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IrO3 is alpha Rhenium trioxide structured and crystallizes in the cubic Pm-3m space group. The structure is three-dimensional. Ir is bonded to six equivalent O atoms to form corner-sharing IrO6 octahedra. The corner-sharing octahedral tilt angles are 0°. All Ir–O bond lengths are 1.90 Å. O is bonded in a linear geometry to two equivalent Ir atoms.
IrO3 crystallizes in the orthorhombic Amm2 space group. The structure is three-dimensional. Ir is bonded to six O atoms to form corner-sharing IrO6 octahedra. The corner-sharing octahedra tilt angles range from 2–41°. There are a spread of Ir–O bond distances ranging from 1.88–1.97 Å. There are four inequivalent O sites. In the first O site, O is bonded in a bent 150 degrees geometry to two equivalent Ir atoms. In the second O site, O is bonded in a bent 150 degrees geometry to two equivalent Ir atoms. In the third O site, O is bonded in a bent 150 degrees geometry to two equivalent Ir atoms. In the fourth O site, O is bonded in a linear geometry to two equivalent Ir atoms.
Ba2Ir3O9 crystallizes in the cubic I23 space group. The structure is three-dimensional. Ba2+ is bonded in a 9-coordinate geometry to nine O2- atoms. There are a spread of Ba–O bond distances ranging from 2.65–3.04 Å. Ir+4.67+ is bonded to six O2- atoms to form a mixture of edge and corner-sharing IrO6 octahedra. The corner-sharing octahedral tilt angles are 49°. There are a spread of Ir–O bond distances ranging from 1.98–2.01 Å. There are two inequivalent O2- sites. In the first O2- site, O2- is bonded in a 4-coordinate geometry to two equivalent Ba2+ and two equivalent Ir+4.67+ atoms. In the second O2- site, O2- is bonded in a distorted L-shaped geometry to two equivalent Ba2+ and two equivalent Ir+4.67+ atoms.
CaCu3Ir4O12 crystallizes in the cubic Im-3 space group. The structure is three-dimensional. Ca2+ is bonded to twelve equivalent O2- atoms to form CaO12 cuboctahedra that share faces with eight equivalent IrO6 octahedra. All Ca–O bond lengths are 2.63 Å. Ir+4.75+ is bonded to six equivalent O2- atoms to form IrO6 octahedra that share corners with six equivalent IrO6 octahedra and faces with two equivalent CaO12 cuboctahedra. The corner-sharing octahedral tilt angles are 43°. All Ir–O bond lengths are 2.02 Å. Cu1+ is bonded in a square co-planar geometry to four equivalent O2- atoms. All Cu–O bond lengths are 1.94 Å. O2- is bonded in a 4-coordinate geometry to one Ca2+, two equivalent Ir+4.75+, and one Cu1+ atom.
The discovery of high-performing and stable materials for sustainable energy applications is a pressing goal in catalysis and materials science. Understanding the relationship between a material’s structure and functionality is an important step in the process, such that viable polymorphs for a given chemical composition need to be identified. Machine-learning-based surrogate models have the potential to accelerate the search for polymorphs that target specific applications. Herein, we report a readily generalizable active-learning (AL) accelerated algorithm for identification of electrochemically stable iridium oxide polymorphs of IrO 2 and IrO 3 . The search is coupled to a subsequent analysis of the electrochemical stability of the discovered structures for the acidic oxygen evolution reaction (OER). Structural candidates are generated by identifying all 956 structurally unique AB2 and AB3 prototypes in existing materials databases (more than 38000). Next, using an active learning approach, we find 196 IrO 2 polymorphs within the thermodynamic amorphous synthesizability limit and reaffirm the global stability of the rutile structure. We find 75 synthesizable IrO 3 polymorphs and report a previously unknown FeF 3 -type structure as the most stable, termed α-IrO 3 . To test the algorithms performance, we compare to a random search of the candidate space and report at least a 2-fold increase in the rate of discovery. Additionally, the AL approach can acquire the most stable polymorphs of IrO 2 and IrO 3 with fewer than 30 density functional theory optimizations. Analysis of the structural properties of the discovered polymorphs reveals that octahedral local coordination environments are preferred for nearly all low-energy structures. Subsequent Pourbaix Ir–H 2 O analysis shows that α-IrO3 is the globally stable solid phase under acidic OER conditions and supersedes the stability of rutile IrO 2 . Calculation of theoretical OER surface activities reveal ideal weaker binding of the OER intermediates on α-IrO 3 than on any other considered iridium oxide. We emphasize that the proposed AL algorithm can be easily generalized to search for any binary metal oxide structure with a defined stoichiometry.