Soft-Chemical Synthesis, Structure Evolution, and Insulator-to-Metal Transition in a Prototypical Metal Oxide, λ-RhO_2

λ-RhO_2, a prototype 4d transition metal oxide, has been prepared by oxidative delithiation of spinel LiRh_2O_4 using ceric ammonium nitrate. Average-structure studies of this RhO_2 polytype, including synchrotron powder X-ray diffraction and electron diffraction, indicate the room temperature structure to be tetragonal, in the space group I41/amd, with a first-order structural transition to cubic Fd-3m at T = 345 K on warming. Synchrotron X-ray pair distribution function analysis and ^7Li 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 non-magnetic 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 surprisingly simple binary oxide compound.