Identifying the structure of La3Ni2O7 in the pressurized superconducting state

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.

Figures (4)

• Figure 1: Crystal structures of La$_3$Ni$_2$O$_7$. (a) The low-pressure orthorhombic unit cell ($Amam$ or $Fmmm$ space groups), characterized by a Ni--O--Ni bond angle of 168$^\circ$ or 180$^\circ$ along the $c$ axis, respectively. (b) The high-pressure tetragonal unit cell ($I4/mmm$ space group).
• Figure 2: High-pressure Raman spectra of La$_3$Ni$_2$O$_7$. Raman spectra measured from 0 to 32.7 GPa, offset vertically for clarity. At ambient pressure, the spectrum corresponds to the orthorhombic $Amam$ phase. Between 3.1 and 12.2 GPa, the system enters a mixed phase of $Amam$ and $Fmmm$. Above 14.5 GPa, a transition to the tetragonal $I4/mmm$ phase occurs, accompanied by a reorganization of phonon modes. All spectra were acquired under identical experimental conditions.
• Figure 3: Raman spectral analysis of La$_3$Ni$_2$O$_7$ under pressure. (a) Ambient-pressure spectrum with a linear combination of Lorentzian and Gaussian fits (pink). The phonon modes are deconvoluted into eight peaks (A1--A8), indicated by distinct colored areas. Vertical dashed lines indicate calculated $ab$-plane phonon frequencies. (b)--(d) Fitted spectra at 6.4, 12.2, and 19.2 GPa (300 K), respectively. Peaks associated with the $Fmmm$ phase (F1--F4) and the $I4/mmm$ phase (I1--I5) are highlighted with colored areas. The vertical lines at the bottom show calculated frequencies for each phase. (e) Spectrum measured at 17.3 GPa and 3 K. For a better visualization, the weak signals in the dashed box were magnified by a factor of (a, b) 15, (c) 7.5. The whole spectra in (d) and (e) were magnified 15 and 1.5, respectively. (f) Pressure dependence of the Raman shifts for all identified phonon modes, labeled according to their respective structural phases: $Amam$ (A1--A8), $Fmmm$ (F1--F4), and $I4/mmm$ (I1--I5). (g)--(i) Pressure evolution of the intensity and full width at half maximum (FWHM) for selected Raman-active phonons.
• Figure 4: Visualization of selected phonon mode vibrations. (a) Three-dimensional view of the atomic displacements for phonon modes A7 (magenta arrows) and A8 (azure arrows) in the $Amam$ phase. (b, c) Comparative visualization of a characteristic in-phase vibrational mode in the (b) $Fmmm$ (peak F2) and (c) $I4/mmm$ (peak I4) phases. Atomic displacements are indicated by arrows. For clarity, the illustrations are restricted to the atoms of a single bilayer within the unit cell. The red balls represent O atoms and the grey balls represent Ni atoms. La atoms are omitted as they exhibit negligible displacement in these modes. The corresponding wavenumbers are derived from theoretical calculations.