K3Mg2Cl7 crystallizes in the tetragonal I4/mmm space group. There are two inequivalent K sites. In the first K site, K(1) is bonded in a 9-coordinate geometry to four equivalent Cl(1) and five equivalent Cl(2) atoms. In the second K site, K(2) is bonded to four equivalent Cl(3) and eight equivalent Cl(1) atoms to form KCl12 cuboctahedra that share corners with four equivalent K(2)Cl12 cuboctahedra, faces with four equivalent K(2)Cl12 cuboctahedra, and faces with eight equivalent Mg(1)Cl6 octahedra. Mg(1) is bonded to one Cl(2), one Cl(3), and four equivalent Cl(1) atoms to form MgCl6 octahedra that share corners with five equivalent Mg(1)Cl6 octahedra and faces with four equivalent K(2)Cl12 cuboctahedra. The corner-sharing octahedral tilt angles range from 0-4°. There are three inequivalent Cl sites. In the first Cl site, Cl(1) is bonded to two equivalent K(1), two equivalent K(2), and two equivalent Mg(1) atoms to form distorted ClK4Mg2 octahedra that share corners with six equivalent Cl(2)K5Mg octahedra, corners with eight equivalent Cl(1)K4Mg2 octahedra, edges with three equivalent Cl(1)K4Mg2 octahedra, faces with two equivalent Cl(2)K5Mg octahedra, and faces with four equivalent Cl(1)K4Mg2 octahedra. The corner-sharing octahedral tilt angles range from 0-61°. In the second Cl site, Cl(2) is bonded to five equivalent K(1) and one Mg(1) atom to form distorted ClK5Mg octahedra that share corners with four equivalent Cl(2)K5Mg octahedra, corners with twelve equivalent Cl(1)K4Mg2 octahedra, edges with eight equivalent Cl(2)K5Mg octahedra, and faces with four equivalent Cl(1)K4Mg2 octahedra. The corner-sharing octahedral tilt angles range from 12-53°. In the third Cl site, Cl(3) is bonded in a distorted linear geometry to four equivalent K(2) and two equivalent Mg(1) atoms.
K3Mg2Cl7 crystallizes in the tetragonal I4/mmm space group. There are two inequivalent K sites. In the first K site, K(1) is bonded in a 9-coordinate geometry to four equivalent Cl(1) and five equivalent Cl(2) atoms. All K(1)-Cl(1) bond lengths are 3.30 Å. There is one shorter (3.08 Å) and four longer (3.52 Å) K(1)-Cl(2) bond lengths. In the second K site, K(2) is bonded to four equivalent Cl(3) and eight equivalent Cl(1) atoms to form KCl12 cuboctahedra that share corners with four equivalent K(2)Cl12 cuboctahedra, faces with four equivalent K(2)Cl12 cuboctahedra, and faces with eight equivalent Mg(1)Cl6 octahedra. All K(2)-Cl(3) bond lengths are 3.50 Å. All K(2)-Cl(1) bond lengths are 3.47 Å. Mg(1) is bonded to one Cl(2), one Cl(3), and four equivalent Cl(1) atoms to form MgCl6 octahedra that share corners with five equivalent Mg(1)Cl6 octahedra and faces with four equivalent K(2)Cl12 cuboctahedra. The corner-sharing octahedral tilt angles range from 0-4°. The Mg(1)-Cl(2) bond length is 2.46 Å. The Mg(1)-Cl(3) bond length is 2.52 Å. All Mg(1)-Cl(1) bond lengths are 2.48 Å. There are three inequivalent Cl sites. In the first Cl site, Cl(1) is bonded to two equivalent K(1), two equivalent K(2), and two equivalent Mg(1) atoms to form distorted ClK4Mg2 octahedra that share corners with six equivalent Cl(2)K5Mg octahedra, corners with eight equivalent Cl(1)K4Mg2 octahedra, edges with three equivalent Cl(1)K4Mg2 octahedra, faces with two equivalent Cl(2)K5Mg octahedra, and faces with four equivalent Cl(1)K4Mg2 octahedra. The corner-sharing octahedral tilt angles range from 0-61°. In the second Cl site, Cl(2) is bonded to five equivalent K(1) and one Mg(1) atom to form distorted ClK5Mg octahedra that share corners with four equivalent Cl(2)K5Mg octahedra, corners with twelve equivalent Cl(1)K4Mg2 octahedra, edges with eight equivalent Cl(2)K5Mg octahedra, and faces with four equivalent Cl(1)K4Mg2 octahedra. The corner-sharing octahedral tilt angles range from 12-53°. In the third Cl site, Cl(3) is bonded in a distorted linear geometry to four equivalent K(2) and two equivalent Mg(1) atoms.