  The Ruddlesden-Popper (R-P) bilayer nickelate, La3Ni2O7, was recently found
to show signatures of high-temperature superconductivity (HTSC) at pressures
above 14 GPa. Subsequent investigations achieved zero resistance in single- and
poly-crystalline samples under hydrostatic pressure conditions. Yet, obvious
diamagnetic signals, the other hallmark of superconductors, are still lacking
owing to the filamentary nature with low superconducting volume fraction. The
presence of a novel "1313" polymorph and competing R-P phases obscured proper
identification of the phase for HTSC. Thus, achieving bulk HTSC and identifying
the phase at play are the most prominent tasks at present. Here, we address
these issues in the praseodymium (Pr)-doped La2PrNi2O7 polycrystalline samples.
We find that the substitutions of Pr for La effectively inhibits the
intergrowth of different R-P phases, resulting in nearly pure bilayer
structure. For La2PrNi2O7, pressure-induced orthorhombic-to-tetragonal
structural transition takes place at Pc ~ 11 GPa, above which HTSC emerges
gradually upon further compression. The superconducting transition temperatures
at 18-20 GPa reach Tconset = 82.5 K and Tczero = 60 K, which are the highest
values among known nickelate superconductors. More importantly, bulk HTSC was
testified by detecting clear diamagnetic signals below ~75 K corresponding to
an estimated superconducting volume fraction ~ 57(5)% at 20 GPa. Our results
not only resolve the existing controversies but also illuminate directions for
exploring bulk HTSC in the bilayer nickelates.
