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

Abstract

We have investigated the crystal structure, nuclearand electron-density distributions, electronic structure, and oxygen p er m ea t io n r a t e o f t h r e e K 2 N i F 4 -t y p e o x i d e s o f Pr 2 (Ni 0.75 Cu 0.25 ) 0.95 Ga 0.05 O 4+δ , Pr 2 Ni 0.75 Cu 0.25 O 4+δ , and Sr 2 Ti 0.9 Co 0.1 O 4−ε , in order to study the role of d 10 Ga 3+ , Jahn− Teller Cu 2+ , and interstitial oxygen O3 in the high oxygen diffusivity of Pr 2 (Ni 0.75 Cu 0.25 ) 0.95 Ga 0.05 O 4+δ . The composition Pr 2 (Ni 0.75 Cu 0.25 ) 0.95 Ga 0.05 O 4+δ has a larger amount of interstitial oxygen O3 atoms (δ = 0.31 at room temperature (RT)) compared with Pr 2 Ni 0.75 Cu 0.25 O 4+δ (δ = 0.19 at RT) and the oxygen deficient Sr 2 Ti 0.9 Co 0.1 O 4−ε (ε = 0.02 at RT). The interstitial O3 atom is stabilized by (1) the substitution of (Ni,Cu) 2+ by higher valence Ga 3+ , (2) static atomic displacements of the apical O2 oxygen, and (3) local relaxation near d 10 Ga 3+ . Nuclear-density distributions of Pr 2 (Ni 0.75 Cu 0.25 ) 0.95 Ga 0.05 O 4+δ and Pr 2 Ni 0.75 Cu 0.25 O 4+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 Cu 2+ in Pr 2 NiO 4+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 ρ N (T) is a useful microscopic parameter for the oxygen diffusivity. The ρ N (T) is regarded as the oxygen probability density at the bottleneck for diffusion. The oxygen permeation rate ρ P (T) increases with an increase of ρ N (T). The activation energy for oxygen diffusion estimated by the plots of log (the normalized oxygen permeation rate ρ P (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(ρ N (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 Ga 3+ at (Ni,Cu) 2+ site in Pr 2 Ni 0.75 Cu 0.25 O 4+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 Pr 2 (Ni 0.75 Cu 0.25 ) 0.95 Ga 0.05 O 4.13 at 900 °C. The present work demonstrates the design concept of interstitial ion conductors through the higher-valence d 10 dopant and Jahn−Teller effect.

Oxide ion conductivity in defect perovskite, Pr₂NiO₄ and its application for solid oxide fuel cells

“…Considering the Xray diffraction patterns, the crystal structure of this phase may change from a triclinic to a tetragonal lattice at x = 0.05 and 0.1. A similar phase change has been reported in the case of Nd 2 NiO 4 ; the phase change was also detected in this study from the sharpness of the main diffraction peaks [24,25]. Figure 3 shows the oxygen permeation rate from air to He in Pr 2¹X Ni 0.76¹X Cu 0.24 Ga X O 4+¤ with La 0.1 Sr 0.9 Co 0.8 Fe 0.2 O 3 -(LSCF1982) catalyst as a function of X value.…”

“…This small P O2 dependence of total conductivity in bulk sample might be explained by saturated amount of O i 00 , or the decreased mobility or trapping hole at the extremely large hole concentration in PNCG. It was reported that PNCG shows high oxide ion conductivity by interstitial oxygen in a rock salt block 5), 24) and so improved hole concentration suggests that the oxide ion conductivity is also improved by increasing the interstitial oxygen amount. Since the estimated activation energy for hole formation is almost the same with that of bulk sample, introduction of interstitial oxygen is expected and the oxidation state of Pr or Ni might be varied in the film because of a tensile strain, which is suggested by the expanded lattice parameter.…”

Advances in Cathode Materials for Solid Oxide Fuel Cells: Complex Oxides without Alkaline Earth Metal Elements

“…[ 333 ] Yashima et al explored the role of Cu 2+ and Ga 3+ in a Pr 2 NiO 4 -based oxide in depth. [ 185 ] The Cu 2+ introduced the Jahn-Teller effect into the crystal, which could stabilize the tetragonal structure of the material at high temperature and made the mobility of apical oxygen, which participated in the diffusion of oxygen in the lattice bulk. The higher symmetry of the tetragonal I 4/ mmm PLNCG is a factor of the lower activation energy for oxygen permeation rate, compared to the lower symmetry of the orthorhombic Bmab Pr 2 NiO 4+ δ .…”

Oxygen electrode characteristics of Pr2NiO4+δ-infiltrated porous (La0.9Sr0.1)(Ga0.8Mg0.2)O3–δ

“…PNO based cathodes on CGO [24] have been reported to yield R P as low as 0.08 Ω • cm 2 at 600°C (although more recent reports by the same group give a value of 0.28 Ω•cm 2 ) [14]; these relatively low resistances are somewhat surprising given that the nickelate is unstable and may react with the electrolyte as suggested by the appearance of NiO peaks in X-ray [14]. Substitution of Sr [26], La, Cu or Ga [27,28] in Pr 2 NiO 4 has been shown to improve stability, polarization resistance, and oxygen diffusivity.…”