DOE OSTI2020
Tm4CdSe7 crystallizes in the monoclinic Cm space group. The structure is three-dimensional. there are four inequivalent Tm3+ sites. In the first Tm3+ site, Tm3+ is bonded to seven Se2- atoms to form distorted TmSe7 pentagonal bipyramids that share corners with four TmSe6 octahedra, corners with four equivalent CdSe6 octahedra, an edgeedge with one CdSe6 octahedra, edges with three equivalent TmSe6 octahedra, edges with two equivalent TmSe7 pentagonal bipyramids, and faces with two equivalent TmSe7 pentagonal bipyramids. The corner-sharing octahedra tilt angles range from 23–68°. There are a spread of Tm–Se bond distances ranging from 2.82–3.15 Å. In the second Tm3+ site, Tm3+ is bonded to seven Se2- atoms to form distorted TmSe7 pentagonal bipyramids that share corners with two equivalent CdSe6 octahedra, corners with six TmSe6 octahedra, edges with four TmSe6 octahedra, edges with two equivalent TmSe7 pentagonal bipyramids, and faces with two equivalent TmSe7 pentagonal bipyramids. The corner-sharing octahedra tilt angles range from 22–68°. There are a spread of Tm–Se bond distances ranging from 2.81–3.08 Å. In the third Tm3+ site, Tm3+ is bonded to six Se2- atoms to form TmSe6 octahedra that share a cornercorner with one TmSe6 octahedra, a cornercorner with one CdSe6 octahedra, corners with four TmSe7 pentagonal bipyramids, edges with two equivalent TmSe6 octahedra, and edges with six TmSe7 pentagonal bipyramids. The corner-sharing octahedra tilt angles range from 54–56°. There are a spread of Tm–Se bond distances ranging from 2.76–2.84 Å. In the fourth Tm3+ site, Tm3+ is bonded to six Se2- atoms to form TmSe6 octahedra that share a cornercorner with one TmSe6 octahedra, corners with two equivalent CdSe6 octahedra, corners with six TmSe7 pentagonal bipyramids, edges with two equivalent TmSe6 octahedra, edges with three equivalent CdSe6 octahedra, and an edgeedge with one TmSe7 pentagonal bipyramid. The corner-sharing octahedra tilt angles range from 3–54°. There are a spread of Tm–Se bond distances ranging from 2.74–2.88 Å. Cd2+ is bonded to six Se2- atoms to form CdSe6 octahedra that share corners with three TmSe6 octahedra, corners with six TmSe7 pentagonal bipyramids, edges with two equivalent CdSe6 octahedra, edges with three equivalent TmSe6 octahedra, and an edgeedge with one TmSe7 pentagonal bipyramid. The corner-sharing octahedra tilt angles range from 3–56°. There are a spread of Cd–Se bond distances ranging from 2.77–2.90 Å. There are seven inequivalent Se2- sites. In the first Se2- site, Se2- is bonded to four Tm3+ and one Cd2+ atom to form distorted SeTm4Cd trigonal bipyramids that share corners with five SeTm3Cd tetrahedra, corners with four SeTm5 trigonal bipyramids, an edgeedge with one SeTm4 tetrahedra, and edges with five SeTm4Cd trigonal bipyramids. In the second Se2- site, Se2- is bonded to three Tm3+ and two equivalent Cd2+ atoms to form distorted SeTm3Cd2 trigonal bipyramids that share corners with five SeTm3Cd tetrahedra, corners with four SeTm5 trigonal bipyramids, an edgeedge with one SeTm3Cd tetrahedra, and edges with five SeTm4Cd trigonal bipyramids. In the third Se2- site, Se2- is bonded to five Tm3+ atoms to form distorted SeTm5 trigonal bipyramids that share corners with four equivalent SeTm3Cd tetrahedra, corners with six SeTm4Cd trigonal bipyramids, edges with three SeTm3Cd tetrahedra, and edges with six SeTm3Cd2 trigonal bipyramids. In the fourth Se2- site, Se2- is bonded to five Tm3+ atoms to form distorted SeTm5 trigonal bipyramids that share corners with four equivalent SeTm4 tetrahedra, corners with six SeTm4Cd trigonal bipyramids, edges with three SeTm3Cd tetrahedra, and edges with six SeTm4Cd trigonal bipyramids. In the fifth Se2- site, Se2- is bonded to three Tm3+ and one Cd2+ atom to form distorted SeTm3Cd tetrahedra that share corners with three SeTm3Cd tetrahedra, corners with nine SeTm4Cd trigonal bipyramids, and edges with four SeTm3Cd2 trigonal bipyramids. In the sixth Se2- site, Se2- is bonded to four Tm3+ atoms to form distorted SeTm4 tetrahedra that share corners with three SeTm3Cd tetrahedra, corners with nine SeTm4Cd trigonal bipyramids, and edges with four SeTm4Cd trigonal bipyramids. In the seventh Se2- site, Se2- is bonded in a rectangular see-saw-like geometry to two equivalent Tm3+ and two equivalent Cd2+ atoms.