Phase formation, crystallography and ionic conductivity of lithium manganese orthosilicates

On the orthosilicate join, Li4SiO4–Mn2SiO4, the new phase Li3Mn0.5SiO4 and a range of Li2+2xMn1–xSiO4 solid solutions with ∼0.76 ≤ x ≤ 1 have been prepared by high-temperature, solid-state reaction and characterized. Li3Mn0.5SiO4 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 γII 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.