Mg14NbC crystallizes in the orthorhombic Amm2 space group. There are seven inequivalent Mg sites. In the first Mg site, Mg(1) is bonded to two equivalent Mg(2), two equivalent Mg(6), four equivalent Mg(4), and four equivalent Mg(5) atoms to form MgMg12 cuboctahedra that share corners with six equivalent Mg(1)Mg12 cuboctahedra, edges with two equivalent Mg(2)Mg12 cuboctahedra, edges with four equivalent Mg(6)Mg11Nb cuboctahedra, edges with four equivalent Mg(4)Mg12 cuboctahedra, faces with two equivalent Mg(6)Mg11Nb cuboctahedra, faces with two equivalent Mg(2)Mg12 cuboctahedra, faces with two equivalent Nb(1)Mg10C2 cuboctahedra, and faces with four equivalent Mg(4)Mg12 cuboctahedra. In the second Mg site, Mg(2) is bonded to two equivalent Mg(1), two equivalent Mg(7), four equivalent Mg(4), and four equivalent Mg(5) atoms to form MgMg12 cuboctahedra that share corners with four equivalent Nb(1)Mg10C2 cuboctahedra, corners with six equivalent Mg(2)Mg12 cuboctahedra, edges with two equivalent Mg(1)Mg12 cuboctahedra, edges with four equivalent Mg(4)Mg12 cuboctahedra, faces with two equivalent Mg(1)Mg12 cuboctahedra, faces with four equivalent Mg(4)Mg12 cuboctahedra, and faces with six equivalent Mg(6)Mg11Nb cuboctahedra. In the third Mg site, Mg(3) is bonded in a 10-coordinate geometry to two equivalent Mg(3), two equivalent Mg(5), two equivalent Mg(6), two equivalent Mg(7), and two equivalent Nb(1) atoms. In the fourth Mg site, Mg(4) is bonded to two equivalent Mg(1), two equivalent Mg(2), two equivalent Mg(4), two equivalent Mg(5), two equivalent Mg(6), and two equivalent Mg(7) atoms to form MgMg12 cuboctahedra that share corners with four equivalent Nb(1)Mg10C2 cuboctahedra, corners with six equivalent Mg(4)Mg12 cuboctahedra, edges with two equivalent Mg(1)Mg12 cuboctahedra, edges with two equivalent Mg(2)Mg12 cuboctahedra, edges with two equivalent Mg(4)Mg12 cuboctahedra, edges with four equivalent Mg(6)Mg11Nb cuboctahedra, faces with two equivalent Mg(6)Mg11Nb cuboctahedra, faces with two equivalent Mg(1)Mg12 cuboctahedra, faces with two equivalent Mg(2)Mg12 cuboctahedra, and faces with two equivalent Mg(4)Mg12 cuboctahedra. In the fifth Mg site, Mg(5) is bonded in a 12-coordinate geometry to one Mg(1), one Mg(2), one Mg(3), one Mg(4), two equivalent Mg(5), two equivalent Mg(6), two equivalent Mg(7), one Nb(1), and one C(1) atom. In the sixth Mg site, Mg(6) is bonded to one Mg(1), two equivalent Mg(3), two equivalent Mg(4), two equivalent Mg(7), four equivalent Mg(5), and one Nb(1) atom to form distorted MgMg11Nb cuboctahedra that share corners with six equivalent Mg(6)Mg11Nb cuboctahedra, edges with two equivalent Mg(1)Mg12 cuboctahedra, edges with two equivalent Nb(1)Mg10C2 cuboctahedra, edges with four equivalent Mg(4)Mg12 cuboctahedra,  a faceface with one Mg(1)Mg12 cuboctahedra,  a faceface with one Nb(1)Mg10C2 cuboctahedra, faces with two equivalent Mg(6)Mg11Nb cuboctahedra, faces with two equivalent Mg(4)Mg12 cuboctahedra, and faces with three equivalent Mg(2)Mg12 cuboctahedra. In the seventh Mg site, Mg(7) is bonded in a 12-coordinate geometry to one Mg(2), two equivalent Mg(3), two equivalent Mg(4), two equivalent Mg(6), four equivalent Mg(5), and one C(1) atom. Nb(1) is bonded to two equivalent Mg(6), four equivalent Mg(3), four equivalent Mg(5), and two equivalent C(1) atoms to form NbMg10C2 cuboctahedra that share corners with four equivalent Mg(2)Mg12 cuboctahedra, corners with six equivalent Nb(1)Mg10C2 cuboctahedra, corners with eight equivalent Mg(4)Mg12 cuboctahedra, edges with four equivalent Mg(6)Mg11Nb cuboctahedra, faces with two equivalent Mg(6)Mg11Nb cuboctahedra, and faces with two equivalent Mg(1)Mg12 cuboctahedra. C(1) is bonded in a 8-coordinate geometry to two equivalent Mg(7), four equivalent Mg(5), and two equivalent Nb(1) atoms.
Mg14NbC crystallizes in the orthorhombic Amm2 space group. There are seven inequivalent Mg sites. In the first Mg site, Mg(1) is bonded to two equivalent Mg(2), two equivalent Mg(6), four equivalent Mg(4), and four equivalent Mg(5) atoms to form MgMg12 cuboctahedra that share corners with six equivalent Mg(1)Mg12 cuboctahedra, edges with two equivalent Mg(2)Mg12 cuboctahedra, edges with four equivalent Mg(6)Mg11Nb cuboctahedra, edges with four equivalent Mg(4)Mg12 cuboctahedra, faces with two equivalent Mg(6)Mg11Nb cuboctahedra, faces with two equivalent Mg(2)Mg12 cuboctahedra, faces with two equivalent Nb(1)Mg10C2 cuboctahedra, and faces with four equivalent Mg(4)Mg12 cuboctahedra. Both Mg(1)-Mg(2) bond lengths are 3.06 Å. Both Mg(1)-Mg(6) bond lengths are 3.07 Å. There are two shorter (3.06 Å) and two longer (3.16 Å) Mg(1)-Mg(4) bond lengths. All Mg(1)-Mg(5) bond lengths are 3.21 Å. In the second Mg site, Mg(2) is bonded to two equivalent Mg(1), two equivalent Mg(7), four equivalent Mg(4), and four equivalent Mg(5) atoms to form MgMg12 cuboctahedra that share corners with four equivalent Nb(1)Mg10C2 cuboctahedra, corners with six equivalent Mg(2)Mg12 cuboctahedra, edges with two equivalent Mg(1)Mg12 cuboctahedra, edges with four equivalent Mg(4)Mg12 cuboctahedra, faces with two equivalent Mg(1)Mg12 cuboctahedra, faces with four equivalent Mg(4)Mg12 cuboctahedra, and faces with six equivalent Mg(6)Mg11Nb cuboctahedra. Both Mg(2)-Mg(7) bond lengths are 3.16 Å. There are two shorter (3.10 Å) and two longer (3.12 Å) Mg(2)-Mg(4) bond lengths. All Mg(2)-Mg(5) bond lengths are 3.18 Å. In the third Mg site, Mg(3) is bonded in a 10-coordinate geometry to two equivalent Mg(3), two equivalent Mg(5), two equivalent Mg(6), two equivalent Mg(7), and two equivalent Nb(1) atoms. There is one shorter (2.99 Å) and one longer (3.14 Å) Mg(3)-Mg(3) bond length. Both Mg(3)-Mg(5) bond lengths are 2.95 Å. Both Mg(3)-Mg(6) bond lengths are 3.05 Å. Both Mg(3)-Mg(7) bond lengths are 2.98 Å. There is one shorter (3.04 Å) and one longer (3.18 Å) Mg(3)-Nb(1) bond length. In the fourth Mg site, Mg(4) is bonded to two equivalent Mg(1), two equivalent Mg(2), two equivalent Mg(4), two equivalent Mg(5), two equivalent Mg(6), and two equivalent Mg(7) atoms to form MgMg12 cuboctahedra that share corners with four equivalent Nb(1)Mg10C2 cuboctahedra, corners with six equivalent Mg(4)Mg12 cuboctahedra, edges with two equivalent Mg(1)Mg12 cuboctahedra, edges with two equivalent Mg(2)Mg12 cuboctahedra, edges with two equivalent Mg(4)Mg12 cuboctahedra, edges with four equivalent Mg(6)Mg11Nb cuboctahedra, faces with two equivalent Mg(6)Mg11Nb cuboctahedra, faces with two equivalent Mg(1)Mg12 cuboctahedra, faces with two equivalent Mg(2)Mg12 cuboctahedra, and faces with two equivalent Mg(4)Mg12 cuboctahedra. There is one shorter (3.03 Å) and one longer (3.10 Å) Mg(4)-Mg(4) bond length. Both Mg(4)-Mg(5) bond lengths are 3.22 Å. Both Mg(4)-Mg(6) bond lengths are 3.10 Å. Both Mg(4)-Mg(7) bond lengths are 3.22 Å. In the fifth Mg site, Mg(5) is bonded in a 12-coordinate geometry to one Mg(1), one Mg(2), one Mg(3), one Mg(4), two equivalent Mg(5), two equivalent Mg(6), two equivalent Mg(7), one Nb(1), and one C(1) atom. There is one shorter (3.00 Å) and one longer (3.13 Å) Mg(5)-Mg(5) bond length. There is one shorter (3.07 Å) and one longer (3.16 Å) Mg(5)-Mg(6) bond length. There is one shorter (2.99 Å) and one longer (3.23 Å) Mg(5)-Mg(7) bond length. The Mg(5)-Nb(1) bond length is 2.95 Å. The Mg(5)-C(1) bond length is 2.90 Å. In the sixth Mg site, Mg(6) is bonded to one Mg(1), two equivalent Mg(3), two equivalent Mg(4), two equivalent Mg(7), four equivalent Mg(5), and one Nb(1) atom to form distorted MgMg11Nb cuboctahedra that share corners with six equivalent Mg(6)Mg11Nb cuboctahedra, edges with two equivalent Mg(1)Mg12 cuboctahedra, edges with two equivalent Nb(1)Mg10C2 cuboctahedra, edges with four equivalent Mg(4)Mg12 cuboctahedra,  a faceface with one Mg(1)Mg12 cuboctahedra,  a faceface with one Nb(1)Mg10C2 cuboctahedra, faces with two equivalent Mg(6)Mg11Nb cuboctahedra, faces with two equivalent Mg(4)Mg12 cuboctahedra, and faces with three equivalent Mg(2)Mg12 cuboctahedra. Both Mg(6)-Mg(7) bond lengths are 3.07 Å. The Mg(6)-Nb(1) bond length is 3.01 Å. In the seventh Mg site, Mg(7) is bonded in a 12-coordinate geometry to one Mg(2), two equivalent Mg(3), two equivalent Mg(4), two equivalent Mg(6), four equivalent Mg(5), and one C(1) atom. The Mg(7)-C(1) bond length is 2.89 Å. Nb(1) is bonded to two equivalent Mg(6), four equivalent Mg(3), four equivalent Mg(5), and two equivalent C(1) atoms to form NbMg10C2 cuboctahedra that share corners with four equivalent Mg(2)Mg12 cuboctahedra, corners with six equivalent Nb(1)Mg10C2 cuboctahedra, corners with eight equivalent Mg(4)Mg12 cuboctahedra, edges with four equivalent Mg(6)Mg11Nb cuboctahedra, faces with two equivalent Mg(6)Mg11Nb cuboctahedra, and faces with two equivalent Mg(1)Mg12 cuboctahedra. Both Nb(1)-C(1) bond lengths are 3.06 Å. C(1) is bonded in a 8-coordinate geometry to two equivalent Mg(7), four equivalent Mg(5), and two equivalent Nb(1) atoms.