TlIrO: A Pauli Paramagnetic Metal, Proximal to a Metal Insulator Transition.

A polycrystalline sample of TlIrO was synthesized by high-pressure and high-temperature methods. TlIrO crystallizes in the cubic pyrochlore structure with space group 3̅ (No. 227). The Ir oxidation state is confirmed by Ir- X-ray absorption near-edge spectroscopy. Combined temperature-dependent magnetic susceptibility, resistivity, specific heat, and DFT+DMFT calculation data show that TlIrO is a Pauli paramagnetic metal, but it is close to a metal-insulator transition. The effective ionic size of Tl is much smaller than that of Pr in metallic PrIrO; hence, TlIrO would be expected to be insulating according to the established phase diagram of the pyrochlore iridate compounds, AIrO. Our experimental and theoretical studies indicate that TlIrO is uniquely different from the current AIrO phase diagram. This uniqueness is attributed primarily to the electronic configuration difference between Tl and rare-earth ions, which plays a substantial role in determining the Ir-O-Ir bond angle, and the corresponding electrical and magnetic properties.