Stabilization of a metastable tunnel-structured orthorhombic phase of VO₂ upon iridium doping

Metastable compounds accessible through kinetic stabilization or under conditions of constrained equilibrium represent a richly varied landscape of structures, properties, and function that are oftentimes entirely inaccessible in thermodynamic minima. The multiple redox states of vanadium and the ability to modulate the connectivity of vanadium and oxygen atoms yields a rich diversity of structures for binary vanadium oxides. Here, it is demonstrated that an orthorhombic quasi-1D polymorph of VO₂, characterized by extended tunnels with a rectangular cross-section, can be stabilized through the substitutional doping of iridium on the vanadium sublattice. The obtained structure is considerably distorted from a previously reported paramontroseite mineral phase. The metastable phase is obtained in nanoplatelet form and is stable with respect to the energetically proximate monoclinic/rutile thermodynamic minima up to a temperature of 350 C. The open framework structure and the accessibility of multiple redox states at the vanadium center suggests that the polymorph could potentially serve as an intercalation host. Beyond the solubility limit of iridium on the vanadium sublattice (1.28-3.15 at.% depending on the precursor concentration), metallic Ir nanocrystals are found to be homogeneously dispersed on the surfaces of the nanoplatelets indicating a strong interaction between the metal nanocrystals and oxide lattice. (copyright 2018 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim)