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. 2025 Jan 10;64(2):828–834. doi: 10.1021/acs.inorgchem.4c03042

Temperature-Dependent Structural Evolution of Ruddlesden–Popper Bilayer Nickelate La3Ni2O7

Haozhe Wang †, Haidong Zhou ‡, Weiwei Xie †,*
PMCID: PMC11752513  PMID: 39791441

Abstract

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A recent article (J. Am. Chem. Soc. 2024, 146, 7506–7514 ) details a pressure–temperature (P–T) phase diagram for the Ruddlesden–Popper bilayer nickelate La3Ni2O7 (LNO-2222) using synchrotron X-ray diffraction. This study identifies a phase transition from Amam (#63) to Fmmm (#69) within the temperature range of 104–120 K under initial pressure and attributes the I4/mmm (#139) space group to the structure responsible for the superconductivity of LNO-2222. Herein, we examine the temperature-dependent structural evolution of LNO-2222 single crystals at ambient pressure. Contrary to the symmetry increase and the established Amam–Fmmm phase boundary, we observe an enhancement in the Amam reflections as temperature decreases. This work not only delivers high-quality crystallographic data of LNO-2222 using laboratory X-rays across various temperatures but also enhances the understanding of the complex crystallographic behavior of this system, contributing insights to further experimental and theoretical explorations.

Short abstract

This study employs laboratory single-crystal X-ray diffraction to explore the temperature-dependent structural evolution of La3Ni2O7 (LNO-2222) at ambient pressure. Contrary to previous synchrotron data that established an Amam–Fmmm phase transition, enhanced Cmcm reflections are observed with decreasing temperature. Our findings offer new insights into its complex crystallographic behavior, aiding future experimental and theoretical investigations on Ruddlesden−Popper nickelates.

Introduction

Nickelates have emerged as promising candidates for high-temperature superconductivity, drawing parallel with the cuprates first discovered in the 1980s,1 due to their analogous crystal and electronic structures.2 Notably, superconductivity was previously identified in epitaxial thin films of reduced square-planar phases,3−9 characterized by an ultralow valence state of Ni1+, isostructural to Cu2+. Continuing this trajectory, a significant breakthrough was reported last year with the observation of superconductivity signatures in Ruddlesden–Popper bilayer nickelate La3Ni2O7 (LNO-2222), which exhibited a TC up to 80 K within a pressure range of 14.0–43.5 GPa.10 This discovery was also marked by a structural transition from the ambient pressure Amam (#63) to the high-pressure Fmmm (#69) above 15.0 GPa, aligning with the onset of superconductivity. Further advancements have been achieved through in situ low-temperature high-pressure synchrotron X-ray diffraction (XRD), which has effectively mapped the pressure–temperature (P–T) structure phase diagram of LNO-2222, clarifying the phase boundaries among the Amam, Fmmm, and I4/mmm (#139) space group.11

The initial determination of the crystal structure of LNO-2222, classified in the F-centered orthorhombic Fmmm space group, was performed using powder XRD and Rietveld refinement on polycrystalline samples.12 Recognizing the substantial uncertainty in determining oxygen coordinates, neutron powder diffraction (NPD) was subsequently employed.13,14 The results indicated that the Fmmm space group was inappropriate, as it failed to account for extra weak peaks, suggesting lower symmetry. Consequently, a C-centered orthorhombic lattice in the space group Cmcm (#63, symmetry equivalent to Amam) was proposed.13 Challenges related to lattice centering, the impact of oxygen vacancies, and the coexistence of Ruddlesden–Popper bilayer and trilayer phases highlight the complexities in structure determination, underscoring the necessity for pure LNO-2222 single crystals. Recent advances in the high-pressure floating zone method, which allows for a 100% O2 atmosphere with controllable gas pressure, have facilitated the growth of high-purity Ruddlesden–Popper nickelate single crystals, enabling precise structure determination using laboratory X-rays.15−17 Furthermore, a previously unrecognized phase of La3Ni2O7 with distinct layer stacking, LNO-1313, was first reported by Chen et al.16 in the growth of LNO-2222, and supported by later studies.17,18

The referenced study11 caught our attention because a phase transition from Amam to Fmmm was observed within the temperature range of 104–120 K under initial pressure conditions. However, this symmetry increase contradicts our previous observations in high-quality Sr-doped LNO-2222 single crystals synthesized under high pressure.19 The absence of detailed crystal structure data targeting the Amam to Fmmm transition in the study prompted further investigation into the effects of temperature on this specific symmetry change. Meanwhile, the structure determination in the referenced study11 was conducted using powder XRD refinements, achieving a resolution limit of approximately 1.14 Å. While it is recognized that obtaining higher resolution and enhanced data quality under high pressure presents significant challenges, in the case of LNO-2222, precise determination of lattice centering and space groups critically depends on the observation of some weak peaks. Therefore, a more dedicated focus on studying the crystal structure, particularly through the use of single crystals, is essential for comprehensively resolving the complexities of this material.

On the other hand, increasing efforts have been directed toward theoretical investigations of the mechanism behind high-TC superconductivity in LNO-2222. The accuracy of these theoretical models might be compromised without precisely determining crystal structures. Notably, conflicting suggestions regarding the size of A-site cations and their role in stabilizing the superconducting phase at ambient pressure have been reported.20,21

We conducted temperature-dependent single-crystal XRD experiments on LNO-2222, spanning a temperature range from 80 to 400 K at ambient pressure. Our results indicate that with decreasing temperature, the tilt of octahedra in LNO-2222 is enhanced, and the Amam space group becomes increasingly favorable. This work aims to establish a benchmark method for single-crystal structure studies of LNO-2222 by using laboratory X-rays. Further investigations exploring the crystal structure and structure–property relationship in LNO-2222 will likely require much more high-quality data, utilizing advanced techniques such as synchrotron X-rays and neutrons.

Experimental Section

The crystals of LNO-2222 used in this study were selected from the same batch as previously reported17 and grown using the floating zone method under 100% O2 at a pressure of 14–15 bar at the University of Tennessee. A single crystal with dimensions of 0.069 × 0.045 × 0.014 mm3 was picked up, mounted on a nylon loop with paratone oil, and measured using an XtalLAB Synergy, Dualflex, Hypix single-crystal X-ray diffractometer equipped with an Oxford Cryosystems 800 low-temperature device. The temperature protocol commenced with cooling the sample to 80(2) K, followed by sequential heating to 400(2) K in increments of 40 K. A 10 min stabilization period was allowed between each temperature scan, with continuous monitoring and adjustment of crystal centering as needed throughout the process. Data acquisition was performed using ω scans with Mo Kα radiation (λ = 0.71073 Å, microfocus sealed X-ray tube, 50 kV, 1 mA). The measurement strategy, including the total number of runs and images, was determined using the strategy calculation feature in CrysAlisPro software (version 1.171.43.104a, Rigaku OD, 2023), which was established at 80(2) K and consistently applied across all temperature steps. Data reduction induced a correction for Lorentz polarization. Numerical absorption correction was based on Gaussian integration over a multifaceted crystal model. Empirical absorption correction was applied using spherical harmonics implemented in the SCALE3 ABSPACK scaling algorithm. Structure solution and refinement were conducted using the Bruker SHELXTL Software Package.22,23

Prior to temperature-varied data collection, the quality of the crystal was examined at room temperature, confirming bilayer stacking, as well as single-crystallographic domain behavior without significant twinning or disorder. This ensures the integrity of the structural analysis.

Results and Discussion

In the referenced low-temperature high-pressure study,11 multiple instances of twins and severe texture development were reported during pressure increase across multiple runs, yet these claims lacked experimental evidence. If the sample quality was thoroughly examined prior to pressurization, it would be essential to document at which pressure and temperature conditions these multiple twins (or texture development) occurred in LNO-2222, their intrinsic nature, and their impacts on the results of structure determination. The powder XRD refinement results remain ambiguous, with “atomic positions optimized theoretically” mentioned but without additional details provided.

The presence of oxygen vacancies significantly influences the determination of the crystal structure, often leading to a symmetry increase. For example, polycrystalline samples of La3Ni2O6.92-2222 and La3Ni2O6.94-2222 were determined to be Fmmm by powder XRD,12,24 while La3Ni2O6-2222 and La3Ni2O6.35-2222 was identified as I4/mmm by NPD.25,26

Concerning the tuning of LNO-2222 crystal structures through A-site doping, there are claims that using smaller atoms might induce a chemical precompression effect.11 One argument presented is the comparison of the A/B atomic size ratio between tetragonal Sr3Ti2O7 and LNO-2222. However, the lack of a reference source for atomic size data undermines the credibility of this comparison. Whether using an atomic size or ionic size is more appropriate remains unclear. According to the authors’ logic, a rough calculation reveals that Sr2+/Ti4+ = 132/74.5 = 1.772, while La3+/(0.5 × (Ni2+ + Ni3+)) = 117.2/(0.5 × (83 + 70)) = 1.532.27 Even considering Ni2+/3+ as Ni3+ exclusively, the upper boundary for La3+/Ni2+/3+ would be 117.2/70 = 1.674, still smaller than the Sr2+/Ti4+ ratio, suggesting an opposite conclusion. Furthermore, in the case of La2–2xSr1+2xMn2O7, detailed crystallographic and magnetic phase diagram provided by temperature-dependent neutron powder diffraction shows a tetragonal-to-orthorhombic phase transition, peaking at x = 0.80.28 However, these findings were never related to the size of the A-site in the original report. The persistence of the tetragonal I4/mmm structure to 35 GPa in LaSr2Mn2O729 (x = 1.00) contradicts the claims, as the size of Sr2+ is larger than La3+.

Our previous report19 included a temperature-dependent crystal structure study of high-quality Sr-doped LNO-2222 single crystals (formula La2.80(1)Sr0.20(1)Ni2O6.95(1), denoted as Sr-LNO-2222), obtained via high-pressure synthesis. A comparison of the out-of-plane Ni–O–Ni bond angles between undoped LNO-2222 and Sr-LNO-2222 reveals less magnitude of octahedral tilts exhibited in Sr-LNO-2222, which supports the hypothesis that the incorporation of larger A-site atoms contributes to a potential rise in symmetry.

Figure 1 illustrates the crystal structure of LNO-2222 in space groups Amam, Fmmm, and I4/mmm, providing a detailed view of their structural variations. Central to this discussion are the group–subgroup relationships that explain how LNO-2222 crystallizes in these specific space groups. In the Amam space group, the structure can be interpreted as I4/mmm with two octahedral tilts about the [011] and [01–1] axes. These tilts are highlighted in Figure 1d with orange and red arrows indicating different directions of octahedral tilts, while an asterisk marks the consistent octahedral direction across different views. Transitioning to the Fmmm space group, the octahedral tilts are absent, resulting in a structure that might otherwise resemble I4/mmm symmetry. However, as shown in Figure 1e, lattice distortions along the a and b axes prevent this. Furthermore, it is worth mentioning that the I4/mmm structure does not display a perfect square lattice. This difference is primarily due to a slight displacement of oxygen along the c-axis, as represented in Figure 1f, which disrupts the planarity between the oxygen and nickel atoms, inducing nonlinear “in-plane” Ni–O–Ni bond angles.

Figure 1.

Figure 1

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Crystal structure of LNO-2222. (a–c) Layer stacking views in the space group Amam, Fmmm, and I4/mmm, respectively. Green, gray, and red represent La, Ni, and O atoms. In the Amam space group, crystallographically unique atoms are labeled. (d) Two octahedral tilts about the [110] and [1–10] axes in the Amam space group. (e) Lattice distortion along the a and b axes in the Fmmm space group. (f) Oxygen displacements along the c axis in the I4/mmm space group lead to nonlinear “in-plane” Ni–O–Ni bond angles.

The crystallographic data and structure refinements for LNO-2222 at 80 and 280 K are summarized in Tables 1–3. Our refinements revealed no instances of oxygen vacancies. A slight lattice expansion was observed at 280 K when compared to the structure at 80 K as expected for thermal expansion. Our experimental reciprocal lattice planes, (0kl), (h1l), and (hk0) at temperatures of 80, 280, and 400 K, are presented in Figure 2. Laue symmetry mmm was applied in the regeneration of these (hkl) planes. The reflection conditions for the Amam space group are defined as k + l = 2n for all reflections, and h = 2n specifically for reflections on the (h0l) plane. For the F-centering lattice, the reflection conditions stipulate that h, k, and l must all be either even or odd (“unmixed”). In our analysis, additional reflections that characterize the Amam space group and signify violations for the Fmmm space group were observed and labeled on each reciprocal lattice plane. These reflections progressively become weaker as the temperatures increased. Further details on the crystallographic data, structure refinements, and reciprocal lattice planes at other temperatures studied are available in Figures S1 and S2 and Tables S1–S14.

Table 1. Crystal Data and Structure Refinement of LNO-2222 at 80 and 280 K.

chemical formula La3Ni2O7-2222 La3Ni2O7-2222
temperature 80(2) K 280(2) K
formula weight 646.15 g/mol 646.15 g/mol
space group Amam Amam
unit cell dimensions a = 5.37729(18) Å a = 5.38996(19) Å
b = 5.44849(17) Å b = 5.44719(18) Å
c = 20.4851(7) Å c = 20.5305(6) Å
volume 600.18(3) Å3 602.78(3) Å3
Z 4 4
density (calculated) 7.151 g/cm3 7.120 g/cm3
absorption coefficient 26.980 mm–1 26.895 mm–1
F(000) 1132 1132
θ range 3.87–40.66° 3.87–40.71°
reflections collected 18,594 18,596
independent reflections 1049 [Rint = 0.0775] 1055 [Rint = 0.0683]
refinement method full-matrix least-squares on F2 full-matrix least-squares on F2
data/restraints/parameters 1049/0/37 1055/0/37
final R indices R1 (I > 2σ(I)) = 0.0291; wR2 (I > 2σ(I)) = 0.0724 R1 (I > 2σ(I)) = 0.0280; wR2 (I > 2σ(I)) = 0.0687
  R1 (all) = 0.0350; wR2 (all) = 0.0751 R1 (all) = 0.0372; wR2 (all) = 0.0729
largest diff. peak and hole +4.914 and –2.700 e/Å3 +5.847 and –2.023 e/Å3
R.M.S. deviation from mean 0.502 e/Å3 0.482 e/Å3
goodness-of-fit on F2 1.199 1.120
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Table 3. Atomic Coordinates and Equivalent Isotropic Atomic Displacement Parameters (Å2) of LNO-2222 at 280 Ka.

  Wyck. x y z Occ. Ueq
La1 4c 1/4 0.24943(5) 0 1 0.00696(8)
La2 8g 1/4 0.24208(3) 0.17980(2) 1 0.00603(7)
Ni 8g 1/4 0.74758(7) 0.90411(3) 1 0.00444(10)
O1 8e 0 0 0.08927(16) 1 0.0104(6)
O2 8e 0 1/2 0.10472(17) 1 0.0111(5)
O3 8g 3/4 0.2173(6) 0.20441(16) 1 0.0121(6)
O4 4c 1/4 0.7087(8) 0 1 0.0111(7)
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a

Ueq is defined as one-third of the trace of the orthogonalized Uij tensor.

Figure 2.

Figure 2

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Reciprocal lattice planes of LNO-2222. (a–c) (0kl), (h1l), and (hk0) planes at 80 K. (d–f) (0kl), (h1l), and (hk0) planes at 280 K. (g–i) (0kl), (h1l), and (hk0) planes at 400 K. Laue symmetry mmm has been applied in the regeneration of these (hkl) planes.

Table 2. Atomic Coordinates and Equivalent Isotropic Atomic Displacement Parameters (Å2) of LNO-2222 at 80 Ka.

  Wyck. x y z Occ. Ueq
La1 4c 1/4 0.24909(5) 0 1 0.00333(8)
La2 8g 1/4 0.24129(4) 0.17975(2) 1 0.00320(7)
Ni 8g 1/4 0.74744(8) 0.90411(3) 1 0.00267(10)
O1 8e 0 0 0.08874(16) 1 0.0057(5)
O2 8e 0 1/2 0.10540(17) 1 0.0059(5)
O3 8g 3/4 0.2138(7) 0.20447(16) 1 0.0077(5)
O4 4c 1/4 0.7055(9) 0 1 0.0063(7)
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a

Ueq is defined as one-third of the trace of the orthogonalized Uij tensor.

Figure 3a,b displays the temperature-dependent evolution of the lattice parameters, a, b, c, and the unit cell volume of LNO-2222. These parameters generally follow the expected thermal expansion behavior as the temperature increases. Notably, the evolution of the lattice parameter b exhibits distinct anomalies below 160 K, potentially due to structure modulation, which is also reflected in the lattice parameter c and the unit cell volume. This aligns with previous reports of electrical resistance of LNO-2222 that two transition-like kinks were observed at ∼122 K and ∼153 K,16,30 as well as a very recent observation of density-wave-like gap at ∼151 K using ultrafast optical spectroscopy.31Figure 3c,d provides more structure details about the in-plane and out-of-plane Ni–O–Ni bond angles. As temperature decreases, these bond angles increasingly deviate from 180°, indicative of enhanced octahedral tilts. Such behavior points to potential signs of symmetry lowering with respect to Fmmm. Outlined in Figure 3e is the methodology for analyzing experimental reflection data. We identify and focus on specific reflections characteristic of A-centering and violate F-centering norms, which are expected to be weak, as well as reflections common to both A- and F-centering. Due to data redundancy, the same reflections are observed multiple times during data collection. We also merge reflections equivalent under Laue symmetry to streamline the analysis. Moreover, we define the average intensity ratio of specific reflections from these two classes, for example, 140 and 200 here, as a metric to evaluate potential changes in symmetry. The unbiased selection of reflections has been validated by incorporating an additional A-centering characteristic reflection, 033, which is among the most intense. The results are presented in Figure S3, demonstrating the consistency and reliability of our methodology. Figure S3 also gives in-plane and out-of-plane Ni–O bond lengths across various temperatures. Our comprehensive analysis confirms that at low temperatures, down to 80 K, the Amam space group becomes increasingly favorable, aligning with the observed changes in the out-of-plane Ni–O4–Ni bond angle as detailed in Figure 3c. This correlation emphasizes the structural dynamics of LNO-2222 under varying thermal conditions.

Figure 3.

Figure 3

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Temperature-dependent structure evolution of LNO-2222. (a) Lattice parameters a and b. Color filling highlights the temperature ranges with anomalous behavior observed. (b) Lattice parameter c and unit cell volume. (c) Out-of-plane Ni–O4–Ni bond angle and the observed intensity ratio of the 140 and 200 reflections normalized to 80 K. Error bars are indicated by color filling. (d) In-plane Ni–O1–Ni and Ni–O2–Ni bond angles. (e) Methodology for obtaining the observed intensity ratios is presented in (c).

Given the observed enhancement of Amam reflections at low temperatures, considering external pressure as a tuning parameter offers a new perspective on the structure behavior of LNO-2222. We can extrapolate its temperature behavior at ambient pressure to at least slightly higher pressures under the assumption that the Amam–Fmmm transition at low temperatures and high pressures is gradual rather than abrupt. This assumption is reasonable, as no formation or breaking of chemical bonds is involved. In terms of crystal structure dynamics, for the transition to occur, the out-of-plane Ni–O–Ni bond angle must approach exactly 180 deg. Based on this requirement, we hypothesize that, compared to room temperature, a higher pressure would be necessary to induce this transition at lower temperatures. Figure 4 presents our sketch structure phase diagram of LNO-2222, where the Amam–Fmmm phase boundary is indicated by a dashed line. This boundary, with a negative dT/dP slope, is notably inconsistent with the reported one.11 Additionally, it is plausible that sufficiently high temperatures could also facilitate this phase transition, although such conditions may extend beyond the scope of our current discussion.

Figure 4.

Figure 4

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Structure phase diagram of LNO-2222. The dashed line indicates the phase boundary with negative dT/dP values between the Amam and Fmmm space groups.

The evaluation of the Fmmm-I4/mmm phase boundary, as reported in the referenced study,11 presents distinct challenges in the evaluation of reciprocal lattice planes due to inconsistencies in unit cell selection, necessitating the use of √2 super reciprocal vectors in the hk plane for accurate analysis. The detection of very weak difference reflections, which appear on half-integral reciprocal lattice planes when using original sub-cell axes, requires specialized efforts. These reflections are critical for confirming the phase transition but may not be readily observable under high-pressure conditions due to the inherent experimental challenges. Further detailed experiments involving low-temperature and high-pressure single-crystal XRD will be essential to fully elucidate the complex crystallographic behavior of LNO-2222.

Conclusions

In conclusion, we present our investigation into the temperature-dependent structural evolution of LNO-2222 single crystals at ambient pressure. Our results highlight the enhancement of Amam reflections as temperature decreases. Additionally, we have developed our sketch structure phase diagram for LNO-2222 that differs from that reported, particularly concerning the Amam–Fmmm phase boundary. This study delivers high-quality crystallographic data of LNO-2222 across various temperatures using laboratory X-rays.32 More importantly, our work enhances our understanding of the complex crystallographic behavior of this system, laying a solid foundation for further experimental and theoretical investigations.

Acknowledgments

The work at Michigan State University was supported by U.S. DOE-BES under Contract DE-SC0023648. The work at the University of Tennessee (crystal growth) was supported by the Air Force Office of Scientific Research under Grant No. FA9550-23-1-0502. H.W. appreciates helpful discussions with Dr. Xinglong Chen (Argonne National Laboratory).

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.inorgchem.4c03042.

  • Reciprocal lattice planes of LNO-2222 at temperatures of 120, 160, 200, 240, 320, and 360 K; temperature-dependent structure evolution of LNO-2222; crystal data and structure refinement, atomic coordinates, and equivalent isotropic atomic displacement parameters of LNO-2222 at temperatures of 120, 160, 200, 240, 320, 360, and 400 K (PDF)

The authors declare no competing financial interest.

Supplementary Material

ic4c03042_si_001.pdf (685KB, pdf)

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