN = 6). It was found that the minimum nuclear density on the O2–O3 pathway ρN(T) is a useful microscopic parameter for the oxygen diffusivity. The ρN(T) is regarded as the oxygen probability density at the bottleneck for diffusion. The oxygen permeation rate ρP(T) increases with an increase of ρN(T). The activation energy for oxygen diffusion estimated by the plots of log (the normalized oxygen permeation rate ρP(T)/δ) against T–1 (reciprocal of absolute temperature) is relatively independent of temperature as well as the formation energy of oxygen atoms at the bottleneck from the plots of log­(ρN(T)/δ) against T–1. These results indicate that the amount of interstitial oxygen δ is proportional to the carrier concentration for the oxide-ion diffusion. Doping of higher-valence Ga3+ at (Ni,Cu)2+ site in Pr2Ni0.75Cu0.25O4+δ does not change largely the activation energy for the oxygen permeation and formation energy of oxygen atoms at the bottleneck but increases the amount of excess interstitial oxygen (carrier concentration), which yields the high oxygen permeation rate of 262 μ mol min–1 cm–2 in Pr2(Ni0.75Cu0.25)0.95Ga0.05O4.13 at 900 °C. The present work demonstrates the design concept of interstitial ion conductors through the higher-valence d10 dopant and Jahn–Teller effect