DOE OSTI2020
Ca2Mn3O8 is Pyrite-derived structured and crystallizes in the monoclinic C2/m space group. The structure is three-dimensional. Ca2+ is bonded to six O2- atoms to form distorted CaO6 pentagonal pyramids that share corners with seven MnO6 octahedra, edges with three MnO6 octahedra, and edges with two equivalent CaO6 pentagonal pyramids. The corner-sharing octahedra tilt angles range from 20–56°. There are a spread of Ca–O bond distances ranging from 2.28–2.45 Å. There are two inequivalent Mn4+ sites. In the first Mn4+ site, Mn4+ is bonded to six O2- atoms to form MnO6 octahedra that share corners with six equivalent CaO6 pentagonal pyramids, edges with four equivalent MnO6 octahedra, and edges with two equivalent CaO6 pentagonal pyramids. There is four shorter (1.93 Å) and two longer (1.95 Å) Mn–O bond length. In the second Mn4+ site, Mn4+ is bonded to six O2- atoms to form MnO6 octahedra that share corners with four equivalent CaO6 pentagonal pyramids, edges with four MnO6 octahedra, and edges with two equivalent CaO6 pentagonal pyramids. There are a spread of Mn–O bond distances ranging from 1.90–2.03 Å. There are three inequivalent O2- sites. In the first O2- site, O2- is bonded in a distorted trigonal planar geometry to one Ca2+ and two equivalent Mn4+ atoms. In the second O2- site, O2- is bonded to one Ca2+ and three Mn4+ atoms to form a mixture of distorted edge and corner-sharing OCaMn3 trigonal pyramids. In the third O2- site, O2- is bonded to two equivalent Ca2+ and two Mn4+ atoms to form a mixture of distorted edge and corner-sharing OCa2Mn2 trigonal pyramids.