DOE OSTI2020
Ca5Ga2As6 crystallizes in the orthorhombic Pbam space group. The structure is three-dimensional. there are three inequivalent Ca2+ sites. In the first Ca2+ site, Ca2+ is bonded to six As+2.67- atoms to form CaAs6 octahedra that share corners with five CaAs6 octahedra, corners with seven equivalent CaAs7 pentagonal bipyramids, edges with two equivalent CaAs6 octahedra, edges with two equivalent CaAs7 pentagonal bipyramids, edges with four equivalent GaAs4 tetrahedra, a faceface with one 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 3.00–3.04 Å. In the second Ca2+ site, Ca2+ is bonded to seven As+2.67- atoms to form CaAs7 pentagonal bipyramids that share corners with nine CaAs6 octahedra, corners with four equivalent GaAs4 tetrahedra, edges with two equivalent CaAs6 octahedra, edges with two equivalent CaAs7 pentagonal bipyramids, edges with two equivalent GaAs4 tetrahedra, faces with two CaAs6 octahedra, and faces with three equivalent CaAs7 pentagonal bipyramids. The corner-sharing octahedra tilt angles range from 29–51°. There are a spread of Ca–As bond distances ranging from 3.06–3.23 Å. In the third Ca2+ site, Ca2+ is bonded to six As+2.67- atoms to form CaAs6 octahedra that share corners with six equivalent CaAs6 octahedra, corners with four equivalent CaAs7 pentagonal bipyramids, corners with eight equivalent GaAs4 tetrahedra, edges with two equivalent CaAs6 octahedra, faces with two equivalent CaAs6 octahedra, and faces with two equivalent CaAs7 pentagonal bipyramids. The corner-sharing octahedra tilt angles range from 35–52°. There are two shorter (3.02 Å) and four longer (3.03 Å) Ca–As bond lengths. Ga3+ is bonded to four As+2.67- atoms to form GaAs4 tetrahedra that share corners with four equivalent CaAs6 octahedra, corners with four equivalent CaAs7 pentagonal bipyramids, corners with two equivalent GaAs4 tetrahedra, edges with four equivalent CaAs6 octahedra, and edges with two equivalent CaAs7 pentagonal bipyramids. The corner-sharing octahedra tilt angles range from 44–61°. There are a spread of Ga–As bond distances ranging from 2.50–2.70 Å. There are three inequivalent As+2.67- sites. In the first As+2.67- site, As+2.67- is bonded in a 6-coordinate geometry to four Ca2+ and two equivalent Ga3+ atoms. In the second As+2.67- site, As+2.67- is bonded in a 8-coordinate geometry to six Ca2+, one Ga3+, and one As+2.67- atom. The As–As bond length is 2.53 Å. In the third As+2.67- site, As+2.67- is bonded to six Ca2+ and one Ga3+ atom to form a mixture of distorted edge, face, and corner-sharing AsCa6Ga pentagonal bipyramids.