DOE OSTI2020
Ca21Zn4As18 crystallizes in the monoclinic C2/m space group. The structure is three-dimensional. there are twelve inequivalent Ca2+ sites. In the first Ca2+ site, Ca2+ is bonded to six As+2.78- atoms to form distorted CaAs6 octahedra that share corners with seven CaAs6 octahedra, a cornercorner with one CaAs7 pentagonal bipyramid, corners with two ZnAs4 tetrahedra, edges with three CaAs6 octahedra, edges with two ZnAs4 tetrahedra, faces with two CaAs6 octahedra, and a faceface with one CaAs7 pentagonal bipyramid. The corner-sharing octahedra tilt angles range from 34–60°. There are a spread of Ca–As bond distances ranging from 2.90–3.17 Å. In the second Ca2+ site, Ca2+ is bonded to six As+2.78- atoms to form distorted CaAs6 octahedra that share corners with eight CaAs6 octahedra, a cornercorner with one CaAs7 pentagonal bipyramid, corners with two ZnAs4 tetrahedra, edges with two CaAs6 octahedra, an edgeedge with one ZnAs4 tetrahedra, faces with three CaAs6 octahedra, a faceface with one CaAs7 pentagonal bipyramid, and a faceface with one ZnAs4 tetrahedra. The corner-sharing octahedra tilt angles range from 35–52°. There are a spread of Ca–As bond distances ranging from 2.97–3.26 Å. In the third Ca2+ site, Ca2+ is bonded in a 6-coordinate geometry to six As+2.78- atoms. There are a spread of Ca–As bond distances ranging from 2.92–3.59 Å. In the fourth Ca2+ site, Ca2+ is bonded in a 7-coordinate geometry to six As+2.78- atoms. There are a spread of Ca–As bond distances ranging from 2.91–3.52 Å. In the fifth Ca2+ site, Ca2+ is bonded to six As+2.78- atoms to form CaAs6 octahedra that share corners with eight CaAs6 octahedra, a cornercorner with one CaAs7 pentagonal bipyramid, corners with three ZnAs4 tetrahedra, edges with two equivalent CaAs6 octahedra, faces with three CaAs6 octahedra, and a faceface with one CaAs7 pentagonal bipyramid. The corner-sharing octahedra tilt angles range from 34–60°. There are a spread of Ca–As bond distances ranging from 2.97–3.19 Å. In the sixth Ca2+ site, Ca2+ is bonded in a 6-coordinate geometry to six As+2.78- atoms. There are a spread of Ca–As bond distances ranging from 2.87–3.53 Å. In the seventh Ca2+ site, Ca2+ is bonded to six As+2.78- atoms to form distorted CaAs6 octahedra that share corners with six CaAs6 octahedra, a cornercorner with one CaAs7 pentagonal bipyramid, corners with four ZnAs4 tetrahedra, edges with three CaAs6 octahedra, an edgeedge with one ZnAs4 tetrahedra, faces with two CaAs6 octahedra, and a faceface with one CaAs7 pentagonal bipyramid. The corner-sharing octahedra tilt angles range from 36–73°. There are a spread of Ca–As bond distances ranging from 3.03–3.42 Å. In the eighth Ca2+ site, Ca2+ is bonded to six As+2.78- atoms to form distorted CaAs6 octahedra that share corners with seven CaAs6 octahedra, a cornercorner with one CaAs7 pentagonal bipyramid, corners with three ZnAs4 tetrahedra, edges with three CaAs6 octahedra, an edgeedge with one ZnAs4 tetrahedra, faces with two CaAs6 octahedra, and a faceface with one CaAs7 pentagonal bipyramid. The corner-sharing octahedra tilt angles range from 35–52°. There are a spread of Ca–As bond distances ranging from 2.91–3.34 Å. In the ninth Ca2+ site, Ca2+ is bonded to six As+2.78- atoms to form distorted CaAs6 octahedra that share corners with seven CaAs6 octahedra, corners with two CaAs7 pentagonal bipyramids, a cornercorner with one ZnAs4 tetrahedra, edges with three CaAs6 octahedra, an edgeedge with one ZnAs4 tetrahedra, faces with four CaAs6 octahedra, and a faceface with one CaAs7 pentagonal bipyramid. The corner-sharing octahedra tilt angles range from 34–50°. There are a spread of Ca–As bond distances ranging from 2.91–3.18 Å. In the tenth Ca2+ site, Ca2+ is bonded to seven As+2.78- atoms to form distorted CaAs7 pentagonal bipyramids that share corners with eight CaAs6 octahedra, corners with two ZnAs4 tetrahedra, an edgeedge with one CaAs7 pentagonal bipyramid, faces with six CaAs6 octahedra, and faces with two equivalent ZnAs4 tetrahedra. The corner-sharing octahedra tilt angles range from 39–55°. There are a spread of Ca–As bond distances ranging from 3.08–3.43 Å. In the eleventh Ca2+ site, Ca2+ is bonded to seven As+2.78- atoms to form CaAs7 pentagonal bipyramids that share corners with six CaAs6 octahedra, a cornercorner with one ZnAs4 tetrahedra, an edgeedge with one CaAs7 pentagonal bipyramid, an edgeedge with one ZnAs4 tetrahedra, faces with six CaAs6 octahedra, and faces with two equivalent ZnAs4 tetrahedra. The corner-sharing octahedra tilt angles range from 38–50°. There are a spread of Ca–As bond distances ranging from 3.05–3.46 Å. In the twelfth Ca2+ site, Ca2+ is bonded in a 7-coordinate geometry to seven As+2.78- atoms. There are a spread of Ca–As bond distances ranging from 2.87–3.66 Å. There are three inequivalent Zn2+ sites. In the first Zn2+ site, Zn2+ is bonded to four As+2.78- atoms to form ZnAs4 tetrahedra that share corners with eight CaAs6 octahedra, a cornercorner with one ZnAs4 tetrahedra, edges with four CaAs6 octahedra, an edgeedge with one ZnAs4 tetrahedra, and faces with two CaAs7 pentagonal bipyramids. The corner-sharing octahedra tilt angles range from 36–56°. There are a spread of Zn–As bond distances ranging from 2.56–2.71 Å. In the second Zn2+ site, Zn2+ is bonded to four As+2.78- atoms to form ZnAs4 tetrahedra that share corners with six CaAs6 octahedra, a cornercorner with one CaAs7 pentagonal bipyramid, corners with two equivalent ZnAs4 tetrahedra, edges with two equivalent CaAs6 octahedra, an edgeedge with one CaAs7 pentagonal bipyramid, an edgeedge with one ZnAs4 tetrahedra, and faces with two equivalent CaAs6 octahedra. The corner-sharing octahedra tilt angles range from 43–96°. There are a spread of Zn–As bond distances ranging from 2.54–2.62 Å. In the third Zn2+ site, Zn2+ is bonded to four As+2.78- atoms to form ZnAs4 tetrahedra that share corners with eight CaAs6 octahedra, corners with two CaAs7 pentagonal bipyramids, edges with two equivalent CaAs6 octahedra, and an edgeedge with one ZnAs4 tetrahedra. The corner-sharing octahedra tilt angles range from 38–61°. All Zn–As bond lengths are 2.61 Å. There are fourteen inequivalent As+2.78- sites. In the first As+2.78- site, As+2.78- is bonded in a 9-coordinate geometry to seven Ca2+ and two equivalent Zn2+ atoms. In the second As+2.78- site, As+2.78- is bonded in a 9-coordinate geometry to eight Ca2+ and one Zn2+ atom. In the third As+2.78- site, As+2.78- is bonded in a 8-coordinate geometry to eight Ca2+ atoms. In the fourth As+2.78- site, As+2.78- is bonded in a 8-coordinate geometry to eight Ca2+ atoms. In the fifth As+2.78- site, As+2.78- is bonded in a 8-coordinate geometry to seven Ca2+ and one As+2.78- atom. The As–As bond length is 2.50 Å. In the sixth As+2.78- site, As+2.78- is bonded in a 7-coordinate geometry to five Ca2+, one Zn2+, and one As+2.78- atom. The As–As bond length is 2.51 Å. In the seventh As+2.78- site, As+2.78- is bonded in a 9-coordinate geometry to eight Ca2+ and one As+2.78- atom. The As–As bond length is 2.55 Å. In the eighth As+2.78- site, As+2.78- is bonded in a 9-coordinate geometry to eight Ca2+ and one As+2.78- atom. The As–As bond length is 2.57 Å. In the ninth As+2.78- site, As+2.78- is bonded in a 8-coordinate geometry to eight Ca2+ atoms. In the tenth As+2.78- site, As+2.78- is bonded in a 9-coordinate geometry to seven Ca2+ and one Zn2+ atom. In the eleventh As+2.78- site, As+2.78- is bonded in a 8-coordinate geometry to seven Ca2+ and one Zn2+ atom. In the twelfth As+2.78- site, As+2.78- is bonded in a 8-coordinate geometry to six Ca2+ and two Zn2+ atoms. In the thirteenth As+2.78- site, As+2.78- is bonded in a 9-coordinate geometry to six Ca2+ and three Zn2+ atoms. In the fourteenth As+2.78- site, As+2.78- is bonded in a 9-coordinate geometry to eight Ca2+ and one Zn2+ atom.