N. Bassot¹, J.-B. de Vaulx¹, A. Ahmad¹, D. Valenti¹, M. Legendre¹, C. Goujon¹, G. Garbarino², F. Alabarse³, P. Fertey⁴, S. Savvin⁵, D. Braithwaite⁶, Q. Meier¹, A. Cano¹, C. Lepoittevin¹, V. Olevano¹, P. Toulemonde¹

¹Institut Néel CNRS & UGA, 25 avenue des Martyrs 38042 Grenoble, France

²ESRF, 38000 Grenoble, France

³Elettra, Trieste, Italy

⁴SOLEIL, Saclay, France

⁵ILL, 38000 Grenoble, France

⁶UGA and CEA Grenoble IRIG-PHELIQS F-38000 Grenoble, France.

The discovery of unconventional superconductivity in thin film of hole-doped infinite‑layer nickelate Nd_1-xSr_xNiO₂ below T_c = 15-20 K in 2019 has suddenly refreshed the field of high T_c superconductivity research. So far, no superconducting bulk nickelate was discovered until the recent report of superconductivity near 80 K in La₃Ni₂O₇ (La-327) and 30K in La₄Ni₃O₁₀ (La4310) above a pressure P* ~ 15 GPa. Instead of previous 2D systems with NiO₂ planes, La_n+1Ni_nO_3n+1 compounds contain apical oxygens connecting NiO₂ planes through 3d_z² Ni orbitals which, under pressure, based on theoretical DFT calculations, contribute to supplementary pockets at the Fermi surface, in addition to the usual one derived from 3d_x²_−y² orbitals, which seem essential for the occurrence of superconductivity. For La-327, a structural transition between two orthorhombic crystal structures (Amam & Fmmm) has been reported in early works, concomitantly, around 10‑15 GPa at 300 K, accompanied by a change of the Ni‑O_apical-Ni angle from 168° to 180°, when solid or liquid pressure transmitting medium (PTM) is used.

In this study, we revisited the P, T phase diagram of La-327, on polycrystalline superconducting samples, in particular in the low T range, using x-ray and neutron diffraction, and helium, a better hydrostatic PTM. At 300 K a phase transition from orthorhombic to I4/mmm tetragonal symmetry (i.e. not the orthorhombic Fmmm one) is found around P* = 7.5-8 GPa (i.e. at lower pressure than with other PTM). We were also able to follow this transition line down to 5 K (and under pressure) where P* is increased to ~ 10-11 GPa. This work establishes more precisely the localization of the tetragonal phase which is the right symmetry adopted in the superconducting state.