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

A polycrystalline sample of Tl 2 Ir 2 O 7 was synthesized by high-pressure and high-temperature methods. Tl 2 Ir 2 O 7 crystallizes in the cubic pyrochlore structure with space group Fd 3̅ m (No. 227). The Ir 4+ oxidation state is confirmed by Ir- L 3 X-ray absorption near-edge spectroscopy. Combined temperature-dependent magnetic susceptibility, resistivity, specific heat, and DFT+DMFT calculation data show that Tl 2 Ir 2 O 7 is a Pauli paramagnetic metal, but it is close to a metal-insulator transition. The effective ionic size of Tl 3+ is much smaller than that of Pr 3+ in metallic Pr 2 Ir 2 O 7 ; hence, Tl 2 Ir 2 O 7 would be expected to be insulating according to the established phase diagram of the pyrochlore iridate compounds, A 3+ 2 Ir 4+ 2 O 7 . Our experimental and theoretical studies indicate that Tl 2 Ir 2 O 7 is uniquely different from the current A 3 + 2 Ir 4+ 2 O 7 phase diagram. This uniqueness is attributed primarily to the electronic configuration difference between Tl 3+ and rare-earth ions, which plays a substantial role in determining the Ir-O-Ir bond angle, and the corresponding electrical and magnetic properties.