Pressure-induced collapse of the spin-orbital Mott state in the hyperhoneycomb iridate β-Li₂IrO₃

Hyperhoneycomb iridate β-Li₂IrO₃ is a three-dimensional analog of two-dimensional honeycomb iridates, such as α-Li₂IrO₃, which recently appeared as another playground for the physics of Kitaev-type spin liquid. β-Li₂IrO₃ shows a noncollinear spiral ordering of spin-orbital-entangled J_eff = 1/2 moments at low temperatures below 38 K, which is known to be suppressed under a pressure of similar to ∼2 GPa. In addition, a structural transition is observed at P_s ∼ 4 GPa at room temperature. Using the neutron powder diffraction technique, the crystal structure in the high-pressure phase of β-Li₂IrO₃ above P_S was refined, which indicates the formation of Ir₂ dimers on the zigzag chains, with an Ir-Ir distance of ∼ 2.66 Å, even shorter than that of metallic Ir. We argue that the strong dimerization stabilizes the bonding molecular-orbital state comprising the two local d_zx, orbitals in the Ir-O₂ -Ir bond plane, which conflicts with the equal superposition of d_xy, d_yz, and d_zx, orbitals in the J_eff = 1/2 wave function produced by strong spin-orbit coupling. The results of resonant inelastic x-ray scattering measurements and the electronic structure calculations are fully consistent with the collapse of the J_eff = 1/2 state. The competition between the spin-orbital-entangled J_eff = 1/2 state and molecular-orbital formation is most likely universal in honeycomb-based Kitaev materials.