La3Ni2O7: Structure and Superconductivity
- La3Ni2O7 is a bilayer Ruddlesden–Popper nickelate that shows high-temperature superconductivity under pressure, with oxygen deficiencies tuning its carrier density.
- Distinct pressure-driven structural transitions—from orthorhombic to tetragonal phases—significantly enhance Ni–O orbital overlap and influence superconducting behavior.
- Superconductivity in La3Ni2O7 is characterized by a predominantly nodal gap revealed through transport and spectroscopic studies, underscoring complex charge-transfer and correlation effects.
Searching arXiv for papers on La3Ni2O7 to ground the article with current literature. Searching arXiv for papers on La3Ni2O7 to ground the article with current literature. LaNiO and its oxygen-deficient form LaNiO are bilayer Ruddlesden–Popper nickelates that have become central to the study of high-temperature superconductivity under pressure. The compound combines NiO bilayers, strong orbital selectivity, charge-transfer physics, and pressure-tunable structural phases, and it has been reported to exhibit superconductivity with transition temperatures approaching or exceeding liquid-nitrogen temperature under compression. At the same time, its literature contains unresolved issues concerning the high-pressure crystal structure, the superconducting volume fraction, the role of oxygen stoichiometry, and the relation between magnetism and pairing (Hou et al., 2023, Cao et al., 16 Sep 2025, Zhou et al., 2023).
1. Composition, stoichiometry, and lattice architecture
LaNiO is a Ruddlesden–Popper-type bilayer perovskite with alternating LaO rock-salt and NiO0 perovskite layers. The “1” notation denotes apical-oxygen deficiencies 2, and the missing oxygen atoms are reported to occur predominantly at the inner apical sites that bridge the two NiO3 sheets of a bilayer. Small 4 changes the Ni valence and carrier density and is believed to tune the superconducting state (Cao et al., 16 Sep 2025, Dong et al., 2023).
At ambient pressure, La5Ni6O7 is widely described as an orthorhombic 8 Ruddlesden–Popper phase with space group 9. In this phase the NiO0 octahedra form bilayers separated by La–O slabs, and the inter-bilayer Ni–O–Ni bond angle is buckled. Powder refinement for polycrystalline La1Ni2O3 with 4 gave 5 Å, 6 Å, and 7 Å, while single-crystal X-ray diffraction at 8 K in an ambient-pressure study gave 9 Å, 0 Å, and 1 Å (Wang et al., 2023, Huo et al., 27 Jan 2025).
The oxygen sublattice is not a passive structural detail. Energy-filtered multislice electron ptychography and O-2-edge EELS showed that planar and outer-apical oxygen sites remain fully occupied within uncertainty, whereas inner-apical sites host most of the vacancies. Three representative 3 regions gave 4, 5, and 6, demonstrating nanoscale inhomogeneity in stoichiometry and electronic structure (Dong et al., 2023).
2. Pressure-driven structural evolution
A central issue in La7Ni8O9 research is the structure of the superconducting state under pressure. Early high-pressure X-ray diffraction identified a first-order structural transition above 0 GPa from orthorhombic 1 to higher-symmetry orthorhombic 2, with a coexistence range of 3–4 GPa. In the 5 phase the Ni–O–Ni bond angle within the bilayers straightens, enhancing Ni 6–O 7 overlap (Hou et al., 2023).
Low-temperature synchrotron XRD subsequently reported a tetragonal 8 phase when the sample was compressed to 9 GPa at 0 K, under conditions where superconductivity takes place. In that study the 1 transition occurred at 2 GPa at 3 K, with a 4 drop in unit-cell volume. The tetragonal phase was described as having nearly regular NiO5 octahedra, in-plane Ni–O–Ni bond angle 6 at 7 GPa, and low-energy states dominated by the Ni 8 orbitals 9 and 0 (Wang et al., 2023).
A later Raman and first-principles study proposed a more explicit sequence: orthorhombic 1 below 2 GPa, mixed 3 for 4 GPa, and tetragonal 5 above 6 GPa. In that account, the emergence of bulk superconductivity coincided precisely with the transition to 7, and a new 8 Raman mode at 9 appeared abruptly as the orthorhombic oxygen modes disappeared (Zhang et al., 19 Nov 2025).
These results frame a major controversy in the field. Orthorhombic 0 and tetragonal 1 have both been advanced as the relevant high-pressure structure. The later Raman work explicitly argued that 2 is the host of superconductivity, whereas earlier transport and diffraction studies emphasized 3. The disagreement is one of the principal structural questions surrounding La4Ni5O6 under pressure (Hou et al., 2023, Zhang et al., 19 Nov 2025).
3. Superconductivity under compression
Transport studies established that superconductivity in La7Ni8O9 is pressure stabilized, but the threshold pressure and the degree of bulk character depend strongly on pressure conditions, sample form, and stoichiometry. One single-crystal study reported that bulk superconductivity emerges only under true hydrostatic pressures above 0 GPa, and focused on 1–2 GPa using a symmetric diamond-anvil cell with condensed helium as the pressure-transmitting medium. Four-probe resistance measurements in the same cell showed a sharp onset of zero resistance at 3–4 K, with midpoint 5 K for Sample #1 at 6 GPa and transition width 7 (Cao et al., 16 Sep 2025).
Other high-pressure transport studies reported lower-pressure onsets. In cubic-anvil-cell measurements with liquid glycerol as pressure medium, sample #2 developed a 8 resistance drop below 9 K at 0 GPa, and sample #3 reached zero resistance below 1 K at 2 GPa and 3 K at 4 GPa. From 5 and 6 criteria, 7 was estimated as 8 T and 9 T, respectively, using the Ginzburg–Landau form
00
The same study associated the onset of superconductivity with the 01 transition and a dome reaching 02–03 K at 04 GPa (Hou et al., 2023).
Polycrystalline La05Ni06O07 also shows pressure-induced superconductivity. In a cubic-anvil cell with glycerol, the density-wave-like anomaly in 08 was progressively suppressed with increasing pressure, a resistivity drop appeared at 09 GPa, zero resistivity was achieved at 10 GPa below 11 K, and 12 rose to 13 K at 14 GPa. The onset temperature 15 reached 16 K at 17 GPa (Wang et al., 2023).
The interpretation of these superconducting transitions is debated. Modulated ac susceptibility on compressed single crystals found no superconducting anomaly below 18 GPa, then a broad diamagnetic peak centered at 19 K at 20 GPa, and a superconducting volume fraction 21. That work concluded that the superconductivity is filamentary-like rather than bulk (Zhou et al., 2023). By contrast, the hydrostatic point-contact spectroscopy study described the superconducting state at 22–23 GPa as pressure-stabilized bulk La24Ni25O26 (Cao et al., 16 Sep 2025). The coexistence of these claims defines a second major controversy in the literature.
4. Superconducting gap symmetry and pairing constraints
The most direct spectroscopic evidence for the pairing state in pressurized La27Ni28O29 came from in situ directional point-contact spectroscopy under truly hydrostatic pressure. A four-electrode geometry in a diamond-anvil cell formed nanoscale ballistic Au/La30Ni31O32 junctions in situ, allowing comparison of c-axis and in-plane spectra at 33–34 GPa (Cao et al., 16 Sep 2025).
Along the c-axis, 35 showed symmetric coherence peaks at 36 mV bracketing a V-shaped dip at zero bias. In the ab plane, antinodal orientation gave partially developed symmetric gap-edge features plus a moderate zero-bias enhancement, whereas nodal orientation produced a pronounced zero-bias conductance peak, ascribed to zero-energy Andreev bound states at a 37-wave node. The spectra were reported to agree qualitatively with 2D-BTK modeling for a 38 gap with high barrier strength 39, and the combination of a V-shaped gap and ZBCP was stated to rule out simple isotropic 40-wave or sign-changing 41 proposals unless accidental nodes are invoked (Cao et al., 16 Sep 2025).
The c-axis coherence peaks at 42 meV imply 43 meV. Using 44 K, the reported gap ratio was
45
which exceeds the BCS weak-coupling 46-wave value 47 and was identified as typical of strongly coupled, nodal superconductors (Cao et al., 16 Sep 2025).
Theoretical work has nevertheless emphasized that pairing channels are close in energy. In a 31-orbital Ni 48–O 49 Hubbard model treated within RPA, a transition between 50-wave and sign-changing 51-wave pairing states was found as a function of pressure and interaction parameters. At realistic parameters, incommensurate spin-fluctuations near 52 and 53 cooperatively stabilized a 54 order parameter, while the close competition between 55 and 56 was proposed as an explanation for why simplified models have produced conflicting gap symmetries (Xu et al., 9 Jan 2025). Taken together, the experimental and theoretical record places the strongest current constraint on a predominantly nodal superconducting state, while preserving a parameter-sensitive competition among candidate pairing channels.
5. Electronic structure, charge dynamics, and magnetism
At ambient pressure, ARPES and DFT+57 established La58Ni59O60 as a multiorbital correlated metal with strong orbital selectivity. High-resolution ARPES found two main Fermi-surface sheets associated primarily with Ni 61, while a nearly flat Ni 62 band lay 63–64 meV below 65. The renormalization is substantially larger for 66 than for 67: one study reported 68 and 69, and another reported 70–71 for 72 and 73–74 for the 75 flat band (Li et al., 2024, Yang et al., 2023).
The same ARPES work also reported pseudogap-like behavior in the 76 band: upon cooling from 77 K to 78 K, the leading-edge midpoint at 79 shifted away from 80 by up to 81 meV, with onset near 82 K and no full gap opening or coherence peak. Under high pressure, DFT+83 predicted a 84 broadening of all bands and motion of the flat 85 band to 86, creating a van Hove singularity near 87 (Li et al., 2024).
Spectroscopy of oxygen states places stoichiometric La88Ni89O90 in the charge-transfer regime. O-91-edge EELS identified a pre-edge at 92 eV associated with O 93 states hybridized with Ni 94, and its suppression with increasing 95 was summarized by 96. Ligand holes were found on inner-apical 97 and planar 98 orbitals, while outer-apical oxygen was described as less relevant to low-energy physics and safely disregarded in theoretical models (Dong et al., 2023). A film study similarly described La99Ni00O01 as a charge-transfer system with both in-plane and out-of-plane Zhang–Rice singlets and a strong Ni–O02–Ni interlayer coupling (Ren et al., 2024).
Optical and transport measurements reinforce the picture of an anisotropic multiband metal. Optical conductivity showed a coherent-to-incoherent crossover of c-axis charge dynamics around 03 K, with 04 Å 05 Å but 06 Å 07, while the ab-plane remained coherent from 08 to 09 K with 10. At 11 K, the Drude-weight ratio 12 and the dc conductivity ratio 13 (Su et al., 2024). Under pressure and high magnetic field, magnetoresistance was found to be quasi-quadratic with exponent 14, to follow extended Kohler scaling, and to indicate a multiband metallic normal state (Chen et al., 2024).
The magnetic ground state remains actively discussed. 15La NMR on La16Ni17O18 found a spin-density-wave transition at 19 K and proposed a possible double spin stripe with moments aligned along the 20-axis (Zhao et al., 2024). Resonant X-ray scattering on films detected a superlattice reflection at 21, with charge-anisotropy order setting in at 22 K and collinear antiferromagnetic order at 23 K (Ren et al., 2024). Neutron scattering on single crystals later resolved well defined spin excitations at 24, a spin gap of 25 meV, and a bilayer Heisenberg spectrum with strong interlayer exchange 26 meV and total fluctuating moment 27 (Chen et al., 5 May 2026). A plausible implication is that oxygen stoichiometry, phase purity, and sample type materially affect the observed magnetic phenomenology.
6. Reproducibility, interfaces, and related phases
Reproducibility is strongly tied to oxygen content and microstructure. In polycrystalline La28Ni29O30, superconductivity under pressure was observed only for 31; outside this corridor neither a diamagnetic transition nor zero resistance was seen. Within the corridor the maximum 32 remained essentially constant at 33 K, and a narrower region near 34 showed a metallic normal state down to low temperature (Zhou et al., 2023).
The same study argued that the superconductivity is filamentary-like and most likely emerges at interfaces between the dominant “327” La35Ni36O37 phase and a minor “4310” La38Ni39O40 phase. STEM showed that samples are dominated by the 327 phase at 41, but contain thin extended interfaces where bilayer stacking changes to trilayer stacking. Because superconductivity was seen only in the presence of these intergrowths and never in phase-pure 327 crystals, the authors attributed the superconducting paths to two-dimensional interfacial channels (Zhou et al., 2023).
Ambient-pressure superconducting signatures have also been reported, but again with very small volume fraction. In oxygen-post-annealed single crystals, dc magnetization showed a diamagnetic downturn below 42 K, absent in as-grown material and strongly dependent on annealing time. The superconducting volume fraction was estimated as 43 at 44 Oe in the best sample, while resistivity displayed an additional broad drop of 45 between 46 K and 47 K (Huo et al., 27 Jan 2025). That work connected the effect to high-pressure O48 annealing, inner-apical oxygen vacancies, and strain at structural domain boundaries.
A distinct polymorph broadens the context of La49Ni50O51 physics. The long-range-ordered hybrid “1313” type, with alternating single-layer and trilayer blocks and ambient-pressure 52 symmetry, shows semiconducting behavior and a spin-density-wave transition at 53 K evidenced by susceptibility, specific heat, and 54La NMR. Under pressure it metallizes above 55 GPa but shows no discernible traces of superconductivity up to 56 GPa (Zhang et al., 19 Jan 2026). This provides a counterexample within the broader La57Ni58O59 family: pressure-induced metallization alone is not sufficient for superconductivity, and the bilayer structural motif of the 327 phase remains a special electronic setting.
Across these studies, La60Ni61O62 emerges as a pressure-tuned, stoichiometry-sensitive, structurally polymorphic nickelate in which superconductivity, magnetism, and charge-transfer physics are tightly entangled. The most stable current experimental constraints are the bilayer Ruddlesden–Popper lattice, the decisive role of inner-apical oxygen, the proximity of spin and density-wave tendencies, and a predominantly nodal superconducting gap under hydrostatic high pressure (Cao et al., 16 Sep 2025). The main unsettled questions remain the exact high-pressure host structure, the extent of bulk versus filamentary superconductivity across sample classes, and the microscopic route by which multiorbital charge-transfer physics produces high-63 pairing.