Structural stability and electronic structure of the single-layer+trilayer La₃Ni₂O_7 polymorph

The discovery of superconductivity in Ruddlesden-Popper (RP) nickelates has drawn great attention. A polymorph of the bilayer RP nickelate La₃Ni₂O₇ that displays an alternating single-layer and trilayer (1313) stacking pattern has recently been discovered. Signatures of superconductivity under pressure have also been found in this novel phase. At ambient pressure,  this 1313 polymorph has been reported to crystallize in three different space group symmetries  Cmmm, Imma, and Fmmm. Unlike the commonly observed tilted NiO₆ octahedra in perovskite nickelates, the Cmmm phase exhibits no tilts, implying that this structural feature alone may be insufficient to give rise to superconductivity in RP nickelates. Here, we employ first-principles calculations and group theory analysis to study the pressure dependence of the structural instabilities in this 1313-RP polymorph. At ambient pressure, we identify multiple unstable phonon branches in the Cmmm structure. Distortions associated with these instabilities can give rise to an Imma structure that seems to be the most stable space group at ambient pressure from enthalpy considerations. The electronic structure of the 1313-RP at ambient pressure in its magnetic ground state is dominated by the trilayer block, as the single-layer is in a Mott-insulating regime.