Pressure Effect on the Structural Transition and Suppression of the High-Spin State in the Triple-Layer T′-La4Ni3O8

We report a comprehensive high-pressure study on the triple-layer T-'-La4Ni3O8 with a suite of experimental probes, including structure determination, magnetic, and transport properties up to 50 GPa. Consistent with a recent ab inito calculation, application of hydrostatic pressure suppresses an insulator-metal spin-state transition at P-C approximate to 6 GPa. However, a low-spin metallic phase does not emerge after the high-spin state is suppressed to the lowest temperature. For P > 20 GPa, the ambient T-' structure transforms gradually to a T-dagger-type structure, which involves a structural reconstruction from fluorite La-O-2-La blocks under low pressures to rock-salt LaO-LaO blocks under high pressures. Absence of the metallic phase under pressure has been discussed in terms of local displacements of O(2-)ions in the fluorite block under pressure before a global T-dagger phase is established.