DOE OSTI2020
Mg21Sm3Sb16 crystallizes in the trigonal P-3m1 space group. The structure is three-dimensional. there are eleven inequivalent Mg2+ sites. In the first Mg2+ site, Mg2+ is bonded to six equivalent Sb3- atoms to form MgSb6 octahedra that share corners with twelve MgSb4 tetrahedra, edges with six equivalent MgSb6 octahedra, and edges with six MgSb4 tetrahedra. All Mg–Sb bond lengths are 3.13 Å. In the second Mg2+ site, Mg2+ is bonded to six Sb3- atoms to form MgSb6 octahedra that share corners with twelve MgSb4 tetrahedra, edges with six equivalent MgSb6 octahedra, and edges with six MgSb4 tetrahedra. There are three shorter (3.09 Å) and three longer (3.16 Å) Mg–Sb bond lengths. In the third Mg2+ site, Mg2+ is bonded to four Sb3- atoms to form MgSb4 tetrahedra that share corners with three equivalent MgSb6 octahedra, corners with three equivalent SmSb6 octahedra, corners with six equivalent MgSb4 tetrahedra, edges with three equivalent SmSb6 octahedra, and edges with three equivalent MgSb4 tetrahedra. The corner-sharing octahedra tilt angles range from 14–59°. There are three shorter (2.86 Å) and one longer (2.90 Å) Mg–Sb bond lengths. In the fourth Mg2+ site, Mg2+ is bonded to four Sb3- atoms to form MgSb4 tetrahedra that share corners with three equivalent MgSb6 octahedra, corners with three equivalent SmSb6 octahedra, corners with six equivalent MgSb4 tetrahedra, edges with three equivalent SmSb6 octahedra, and edges with three equivalent MgSb4 tetrahedra. The corner-sharing octahedra tilt angles range from 14–59°. There are three shorter (2.86 Å) and one longer (2.90 Å) Mg–Sb bond lengths. In the fifth Mg2+ site, Mg2+ is bonded to four Sb3- atoms to form MgSb4 tetrahedra that share corners with three equivalent MgSb6 octahedra, corners with three equivalent SmSb6 octahedra, corners with six equivalent MgSb4 tetrahedra, edges with three equivalent SmSb6 octahedra, and edges with three equivalent MgSb4 tetrahedra. The corner-sharing octahedra tilt angles range from 13–58°. All Mg–Sb bond lengths are 2.87 Å. In the sixth Mg2+ site, Mg2+ is bonded to four Sb3- atoms to form MgSb4 tetrahedra that share corners with six MgSb6 octahedra, corners with six equivalent MgSb4 tetrahedra, edges with three equivalent MgSb6 octahedra, and edges with three equivalent MgSb4 tetrahedra. The corner-sharing octahedra tilt angles range from 12–60°. There are three shorter (2.87 Å) and one longer (2.97 Å) Mg–Sb bond lengths. In the seventh Mg2+ site, Mg2+ is bonded to six equivalent Sb3- atoms to form MgSb6 octahedra that share corners with twelve MgSb4 tetrahedra, edges with six equivalent MgSb6 octahedra, and edges with six equivalent MgSb4 tetrahedra. All Mg–Sb bond lengths are 3.21 Å. In the eighth Mg2+ site, Mg2+ is bonded to four Sb3- atoms to form MgSb4 tetrahedra that share corners with three equivalent MgSb6 octahedra, corners with three equivalent SmSb6 octahedra, corners with six equivalent MgSb4 tetrahedra, edges with three equivalent MgSb6 octahedra, and edges with three equivalent MgSb4 tetrahedra. The corner-sharing octahedra tilt angles range from 12–56°. There are three shorter (2.84 Å) and one longer (2.97 Å) Mg–Sb bond lengths. In the ninth Mg2+ site, Mg2+ is bonded to four Sb3- atoms to form MgSb4 tetrahedra that share corners with three equivalent MgSb6 octahedra, corners with three equivalent SmSb6 octahedra, corners with six equivalent MgSb4 tetrahedra, edges with three equivalent MgSb6 octahedra, and edges with three equivalent MgSb4 tetrahedra. The corner-sharing octahedra tilt angles range from 12–56°. There are three shorter (2.84 Å) and one longer (2.97 Å) Mg–Sb bond lengths. In the tenth Mg2+ site, Mg2+ is bonded to four Sb3- atoms to form MgSb4 tetrahedra that share corners with three equivalent MgSb6 octahedra, corners with three equivalent SmSb6 octahedra, corners with six equivalent MgSb4 tetrahedra, edges with three equivalent MgSb6 octahedra, and edges with three equivalent MgSb4 tetrahedra. The corner-sharing octahedra tilt angles range from 13–56°. There are three shorter (2.83 Å) and one longer (2.98 Å) Mg–Sb bond lengths. In the eleventh Mg2+ site, Mg2+ is bonded to four Sb3- atoms to form MgSb4 tetrahedra that share corners with six MgSb6 octahedra, corners with six equivalent MgSb4 tetrahedra, edges with three equivalent MgSb6 octahedra, and edges with three equivalent MgSb4 tetrahedra. The corner-sharing octahedra tilt angles range from 9–57°. There are three shorter (2.87 Å) and one longer (2.97 Å) Mg–Sb bond lengths. There are two inequivalent Sm2+ sites. In the first Sm2+ site, Sm2+ is bonded to six Sb3- atoms to form SmSb6 octahedra that share corners with twelve MgSb4 tetrahedra, edges with six equivalent SmSb6 octahedra, and edges with six MgSb4 tetrahedra. There are three shorter (3.23 Å) and three longer (3.24 Å) Sm–Sb bond lengths. In the second Sm2+ site, Sm2+ is bonded to six equivalent Sb3- atoms to form SmSb6 octahedra that share corners with twelve MgSb4 tetrahedra, edges with six equivalent SmSb6 octahedra, and edges with six equivalent MgSb4 tetrahedra. All Sm–Sb bond lengths are 3.24 Å. There are seven inequivalent Sb3- sites. In the first Sb3- site, Sb3- is bonded in a 7-coordinate geometry to seven Mg2+ atoms. In the second Sb3- site, Sb3- is bonded in a 7-coordinate geometry to seven Mg2+ atoms. In the third Sb3- site, Sb3- is bonded to seven Mg2+ atoms to form distorted edge-sharing SbMg7 pentagonal bipyramids. In the fourth Sb3- site, Sb3- is bonded to four Mg2+ and three equivalent Sm2+ atoms to form a mixture of distorted corner and edge-sharing SbSm3Mg4 pentagonal bipyramids. In the fifth Sb3- site, Sb3- is bonded to four Mg2+ and three equivalent Sm2+ atoms to form a mixture of distorted corner and edge-sharing SbSm3Mg4 pentagonal bipyramids. In the sixth Sb3- site, Sb3- is bonded to four Mg2+ and three equivalent Sm2+ atoms to form a mixture of distorted corner and edge-sharing SbSm3Mg4 pentagonal bipyramids. In the seventh Sb3- site, Sb3- is bonded to seven Mg2+ atoms to form a mixture of distorted corner and edge-sharing SbMg7 pentagonal bipyramids.