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.