High-pressure X-ray diffraction reveals hidden layers in superconducting nickelates

Superconductors – materials that carry electrical current without energy loss – offer vast opportunities for technological advances in energy transmission, quantum computing, and medical imaging devices. Recently, high-pressure superconductivity was observed in the Ruddlesden-Popper (RP) phase nickelate La₃Ni₂O₇, with a transition temperature (T_c) of 80 K [1]. This remarkably high T_c exceeds the liquefaction point of nitrogen (77 K), positioning La₃Ni₂O₇ as a potential bridge between other nickelates with lower T_c values [2] and the widely studied cuprate high-temperature superconductors. 

Despite these promising properties, the microscopic mechanism driving superconductivity in RP nickelates remains a topic of debate. In addition, inconsistencies have been reported in the electronic transport properties of La₃Ni₂O₇ samples, including variations in superconducting onset temperatures and discrepancies regarding whether zero resistance is achieved. These differences have been attributed to factors such as stacking disorder involving mixed RP phases within individual crystals and/or oxygen off-stoichiometries in La₃Ni₂O_7±δ.

In this study, researchers reassessed the structure of La₃Ni₂O₇ single crystals that exhibit signs of filamentary superconductivity. Using scanning transmission electron microscopy (STEM) and single-crystal X-ray diffraction (XRD) under high-pressure conditions at the ID27 beamline, they identified a significant deviation from the previously proposed bilayer structure [1].

In high-quality single crystals grown using the optical floating zone technique, the dominant crystallographic phase was found to consist of alternating monolayer (ML) and trilayer (TL) blocks of NiO₆ octahedra. This novel stacking order represents an unrecognized polymorph of the conventional RP phases, offering a new perspective on the class of layered nickelates.

Additionally, density functional theory was used to disentangle the individual contributions of the ML and TL structural units to the electronic band structure of La₃Ni₂O₇, providing a foundation for advanced theoretical modelling and future evaluations of the ML-TL structure’s potential to host superconductivity.

Using a membrane-driven diamond anvil cell with helium as the pressure-transmitting medium, along with the highly focused X-ray beam at ID27, researchers were able to probe the crystal structure locally. STEM imaging and XRD data refinement revealed detailed insights into the crystal structure (Figure 1).

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Fig. 1: Crystal structure of the monolayer (ML) trilayer (TL) polymorph of La₃Ni₂O₇. a) Atomic-resolution STEM images of a La₃Ni₂O₇ single crystal, grown using the optical floating zone (OFZ) method, exhibiting a stacking sequence of alternating ML and TL units. b) Schematic representation of the ML-TL crystal structure with an orthorhombic Fmmm unit cell, determined at 0.7 GPa from single-crystal XRD Rietveld refinements. c) Schematic of the tetragonal P4/mmm unit cell, obtained at 15 GPa from refinements. Panels adapted from the original publication.

At pressures above 12.3 GPa, the symmetry of the ML-TL unit cell transitions from orthorhombic to tetragonal, accompanied by a decrease in lattice constants (Figure 2). This symmetry change coincides with a pronounced sharpening of the transition in the electronic transport observed between 12.2 and 13.9 GPa.

Furthermore, within the TL units, the Ni─O─Ni bond angles along the c-axis remain nearly constant at 170° for pressures up to ~6 GPa but rapidly approach 180° at 12.3 GPa. This evolution of the Ni─O─Ni bond angle correlated strongly with the increase in T_c observed in the transport data, highlighting the intricate relationship between structural and electronic properties under compression.

In the ML unit, the octahedral distortions, particularly the asymmetry between the apical Ni─O bond length and the in-plane bond lengths, are comparable to those of the ML in La₂NiO₄. In the TL unit, while the asymmetry between the outermost apical Ni─O bond and the in-plane distances is similar to that of the ML unit, its magnitude gradually decreases for Ni─O bonds approaching the centre of the TL unit.

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Fig. 2: Evolution of the crystal structure under pressure. a) Left and right panels illustrate the symmetry transition from Fmmm to P4/mmm. b) Ni─O─Ni bond angle along the c-axis direction in the TL unit. At 180°, the bond angle is intrinsic to the P4=mmm unit cell refinements. c) Ni-apical O bond distances in the ML (yellow symbols) and TL units (dark to light blue symbols). d) Ni-in-plane oxygen bond distances along the a- and b-axis directions, colour-coded in orange and green, respectively. Panels adapted from the original publication.

The discovery of ML-TL stacking in La₃Ni₂O₇ single crystals introduces a new form of structural complexity in RP-phase materials. This stacking, consisting of alternating ML and TL blocks extending over macroscopic length scales, is distinct from the commonly observed intergrowth in RP crystals, where micrometre-sized lamellae of different phases grow on top of each other.

This newly identified polymorph explains previously observed variations in electrical transport properties and necessitates a a re-evaluation of theoretical models for superconductivity in La₃Ni₂O₇. However, the possibility that filamentary superconductivity arises from a minority phase – such as undetected bilayer (BL) stacks, a combination of ML, BL, and TL units, or interfaces between these units – cannot be completely ruled out. 

Despite this uncertainty, the findings present intriguing opportunities for identifying the superconducting phase in La₃Ni₂O₇. They also pave the way for further investigations using advanced diffraction and spectroscopic techniques to explore the relationship between structure and superconductivity, potentially leading to the discovery of new superconducting materials with tailored properties.

 
Principal publication and authors
Unconventional Crystal Structure of the High-Pressure Superconductor La₃Ni₂O₇, P. Puphal (a), P. Reiss (a), N. Enderlein (b), Y.-M. Wu (a), G. Khaliullin (a), V. Sundaramurthy (a), T. Priessnitz (a), M. Knauft (a), A. Suthar (a), L. Richter (c), M. Isobe (a), P.A. van Aken (a), H. Takagi (a), B. Keimer (a), Y.E. Suyolcu (a), B. Wehinger (d), P. Hansmann (b), M. Hepting (a), Phys. Rev. Lett. 133, 146002 (2024); https://doi.org/10.1103/PhysRevLett.133.146002
(a) Max-Planck-Institute for Solid State Research, Stuttgart (Germany)
(b) Department of Physics, Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU), Erlangen (Germany)
(c) Max-Planck-Institute for Chemical Physics of Solids, Dresden (Germany)
(d) ESRF

References
[1] H. Sun et al., Nature 621, 493 (2023).
[2] D. Li et al., Nature 572, 624 (2019).