Structure and properties of ordered Li2IrO3 and Li2PtO3

The structures of Li₂MO₃ (M=Ir, Pt) can be derived from the well-known Li-ion battery cathode material, LiCoO₂, through ordering of Li⁺ and M⁴⁺ ions in the layers that are exclusively occupied by cobalt in LiCoO₂. The additional cation ordering lowers the symmetry from rhombohedral (R-3m) to monoclinic (C2/m). Unlike Li₂RuO₃ no evidence is found for a further distortion of the structure driven by formation of metal-metal bonds. Thermal analysis studies coupled with both ex-situ and in-situ X-ray diffraction measurements show that these compounds are stable up to temperatures approaching 1375 K in O₂, N₂, and air, but decompose at much lower temperatures in forming gas (5% H₂:95% N₂) due to reduction of the transition metal to its elemental form. Li₂IrO₃ undergoes a slightly more complicated decomposition in reducing atmospheres, which appears to involve loss of oxygen prior to collapse of the layered Li₂IrO₃ structure. Electrical measurements, UV-visible reflectance spectroscopy and electronic band structure calculations show that Li₂IrO₃ is metallic, while Li₂PtO₃ is a semiconductor, with a band gap of 2.3 eV.