Polymorphism in Ruddlesden-Popper $La_{3}Ni_{2}O_{7}$: Discovery of a Hidden Phase with Distinctive Layer Stacking

TL;DR

This paper reports the discovery of a novel polymorph of La3Ni2O7 with a distinctive layer stacking sequence, exhibiting metallic behavior and a charge density wave transition, potentially impacting the understanding of RP materials and high-temperature superconductivity.

Contribution

It introduces a new long-range polymorph of La3Ni2O7 with a unique stacking sequence, expanding the known structural diversity of Ruddlesden-Popper oxides.

Findings

Discovered a new stacking sequence in La3Ni2O7 polymorph.

Observed metallic behavior with a charge density wave transition at 134 K.

Identified evidence of a competing space group variant.

Abstract

We report the discovery of a novel form of Ruddlesden-Popper (RP) oxide, which 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 $La_{3}Ni_{2}O_{7}$adopts a novel stacking sequence in which single and trilayer blocks of $NiO_{6}$ octahedra alternate in a 1313 sequence. Crystals of this new polymorph are described in space group Cmmm, although we note evidence for a competing Imcm variant. Transport measurements at ambient pressure reveal metallic character with evidence of a charge density wave transition with onset at T = 134 K, which lies intermediate between that of the standard 2222 polymorph of $La_{3}Ni_{2}O_{7}$ (space group Amam) and the trilayer RP phase, $La_{4}Ni_{3}O_{10}$. 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 with $T_{c}$ as high as 80 K above 14 GPa in bilayer $La_{3}Ni_{2}O_{7}$.