Single-crystalline thin film of an iridium dioxide polymorph Ir2 O4 has been fabricated by the pulsed laser deposition of Li x Ir2 O4 precursor and the subsequent Li-deintercalation using soft chemistry. Ir2 O4 crystallizes in a spinel (A B2 O4) without A cations in the tetrahedral site, which is isostructural to λ -MnO2. Ir ions form a pyrochlore sublattice, which is known to give rise to a strong geometrical frustration. This Ir spinel was found to be a narrow gap insulator, in remarkable contrast to the metallic ground state of rutile-type IrO2. We argue that an interplay of a strong spin-orbit coupling and a Coulomb repulsion gives rise to an insulating ground state as in a layered perovskite Sr2 IrO4.
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