DOE OSTI2020
CaCr2O4 is Spinel-like structured and crystallizes in the monoclinic Cm space group. The structure is three-dimensional. there are eight inequivalent Ca2+ sites. In the first Ca2+ site, Ca2+ is bonded to four O2- atoms to form CaO4 tetrahedra that share corners with three CaO6 octahedra and corners with nine CrO6 octahedra. The corner-sharing octahedra tilt angles range from 56–72°. There are a spread of Ca–O bond distances ranging from 2.16–2.28 Å. In the second Ca2+ site, Ca2+ is bonded to six O2- atoms to form CaO6 octahedra that share corners with three CaO4 tetrahedra, corners with three CrO4 tetrahedra, and edges with six CrO6 octahedra. There are three shorter (2.25 Å) and three longer (2.31 Å) Ca–O bond lengths. In the third Ca2+ site, Ca2+ is bonded to four O2- atoms to form CaO4 tetrahedra that share corners with three CaO6 octahedra and corners with nine CrO6 octahedra. The corner-sharing octahedra tilt angles range from 55–70°. There are a spread of Ca–O bond distances ranging from 2.15–2.28 Å. In the fourth Ca2+ site, Ca2+ is bonded to six O2- atoms to form CaO6 octahedra that share corners with two equivalent CaO4 tetrahedra, corners with four CrO4 tetrahedra, an edgeedge with one CaO6 octahedra, and edges with five CrO6 octahedra. There are a spread of Ca–O bond distances ranging from 2.23–2.33 Å. In the fifth Ca2+ site, Ca2+ is bonded to six O2- atoms to form CaO6 octahedra that share corners with six CrO4 tetrahedra, edges with two CaO6 octahedra, and edges with four equivalent CrO6 octahedra. There are four shorter (2.27 Å) and two longer (2.32 Å) Ca–O bond lengths. In the sixth Ca2+ site, Ca2+ is bonded to six O2- atoms to form CaO6 octahedra that share corners with six CrO4 tetrahedra, edges with two CaO6 octahedra, and edges with four CrO6 octahedra. There are a spread of Ca–O bond distances ranging from 2.27–2.33 Å. In the seventh Ca2+ site, Ca2+ is bonded to six O2- atoms to form CaO6 octahedra that share a cornercorner with one CaO4 tetrahedra, corners with five CrO4 tetrahedra, an edgeedge with one CaO6 octahedra, and edges with five CrO6 octahedra. There are a spread of Ca–O bond distances ranging from 2.23–2.31 Å. In the eighth Ca2+ site, Ca2+ is bonded to six O2- atoms to form CaO6 octahedra that share corners with six CrO4 tetrahedra, edges with two CaO6 octahedra, and edges with four CrO6 octahedra. There are a spread of Ca–O bond distances ranging from 2.27–2.33 Å. There are twelve inequivalent Cr3+ sites. In the first Cr3+ site, Cr3+ is bonded to six O2- atoms to form CrO6 octahedra that share corners with three CaO4 tetrahedra, corners with three CrO4 tetrahedra, edges with two equivalent CaO6 octahedra, and edges with four CrO6 octahedra. There are a spread of Cr–O bond distances ranging from 1.97–2.15 Å. In the second Cr3+ site, Cr3+ is bonded to six O2- atoms to form CrO6 octahedra that share corners with three equivalent CaO4 tetrahedra, corners with three equivalent CrO4 tetrahedra, edges with two CaO6 octahedra, and edges with four CrO6 octahedra. There are a spread of Cr–O bond distances ranging from 1.96–2.12 Å. In the third Cr3+ site, Cr3+ is bonded to four O2- atoms to form CrO4 tetrahedra that share corners with six CaO6 octahedra and corners with six CrO6 octahedra. The corner-sharing octahedra tilt angles range from 51–63°. There are a spread of Cr–O bond distances ranging from 1.90–2.12 Å. In the fourth Cr3+ site, Cr3+ is bonded to six O2- atoms to form CrO6 octahedra that share corners with two equivalent CaO4 tetrahedra, corners with four CrO4 tetrahedra, edges with three CaO6 octahedra, and edges with three CrO6 octahedra. There are a spread of Cr–O bond distances ranging from 1.96–2.13 Å. In the fifth Cr3+ site, Cr3+ is bonded to four O2- atoms to form CrO4 tetrahedra that share corners with three CaO6 octahedra and corners with nine CrO6 octahedra. The corner-sharing octahedra tilt angles range from 56–62°. There are a spread of Cr–O bond distances ranging from 1.91–2.37 Å. In the sixth Cr3+ site, Cr3+ is bonded to four O2- atoms to form CrO4 tetrahedra that share corners with six CaO6 octahedra and corners with six CrO6 octahedra. The corner-sharing octahedra tilt angles range from 49–65°. There are a spread of Cr–O bond distances ranging from 1.89–2.05 Å. In the seventh Cr3+ site, Cr3+ is bonded to six O2- atoms to form CrO6 octahedra that share corners with six CrO4 tetrahedra, edges with two equivalent CrO6 octahedra, and edges with four CaO6 octahedra. There are a spread of Cr–O bond distances ranging from 2.04–2.10 Å. In the eighth Cr3+ site, Cr3+ is bonded to four O2- atoms to form CrO4 tetrahedra that share corners with six CaO6 octahedra and corners with six CrO6 octahedra. The corner-sharing octahedra tilt angles range from 50–65°. There are a spread of Cr–O bond distances ranging from 1.90–2.09 Å. In the ninth Cr3+ site, Cr3+ is bonded to six O2- atoms to form CrO6 octahedra that share a cornercorner with one CaO4 tetrahedra, corners with five CrO4 tetrahedra, edges with three CaO6 octahedra, and edges with three CrO6 octahedra. There are a spread of Cr–O bond distances ranging from 1.98–2.15 Å. In the tenth Cr3+ site, Cr3+ is bonded to four O2- atoms to form CrO4 tetrahedra that share corners with six CaO6 octahedra and corners with six CrO6 octahedra. The corner-sharing octahedra tilt angles range from 50–63°. There are a spread of Cr–O bond distances ranging from 1.90–2.11 Å. In the eleventh Cr3+ site, Cr3+ is bonded to six O2- atoms to form CrO6 octahedra that share corners with three equivalent CaO4 tetrahedra, corners with three equivalent CrO4 tetrahedra, edges with two CaO6 octahedra, and edges with four CrO6 octahedra. There are a spread of Cr–O bond distances ranging from 1.97–2.10 Å. In the twelfth Cr3+ site, Cr3+ is bonded to four O2- atoms to form CrO4 tetrahedra that share corners with three CaO6 octahedra and corners with nine CrO6 octahedra. The corner-sharing octahedra tilt angles range from 55–60°. There are a spread of Cr–O bond distances ranging from 1.91–2.42 Å. There are twenty-four inequivalent O2- sites. In the first O2- site, O2- is bonded to two Ca2+ and two Cr3+ atoms to form distorted OCa2Cr2 trigonal pyramids that share a cornercorner with one OCa2Cr2 tetrahedra, corners with four OCr4 trigonal pyramids, and edges with three OCa2Cr2 trigonal pyramids. In the second O2- site, O2- is bonded to two Ca2+ and two equivalent Cr3+ atoms to form distorted OCa2Cr2 trigonal pyramids that share corners with five OCr4 trigonal pyramids and edges with three OCa2Cr2 trigonal pyramids. In the third O2- site, O2- is bonded to one Ca2+ and three Cr3+ atoms to form distorted OCaCr3 trigonal pyramids that share a cornercorner with one OCa2Cr2 tetrahedra, corners with five OCr4 trigonal pyramids, and edges with three OCa2Cr2 trigonal pyramids. In the fourth O2- site, O2- is bonded to four Cr3+ atoms to form distorted corner-sharing OCr4 trigonal pyramids. In the fifth O2- site, O2- is bonded to two Ca2+ and two Cr3+ atoms to form distorted OCa2Cr2 trigonal pyramids that share corners with four OCa2Cr2 trigonal pyramids and edges with two OCaCr3 trigonal pyramids. In the sixth O2- site, O2- is bonded in a rectangular see-saw-like geometry to one Ca2+ and three Cr3+ atoms. In the seventh O2- site, O2- is bonded in a rectangular see-saw-like geometry to one Ca2+ and three Cr3+ atoms. In the eighth O2- site, O2- is bonded in a rectangular see-saw-like geometry to one Ca2+ and three Cr3+ atoms. In the ninth O2- site, O2- is bonded to one Ca2+ and three Cr3+ atoms to form a mixture of distorted edge and corner-sharing OCaCr3 trigonal pyramids. In the tenth O2- site, O2- is bonded in a rectangular see-saw-like geometry to two Ca2+ and two Cr3+ atoms. In the eleventh O2- site, O2- is bonded in a rectangular see-saw-like geometry to one Ca2+ and three Cr3+ atoms. In the twelfth O2- site, O2- is bonded in a rectangular see-saw-like geometry to two Ca2+ and two Cr3+ atoms. In the thirteenth O2- site, O2- is bonded in a rectangular see-saw-like geometry to one Ca2+ and three Cr3+ atoms. In the fourteenth O2- site, O2- is bonded in a distorted rectangular see-saw-like geometry to one Ca2+ and three Cr3+ atoms. In the fifteenth O2- site, O2- is bonded in a rectangular see-saw-like geometry to one Ca2+ and three Cr3+ atoms. In the sixteenth O2- site, O2- is bonded in a rectangular see-saw-like geometry to one Ca2+ and three Cr3+ atoms. In the seventeenth O2- site, O2- is bonded in a rectangular see-saw-like geometry to two Ca2+ and two Cr3+ atoms. In the eighteenth O2- site, O2- is bonded in a rectangular see-saw-like geometry to two Ca2+ and two Cr3+ atoms. In the nineteenth O2- site, O2- is bonded to two Ca2+ and two equivalent Cr3+ atoms to form corner-sharing OCa2Cr2 tetrahedra. In the twentieth O2- site, O2- is bonded in a rectangular see-saw-like geometry to one Ca2+ and three Cr3+ atoms. In the twenty-first O2- site, O2- is bonded in a rectangular see-saw-like geometry to one Ca2+ and three Cr3+ atoms. In the twenty-second O2- site, O2- is bonded to four Cr3+ atoms to form distorted corner-sharing OCr4 trigonal pyramids. In the twenty-third O2- site, O2- is bonded in a rectangular see-saw-like geometry to one Ca2+ and three Cr3+ atoms. In the twenty-fourth O2- site, O2- is bonded in a rectangular see-saw-like geometry to one Ca2+ and three Cr3+ atoms.