Which $\mathrm{La_3Ni_2O_7}$ polymorph carries bulk superconductivity under pressure?
This text mentions work from 2024 on. Those parts were partly written without a live literature search and have not been checked by an editor yet.
In plain words
Early pressure experiments found only a small superconducting volume, and $\mathrm{La_3Ni_2O_7}$ crystals contain two stacking patterns of nickel-oxide layers, so which structure superconducts in bulk was unclear.
Precise statement
Determine which structural polymorph of $\mathrm{La_3Ni_2O_7}$ 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: it names recent work and was partly written without a live literature search; it awaits a check by an editor.
Bulk superconductivity was reported in $\mathrm{La_2PrNi_2O_7}$ 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).
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