Identifying the structure of La3Ni2O7 in the pressurized superconducting state

TL;DR

This study uses high-pressure Raman spectroscopy and first-principles calculations to determine the crystal structure of La3Ni2O7 in its superconducting state, revealing a transition to a tetragonal phase that hosts superconductivity.

Contribution

It definitively establishes the tetragonal I4/mmm structure as the superconducting phase of La3Ni2O7, resolving previous structural debates.

Findings

Structural transition from orthorhombic to tetragonal phase at 14.5 GPa

Superconductivity coincides with the transition to the tetragonal phase

Phonon renormalization signals the structural transition

Abstract

The precise crystal structure of La3Ni2O7 in its high-pressure superconducting state has been a subject of intense debate, with proposed models including both orthorhombic and tetragonal symmetries. Using high-pressure Raman spectroscopy combined with frst-principles calculations, we unravel the structural evolution of La3Ni2O7 under pressure up to 32.7 GPa. We identify a clear structural transition sequence: from the orthorhombic Amam phase to a mixed Amam+Fmmm phase at 4 GPa, followed by a complete transition to the tetragonal I4/mmm phase at 14.5 GPa, which is signaled by a pronounced phonon renormalization. The emergence of bulk superconductivity is found to coincide precisely with this transition to the I4/mmm phase. Our results de nitively establish the tetragonal I4/mmm structure as the host of superconductivity in La3Ni2O7, resolving a central controversy and providing a critical foundation for understanding the superconducting mechanism in nickelates.