DOE OSTI2020
CaCo2O4 crystallizes in the triclinic P1 space group. The structure is three-dimensional. there are two inequivalent Ca2+ sites. In the first Ca2+ site, Ca2+ is bonded in a 7-coordinate geometry to seven O2- atoms. There are a spread of Ca–O bond distances ranging from 2.29–2.67 Å. In the second Ca2+ site, Ca2+ is bonded in a 7-coordinate geometry to seven O2- atoms. There are a spread of Ca–O bond distances ranging from 2.28–2.68 Å. There are four inequivalent Co3+ sites. In the first Co3+ site, Co3+ is bonded to five O2- atoms to form CoO5 square pyramids that share corners with four CoO6 octahedra and edges with two equivalent CoO5 square pyramids. The corner-sharing octahedra tilt angles range from 53–54°. There are a spread of Co–O bond distances ranging from 1.84–2.01 Å. In the second Co3+ site, Co3+ is bonded to six O2- atoms to form CoO6 octahedra that share corners with four CoO5 square pyramids and edges with four CoO6 octahedra. There are a spread of Co–O bond distances ranging from 1.86–2.00 Å. In the third Co3+ site, Co3+ is bonded to six O2- atoms to form CoO6 octahedra that share corners with four CoO5 square pyramids and edges with four CoO6 octahedra. There are a spread of Co–O bond distances ranging from 1.86–2.00 Å. In the fourth Co3+ site, Co3+ is bonded to five O2- atoms to form CoO5 square pyramids that share corners with four CoO6 octahedra and edges with two equivalent CoO5 square pyramids. The corner-sharing octahedra tilt angles range from 53–54°. There are a spread of Co–O bond distances ranging from 1.85–2.00 Å. There are eight inequivalent O2- sites. In the first O2- site, O2- is bonded in a trigonal planar geometry to three Co3+ atoms. In the second O2- site, O2- is bonded to one Ca2+ and three Co3+ atoms to form distorted OCaCo3 trigonal pyramids that share corners with two equivalent OCa2Co3 square pyramids, corners with two equivalent OCaCo3 trigonal pyramids, and edges with three OCa2Co3 square pyramids. In the third O2- site, O2- is bonded to one Ca2+ and three Co3+ atoms to form distorted OCaCo3 trigonal pyramids that share corners with two equivalent OCa2Co3 square pyramids, corners with two equivalent OCaCo3 trigonal pyramids, and edges with three OCa2Co3 square pyramids. In the fourth O2- site, O2- is bonded in a trigonal planar geometry to three Co3+ atoms. In the fifth O2- site, O2- is bonded in a 6-coordinate geometry to four Ca2+ and two equivalent Co3+ atoms. In the sixth O2- site, O2- is bonded to two equivalent Ca2+ and three Co3+ atoms to form OCa2Co3 square pyramids that share corners with two equivalent OCaCo3 trigonal pyramids, edges with four OCa2Co3 square pyramids, and edges with three OCaCo3 trigonal pyramids. In the seventh O2- site, O2- is bonded in a 6-coordinate geometry to four Ca2+ and two equivalent Co3+ atoms. In the eighth O2- site, O2- is bonded to two equivalent Ca2+ and three Co3+ atoms to form OCa2Co3 square pyramids that share corners with two equivalent OCaCo3 trigonal pyramids, edges with four OCa2Co3 square pyramids, and edges with three OCaCo3 trigonal pyramids.