Crystal Chemistry of a Nickelate Superconductor: La/Pr Distribution, Oxygen Stoichiometry, and High-Pressure...

We report a comprehensive structural characterization of La2PrNi2O7, a bilayer Ruddlesden-Popper nickelate that has been described to exhibit high-temperature superconductivity under hydrostatic pressure. Synchrotron X-ray diffraction (SXRD) data and high-resolution neutron powder diffraction (NPD) data, collected at ambient pressure, together with NPD data recorded under pressures up to 8.42 GPa in a Paris-Edinburgh cell, have been analyzed by the Rietveld method in the orthorhombic Amam space group (No. 63). This work aims at providing precise bulk structural benchmarks─site-selective cation distribution, oxygen stoichiometry, and the pressure evolution of the Ni-O-Ni bond geometry─that are essential inputs for theoretical models. High-resolution NPD, which offers a strong La/Pr scattering-length contrast, yields refined La/Pr occupancies that suggest a slight preference of Pr for the outer rock-salt site (La2/Pr2), although the deviation from a random distribution does not exceed three standard deviations and is therefore not statistically conclusive. NPD also assesses the full oxygen stoichiometry of the specimen under study. Under applied pressure up to 8.42 GPa (below the superconducting critical pressure Pc ≈ 11 GPa), the orthorhombic structure is retained with a systematic compression of all lattice parameters, a reduction of the unit-cell volume of ∼4.9%, and a progressive straightening of the out-of-plane Ni-O-Ni bond angle. No structural phase transition to the tetragonal I4/mmm symmetry is observed in the pressure range studied, consistent with the superconducting onset occurring above Pc ≈ 11 GPa. Cooling to 5 K at 8.42 GPa produces no measurable additional structural change, indicating that precursor distortions to the superconducting state are subtle.