DOE OSTI2020
MgTm2Se4 crystallizes in the orthorhombic Pca2_1 space group. The structure is three-dimensional. there are two inequivalent Mg2+ sites. In the first Mg2+ site, Mg2+ is bonded to six Se2- atoms to form MgSe6 octahedra that share a cornercorner with one TmSe6 octahedra, corners with two equivalent MgSe6 octahedra, corners with four TmSe7 pentagonal bipyramids, an edgeedge with one MgSe6 octahedra, edges with four TmSe6 octahedra, and edges with three TmSe7 pentagonal bipyramids. The corner-sharing octahedra tilt angles range from 13–54°. There are a spread of Mg–Se bond distances ranging from 2.67–2.96 Å. In the second Mg2+ site, Mg2+ is bonded to six Se2- atoms to form MgSe6 octahedra that share a cornercorner with one TmSe6 octahedra, corners with two equivalent MgSe6 octahedra, corners with four TmSe7 pentagonal bipyramids, an edgeedge with one MgSe6 octahedra, edges with four TmSe6 octahedra, and edges with three TmSe7 pentagonal bipyramids. The corner-sharing octahedra tilt angles range from 13–54°. There are a spread of Mg–Se bond distances ranging from 2.69–2.86 Å. There are four inequivalent Tm3+ sites. In the first Tm3+ site, Tm3+ is bonded to six Se2- atoms to form TmSe6 octahedra that share a cornercorner with one MgSe6 octahedra, corners with two equivalent TmSe6 octahedra, corners with four TmSe7 pentagonal bipyramids, an edgeedge with one TmSe6 octahedra, edges with four MgSe6 octahedra, and edges with three TmSe7 pentagonal bipyramids. The corner-sharing octahedra tilt angles range from 12–54°. There are a spread of Tm–Se bond distances ranging from 2.79–2.86 Å. In the second Tm3+ site, Tm3+ is bonded to six Se2- atoms to form TmSe6 octahedra that share a cornercorner with one MgSe6 octahedra, corners with two equivalent TmSe6 octahedra, corners with four TmSe7 pentagonal bipyramids, an edgeedge with one TmSe6 octahedra, edges with four MgSe6 octahedra, and edges with three TmSe7 pentagonal bipyramids. The corner-sharing octahedra tilt angles range from 12–54°. There are a spread of Tm–Se bond distances ranging from 2.78–2.88 Å. In the third Tm3+ site, Tm3+ is bonded to seven Se2- atoms to form distorted TmSe7 pentagonal bipyramids that share corners with four MgSe6 octahedra, corners with four TmSe6 octahedra, edges with three MgSe6 octahedra, edges with three TmSe6 octahedra, and faces with two equivalent TmSe7 pentagonal bipyramids. The corner-sharing octahedra tilt angles range from 16–68°. There are a spread of Tm–Se bond distances ranging from 2.83–3.13 Å. In the fourth Tm3+ site, Tm3+ is bonded to seven Se2- atoms to form distorted TmSe7 pentagonal bipyramids that share corners with four MgSe6 octahedra, corners with four TmSe6 octahedra, edges with three MgSe6 octahedra, edges with three TmSe6 octahedra, and faces with two equivalent TmSe7 pentagonal bipyramids. The corner-sharing octahedra tilt angles range from 17–67°. There are a spread of Tm–Se bond distances ranging from 2.82–3.15 Å. There are eight inequivalent Se2- sites. In the first Se2- site, Se2- is bonded to two Mg2+ and three Tm3+ atoms to form distorted SeTm3Mg2 trigonal bipyramids that share corners with two equivalent SeTm3Mg2 square pyramids, corners with three equivalent SeTm3Mg tetrahedra, corners with two equivalent SeTm3Mg2 trigonal bipyramids, edges with five SeTm3Mg2 square pyramids, and edges with three SeTm3Mg2 trigonal bipyramids. In the second Se2- site, Se2- is bonded to two Mg2+ and three Tm3+ atoms to form distorted SeTm3Mg2 trigonal bipyramids that share corners with six SeTm3Mg2 square pyramids, corners with two equivalent SeTm3Mg tetrahedra, corners with two equivalent SeTm3Mg2 trigonal bipyramids, edges with three SeTm3Mg2 square pyramids, an edgeedge with one SeTm3Mg tetrahedra, and edges with three SeTm3Mg2 trigonal bipyramids. In the third Se2- site, Se2- is bonded to one Mg2+ and three Tm3+ atoms to form a mixture of distorted edge and corner-sharing SeTm3Mg tetrahedra. In the fourth Se2- site, Se2- is bonded in a distorted rectangular see-saw-like geometry to one Mg2+ and three Tm3+ atoms. In the fifth Se2- site, Se2- is bonded to two Mg2+ and three Tm3+ atoms to form distorted SeTm3Mg2 square pyramids that share a cornercorner with one SeTm3Mg tetrahedra, corners with eight SeTm3Mg2 trigonal bipyramids, edges with four SeTm3Mg2 square pyramids, an edgeedge with one SeTm3Mg tetrahedra, and edges with two SeTm3Mg2 trigonal bipyramids. In the sixth Se2- site, Se2- is bonded to two Mg2+ and three Tm3+ atoms to form distorted SeTm3Mg2 square pyramids that share corners with two equivalent SeTm4Mg square pyramids, a cornercorner with one SeTm3Mg tetrahedra, corners with six SeTm3Mg2 trigonal bipyramids, edges with three SeTm3Mg2 square pyramids, an edgeedge with one SeTm3Mg tetrahedra, and edges with three SeTm3Mg2 trigonal bipyramids. In the seventh Se2- site, Se2- is bonded to one Mg2+ and four Tm3+ atoms to form distorted SeTm4Mg square pyramids that share corners with two equivalent SeTm3Mg2 square pyramids, corners with two equivalent SeTm3Mg tetrahedra, corners with two equivalent SeTm4Mg trigonal bipyramids, edges with three SeTm3Mg2 square pyramids, an edgeedge with one SeTm3Mg tetrahedra, and edges with five SeTm3Mg2 trigonal bipyramids. In the eighth Se2- site, Se2- is bonded to one Mg2+ and four Tm3+ atoms to form distorted SeTm4Mg trigonal bipyramids that share corners with eight SeTm3Mg2 square pyramids, corners with three equivalent SeTm3Mg tetrahedra, edges with two SeTm3Mg2 square pyramids, and edges with four SeTm3Mg2 trigonal bipyramids.