Bonding character and octahedral geometry as key determinants of solute miscibility in Ir-based oxides

The miscibility between two oxides can be estimated using the Hume-Rothery criteria, which consider crystal structure, ionic radius, and cation valence. However, rutile-based IrO2−TiO2 and IrO2−SnO2 systems show unexpectedly limited miscibility despite fully satisfying these criteria. This puzzling phenomenon has not been explained since their phase diagrams were first reported in 1967. Here we find that d0 and d10 M cations (such as 3d0 Ti4+ and 4d10 Sn4+) form primarily ionic M–O bonds due to the lack of overlap between their d states and O 2p states, while Ir–O bonds in IrO2 exhibit highly covalent bonding characteristics. Consequently, these two distinct bonding types at a shared oxygen site in the edge-sharing rutile structure lead to geometrically frustrated electron density localization, resulting in the immiscibility of M in IrO2. However, this frustration is significantly alleviated when the bonds meet in a corner-sharing octahedral geometry with a nearly linear Ir–O–M configuration. Complete solid solutions across the entire composition range can thus be achieved in cubic-perovskite-based Ir oxides with exclusively corner-sharing octahedral geometry. Our findings highlight the crucial role of bonding characteristics and octahedral geometries in determining oxide miscibility. Beyond the Hume-Rothery rules, covalent versus ionic metal–oxygen bonding and octahedral connectivity govern solute miscibility in iridium oxides, enabling complete solid solutions in corner-sharing perovskite-based iridates.