The recent discovery of superconductivity in thin films of doped NdNiO2 confirmed that Ni1+/Ni2+ nickelates can exhibit physics similar to the superconducting cuprates. All known Ni1+/Ni2+ nickelates with infinite NiO2 layers belongs to the T'-type Ln(n+1)Ni(n)O(2n+2) (n = 2, 3, infinity) homologous series. Such nickelates are metastable and can be prepared only by the low-temperature reduction of the appropriate parent phases. La3Ni2O6 was prepared for the first time by the reaction of La3Ni2O7 with calcium hydride (CaH2) at 350°C. Single-phase Ln4Ni3O8 (Ln = La, Nd) nickelates were synthesized by hydrogen reduction of the parent Ruddlesden-Popper phases. The crystal structures of the reduced nickelates were determined by Rietveld refinement of powder neutron diffraction data. The structures of these phases can be described as an intergrowth of LnO2 fluorite layers with NiO2 infinite-layer structural blocks. La3Ni2O6 crystallizes in a unique double T'-type structure, which has not been observed earlier. The 3d9/3d8 electronic configuration of Ni1+/Ni2+ is the same as that of Cu2+/Cu3+ in the high-temperature superconductors. Except for LnNiO2, these are the only known Ni1+/Ni2+ oxide phases with the infinite NiO2 layer as a structural element. Such square planar Ni coordination is believed to be crucial to force Ni2+ in the low - spin configuration. Physical properties, as well as density functional theory calculations for the T'-type nickelates, are discussed and compared with those of cuprates. X-ray absorption spectra near the K-edge of Ni suggest that the charge density on the nickel ion is similar for all members of the T'-type homologous series, which correlates with nearly constant Ni-O bond lengths observed in all the reduced nickelates. The synthesis, structures, and magnetic properties of two new oxyfluorides La4Ni3O8F1 and La4Ni3O8F2 are reported. These phases were prepared by the low-temperature fluorination of La4Ni3O8. Rietveld structure refinement and electron diffraction data for oxyfluorides will be presented.

Ni1+/Ni2+ (d9/d8) nickelates with infinite NiO2 square-planar layers analogues to the high-temperature superconducting cuprates

ACS Spring Meeting 2021

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