Theoretical study of the crystal structure of the bilayer nickel oxychloride Sr3Ni2O5Cl2 and analysis of possible unconventional superconductivity

The discovery of superconductivity under high pressure with Tc exceeding 80 K in a bilayer nickelate La3Ni2O7 has led to a strong desire to realize similar high Tc phenomena at ambient pressure. As one possible path toward realizing superconductivity at ambient pressure, we here propose to consider Sr3Ni2O5Cl2 as a possible candidate. In this study, we theoretically investigate the electronic structure of Sr3Ni2O5Cl2 and its structural stability. Our phonon calculation shows that this compound with the I4/mmm tetragonal structure is dynamically stable even at ambient pressure. The characteristic crystal field in this compound lowers the Ni-d3z2-r2 orbital energy, by which the Ni-d3z2-r2 orbital becomes rather closer to the half filling in Sr3Ni2O5Cl2 than La3Ni2O7. As a result, we find that superconductivity is enhanced even though a relatively strong orbital hybridization between the t2g and eg orbitals is somewhat detrimental for superconductivity. We also check the formation enthalpy, which shows that the high-pressure synthesis can be a good way to actually produce Sr3Ni2O5Cl2. We find that Sr3Ni2O5Cl2 is a promising new candidate of bilayer-nickelate superconductors, which can possess even higher Tc than pressurized La3Ni2O7, at ambient pressure.