---
title: 'La3Ni2O7: Structure and Superconductivity'
url: https://www.emergentmind.com/topics/la3ni2o7
type: topic
---

# La3Ni2O7: Structure and Superconductivity

Searching arXiv for recent papers on La3Ni2O7 to ground the article with current literature.
Searching arXiv for recent papers on La3Ni2O7 to ground the article with current literature.
La\(_3\)Ni\(_2\)O\(_7\) and its oxygen-deficient form La\(_3\)Ni\(_2\)O\(_{7-\delta}\) are bilayer Ruddlesden–Popper nickelates that have become central to the study of high-temperature superconductivity under pressure. The compound combines NiO\(_2\) 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 [2307.09865] [2509.12606] [2311.12361].

## 1. Composition, stoichiometry, and lattice architecture

La\(_3\)Ni\(_2\)O\(_{7-\delta}\) is a Ruddlesden–Popper-type bilayer perovskite with alternating LaO rock-salt and NiO\(_2\) perovskite layers. The “\(7-\delta\)” notation denotes apical-oxygen deficiencies \(\delta\), and the missing oxygen atoms are reported to occur predominantly at the inner apical sites that bridge the two NiO\(_2\) sheets of a bilayer. Small \(\delta\) changes the Ni valence and carrier density and is believed to tune the superconducting state [2509.12606] [2312.15727].

At ambient pressure, La\(_3\)Ni\(_2\)O\(_7\) is widely described as an orthorhombic \(n=2\) Ruddlesden–Popper phase with space group \(Amam\). In this phase the NiO\(_6\) octahedra form bilayers separated by La–O slabs, and the inter-bilayer Ni–O–Ni bond angle is buckled. Powder refinement for polycrystalline La\(_3\)Ni\(_2\)O\(_{7-\delta}\) with \(\delta \simeq 0.07\) gave \(a = 5.4763(1)\) Å, \(b = 5.4628(1)\) Å, and \(c = 19.578(2)\) Å, while single-crystal X-ray diffraction at \(80\) K in an ambient-pressure study gave \(a = 5.407(2)\) Å, \(b = 5.4176(18)\) Å, and \(c = 20.490(5)\) Å [2309.17378] [2501.15929].

The oxygen sublattice is not a passive structural detail. Energy-filtered multislice electron ptychography and O-\(K\)-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 \(\sim(10\,\mathrm{nm})^3\) regions gave \(\delta = 0.04 \pm 0.11\), \(0.17 \pm 0.12\), and \(0.34 \pm 0.22\), demonstrating nanoscale inhomogeneity in stoichiometry and electronic structure [2312.15727].

## 2. Pressure-driven structural evolution

A central issue in La\(_3\)Ni\(_2\)O\(_7\) research is the structure of the superconducting state under pressure. Early high-pressure X-ray diffraction identified a first-order structural transition above \(P_s \simeq 14\) GPa from orthorhombic \(Amam\) to higher-symmetry orthorhombic \(Fmmm\), with a coexistence range of \(\sim 13\)–\(16\) GPa. In the \(Fmmm\) phase the Ni–O–Ni bond angle within the bilayers straightens, enhancing Ni \(3d\)–O \(2p\) overlap [2307.09865].

Low-temperature synchrotron XRD subsequently reported a tetragonal \(I4/mmm\) phase when the sample was compressed to \(19\) GPa at \(40\) K, under conditions where superconductivity takes place. In that study the \(Fmmm \rightarrow I4/mmm\) transition occurred at \(P_C \simeq 19\) GPa at \(40\) K, with a \(\sim 1.2\%\) drop in unit-cell volume. The tetragonal phase was described as having nearly regular NiO\(_6\) octahedra, in-plane Ni–O–Ni bond angle \(\theta = 178.38^\circ\) at \(19.6\) GPa, and low-energy states dominated by the Ni \(e_g\) orbitals \(3d_{x^2-y^2}\) and \(3d_{z^2}\) [2311.09186].

A later Raman and first-principles study proposed a more explicit sequence: orthorhombic \(Amam\) below \(P_1 \approx 4\) GPa, mixed \(Amam+Fmmm\) for \(4 \lesssim P \lesssim 14.5\) GPa, and tetragonal \(I4/mmm\) above \(P_2 \approx 14.5\) GPa. In that account, the emergence of bulk superconductivity coincided precisely with the transition to \(I4/mmm\), and a new \(A_{1g}\) Raman mode at \(\omega_{I4} \approx 481\ \mathrm{cm}^{-1}\) appeared abruptly as the orthorhombic oxygen modes disappeared [2511.15265].

These results frame a major controversy in the field. Orthorhombic \(Fmmm\) and tetragonal \(I4/mmm\) have both been advanced as the relevant high-pressure structure. The later Raman work explicitly argued that \(I4/mmm\) is the host of superconductivity, whereas earlier transport and diffraction studies emphasized \(Fmmm\). The disagreement is one of the principal structural questions surrounding La\(_3\)Ni\(_2\)O\(_7\) under pressure [2307.09865] [2511.15265].

## 3. Superconductivity under compression

Transport studies established that superconductivity in La\(_3\)Ni\(_2\)O\(_{7-\delta}\) 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 \(\sim 14\) GPa, and focused on \(20\)–\(22\) 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 \(T_c^{(\mathrm{onset})} \simeq 64\)–\(68\) K, with midpoint \(T_c \simeq 66\) K for Sample \#1 at \(22.0\) GPa and transition width \(\Delta T < 0.1\,T_c\) [2509.12606].

Other high-pressure transport studies reported lower-pressure onsets. In cubic-anvil-cell measurements with liquid glycerol as pressure medium, sample \#2 developed a \(98\%\) resistance drop below \(62\) K at \(15\) GPa, and sample \#3 reached zero resistance below \(\sim 5\) K at \(14\) GPa and \(\sim 10\) K at \(15\) GPa. From \(T_0\) and \(T_{50\%}\) criteria, \(\mu_0 H_{c2}(0)\) was estimated as \(\approx 92\) T and \(\approx 43\) T, respectively, using the Ginzburg–Landau form
\[
\mu_0H_{c2}(T)=\mu_0H_{c2}(0)\,\frac{1-(T/T_c)^2}{1+(T/T_c)^2}.
\]
The same study associated the onset of superconductivity with the \(Amam \rightarrow Fmmm\) transition and a dome reaching \(\approx 62\)–\(64\) K at \(15\) GPa [2307.09865].

Polycrystalline La\(_3\)Ni\(_2\)O\(_7\) also shows pressure-induced superconductivity. In a cubic-anvil cell with glycerol, the density-wave-like anomaly in \(\rho(T)\) was progressively suppressed with increasing pressure, a resistivity drop appeared at \(7\) GPa, zero resistivity was achieved at \(9\) GPa below \(6.6\) K, and \(T_c^{\rm zero}\) rose to \(35.6\) K at \(14.5\) GPa. The onset temperature \(T_c^{\rm onset}\) reached \(72.2\) K at \(14.5\) GPa [2309.17378].

The interpretation of these superconducting transitions is debated. Modulated ac susceptibility on compressed single crystals found no superconducting anomaly below \(\sim 20.7\) GPa, then a broad diamagnetic peak centered at \(T_c \approx 63.4\) K at \(22.0\) GPa, and a superconducting volume fraction \(\lesssim 1\%\). That work concluded that the superconductivity is filamentary-like rather than bulk [2311.12361]. By contrast, the hydrostatic point-contact spectroscopy study described the superconducting state at \(20\)–\(22\) GPa as pressure-stabilized bulk La\(_3\)Ni\(_2\)O\(_{7-\delta}\) [2509.12606]. 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 La\(_3\)Ni\(_2\)O\(_{7-\delta}\) 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/La\(_3\)Ni\(_2\)O\(_{7-\delta}\) junctions in situ, allowing comparison of c-axis and in-plane spectra at \(20\)–\(22\) GPa [2509.12606].

Along the c-axis, \(dI/dV(V)\) showed symmetric coherence peaks at \(V \approx \pm 9.7\) 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 \(d\)-wave node. The spectra were reported to agree qualitatively with 2D-BTK modeling for a \(d_{x^2-y^2}\) gap with high barrier strength \(Z\), and the combination of a V-shaped gap and ZBCP was stated to rule out simple isotropic \(s\)-wave or sign-changing \(s_{\pm}\) proposals unless accidental nodes are invoked [2509.12606].

The c-axis coherence peaks at \(eV=\pm 9.7\) meV imply \(\Delta \simeq 9.7\) meV. Using \(T_c \simeq 66\) K, the reported gap ratio was
\[
\frac{2\Delta}{k_B T_c} \simeq 4.2(5),
\]
which exceeds the BCS weak-coupling \(s\)-wave value \(3.53\) and was identified as typical of strongly coupled, nodal superconductors [2509.12606].

Theoretical work has nevertheless emphasized that pairing channels are close in energy. In a 31-orbital Ni \(3d\)–O \(2p\) Hubbard model treated within RPA, a transition between \(d\)-wave and sign-changing \(s\)-wave pairing states was found as a function of pressure and interaction parameters. At realistic parameters, incommensurate spin-fluctuations near \({\bf q}_1 \approx (\pi/2,\pi/2)\) and \({\bf q}_2 \approx (0.7\pi,0.7\pi)\) cooperatively stabilized a \(d_{x^2-y^2}\) order parameter, while the close competition between \(s_{\pm}\) and \(d\) was proposed as an explanation for why simplified models have produced conflicting gap symmetries [2501.05254]. 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+\(U\) established La\(_3\)Ni\(_2\)O\(_7\) as a multiorbital correlated metal with strong orbital selectivity. High-resolution ARPES found two main Fermi-surface sheets associated primarily with Ni \(3d_{x^2-y^2}\), while a nearly flat Ni \(3d_{z^2}\) band lay \(\sim 30\)–\(50\) meV below \(E_F\). The renormalization is substantially larger for \(d_{z^2}\) than for \(d_{x^2-y^2}\): one study reported \(Z_{x^2-y^2} \approx 1/2.7\) and \(Z_{z^2} \approx 1/5\), and another reported \(Z_{\alpha,\beta} \approx 0.50\)–\(0.55\) for \(d_{x^2-y^2}\) and \(Z_\gamma \approx 0.12\)–\(0.20\) for the \(d_{z^2}\) flat band [2407.07501] [2309.01148].

The same ARPES work also reported pseudogap-like behavior in the \(d_{x^2-y^2}\) band: upon cooling from \(120\) K to \(19\) K, the leading-edge midpoint at \(k_F \approx 0.39\ \mathrm{\AA}^{-1}\) shifted away from \(E_F\) by up to \(\approx 9\) meV, with onset near \(T^* \approx 100\) K and no full gap opening or coherence peak. Under high pressure, DFT+\(U\) predicted a \(\sim 25\%\) broadening of all bands and motion of the flat \(d_{z^2}\) band to \(E_F\), creating a van Hove singularity near \(X\) [2407.07501].

Spectroscopy of oxygen states places stoichiometric La\(_3\)Ni\(_2\)O\(_7\) in the charge-transfer regime. O-\(K\)-edge EELS identified a pre-edge at \(\sim 528\) eV associated with O \(2p\) states hybridized with Ni \(3d\), and its suppression with increasing \(\delta\) was summarized by \(n_h(\delta) \simeq n_h(0)-2\delta\). Ligand holes were found on inner-apical \(p_z\) and planar \(p_{x,y}\) orbitals, while outer-apical oxygen was described as less relevant to low-energy physics and safely disregarded in theoretical models [2312.15727]. A film study similarly described La\(_3\)Ni\(_2\)O\(_{7-\delta}\) as a charge-transfer system with both in-plane and out-of-plane Zhang–Rice singlets and a strong Ni–O\(_{\rm inner}\)–Ni interlayer coupling [2409.04121].

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 \(T^* \approx 150\) K, with \(l_c(10\,\mathrm{K}) \simeq 42\) Å \(> d_c \simeq 20\) Å but \(l_c(300\,\mathrm{K}) \simeq 16\) Å \(< d_c\), while the ab-plane remained coherent from \(10\) to \(300\) K with \(l_{ab} \gg d_{a,b}\). At \(10\) K, the Drude-weight ratio \(S_{ab}/S_c \simeq 6.5\) and the dc conductivity ratio \(\sigma_{1ab}(0)/\sigma_{1c}(0) \simeq 36\) [2411.10786]. Under pressure and high magnetic field, magnetoresistance was found to be quasi-quadratic with exponent \(n \approx 1.9\), to follow extended Kohler scaling, and to indicate a multiband metallic normal state [2412.09375].

The magnetic ground state remains actively discussed. \(^{139}\)La NMR on La\(_3\)Ni\(_2\)O\(_{7-\delta}\) found a spin-density-wave transition at \(T_{SDW} = (150 \pm 3)\) K and proposed a possible double spin stripe with moments aligned along the \(c\)-axis [2402.03952]. Resonant X-ray scattering on films detected a superlattice reflection at \((1/4,1/4,L)\), with charge-anisotropy order setting in at \(T_C \approx 200\) K and collinear antiferromagnetic order at \(T_N \approx 150\) K [2409.04121]. Neutron scattering on single crystals later resolved well defined spin excitations at \(Q=(0,0.5,2.5)\), a spin gap of \(\sim 5\) meV, and a bilayer Heisenberg spectrum with strong interlayer exchange \(SJ_c = 39.86 \pm 0.85\) meV and total fluctuating moment \(\langle m^2\rangle = 1.75 \pm 0.25\ \mu_B^2/{\rm Ni}\) [2605.03448]. 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 La\(_3\)Ni\(_2\)O\(_{7+x}\), superconductivity under pressure was observed only for \(6.89 \le x \le 7.35\); outside this corridor neither a diamagnetic transition nor zero resistance was seen. Within the corridor the maximum \(T_c({\rm onset})\) remained essentially constant at \(\approx 63\) K, and a narrower region near \(x \approx 7.00 \pm 0.05\) showed a metallic normal state down to low temperature [2311.12361].

The same study argued that the superconductivity is filamentary-like and most likely emerges at interfaces between the dominant “327” La\(_3\)Ni\(_2\)O\(_7\) phase and a minor “4310” La\(_4\)Ni\(_3\)O\(_{10}\) phase. STEM showed that samples are dominated by the 327 phase at \(>95\%\), 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 [2311.12361].

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 \(T_c \simeq 80\) K, absent in as-grown material and strongly dependent on annealing time. The superconducting volume fraction was estimated as \(0.2\%\) at \(\mu_0H = 50\) Oe in the best sample, while resistivity displayed an additional broad drop of \(\Delta R/R(300\,\mathrm{K}) \simeq 30\%\) between \(80\) K and \(20\) K [2501.15929]. That work connected the effect to high-pressure O\(_2\) annealing, inner-apical oxygen vacancies, and strain at structural domain boundaries.

A distinct polymorph broadens the context of La\(_3\)Ni\(_2\)O\(_7\) physics. The long-range-ordered hybrid “1313” type, with alternating single-layer and trilayer blocks and ambient-pressure \(Cmmm\) symmetry, shows semiconducting behavior and a spin-density-wave transition at \(170\) K evidenced by susceptibility, specific heat, and \(^{139}\)La NMR. Under pressure it metallizes above \(\sim 40\) GPa but shows no discernible traces of superconductivity up to \(65\) GPa [2601.13090]. This provides a counterexample within the broader La\(_3\)Ni\(_2\)O\(_7\) 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, La\(_3\)Ni\(_2\)O\(_7\) 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 [2509.12606]. 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-\(T_c\) pairing.

Source: https://www.emergentmind.com/topics/la3ni2o7