Polymorphism in the Ruddlesden–Popper Nickelate La₃Ni₂O₇: Discovery of a Hidden Phase with Distinctive Layer Stacking

Summary

We report the discovery of a novel form of Ruddlesden–Popper (RP) nickelate that stands as the first example of long-range, coherent polymorphism in this class of inorganic solids. Rather than the well-known, uniform stacking of perovskite blocks ubiquitously found in RP phases, this newly discovered polymorph of the bilayer RP phase La₃Ni₂O₇ adopts a novel stacking sequence in which single-layer and trilayer blocks of NiO₆ octahedra alternate in a “1313” sequence. Crystals of this new polymorph are described in space group Cmmm, although we note evidence for a competing Imam variant. Transport measurements at ambient pressure reveal metallic character with evidence of a charge density wave transition with an onset at T ≈ 134 K. The discovery of such polymorphism could reverberate to the expansive range of science and applications that rely on RP materials, particularly the recently reported signatures of superconductivity in bilayer La₃Ni₂O₇ with T_c as high as 80 K above 14 GPa.

Materials

• 1313La₃Ni₂O₇

Methods

• X-ray diffraction
• transport measurements

Keywords

• polymorphism
• 1313 stacking
• charge density wave
• metal

Highlights

• First example of long-range, coherent polymorphism in Ruddlesden-Popper nickelates.

Conclusions

• A new polymorph of La₃Ni₂O₇ with alternating single-layer and trilayer stacking ('1313') is discovered, described in space group Cmmm.
• At ambient pressure, it shows metallic behavior with a charge density wave transition at T ≈134 K.

Main claims

• Discovery of a new polymorph of La₃Ni₂O₇ (LNO-1313) with alternating single-layer and trilayer stacking sequences, which is the first example of long-range coherent polymorphism in RP phases.
  • Evidence: SC-XRD shows Cmmm space group with a '1313' stacking; STEM images confirm alternating monolayer and trilayer blocks.
• LNO-1313 exhibits metallic transport with a charge density wave transition at T≈134 K.
  • Evidence: Resistivity measurements show a metal-to-metal transition onset at 134 K, similar to La₄Ni₃O₁₀ and LNO-2222.

Workflow

• Crystal Growth — The new LNO-1313 polymorph is discovered serendipitously.
  • Materials: La₃Ni₂O₇; high oxygen pressure
  • Methods: floating zone technique under high oxygen pressure
  • Observations: competing phases include LNO-2222, La₄Ni₃O₁₀, La₂NiO₄
• Single-Crystal X-ray Diffraction (SC-XRD) — LNO-1313 crystallizes in Cmmm with 1313 stacking sequence.
  • Materials: LNO-1313 single crystals
  • Methods: SC-XRD
  • Observations: Cmmm space group; a=5.4382 Å, b=5.4700 Å, c=20.3598 Å; alternating single-layer and trilayer stacking
• Scanning Transmission Electron Microscopy (STEM) — Real-space atomic structure confirms alternating single-layer/trilayer stacking.
  • Materials: LNO-1313 and LNO-2222 crystals
  • Methods: atomic-resolution HAADF-STEM
  • Observations: clear '1313' and '2222' stacking sequences; low density of stacking faults
• Transport Measurements — LNO-1313 shows metallic character with a CDW transition around 134 K.
  • Materials: LNO-1313 single crystals
  • Methods: electrical resistivity
  • Observations: metallic behavior; anomaly at T≈134 K indicating CDW transition
• First-Principles Calculations — Electronic structure of LNO-1313 is a superposition of La₂NiO₄ and La₄Ni₃O₁₀ subsystems.
  • Materials: LNO-1313
  • Methods: DFT with GGA-PBE
  • Observations: band structure is electronic superposition of single-layer and trilayer subsystems