First-principles investigation on phase stability of BaM2NiO5 precipitated in Ba(Zr,M)O3-δ electrolyte

In protonic ceramic fuel cells using Ba(Zr,M)O_3-δ (M: trivalent dopant elements) as the electrolyte, the precipitation of BaM₂NiO₅ due to Ni diffusion from the co-sintered NiO-based electrode causes degradation of protonic ceramic fuel cells. However, BaM₂NiO₅ itself has been little studied, and even possible stable crystal structures and compositions have not been fully characterized. In this study, we investigated the dynamic and energetic stability of BaM₂NiO₅ for various trivalent M elements by using first-principles calculations. First, dynamically stable crystal structures were determined for all compositions from phonon dispersion analysis. The formation energies showed negative values in the case of M = lanthanide elements, B, Ga, Tl and Y. The contribution of vibrational entropy to the formation energy of BaM₂NiO₅ was insignificant, and the internal energy was dominant. The chemical bonding analysis revealed that in BaM₂NiO₅, the covalent nature of the M-O bond and the ionic nature of the BaO bond are dominant in the stability of the crystal structure. Precipitation of BaM₂NiO₅ in Ba(Zr,M)O_3-δ was suggested to be dominated by a specific threshold value of formation energy. The validity of that assumption was discussed in terms of the relationship between the factors involved in precipitation and the ionic radius of M element. The formation energy of BaM₂NiO₅ in M = lanthanide elements and Y showed a downward convex tendency with M = Pm as the minimum value. The reason for this was discussed in terms of the characteristics of the crystal structure of BaM₂NiO₅, suggesting that the tensile strain in the M-O bonds and the compressive strain in the NiO and BaO bonds relax with the ionic radius of the M element.