Mg14HfC crystallizes in the orthorhombic Amm2 space group. There are seven inequivalent Mg sites. In the first Mg site, Mg(1) is bonded in a 10-coordinate geometry to two equivalent Mg(2), four equivalent Mg(4), and four equivalent Mg(5) atoms. 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 six equivalent Mg(2)Mg12 cuboctahedra, edges with four equivalent Mg(7)HfMg11 cuboctahedra, and faces with two equivalent Mg(7)HfMg11 cuboctahedra. In the third Mg site, Mg(3) is bonded in a single-bond geometry to two equivalent Mg(5), two equivalent Mg(7), and one C(1) atom. In the fourth Mg site, Mg(4) is bonded in a 10-coordinate geometry to two equivalent Mg(1), two equivalent Mg(2), two equivalent Mg(4), two equivalent Mg(5), and two equivalent Mg(7) atoms. In the fifth Mg site, Mg(5) is bonded in a 11-coordinate geometry to one Mg(1), one Mg(2), one Mg(3), one Mg(4), one Mg(6), two equivalent Mg(5), two equivalent Mg(7), one Hf(1), and one C(1) atom. In the sixth Mg site, Mg(6) is bonded in a single-bond geometry to two equivalent Mg(5), two equivalent Mg(7), and one C(1) atom. In the seventh Mg site, Mg(7) is bonded to one Mg(2), two equivalent Mg(3), two equivalent Mg(4), two equivalent Mg(6), four equivalent Mg(5), and one Hf(1) atom to form MgHfMg11 cuboctahedra that share corners with six equivalent Mg(7)HfMg11 cuboctahedra, edges with two equivalent Mg(2)Mg12 cuboctahedra,  a faceface with one Mg(2)Mg12 cuboctahedra, and faces with two equivalent Mg(7)HfMg11 cuboctahedra. Hf(1) is bonded in a 2-coordinate geometry to two equivalent Mg(7), four equivalent Mg(5), and two equivalent C(1) atoms. C(1) is bonded in a 4-coordinate geometry to two equivalent Mg(3), two equivalent Mg(6), four equivalent Mg(5), and two equivalent Hf(1) atoms.
Mg14HfC crystallizes in the orthorhombic Amm2 space group. There are seven inequivalent Mg sites. In the first Mg site, Mg(1) is bonded in a 10-coordinate geometry to two equivalent Mg(2), four equivalent Mg(4), and four equivalent Mg(5) atoms. Both Mg(1)-Mg(2) bond lengths are 2.91 Å. There are two shorter (3.17 Å) and two longer (3.33 Å) Mg(1)-Mg(4) bond lengths. All Mg(1)-Mg(5) bond lengths are 3.18 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 six equivalent Mg(2)Mg12 cuboctahedra, edges with four equivalent Mg(7)HfMg11 cuboctahedra, and faces with two equivalent Mg(7)HfMg11 cuboctahedra. Both Mg(2)-Mg(7) bond lengths are 3.13 Å. There are two shorter (3.22 Å) and two longer (3.28 Å) Mg(2)-Mg(4) bond lengths. All Mg(2)-Mg(5) bond lengths are 3.11 Å. In the third Mg site, Mg(3) is bonded in a single-bond geometry to two equivalent Mg(5), two equivalent Mg(7), and one C(1) atom. Both Mg(3)-Mg(5) bond lengths are 2.89 Å. Both Mg(3)-Mg(7) bond lengths are 3.26 Å. The Mg(3)-C(1) bond length is 2.31 Å. In the fourth Mg site, Mg(4) is bonded in a 10-coordinate geometry to two equivalent Mg(1), two equivalent Mg(2), two equivalent Mg(4), two equivalent Mg(5), and two equivalent Mg(7) atoms. There is one shorter (2.88 Å) and one longer (2.94 Å) Mg(4)-Mg(4) bond length. Both Mg(4)-Mg(5) bond lengths are 3.13 Å. Both Mg(4)-Mg(7) bond lengths are 3.11 Å. In the fifth Mg site, Mg(5) is bonded in a 11-coordinate geometry to one Mg(1), one Mg(2), one Mg(3), one Mg(4), one Mg(6), two equivalent Mg(5), two equivalent Mg(7), one Hf(1), and one C(1) atom. The Mg(5)-Mg(6) bond length is 2.93 Å. There is one shorter (2.85 Å) and one longer (2.96 Å) Mg(5)-Mg(5) bond length. Both Mg(5)-Mg(7) bond lengths are 3.25 Å. The Mg(5)-Hf(1) bond length is 3.21 Å. The Mg(5)-C(1) bond length is 2.96 Å. In the sixth Mg site, Mg(6) is bonded in a single-bond geometry to two equivalent Mg(5), two equivalent Mg(7), and one C(1) atom. Both Mg(6)-Mg(7) bond lengths are 3.15 Å. The Mg(6)-C(1) bond length is 2.38 Å. In the seventh Mg site, Mg(7) is bonded to one Mg(2), two equivalent Mg(3), two equivalent Mg(4), two equivalent Mg(6), four equivalent Mg(5), and one Hf(1) atom to form MgHfMg11 cuboctahedra that share corners with six equivalent Mg(7)HfMg11 cuboctahedra, edges with two equivalent Mg(2)Mg12 cuboctahedra,  a faceface with one Mg(2)Mg12 cuboctahedra, and faces with two equivalent Mg(7)HfMg11 cuboctahedra. The Mg(7)-Hf(1) bond length is 2.98 Å. Hf(1) is bonded in a 2-coordinate geometry to two equivalent Mg(7), four equivalent Mg(5), and two equivalent C(1) atoms. Both Hf(1)-C(1) bond lengths are 3.05 Å. C(1) is bonded in a 4-coordinate geometry to two equivalent Mg(3), two equivalent Mg(6), four equivalent Mg(5), and two equivalent Hf(1) atoms.