Which La3Ni2O7 polymorph carries bulk superconductivity under pressure
In plain words
Early pressure experiments found only a small superconducting volume, and La3Ni2O7 crystals contain two stacking patterns of nickel-oxide layers, so which structure superconducts in bulk was unclear.
Precise statement
Determine which structural polymorph of La3Ni2O7 at high pressure (bilayer 2222 or alternating monolayer-trilayer 1313) carries the $\sim 80\,\mathrm{K}$ superconductivity, and whether the superconducting volume fraction in phase-pure stoichiometric crystals is bulk. An answer combines diffraction under pressure with ac susceptibility volume fractions on the same phase-pure samples.
What would settle it
Shielding-fraction measurements near 100 percent on phase-pure samples of each polymorph with simultaneous structural determination.
Status in the literature
Unverified note
Bulk superconductivity was reported in La2PrNi2O7 under pressure (2024) and in pressurized bilayer nickelate crystals up to about $96\,\mathrm{K}$ (2025), and superconductivity was also reported in the 1313 polymorph (2025).