DOE OSTI2020
MgTb2Se4 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 TbSe6 octahedra, corners with two equivalent MgSe6 octahedra, corners with four TbSe7 pentagonal bipyramids, an edgeedge with one MgSe6 octahedra, edges with four TbSe6 octahedra, and edges with three TbSe7 pentagonal bipyramids. The corner-sharing octahedra tilt angles range from 13–53°. 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 TbSe6 octahedra, corners with two equivalent MgSe6 octahedra, corners with four TbSe7 pentagonal bipyramids, an edgeedge with one MgSe6 octahedra, edges with four TbSe6 octahedra, and edges with three TbSe7 pentagonal bipyramids. The corner-sharing octahedra tilt angles range from 13–54°. There are a spread of Mg–Se bond distances ranging from 2.70–2.87 Å. There are four inequivalent Tb3+ sites. In the first Tb3+ site, Tb3+ is bonded to six Se2- atoms to form TbSe6 octahedra that share a cornercorner with one MgSe6 octahedra, corners with two equivalent TbSe6 octahedra, corners with four TbSe7 pentagonal bipyramids, an edgeedge with one TbSe6 octahedra, edges with four MgSe6 octahedra, and edges with three TbSe7 pentagonal bipyramids. The corner-sharing octahedra tilt angles range from 13–54°. There are a spread of Tb–Se bond distances ranging from 2.83–2.89 Å. In the second Tb3+ site, Tb3+ is bonded to six Se2- atoms to form TbSe6 octahedra that share a cornercorner with one MgSe6 octahedra, corners with two equivalent TbSe6 octahedra, corners with four TbSe7 pentagonal bipyramids, an edgeedge with one TbSe6 octahedra, edges with four MgSe6 octahedra, and edges with three TbSe7 pentagonal bipyramids. The corner-sharing octahedra tilt angles range from 13–53°. There are a spread of Tb–Se bond distances ranging from 2.82–2.93 Å. In the third Tb3+ site, Tb3+ is bonded to seven Se2- atoms to form distorted TbSe7 pentagonal bipyramids that share corners with four MgSe6 octahedra, corners with four TbSe6 octahedra, edges with three MgSe6 octahedra, edges with three TbSe6 octahedra, and faces with two equivalent TbSe7 pentagonal bipyramids. The corner-sharing octahedra tilt angles range from 15–68°. There are a spread of Tb–Se bond distances ranging from 2.87–3.14 Å. In the fourth Tb3+ site, Tb3+ is bonded to seven Se2- atoms to form distorted TbSe7 pentagonal bipyramids that share corners with four MgSe6 octahedra, corners with four TbSe6 octahedra, edges with three MgSe6 octahedra, edges with three TbSe6 octahedra, and faces with two equivalent TbSe7 pentagonal bipyramids. The corner-sharing octahedra tilt angles range from 17–68°. There are a spread of Tb–Se bond distances ranging from 2.86–3.17 Å. There are eight inequivalent Se2- sites. In the first Se2- site, Se2- is bonded to two Mg2+ and three Tb3+ atoms to form distorted SeTb3Mg2 trigonal bipyramids that share corners with two equivalent SeTb3Mg2 square pyramids, corners with three equivalent SeTb3Mg tetrahedra, corners with two equivalent SeTb3Mg2 trigonal bipyramids, edges with five SeTb3Mg2 square pyramids, and edges with three SeTb3Mg2 trigonal bipyramids. In the second Se2- site, Se2- is bonded to two Mg2+ and three Tb3+ atoms to form distorted SeTb3Mg2 trigonal bipyramids that share corners with six SeTb3Mg2 square pyramids, corners with two equivalent SeTb3Mg tetrahedra, corners with two equivalent SeTb3Mg2 trigonal bipyramids, edges with three SeTb3Mg2 square pyramids, an edgeedge with one SeTb3Mg tetrahedra, and edges with three SeTb3Mg2 trigonal bipyramids. In the third Se2- site, Se2- is bonded to one Mg2+ and three Tb3+ atoms to form a mixture of distorted corner and edge-sharing SeTb3Mg tetrahedra. In the fourth Se2- site, Se2- is bonded in a distorted rectangular see-saw-like geometry to one Mg2+ and three Tb3+ atoms. In the fifth Se2- site, Se2- is bonded to two Mg2+ and three Tb3+ atoms to form distorted SeTb3Mg2 square pyramids that share a cornercorner with one SeTb3Mg tetrahedra, corners with eight SeTb3Mg2 trigonal bipyramids, edges with four SeTb3Mg2 square pyramids, an edgeedge with one SeTb3Mg tetrahedra, and edges with two SeTb3Mg2 trigonal bipyramids. In the sixth Se2- site, Se2- is bonded to two Mg2+ and three Tb3+ atoms to form distorted SeTb3Mg2 square pyramids that share corners with two equivalent SeTb4Mg square pyramids, a cornercorner with one SeTb3Mg tetrahedra, corners with six SeTb3Mg2 trigonal bipyramids, edges with three SeTb3Mg2 square pyramids, an edgeedge with one SeTb3Mg tetrahedra, and edges with three SeTb3Mg2 trigonal bipyramids. In the seventh Se2- site, Se2- is bonded to one Mg2+ and four Tb3+ atoms to form distorted SeTb4Mg square pyramids that share corners with two equivalent SeTb3Mg2 square pyramids, corners with two equivalent SeTb3Mg tetrahedra, corners with two equivalent SeTb4Mg trigonal bipyramids, edges with three SeTb3Mg2 square pyramids, an edgeedge with one SeTb3Mg tetrahedra, and edges with five SeTb3Mg2 trigonal bipyramids. In the eighth Se2- site, Se2- is bonded to one Mg2+ and four Tb3+ atoms to form distorted SeTb4Mg trigonal bipyramids that share corners with eight SeTb3Mg2 square pyramids, corners with three equivalent SeTb3Mg tetrahedra, edges with two SeTb3Mg2 square pyramids, and edges with four SeTb3Mg2 trigonal bipyramids.