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

Hyperhoneycomb iridate beta-Li2IrO3 is a three-dimensional analog of two-dimensional honeycomb iridates, such as alpha-Li2IrO3, which recently appeared as another playground for the physics of Kitaev-type spin liquid. beta-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 similar to 4 GPa at room temperature. Using the neutron powder diffraction technique, the crystal structure in the high-pressure phase of beta-Li2IrO3 above P-S was refined, which indicates the formation of Ir-2 dimers on the zigzag chains, with an Ir-Ir distance of similar to 2.66 angstrom, 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-O-2 -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.