DOE OSTI2020
MgLu2Se4 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 LuSe6 octahedra, corners with two equivalent MgSe6 octahedra, corners with four LuSe7 pentagonal bipyramids, an edgeedge with one MgSe6 octahedra, edges with four LuSe6 octahedra, and edges with three LuSe7 pentagonal bipyramids. The corner-sharing octahedra tilt angles range from 15–54°. There are a spread of Mg–Se bond distances ranging from 2.67–2.88 Å. In the second Mg2+ site, Mg2+ is bonded to six Se2- atoms to form MgSe6 octahedra that share a cornercorner with one LuSe6 octahedra, corners with two equivalent MgSe6 octahedra, corners with four LuSe7 pentagonal bipyramids, an edgeedge with one MgSe6 octahedra, edges with four LuSe6 octahedra, and edges with three LuSe7 pentagonal bipyramids. The corner-sharing octahedra tilt angles range from 15–55°. There are a spread of Mg–Se bond distances ranging from 2.71–2.81 Å. There are four inequivalent Lu3+ sites. In the first Lu3+ site, Lu3+ is bonded to six Se2- atoms to form LuSe6 octahedra that share a cornercorner with one MgSe6 octahedra, corners with two equivalent LuSe6 octahedra, corners with four LuSe7 pentagonal bipyramids, an edgeedge with one LuSe6 octahedra, edges with four MgSe6 octahedra, and edges with three LuSe7 pentagonal bipyramids. The corner-sharing octahedra tilt angles range from 14–55°. There are a spread of Lu–Se bond distances ranging from 2.77–2.84 Å. In the second Lu3+ site, Lu3+ is bonded to six Se2- atoms to form LuSe6 octahedra that share a cornercorner with one MgSe6 octahedra, corners with two equivalent LuSe6 octahedra, corners with four LuSe7 pentagonal bipyramids, an edgeedge with one LuSe6 octahedra, edges with four MgSe6 octahedra, and edges with three LuSe7 pentagonal bipyramids. The corner-sharing octahedra tilt angles range from 14–54°. There are a spread of Lu–Se bond distances ranging from 2.76–2.86 Å. In the third Lu3+ site, Lu3+ is bonded to seven Se2- atoms to form distorted LuSe7 pentagonal bipyramids that share corners with four MgSe6 octahedra, corners with four LuSe6 octahedra, edges with three MgSe6 octahedra, edges with three LuSe6 octahedra, and faces with two equivalent LuSe7 pentagonal bipyramids. The corner-sharing octahedra tilt angles range from 16–67°. There are a spread of Lu–Se bond distances ranging from 2.80–3.07 Å. In the fourth Lu3+ site, Lu3+ is bonded to seven Se2- atoms to form distorted LuSe7 pentagonal bipyramids that share corners with four MgSe6 octahedra, corners with four LuSe6 octahedra, edges with three MgSe6 octahedra, edges with three LuSe6 octahedra, and faces with two equivalent LuSe7 pentagonal bipyramids. The corner-sharing octahedra tilt angles range from 18–66°. There are a spread of Lu–Se bond distances ranging from 2.79–3.08 Å. There are eight inequivalent Se2- sites. In the first Se2- site, Se2- is bonded to two Mg2+ and three Lu3+ atoms to form distorted SeLu3Mg2 trigonal bipyramids that share corners with two equivalent SeLu3Mg2 square pyramids, corners with three equivalent SeLu3Mg tetrahedra, corners with two equivalent SeLu3Mg2 trigonal bipyramids, edges with five SeLu3Mg2 square pyramids, and edges with three SeLu3Mg2 trigonal bipyramids. In the second Se2- site, Se2- is bonded to two Mg2+ and three Lu3+ atoms to form distorted SeLu3Mg2 trigonal bipyramids that share corners with six SeLu3Mg2 square pyramids, corners with two equivalent SeLu3Mg tetrahedra, corners with two equivalent SeLu3Mg2 trigonal bipyramids, edges with three SeLu3Mg2 square pyramids, an edgeedge with one SeLu3Mg tetrahedra, and edges with three SeLu3Mg2 trigonal bipyramids. In the third Se2- site, Se2- is bonded to one Mg2+ and three Lu3+ atoms to form a mixture of distorted edge and corner-sharing SeLu3Mg tetrahedra. In the fourth Se2- site, Se2- is bonded in a distorted rectangular see-saw-like geometry to one Mg2+ and three Lu3+ atoms. In the fifth Se2- site, Se2- is bonded to two Mg2+ and three Lu3+ atoms to form distorted SeLu3Mg2 square pyramids that share a cornercorner with one SeLu3Mg tetrahedra, corners with eight SeLu3Mg2 trigonal bipyramids, edges with four SeLu3Mg2 square pyramids, an edgeedge with one SeLu3Mg tetrahedra, and edges with two SeLu3Mg2 trigonal bipyramids. In the sixth Se2- site, Se2- is bonded to two Mg2+ and three Lu3+ atoms to form distorted SeLu3Mg2 square pyramids that share corners with two equivalent SeLu4Mg square pyramids, a cornercorner with one SeLu3Mg tetrahedra, corners with six SeLu3Mg2 trigonal bipyramids, edges with three SeLu3Mg2 square pyramids, an edgeedge with one SeLu3Mg tetrahedra, and edges with three SeLu3Mg2 trigonal bipyramids. In the seventh Se2- site, Se2- is bonded to one Mg2+ and four Lu3+ atoms to form distorted SeLu4Mg square pyramids that share corners with two equivalent SeLu3Mg2 square pyramids, corners with two equivalent SeLu3Mg tetrahedra, corners with two equivalent SeLu4Mg trigonal bipyramids, edges with three SeLu3Mg2 square pyramids, an edgeedge with one SeLu3Mg tetrahedra, and edges with five SeLu3Mg2 trigonal bipyramids. In the eighth Se2- site, Se2- is bonded to one Mg2+ and four Lu3+ atoms to form distorted SeLu4Mg trigonal bipyramids that share corners with eight SeLu3Mg2 square pyramids, corners with three equivalent SeLu3Mg tetrahedra, edges with two SeLu3Mg2 square pyramids, and edges with four SeLu3Mg2 trigonal bipyramids.