Phase Formation, Crystallography, and Ionic Conductivity of Lithium Manganese Orthosilicates

On the orthosilicate join, Li₄SiO₄-Mn₂SiO₄, the new phase Li₃Mn_0.5SiO₄ and a range of Li_{2+2 x}Mn_{1- x}SiO₄ solid solutions with ∼0.76 ≤ x ≤ 1 have been prepared by high-temperature, solid-state reaction and characterized. Li₃Mn_0.5SiO₄ is orthorhombic, space group Pnma, with a = 10.722(3) Å, b = 6.239(2) Å, and c = 5.052(3) Å. A combined analysis of X-ray and neutron powder diffraction data show that its structure is derived from the γ_II tetrahedral structural family typified by Li₃PO₄, but with additional Li⁺ in partially occupied, distorted octahedral sites. These octahedral sites are linked by a combination of edge- and face-sharing, similar to that in the nickel arsenide structure and their partial occupancy is responsible for an Li⁺ ion conductivity of, for example, ∼ 1 × 10^-8 S cm^-1 at 60 °C, with activation energy 0.93(1) eV, which is many orders of magnitude higher than that of Li₂MnSiO₄.