DOE OSTI2020
Sm4Ag2Se7 crystallizes in the monoclinic Pc space group. The structure is three-dimensional. there are four inequivalent Sm3+ sites. In the first Sm3+ site, Sm3+ is bonded to seven Se2- atoms to form distorted SmSe7 pentagonal bipyramids that share corners with three equivalent SmSe6 octahedra, corners with three equivalent SmSe7 pentagonal bipyramids, a cornercorner with one AgSe6 pentagonal pyramid, edges with two equivalent SmSe7 pentagonal bipyramids, an edgeedge with one AgSe6 pentagonal pyramid, a faceface with one SmSe6 octahedra, and a faceface with one AgSe6 pentagonal pyramid. The corner-sharing octahedra tilt angles range from 14–32°. There are a spread of Sm–Se bond distances ranging from 2.93–3.26 Å. In the second Sm3+ site, Sm3+ is bonded to seven Se2- atoms to form distorted SmSe7 pentagonal bipyramids that share a cornercorner with one SmSe6 octahedra, corners with three equivalent SmSe7 pentagonal bipyramids, a cornercorner with one AgSe6 pentagonal pyramid, an edgeedge with one SmSe6 octahedra, edges with two equivalent SmSe7 pentagonal bipyramids, and faces with two equivalent SmSe7 pentagonal bipyramids. The corner-sharing octahedral tilt angles are 15°. There are a spread of Sm–Se bond distances ranging from 2.90–3.14 Å. In the third Sm3+ site, Sm3+ is bonded in a 8-coordinate geometry to eight Se2- atoms. There are a spread of Sm–Se bond distances ranging from 2.99–3.38 Å. In the fourth Sm3+ site, Sm3+ is bonded to six Se2- atoms to form distorted SmSe6 octahedra that share corners with two equivalent SmSe6 octahedra, corners with four SmSe7 pentagonal bipyramids, corners with two equivalent AgSe6 pentagonal pyramids, an edgeedge with one SmSe7 pentagonal bipyramid, a faceface with one SmSe7 pentagonal bipyramid, and faces with two equivalent AgSe6 pentagonal pyramids. The corner-sharing octahedral tilt angles are 56°. There are a spread of Sm–Se bond distances ranging from 2.84–3.07 Å. There are two inequivalent Ag1+ sites. In the first Ag1+ site, Ag1+ is bonded in a 6-coordinate geometry to six Se2- atoms. There are a spread of Ag–Se bond distances ranging from 2.73–3.46 Å. In the second Ag1+ site, Ag1+ is bonded to six Se2- atoms to form distorted AgSe6 pentagonal pyramids that share corners with two equivalent SmSe6 octahedra, corners with two SmSe7 pentagonal bipyramids, an edgeedge with one SmSe7 pentagonal bipyramid, edges with two equivalent AgSe6 pentagonal pyramids, faces with two equivalent SmSe6 octahedra, and a faceface with one SmSe7 pentagonal bipyramid. The corner-sharing octahedra tilt angles range from 32–73°. There are a spread of Ag–Se bond distances ranging from 2.75–3.33 Å. There are seven inequivalent Se2- sites. In the first Se2- site, Se2- is bonded in a 5-coordinate geometry to five Sm3+ atoms. In the second Se2- site, Se2- is bonded in a 5-coordinate geometry to five Sm3+ and one Ag1+ atom. In the third Se2- site, Se2- is bonded to three Sm3+ and two Ag1+ atoms to form a mixture of distorted face and edge-sharing SeSm3Ag2 trigonal bipyramids. In the fourth Se2- site, Se2- is bonded in a 6-coordinate geometry to five Sm3+ and one Ag1+ atom. In the fifth Se2- site, Se2- is bonded in a 6-coordinate geometry to three Sm3+ and three Ag1+ atoms. In the sixth Se2- site, Se2- is bonded to four Sm3+ and two Ag1+ atoms to form a mixture of distorted face and edge-sharing SeSm4Ag2 octahedra. In the seventh Se2- site, Se2- is bonded in a 6-coordinate geometry to three Sm3+ and three Ag1+ atoms.