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

Area of Science:

• Inorganic Chemistry
• Solid-State Physics
• Materials Science

Background:

• Ruddlesden-Popper (RP) phases are inorganic solids known for uniform stacking of perovskite blocks.
• Polymorphism, the existence of multiple crystal structures, is rare in RP phases.
• Bilayer La3Ni2O7 is a known RP phase with recent reports of high-temperature superconductivity.

Purpose of the Study:

• To report the discovery of a novel polymorph of the bilayer RP phase La3Ni2O7.
• To characterize the unique structural features and physical properties of this new material.
• To explore the implications of this discovery for RP materials and superconductivity.

Main Methods:

• Crystallographic analysis to determine the structure and space group.
• Transport measurements to investigate electrical properties.
• Analysis of stacking sequences of NiO6 octahedra.

Main Results:

• Discovery of a novel RP nickelate polymorph with a "1313" stacking sequence of single and trilayer blocks.
• Crystals indexed in space group Cmmm, with evidence for an Imam variant.
• Transport measurements show metallic behavior and a charge density wave transition at T ≈ 134 K.
• This polymorphism could impact RP materials and superconductivity research.

Conclusions:

• The discovery of coherent polymorphism in RP nickelates is a significant finding.
• The novel stacking sequence offers new avenues for tuning material properties.
• This work may advance the understanding and application of RP materials, especially in superconductivity.