DOE OSTI2020
Mg(CoO2)4 crystallizes in the monoclinic Cm space group. The structure is three-dimensional. Mg2+ is bonded to five O2- atoms to form MgO5 square pyramids that share corners with six CoO6 octahedra, edges with three CoO6 octahedra, edges with two equivalent MgO5 square pyramids, and a faceface with one CoO6 octahedra. The corner-sharing octahedra tilt angles range from 11–56°. There are a spread of Mg–O bond distances ranging from 2.04–2.12 Å. There are four inequivalent Co+3.50+ sites. In the first Co+3.50+ site, Co+3.50+ is bonded to six O2- atoms to form CoO6 octahedra that share corners with four CoO6 octahedra, edges with four CoO6 octahedra, and edges with two equivalent MgO5 square pyramids. The corner-sharing octahedra tilt angles range from 49–54°. There are a spread of Co–O bond distances ranging from 1.81–1.99 Å. In the second Co+3.50+ site, Co+3.50+ is bonded to six O2- atoms to form CoO6 octahedra that share corners with four CoO6 octahedra, corners with two equivalent MgO5 square pyramids, edges with four CoO6 octahedra, and a faceface with one MgO5 square pyramid. The corner-sharing octahedra tilt angles range from 49–57°. There are a spread of Co–O bond distances ranging from 1.86–2.02 Å. In the third Co+3.50+ site, Co+3.50+ is bonded to six O2- atoms to form CoO6 octahedra that share corners with four CoO6 octahedra, corners with two equivalent MgO5 square pyramids, and edges with four CoO6 octahedra. The corner-sharing octahedra tilt angles range from 52–54°. There are a spread of Co–O bond distances ranging from 1.79–1.93 Å. In the fourth Co+3.50+ site, Co+3.50+ is bonded to six O2- atoms to form CoO6 octahedra that share corners with four CoO6 octahedra, corners with two equivalent MgO5 square pyramids, edges with four CoO6 octahedra, and an edgeedge with one MgO5 square pyramid. The corner-sharing octahedra tilt angles range from 52–57°. There are a spread of Co–O bond distances ranging from 1.85–1.99 Å. There are eight inequivalent O2- sites. In the first O2- site, O2- is bonded in a distorted trigonal planar geometry to three Co+3.50+ atoms. In the second O2- site, O2- is bonded in a distorted trigonal planar geometry to three Co+3.50+ atoms. In the third O2- site, O2- is bonded in a trigonal planar geometry to three Co+3.50+ atoms. In the fourth O2- site, O2- is bonded to one Mg2+ and three Co+3.50+ atoms to form distorted OMgCo3 trigonal pyramids that share corners with two equivalent OMgCo3 trigonal pyramids, edges with two equivalent OMg2Co3 square pyramids, and edges with two equivalent OMg2Co3 trigonal bipyramids. In the fifth O2- site, O2- is bonded in a distorted trigonal non-coplanar geometry to three Co+3.50+ atoms. In the sixth O2- site, O2- is bonded to two equivalent Mg2+ and three Co+3.50+ atoms to form distorted OMg2Co3 trigonal bipyramids that share corners with two equivalent OMg2Co3 square pyramids, an edgeedge with one OMg2Co3 square pyramid, edges with two equivalent OMg2Co3 trigonal bipyramids, and edges with two equivalent OMgCo3 trigonal pyramids. In the seventh O2- site, O2- is bonded to two equivalent Mg2+ and three Co+3.50+ atoms to form OMg2Co3 square pyramids that share corners with two equivalent OMg2Co3 trigonal bipyramids, edges with two equivalent OMg2Co3 square pyramids, an edgeedge with one OMg2Co3 trigonal bipyramid, and edges with two equivalent OMgCo3 trigonal pyramids. In the eighth O2- site, O2- is bonded in a distorted trigonal non-coplanar geometry to three Co+3.50+ atoms.