DOE OSTI2020
MgSm2Se4 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 SmSe6 octahedra, corners with two equivalent MgSe6 octahedra, corners with four SmSe7 pentagonal bipyramids, an edgeedge with one MgSe6 octahedra, edges with four SmSe6 octahedra, and edges with three SmSe7 pentagonal bipyramids. The corner-sharing octahedra tilt angles range from 13–53°. There are a spread of Mg–Se bond distances ranging from 2.66–3.02 Å. In the second Mg2+ site, Mg2+ is bonded to six Se2- atoms to form MgSe6 octahedra that share a cornercorner with one SmSe6 octahedra, corners with two equivalent MgSe6 octahedra, corners with four SmSe7 pentagonal bipyramids, an edgeedge with one MgSe6 octahedra, edges with four SmSe6 octahedra, and edges with three SmSe7 pentagonal bipyramids. The corner-sharing octahedra tilt angles range from 13–53°. There are a spread of Mg–Se bond distances ranging from 2.68–2.92 Å. There are four inequivalent Sm3+ sites. In the first Sm3+ site, Sm3+ is bonded to six Se2- atoms to form SmSe6 octahedra that share a cornercorner with one MgSe6 octahedra, corners with two equivalent SmSe6 octahedra, corners with four SmSe7 pentagonal bipyramids, an edgeedge with one SmSe6 octahedra, edges with four MgSe6 octahedra, and edges with three SmSe7 pentagonal bipyramids. The corner-sharing octahedra tilt angles range from 12–53°. There are a spread of Sm–Se bond distances ranging from 2.87–2.94 Å. In the second Sm3+ site, Sm3+ is bonded to six Se2- atoms to form SmSe6 octahedra that share a cornercorner with one MgSe6 octahedra, corners with two equivalent SmSe6 octahedra, corners with four SmSe7 pentagonal bipyramids, an edgeedge with one SmSe6 octahedra, edges with four MgSe6 octahedra, and edges with three SmSe7 pentagonal bipyramids. The corner-sharing octahedra tilt angles range from 12–53°. There are a spread of Sm–Se bond distances ranging from 2.86–2.98 Å. In the third Sm3+ site, Sm3+ is bonded to seven Se2- atoms to form distorted SmSe7 pentagonal bipyramids that share corners with four MgSe6 octahedra, corners with four SmSe6 octahedra, edges with three MgSe6 octahedra, edges with three SmSe6 octahedra, and faces with two equivalent SmSe7 pentagonal bipyramids. The corner-sharing octahedra tilt angles range from 14–69°. There are a spread of Sm–Se bond distances ranging from 2.92–3.18 Å. In the fourth Sm3+ site, Sm3+ is bonded to seven Se2- atoms to form distorted SmSe7 pentagonal bipyramids that share corners with four MgSe6 octahedra, corners with four SmSe6 octahedra, edges with three MgSe6 octahedra, edges with three SmSe6 octahedra, and faces with two equivalent SmSe7 pentagonal bipyramids. The corner-sharing octahedra tilt angles range from 16–69°. There are a spread of Sm–Se bond distances ranging from 2.91–3.22 Å. There are eight inequivalent Se2- sites. In the first Se2- site, Se2- is bonded to two Mg2+ and three Sm3+ atoms to form distorted SeSm3Mg2 trigonal bipyramids that share corners with two equivalent SeSm3Mg2 square pyramids, corners with three equivalent SeSm3Mg tetrahedra, corners with two equivalent SeSm3Mg2 trigonal bipyramids, edges with five SeSm3Mg2 square pyramids, and edges with three SeSm3Mg2 trigonal bipyramids. In the second Se2- site, Se2- is bonded to two Mg2+ and three Sm3+ atoms to form distorted SeSm3Mg2 trigonal bipyramids that share corners with six SeSm3Mg2 square pyramids, corners with two equivalent SeSm3Mg tetrahedra, corners with two equivalent SeSm3Mg2 trigonal bipyramids, edges with three SeSm3Mg2 square pyramids, an edgeedge with one SeSm3Mg tetrahedra, and edges with three SeSm3Mg2 trigonal bipyramids. In the third Se2- site, Se2- is bonded to one Mg2+ and three Sm3+ atoms to form a mixture of distorted edge and corner-sharing SeSm3Mg tetrahedra. In the fourth Se2- site, Se2- is bonded in a distorted rectangular see-saw-like geometry to one Mg2+ and three Sm3+ atoms. In the fifth Se2- site, Se2- is bonded to two Mg2+ and three Sm3+ atoms to form distorted SeSm3Mg2 square pyramids that share a cornercorner with one SeSm3Mg tetrahedra, corners with eight SeSm3Mg2 trigonal bipyramids, edges with four SeSm3Mg2 square pyramids, an edgeedge with one SeSm3Mg tetrahedra, and edges with two SeSm3Mg2 trigonal bipyramids. In the sixth Se2- site, Se2- is bonded to two Mg2+ and three Sm3+ atoms to form SeSm3Mg2 square pyramids that share corners with two equivalent SeSm4Mg square pyramids, a cornercorner with one SeSm3Mg tetrahedra, corners with six SeSm3Mg2 trigonal bipyramids, edges with three SeSm3Mg2 square pyramids, an edgeedge with one SeSm3Mg tetrahedra, and edges with three SeSm3Mg2 trigonal bipyramids. In the seventh Se2- site, Se2- is bonded to one Mg2+ and four Sm3+ atoms to form distorted SeSm4Mg square pyramids that share corners with two equivalent SeSm3Mg2 square pyramids, corners with two equivalent SeSm3Mg tetrahedra, corners with two equivalent SeSm4Mg trigonal bipyramids, edges with three SeSm3Mg2 square pyramids, an edgeedge with one SeSm3Mg tetrahedra, and edges with five SeSm3Mg2 trigonal bipyramids. In the eighth Se2- site, Se2- is bonded to one Mg2+ and four Sm3+ atoms to form distorted SeSm4Mg trigonal bipyramids that share corners with eight SeSm3Mg2 square pyramids, corners with three equivalent SeSm3Mg tetrahedra, edges with two SeSm3Mg2 square pyramids, and edges with four SeSm3Mg2 trigonal bipyramids.