Structural stability, electronic structure, and magnetic properties of the single-layer trilayer La3Ni2O7 polymorph

TL;DR

Addressing how structural symmetry and pressure affect the La$_3$Ni$_2$O$_7$-1313 polymorph, the study uses DFT, phonon analysis, and group theory to link lattice instabilities to observed space groups and to the high-pressure phase. The authors find the ambient Cmmm structure is dynamically unstable and distorts toward the Imma phase with octahedral tilts, which is also enthalpically favored; under pressure a tetragonal P4/mmm phase becomes stable around 30 GPa, with Ni-O-Ni angles straightening. Electronically, the single-layer block is driven into a Mott-insulating state while the trilayer block dominates near the Fermi level, with magnetic order comprising SL AFM and TL FM across U values; the unfolded bands resemble that of the trilayer La$_4$Ni$_3$O$_{10}$ and align with ARPES data. Overall, the work clarifies ambient structural ambiguity, ties lattice instabilities to symmetry-allowed distortions, and links structural transitions to the electronic/magnetic properties relevant to superconductivity under pressure.

Abstract

A polymorph of the bilayer nickelate La3Ni2O7 that displays an alternating single-layer (SL) and trilayer (TL; 1313) stacking pattern has recently been discovered. Signatures of superconductivity under pressure have been found in this phase. At ambient pressure, La3Ni2O7-1313 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 NiO6 tilts, implying that this structural feature alone may be insufficient to give rise to superconductivity in Ruddlesden-Popper nickelates. Here, we employ first-principles calculations and group theory analysis to study the pressure dependence of the structural instabilities in this SL-TL La3Ni2O7 polymorph. At ambient pressure, we identify multiple unstable phonon branches in the highest symmetry (Cmmm) structure at various high-symmetry points of the Brillouin zone. Distortions associated with these instabilities lead to one of the other experimentally reported space groups (Imma) that does display octahedral tilts. The magnetic tendencies indicate that the electronic structure of La3Ni2O7-1313 at ambient pressure is dominated by the TL block, as the SL is in a Mott-insulating regime. Under pressure, a tetragonal P4/mmm structure becomes stable, in agreement with experiments.

Figures (8)

• Figure 1: Crystal structure of La$_3$Ni$_2$O$_7$-1313 in the three experimentally reported space groups $Cmmm$, $Imma$, and $Fmmm$. Spheres in purple, gray, and red represent the La, Ni, and O atoms, respectively. The single-layer block is denoted by SL while the outer layer and inner layer within the trilayer are denoted as OL and IL, respectively. The in-plane and out-of-plane Ni-O bond lengths are shown explicitly.
• Figure 2: (a) Phonon dispersion of Cmmm-La$_3$Ni$_2$O$_7$-1313 at ambient pressure. The different unstable modes at various high symmetry points of the Cmmm Brillouin zone are highlighted with different colors. The high symmetry points correspond to: $\Gamma =$ (0.0, 0.0, 0.0), S $=$ (0.0, 0.5, 0.0), Y $=$ (0.5, 0.5, 0.0), Z $=$ (0.0, 0.0, 0.5), R $=$ (0.0, 0.5, 0.5), T $=$ (0.5, 0.5, 0.5). (b) Distortions associated with each of the four irreps of the unstable phonon branches at T that lead to an $Imma$ structure: T$_{4}^{-}$, T$_{3}^{-}$, T$_{3}^{+}$, and T$_{4}^{+}$. (c) (left) Evolution of the lattice parameters of the Cmmm, Imma, and Fmmm structures with pressure. (middle) Evolution of the Ni-O-Ni bond angle across the apical oxygen with pressure for the three different space groups. (right) Enthalpy (H = E + PV) evolution of Imma and Fmmm structures with respect to the Cmmm (left y-scale) and P4/mmm (right y-scale) structures. At ambient pressure the Imma structure has the lowest enthalpy. The $P4/mmm$ space group becomes more stable at $\sim$ 13 GPa, consistent with experiments. (d) Phonon dispersion of the $P4/mmm$ structure at 30 GPa that is dynamically stable. All the first-principles results in this figure correspond to fully relaxed structures and non-spin-polarized calculations.
• Figure 3: Magnetic tendencies of Imma-La$_3$Ni$_2$O$_7$-1313 at ambient pressure. (a) Spin configurations analyzed in this work. SL denotes the single-layer, TL the trilayer Ni atoms. (b) Energetics of the different magnetic configurations as a function of $U$. (c) Band structure for the ground state configuration (SL:AFM; TL:FM) at $U$ = 4 eV in both the majority and minority spin channel. (d) Orbitally ($e_g$) projected density of states for SL, TL:IL (inner-layer) and TL:OL (outer-layer) Ni atoms in the magnetic ground state ($d_{z^2}$ pink, $d_{x^2-y^2}$ blue).
• Figure 4: Structures of (a) La$_2$NiO$_4$ and of La$_4$Ni$_3$O$_{10}$ in both $Bmab$ (b) and $P2_1/c$ (c) symmetries. Relevant bond lengths as obtained from experimental structural data are shown.
• Figure 5: Distortions corresponding to the irreps of the unstable phonon branches of $Cmmm$-La$_{3}$Ni$_{2}$O$_{7}$-1313 at the Y, T, R, and S points of the Brillouin zone not shown in the main text.