Quantum simulations reveal that oxygen insertion into Pr2NiO4 causes structural and half-metal/metal phase transitions.

Recent interest in the structural and electronic properties of Pr2NiO4 stems from its suitability in solid oxide fuel cell (SOFC) applications. It is a mixed ionic and electronic conductor (MIEC) and may be used as a cathode. SOFCs are known to be promising devices for use as a renewable energy source and for generating stable supplies of electrical energy. A known drawback, however, is the high operating temperature, around 1073–1273 K for popularly known electrolyte materials. This introduces problems such as slow start up and a limited range of available cost-efficient materials. In J. Phys.: Condens. Matter 24 405504, we analysed two structures of Pr2NiO4, the high-temperature tetragonal (HTT) structure in the I4/mmm space group and the low-temperature orthorhombic (LTO) structure in the Bmab space group. We then determined the differences in their electronic properties that arise from the difference in their structures—the pronounced tilting of the nickelate octahedral substructures found in the LTO structure.

Using first principles calculations based on density functional theory (DFT), we found a difference in the electronic properties between the two structures. The HTT structure has half-metallic properties as seen from the states that pass through the Fermi level for the majority spin states and a semiconducting gap for the minority spin states. The LTO structure has metallic properties as seen from the states that pass through the Fermi level for both majority and minority spin states (figure 1).

The effect of the non-stoichiometry of Pri2NiO4+δ (δ = 0.031) on the structural and electronic properties of the HTT structure was also addressed in J. Phys.: Condens. Matter 24 405504. Here, we found that the presence of O ions repulsed the oxygen atom at the apical site (Oapical) in its vicinity thereby causing tilting of the nickelate octahedra (figure 2). Apart from that, the electronic properties of the repulsed Oapical atoms have similar electronic properties to that of the Oapical atoms of the LTO structure, strengthening the claim that a major factor of transition from HTT to LTO is the presence of O ions within the bulk.