Mg3Tm is beta Cu3Ti-like structured and crystallizes in the hexagonal P6_3/mmc space group. Mg(1) is bonded to eight equivalent Mg(1) and four equivalent Tm(1) atoms to form MgTm4Mg8 cuboctahedra that share corners with four equivalent Tm(1)Mg12 cuboctahedra, corners with fourteen equivalent Mg(1)Tm4Mg8 cuboctahedra, edges with six equivalent Tm(1)Mg12 cuboctahedra, edges with twelve equivalent Mg(1)Tm4Mg8 cuboctahedra, faces with four equivalent Tm(1)Mg12 cuboctahedra, and faces with sixteen equivalent Mg(1)Tm4Mg8 cuboctahedra. Tm(1) is bonded to twelve equivalent Mg(1) atoms to form TmMg12 cuboctahedra that share corners with six equivalent Tm(1)Mg12 cuboctahedra, corners with twelve equivalent Mg(1)Tm4Mg8 cuboctahedra, edges with eighteen equivalent Mg(1)Tm4Mg8 cuboctahedra, faces with eight equivalent Tm(1)Mg12 cuboctahedra, and faces with twelve equivalent Mg(1)Tm4Mg8 cuboctahedra.
Mg3Tm is beta Cu3Ti-like structured and crystallizes in the hexagonal P6_3/mmc space group. Mg(1) is bonded to eight equivalent Mg(1) and four equivalent Tm(1) atoms to form MgTm4Mg8 cuboctahedra that share corners with four equivalent Tm(1)Mg12 cuboctahedra, corners with fourteen equivalent Mg(1)Tm4Mg8 cuboctahedra, edges with six equivalent Tm(1)Mg12 cuboctahedra, edges with twelve equivalent Mg(1)Tm4Mg8 cuboctahedra, faces with four equivalent Tm(1)Mg12 cuboctahedra, and faces with sixteen equivalent Mg(1)Tm4Mg8 cuboctahedra. There are a spread of Mg(1)-Mg(1) bond distances ranging from 3.18-3.37 Å. There are two shorter (3.22 Å) and two longer (3.30 Å) Mg(1)-Tm(1) bond lengths. Tm(1) is bonded to twelve equivalent Mg(1) atoms to form TmMg12 cuboctahedra that share corners with six equivalent Tm(1)Mg12 cuboctahedra, corners with twelve equivalent Mg(1)Tm4Mg8 cuboctahedra, edges with eighteen equivalent Mg(1)Tm4Mg8 cuboctahedra, faces with eight equivalent Tm(1)Mg12 cuboctahedra, and faces with twelve equivalent Mg(1)Tm4Mg8 cuboctahedra.