Mg3Sb crystallizes in the orthorhombic Imm2 space group. There are two inequivalent Mg sites. In the first Mg site, Mg(1) is bonded to four equivalent Sb(1) atoms to form distorted MgSb4 cuboctahedra that share corners with four equivalent Mg(1)Sb4 cuboctahedra, corners with sixteen equivalent Mg(2)Sb3 cuboctahedra, edges with four equivalent Mg(2)Sb3 cuboctahedra, and edges with four equivalent Mg(1)Sb4 cuboctahedra. In the second Mg site, Mg(2) is bonded to three equivalent Sb(1) atoms to form distorted MgSb3 cuboctahedra that share corners with eight equivalent Mg(1)Sb4 cuboctahedra, corners with thirteen equivalent Mg(2)Sb3 cuboctahedra,  an edgeedge with one Mg(2)Sb3 cuboctahedra, and edges with two equivalent Mg(1)Sb4 cuboctahedra. Sb(1) is bonded to four equivalent Mg(1) and six equivalent Mg(2) atoms to form a mixture of distorted edge, face, and corner-sharing SbMg10 cuboctahedra.
Mg3Sb crystallizes in the orthorhombic Imm2 space group. There are two inequivalent Mg sites. In the first Mg site, Mg(1) is bonded to four equivalent Sb(1) atoms to form distorted MgSb4 cuboctahedra that share corners with four equivalent Mg(1)Sb4 cuboctahedra, corners with sixteen equivalent Mg(2)Sb3 cuboctahedra, edges with four equivalent Mg(2)Sb3 cuboctahedra, and edges with four equivalent Mg(1)Sb4 cuboctahedra. There are two shorter (3.18 Å) and two longer (3.23 Å) Mg(1)-Sb(1) bond lengths. In the second Mg site, Mg(2) is bonded to three equivalent Sb(1) atoms to form distorted MgSb3 cuboctahedra that share corners with eight equivalent Mg(1)Sb4 cuboctahedra, corners with thirteen equivalent Mg(2)Sb3 cuboctahedra,  an edgeedge with one Mg(2)Sb3 cuboctahedra, and edges with two equivalent Mg(1)Sb4 cuboctahedra. There is one shorter (3.16 Å) and two longer (3.22 Å) Mg(2)-Sb(1) bond lengths. Sb(1) is bonded to four equivalent Mg(1) and six equivalent Mg(2) atoms to form a mixture of distorted edge, face, and corner-sharing SbMg10 cuboctahedra.