Tracing the horizon of tetragonal-to-monoclinic distortion in pressurized trilayer nickelate La4Ni3O10

The crux of understanding the superconducting mechanism in pressurized Ruddlesden-Popper nickelates hinges on elucidating their structural phases. Under ambient conditions, the trilayer nickelate La4Ni3O10 stabilizes in a twinned monoclinic structure with space group P21/c. Upon heating, it undergoes a structural transition to the tetragonal I4/mmm phase at Ts ≈ 1030 K, while a second transition associated with the onset of density-wave (DW) ordering emerges upon cooling below TDW ≈ 135 K. Here, from pressure-temperature x-ray diffraction (XRD) on high quality flux-grown single crystals we demonstrate a direct tetragonal-to-monoclinic transition without an intermediate orthorhombic Bmab phase. Ab initio density-functional theory calculations as a function of pressure corroborate the experimental observations. The tetragonal-to-monoclinic transition unfolds as the formation of a two-fold superstructure, as evidenced by the emergence of commensurate superlattice reflections and can be progressively suppressed from 1030 K down to 20 K under 14 GPa. Notably, from XRD we establish the observation of weak incommensurate satellite reflections associated with the DW ordering in flux-grown samples, as previous findings were confined only to crystals grown by the floating-zone technique. This is further reinforced by Raman spectroscopy that reveal the emergence of additional phonon modes below 130 K, concomitant with the onset of the incommensurate DW state. Understanding the superconducting mechanism in pressurized Ruddlesden-Popper nickelates requires insight into their structural phases. Here, the authors use pressure-temperature XRD and ab initio calculations to reveal a direct tetragonal-to-monoclinic transition in La4Ni3O10, highlighting the suppression of this transition under pressure and the presence of intrinsic incommensurate density-wave ordering.