Trigonal polymorph of Li2MnO3

We report the discovery of a trigonal polymorph of the prospective Li-ion battery material ${\mathrm{Li}}_{2}\mathrm{Mn}{\mathrm{O}}_{3}$ and its synthesis in bulk, single-crystal form. Crystal growth of trigonal ${\mathrm{Li}}_{2}\mathrm{Mn}{\mathrm{O}}_{3}$ is strongly dependent upon the quality of a polycrystalline $\mathrm{Li}\mathrm{Mn}{\mathrm{O}}_{2}$ precursor consumed in the synthesis process. The crystal structure of the trigonal phase is composed of ordered honeycomb layers of $\mathrm{Li}{\mathrm{O}}_{6}$ and $\mathrm{Mn}{\mathrm{O}}_{6}$ octahedra segregated by layers of $\mathrm{Li}{\mathrm{O}}_{6}$ octahedra and represents an ordered stacking variant of the known monoclinic polymorph. Diffuse reflectance spectroscopy reveals a direct optical gap of $2.47\ifmmode\pm\else\textpm\fi{}0.11$ eV and a series of charge excitations that are well explained by the expected ${\mathrm{Mn}}^{4+}\phantom{\rule{4pt}{0ex}}3{d}^{3}$ valence. Density functional theory calculations are in excellent agreement with the spectroscopic measurements and find a near degeneracy in the formation energies of the two polymorphs. Our results suggest that the trigonal structure resolves the compositional and structural disorder often manifested in the monoclinic phase.