Bulk high-temperature superconductivity in pressurized tetragonal La₂PrNi₂O₇

The Ruddlesden-Popper (R-P) bilayer nickelate, La₃Ni₂O₇, was recently found to show signatures of high-temperature superconductivity (HTSC) at pressures above 14 GPa (ref. ¹). Subsequent investigations achieved zero resistance in single-crystalline and polycrystalline samples under hydrostatic pressure conditions^2-4. Yet, obvious diamagnetic signals, the other hallmark of superconductors, are still lacking owing to the filamentary nature with low superconducting volume fraction^2,4,5. The presence of a new 1313 polymorph and competing R-P phases obscured proper identification of the phase for HTSC^6-9. Thus, achieving bulk HTSC and identifying the phase at play are the most prominent tasks. Here we address these issues in the praseodymium (Pr)-doped La₂PrNi₂O₇ polycrystalline samples. We find that substitutions of Pr for La effectively inhibit the intergrowth of different R-P phases, resulting in a nearly pure bilayer structure. For La₂PrNi₂O₇, pressure-induced orthorhombic to tetragonal structural transition takes place at P_c ≈ 11 GPa, above which HTSC emerges gradually on further compression. The superconducting transition temperatures at 18-20 GPa reach $$ and $$ , which are the highest values, to our knowledge, among known nickelate superconductors. Importantly, bulk HTSC was testified by detecting clear diamagnetic signals below about 75 K with appreciable superconducting shielding volume fractions at a pressure of above 15 GPa. Our results not only resolve the existing controversies but also provide directions for exploring bulk HTSC in the bilayer nickelates.