Tl₂Ir₂O₇: A Pauli Paramagnetic Metal, Proximal to a Metal Insulator Transition

A polycrystalline sample of Tl₂Ir₂O₇ was synthesized by high-pressure and high-temperature methods. Tl₂Ir₂O₇ crystallizes in the cubic pyrochlore structure with space group Fd3̅m (No. 227). The Ir⁴⁺ oxidation state is confirmed by Ir-L₃ X-ray absorption near-edge spectroscopy. Combined temperature-dependent magnetic susceptibility, resistivity, specific heat, and DFT+DMFT calculation data show that Tl₂Ir₂O₇ is a Pauli paramagnetic metal, but it is close to a metal-insulator transition. The effective ionic size of Tl³⁺ is much smaller than that of Pr³⁺ in metallic Pr₂Ir₂O₇; hence, Tl₂Ir₂O₇ would be expected to be insulating according to the established phase diagram of the pyrochlore iridate compounds, A³⁺₂Ir⁴⁺₂O₇. Our experimental and theoretical studies indicate that Tl₂Ir₂O₇ is uniquely different from the current A³₂Ir⁴⁺₂O₇ phase diagram. This uniqueness is attributed primarily to the electronic configuration difference between Tl³⁺ and rare-earth ions, which plays a substantial role in determining the Ir-O-Ir bond angle, and the corresponding electrical and magnetic properties.