DOE OSTI2020
MgPm2Se4 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 PmSe6 octahedra, corners with two equivalent MgSe6 octahedra, corners with four PmSe7 pentagonal bipyramids, an edgeedge with one MgSe6 octahedra, edges with four PmSe6 octahedra, and edges with three PmSe7 pentagonal bipyramids. The corner-sharing octahedra tilt angles range from 12–53°. There are a spread of Mg–Se bond distances ranging from 2.66–3.04 Å. In the second Mg2+ site, Mg2+ is bonded to six Se2- atoms to form MgSe6 octahedra that share a cornercorner with one PmSe6 octahedra, corners with two equivalent MgSe6 octahedra, corners with four PmSe7 pentagonal bipyramids, an edgeedge with one MgSe6 octahedra, edges with four PmSe6 octahedra, and edges with three PmSe7 pentagonal bipyramids. The corner-sharing octahedra tilt angles range from 12–53°. There are a spread of Mg–Se bond distances ranging from 2.68–2.93 Å. There are four inequivalent Pm3+ sites. In the first Pm3+ site, Pm3+ is bonded to six Se2- atoms to form PmSe6 octahedra that share a cornercorner with one MgSe6 octahedra, corners with two equivalent PmSe6 octahedra, corners with four PmSe7 pentagonal bipyramids, an edgeedge with one PmSe6 octahedra, edges with four MgSe6 octahedra, and edges with three PmSe7 pentagonal bipyramids. The corner-sharing octahedra tilt angles range from 12–53°. There are a spread of Pm–Se bond distances ranging from 2.88–2.96 Å. In the second Pm3+ site, Pm3+ is bonded to six Se2- atoms to form PmSe6 octahedra that share a cornercorner with one MgSe6 octahedra, corners with two equivalent PmSe6 octahedra, corners with four PmSe7 pentagonal bipyramids, an edgeedge with one PmSe6 octahedra, edges with four MgSe6 octahedra, and edges with three PmSe7 pentagonal bipyramids. The corner-sharing octahedra tilt angles range from 12–53°. There are a spread of Pm–Se bond distances ranging from 2.87–2.99 Å. In the third Pm3+ site, Pm3+ is bonded to seven Se2- atoms to form distorted PmSe7 pentagonal bipyramids that share corners with four MgSe6 octahedra, corners with four PmSe6 octahedra, edges with three MgSe6 octahedra, edges with three PmSe6 octahedra, and faces with two equivalent PmSe7 pentagonal bipyramids. The corner-sharing octahedra tilt angles range from 14–70°. There are a spread of Pm–Se bond distances ranging from 2.93–3.19 Å. In the fourth Pm3+ site, Pm3+ is bonded to seven Se2- atoms to form distorted PmSe7 pentagonal bipyramids that share corners with four MgSe6 octahedra, corners with four PmSe6 octahedra, edges with three MgSe6 octahedra, edges with three PmSe6 octahedra, and faces with two equivalent PmSe7 pentagonal bipyramids. The corner-sharing octahedra tilt angles range from 16–70°. There are a spread of Pm–Se bond distances ranging from 2.92–3.23 Å. There are eight inequivalent Se2- sites. In the first Se2- site, Se2- is bonded to two Mg2+ and three Pm3+ atoms to form distorted SePm3Mg2 trigonal bipyramids that share corners with two equivalent SePm3Mg2 square pyramids, corners with three equivalent SePm3Mg tetrahedra, corners with two equivalent SePm3Mg2 trigonal bipyramids, edges with five SePm3Mg2 square pyramids, and edges with three SePm3Mg2 trigonal bipyramids. In the second Se2- site, Se2- is bonded to two Mg2+ and three Pm3+ atoms to form distorted SePm3Mg2 trigonal bipyramids that share corners with six SePm3Mg2 square pyramids, corners with two equivalent SePm3Mg tetrahedra, corners with two equivalent SePm3Mg2 trigonal bipyramids, edges with three SePm3Mg2 square pyramids, an edgeedge with one SePm3Mg tetrahedra, and edges with three SePm3Mg2 trigonal bipyramids. In the third Se2- site, Se2- is bonded to one Mg2+ and three Pm3+ atoms to form a mixture of distorted edge and corner-sharing SePm3Mg tetrahedra. In the fourth Se2- site, Se2- is bonded in a distorted rectangular see-saw-like geometry to one Mg2+ and three Pm3+ atoms. In the fifth Se2- site, Se2- is bonded to two Mg2+ and three Pm3+ atoms to form distorted SePm3Mg2 square pyramids that share a cornercorner with one SePm3Mg tetrahedra, corners with eight SePm3Mg2 trigonal bipyramids, edges with four SePm3Mg2 square pyramids, an edgeedge with one SePm3Mg tetrahedra, and edges with two SePm3Mg2 trigonal bipyramids. In the sixth Se2- site, Se2- is bonded to two Mg2+ and three Pm3+ atoms to form SePm3Mg2 square pyramids that share corners with two equivalent SePm4Mg square pyramids, a cornercorner with one SePm3Mg tetrahedra, corners with six SePm3Mg2 trigonal bipyramids, edges with three SePm3Mg2 square pyramids, an edgeedge with one SePm3Mg tetrahedra, and edges with three SePm3Mg2 trigonal bipyramids. In the seventh Se2- site, Se2- is bonded to one Mg2+ and four Pm3+ atoms to form distorted SePm4Mg square pyramids that share corners with two equivalent SePm3Mg2 square pyramids, corners with two equivalent SePm3Mg tetrahedra, corners with two equivalent SePm4Mg trigonal bipyramids, edges with three SePm3Mg2 square pyramids, an edgeedge with one SePm3Mg tetrahedra, and edges with five SePm3Mg2 trigonal bipyramids. In the eighth Se2- site, Se2- is bonded to one Mg2+ and four Pm3+ atoms to form distorted SePm4Mg trigonal bipyramids that share corners with eight SePm3Mg2 square pyramids, corners with three equivalent SePm3Mg tetrahedra, edges with two SePm3Mg2 square pyramids, and edges with four SePm3Mg2 trigonal bipyramids.