KMg6Nb crystallizes in the orthorhombic Amm2 space group. K(1) is bonded to two equivalent Mg(3), two equivalent Mg(4), four equivalent Mg(1), and four equivalent Mg(2) atoms to form KMg12 cuboctahedra that share corners with four equivalent K(1)Mg12 cuboctahedra, corners with eight equivalent Nb(1)Mg12 cuboctahedra, edges with eight equivalent Mg(1)K2Mg8Nb2 cuboctahedra, faces with two equivalent K(1)Mg12 cuboctahedra, faces with four equivalent Mg(1)K2Mg8Nb2 cuboctahedra, and faces with four equivalent Nb(1)Mg12 cuboctahedra. There are four inequivalent Mg sites. In the first Mg site, Mg(1) is bonded to two equivalent K(1), two equivalent Mg(3), two equivalent Mg(4), four equivalent Mg(2), and two equivalent Nb(1) atoms to form distorted MgK2Mg8Nb2 cuboctahedra that share corners with twelve equivalent Mg(1)K2Mg8Nb2 cuboctahedra, edges with four equivalent K(1)Mg12 cuboctahedra, edges with four equivalent Nb(1)Mg12 cuboctahedra, faces with two equivalent K(1)Mg12 cuboctahedra, faces with two equivalent Nb(1)Mg12 cuboctahedra, and faces with six equivalent Mg(1)K2Mg8Nb2 cuboctahedra. In the second Mg site, Mg(2) is bonded in a 10-coordinate geometry to two equivalent K(1), one Mg(3), one Mg(4), four equivalent Mg(1), and two equivalent Nb(1) atoms. In the third Mg site, Mg(3) is bonded in a 10-coordinate geometry to two equivalent K(1), two equivalent Mg(2), four equivalent Mg(1), and two equivalent Nb(1) atoms. In the fourth Mg site, Mg(4) is bonded in a 10-coordinate geometry to two equivalent K(1), two equivalent Mg(2), four equivalent Mg(1), and two equivalent Nb(1) atoms. Nb(1) is bonded to two equivalent Mg(3), two equivalent Mg(4), four equivalent Mg(1), and four equivalent Mg(2) atoms to form NbMg12 cuboctahedra that share corners with four equivalent Nb(1)Mg12 cuboctahedra, corners with eight equivalent K(1)Mg12 cuboctahedra, edges with eight equivalent Mg(1)K2Mg8Nb2 cuboctahedra, faces with two equivalent Nb(1)Mg12 cuboctahedra, faces with four equivalent K(1)Mg12 cuboctahedra, and faces with four equivalent Mg(1)K2Mg8Nb2 cuboctahedra.
KMg6Nb crystallizes in the orthorhombic Amm2 space group. K(1) is bonded to two equivalent Mg(3), two equivalent Mg(4), four equivalent Mg(1), and four equivalent Mg(2) atoms to form KMg12 cuboctahedra that share corners with four equivalent K(1)Mg12 cuboctahedra, corners with eight equivalent Nb(1)Mg12 cuboctahedra, edges with eight equivalent Mg(1)K2Mg8Nb2 cuboctahedra, faces with two equivalent K(1)Mg12 cuboctahedra, faces with four equivalent Mg(1)K2Mg8Nb2 cuboctahedra, and faces with four equivalent Nb(1)Mg12 cuboctahedra. Both K(1)-Mg(3) bond lengths are 3.43 Å. Both K(1)-Mg(4) bond lengths are 3.50 Å. There are two shorter (3.27 Å) and two longer (3.28 Å) K(1)-Mg(1) bond lengths. All K(1)-Mg(2) bond lengths are 3.46 Å. There are four inequivalent Mg sites. In the first Mg site, Mg(1) is bonded to two equivalent K(1), two equivalent Mg(3), two equivalent Mg(4), four equivalent Mg(2), and two equivalent Nb(1) atoms to form distorted MgK2Mg8Nb2 cuboctahedra that share corners with twelve equivalent Mg(1)K2Mg8Nb2 cuboctahedra, edges with four equivalent K(1)Mg12 cuboctahedra, edges with four equivalent Nb(1)Mg12 cuboctahedra, faces with two equivalent K(1)Mg12 cuboctahedra, faces with two equivalent Nb(1)Mg12 cuboctahedra, and faces with six equivalent Mg(1)K2Mg8Nb2 cuboctahedra. Both Mg(1)-Mg(3) bond lengths are 3.30 Å. Both Mg(1)-Mg(4) bond lengths are 3.23 Å. All Mg(1)-Mg(2) bond lengths are 3.28 Å. There is one shorter (3.25 Å) and one longer (3.31 Å) Mg(1)-Nb(1) bond length. In the second Mg site, Mg(2) is bonded in a 10-coordinate geometry to two equivalent K(1), one Mg(3), one Mg(4), four equivalent Mg(1), and two equivalent Nb(1) atoms. The Mg(2)-Mg(3) bond length is 2.87 Å. The Mg(2)-Mg(4) bond length is 2.90 Å. Both Mg(2)-Nb(1) bond lengths are 3.06 Å. In the third Mg site, Mg(3) is bonded in a 10-coordinate geometry to two equivalent K(1), two equivalent Mg(2), four equivalent Mg(1), and two equivalent Nb(1) atoms. Both Mg(3)-Nb(1) bond lengths are 3.07 Å. In the fourth Mg site, Mg(4) is bonded in a 10-coordinate geometry to two equivalent K(1), two equivalent Mg(2), four equivalent Mg(1), and two equivalent Nb(1) atoms. Both Mg(4)-Nb(1) bond lengths are 3.09 Å. Nb(1) is bonded to two equivalent Mg(3), two equivalent Mg(4), four equivalent Mg(1), and four equivalent Mg(2) atoms to form NbMg12 cuboctahedra that share corners with four equivalent Nb(1)Mg12 cuboctahedra, corners with eight equivalent K(1)Mg12 cuboctahedra, edges with eight equivalent Mg(1)K2Mg8Nb2 cuboctahedra, faces with two equivalent Nb(1)Mg12 cuboctahedra, faces with four equivalent K(1)Mg12 cuboctahedra, and faces with four equivalent Mg(1)K2Mg8Nb2 cuboctahedra.