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Title: Which La3Ni2O7 polymorph carries bulk superconductivity under pressure
Plain: 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: 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.
Settled by: Shielding-fraction measurements near 100 percent on phase-pure samples of each polymorph with simultaneous structural determination.
Kind: well-posed
Answer type: identification
References: (empty)
Literature status: partially-resolved
Status 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).
Posed since: 2023