DOE OSTI2020
Sr5Sn2As6 crystallizes in the orthorhombic Pbam space group. The structure is three-dimensional. there are three inequivalent Sr2+ sites. In the first Sr2+ site, Sr2+ is bonded to six As3- atoms to form SrAs6 octahedra that share corners with five SrAs6 octahedra, corners with six equivalent SrAs7 pentagonal bipyramids, corners with four equivalent SnAs4 tetrahedra, edges with two equivalent SrAs6 octahedra, an edgeedge with one SrAs7 pentagonal bipyramid, edges with two equivalent SnAs4 tetrahedra, a faceface with one SrAs6 octahedra, and faces with two equivalent SrAs7 pentagonal bipyramids. The corner-sharing octahedra tilt angles range from 28–55°. There are a spread of Sr–As bond distances ranging from 3.14–3.25 Å. In the second Sr2+ site, Sr2+ is bonded to seven As3- atoms to form distorted SrAs7 pentagonal bipyramids that share corners with nine SrAs6 octahedra, corners with four equivalent SnAs4 tetrahedra, edges with two SrAs6 octahedra, edges with two equivalent SrAs7 pentagonal bipyramids, edges with two equivalent SnAs4 tetrahedra, faces with two equivalent SrAs6 octahedra, and faces with three equivalent SrAs7 pentagonal bipyramids. The corner-sharing octahedra tilt angles range from 23–52°. There are a spread of Sr–As bond distances ranging from 3.31–3.42 Å. In the third Sr2+ site, Sr2+ is bonded to six As3- atoms to form SrAs6 octahedra that share corners with six equivalent SrAs6 octahedra, corners with six equivalent SrAs7 pentagonal bipyramids, edges with two equivalent SrAs6 octahedra, edges with two equivalent SrAs7 pentagonal bipyramids, edges with four equivalent SnAs4 tetrahedra, and faces with two equivalent SrAs6 octahedra. The corner-sharing octahedra tilt angles range from 44–55°. There are two shorter (3.24 Å) and four longer (3.25 Å) Sr–As bond lengths. Sn4+ is bonded to four As3- atoms to form SnAs4 tetrahedra that share corners with four equivalent SrAs6 octahedra, corners with four equivalent SrAs7 pentagonal bipyramids, corners with two equivalent SnAs4 tetrahedra, edges with four SrAs6 octahedra, and edges with two equivalent SrAs7 pentagonal bipyramids. The corner-sharing octahedra tilt angles range from 42–62°. There are a spread of Sn–As bond distances ranging from 2.65–2.73 Å. There are three inequivalent As3- sites. In the first As3- site, As3- is bonded to six Sr2+ and one Sn4+ atom to form a mixture of distorted corner, edge, and face-sharing AsSr6Sn pentagonal bipyramids. In the second As3- site, As3- is bonded to six Sr2+ and one Sn4+ atom to form a mixture of distorted corner, edge, and face-sharing AsSr6Sn pentagonal bipyramids. In the third As3- site, As3- is bonded in a 6-coordinate geometry to four Sr2+ and two equivalent Sn4+ atoms.