Nickel valence in La3Ni2O7 stays close to 2.5+ from ambient pressure to 40 GPa at 20 K, so pressure-induced superconductivity is tied to a structural transition rather than a change in nickel charge.
Pressure-Induced Phase Transitions in Bilayer La$_3$Ni$_2$O$_7$
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abstract
La$_3$Ni$_2$O$_7$ exists in two polymorphs: an unconventional structure with alternating layers of single- and triple-layered nickel-oxygen octahedra, and a classical double-layered Ruddlesden-Popper phase. In this study, we report the growth of single crystals of classical double-layered La$_3$Ni$_2$O$_7$ using the floating zone method. Structural characterization under pressures up to 15.4 GPa reveals a gradual transition from orthorhombic to tetragonal symmetry near 12 GPa. Additionally, we present pressure and field-dependent electrical resistance measurements under pressures as high as 27.4 GPa, from which we construct a phase diagram.
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Low-temperature mean valence of nickel ions in pressurized La$_3$Ni$_2$O$_7$
Nickel valence in La3Ni2O7 stays close to 2.5+ from ambient pressure to 40 GPa at 20 K, so pressure-induced superconductivity is tied to a structural transition rather than a change in nickel charge.