  Very recently, a new superconductor with Tc = 80 K was reported in nickelate
(La3Ni2O7) at around 15 - 40 GPa conditions (Nature, 621, 493, 2023) [1], which
is the second type of unconventional superconductor, beside the cuprates, with
Tc above liquid nitrogen temperature. However, the phase diagram plotted in
this report was mostly based on the transport measurement at low temperature
and high pressure conditions, and the assumed corresponding X-ray diffraction
(XRD) results was carried out at room temperature. This encouraged us to carry
out in situ high pressure and low temperature synchrotron XRD experiments to
determine which phase is responsible for the high Tc state. In addition to the
phase transition from orthorhombic Amam structure to orthorhombic Fmmm
structure, a tetragonal phase with space group of I4/mmm was discovered when
the sample was compressed to 19 GPa at 40 K where the superconductivity takes
palce in La3Ni2O7. The calculations based on this tetragonal structure reveal
that the electronic states approached to the Fermi energy were mainly dominated
by the eg orbitals (3dz2 and 3dx2-y2) of Ni atoms, which are located in the
oxygen octahedral crystal field. The correlation between Tc and this structural
evolution, especially Ni-O octahedra regularity and the in-plane Ni-O-Ni
bonding angles, are analyzed. This work sheds new lights to identify what is
the most likely phase responsible for superconductivity in the double layered
nickelate.
