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

A recent article ( <i>J. Am. Chem. Soc.</i> 2024, 146, 7506-7514) details a pressure-temperature (<i>P</i>-<i>T</i>) phase diagram for the Ruddlesden-Popper bilayer nickelate La₃Ni₂O₇ (LNO-2222) using synchrotron X-ray diffraction. This study identifies a phase transition from <i>Amam</i> (#63) to <i>Fmmm</i> (#69) within the temperature range of 104-120 K under initial pressure and attributes the <i>I</i>4/<i>mmm</i> (#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 <i>Amam</i>-<i>Fmmm</i> phase boundary, we observe an enhancement in the <i>Amam</i> 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.