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

We report a comprehensive high-pressure study on the triple-layer ${T}^{\ensuremath{'}}\mathrm{\text{\ensuremath{-}}}{\mathrm{La}}_{4}{\mathrm{Ni}}_{3}{\mathrm{O}}_{8}$ 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 ${P}_{c}\ensuremath{\approx}6\text{ }\text{ }\mathrm{GPa}$. However, a low-spin metallic phase does not emerge after the high-spin state is suppressed to the lowest temperature. For $P>20\text{ }\text{ }\mathrm{GPa}$, the ambient ${T}^{\ensuremath{'}}$ structure transforms gradually to a ${T}^{\ifmmode\dagger\else\textdagger\fi{}}$-type structure, which involves a structural reconstruction from fluorite $\mathrm{La}\mathrm{\text{\ensuremath{-}}}{\mathrm{O}}_{2}\mathrm{\text{\ensuremath{-}}}\mathrm{La}$ 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 ${\mathrm{O}}^{2\ensuremath{-}}$ ions in the fluorite block under pressure before a global ${T}^{\ifmmode\dagger\else\textdagger\fi{}}$ phase is established.