Pressure-induced metal-insulator and spin-state transition in low-valence layered nickelates

Ab initio calculations predict a metal-insulator transition at zero temperature to occur in La${}_4$Ni${}_3$O${}_8$ at moderate pressures as a result of a pressure-induced spin-state transition. The spin-state transition that is seen at 105 K at ambient pressure from a low-temperature high-spin state to a high-temperature low-spin state has been observed to be shifted to lower temperatures as pressure is applied. From our calculations we find that a smaller unit cell volume favors the metallic low-spin state, which becomes more stable at 5 GPa. Similar physics should take place in the related compound La${}_3$Ni${}_2$O${}_6$, but on a different energy scale, which may account for why the transition has not been observed in this material.