Soft-Chemical Synthesis, Structure Evolution, and Insulator-to-Metal Transition in Pyrochlore-like -RhO₂

lambda-RhO2, a prototype 4d transition metal oxide, has been prepared by the oxidative delithiation of spinel LiRh2O4 using ceric ammonium nitrate. Average-structure studies of this RhO2 polytype, including synchrotron powder X-ray diffraction and electron diffraction, indicate the room-temperature structure to be tetragonal, in space group I4(1)/amd, with a first-order structural transition to cubic Fd3m at T = 345 K on warming. Synchrotron X-ray pair distribution function analysis and Li-7 solid-state nuclear magnetic resonance measurements suggest that the room-temperature structure displays local Rh-Rh bonding. The formation of these local dimers appears to be associated with a metal-to-insulator transition with a nonmagnetic ground state, as also supported by density functional theory-based electronic structure calculations. This contribution demonstrates the power of soft chemistry to kinetically stabilize a simple binary oxide compound.