Pressure-Induced Phase Transitions in Bilayer La ₃ Ni ₂ O ₇

La3Ni2O7 differs from expectations in part because at ambient pressure, it exists in two polymorphs: an unconventional crystal structure with alternating layers of single- and triple-layered nickel–oxygen octahedra (i.e., La2NiO4 plus La4Ni3O10, with the so-called 1313 crystal structure) and a classical double-layer Ruddlesden–Popper phase (i.e., La3Ni2O7, with the so-called 2222 crystal structure, which dominates at high pressure). In this study, we report the pressure-dependent structural and electrical resistive properties of single crystals of the classical double-layered La3Ni2O7, grown at slightly elevated pressure using the floating-zone method. Structural characterization under pressures up to 15.4 GPa reveals a gradual transition from orthorhombic to tetragonal symmetry that is completed at 12–14 GPa. Additionally, we present pressure and field-dependent electrical resistance measurements up to 27.4 GPa, from which we construct a phase diagram. Although a resistive transition at 80 K is observed at high pressures, the resistive hallmarks of superconductivity are not.