Soft-Chemical Synthesis, Structure Evolution, and Insulator-to-Metal Transition in a Prototypical Metal Oxide, {\lambda}-RhO$_2$

TL;DR

This study synthesizes and characterizes a prototypical 4d transition metal oxide, {5}-RhO, revealing its structural evolution and insulator-to-metal transition driven by local Rh-Rh bonding, using soft-chemical methods and advanced characterization techniques.

Contribution

It demonstrates how soft-chemical synthesis can stabilize a simple binary oxide with unique structural and electronic properties, including a temperature-driven phase transition.

Findings

Room temperature tetragonal structure with local Rh-Rh dimers.

Structural transition to cubic phase at 345 K.

Insulator-to-metal transition linked to Rh-Rh bonding.

Abstract

${\lambda}$-RhO$_2$, a prototype 4d transition metal oxide, has been prepared by oxidative delithiation of spinel LiRh$_2$O$_4$ 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 $^7$Li 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.