MgCa3 crystallizes in the hexagonal P-6m2 space group. There are two inequivalent Ca sites. In the first Ca site, Ca(1) is bonded to six equivalent Ca(1) and six equivalent Ca(2) atoms to form CaCa12 cuboctahedra that share corners with six equivalent Ca(1)Ca12 cuboctahedra, corners with twelve equivalent Ca(2)Ca3Mg3 cuboctahedra, edges with six equivalent Ca(1)Ca12 cuboctahedra, edges with twelve equivalent Ca(2)Ca3Mg3 cuboctahedra, and faces with six equivalent Ca(1)Ca12 cuboctahedra. In the second Ca site, Ca(2) is bonded to three equivalent Ca(1) and three equivalent Mg(1) atoms to form distorted CaCa3Mg3 cuboctahedra that share corners with six equivalent Ca(1)Ca12 cuboctahedra, corners with twelve equivalent Ca(2)Ca3Mg3 cuboctahedra, edges with six equivalent Ca(1)Ca12 cuboctahedra, edges with six equivalent Ca(2)Ca3Mg3 cuboctahedra, and faces with two equivalent Ca(2)Ca3Mg3 cuboctahedra. Mg(1) is bonded in a 6-coordinate geometry to six equivalent Ca(2) atoms.
MgCa3 crystallizes in the hexagonal P-6m2 space group. There are two inequivalent Ca sites. In the first Ca site, Ca(1) is bonded to six equivalent Ca(1) and six equivalent Ca(2) atoms to form CaCa12 cuboctahedra that share corners with six equivalent Ca(1)Ca12 cuboctahedra, corners with twelve equivalent Ca(2)Ca3Mg3 cuboctahedra, edges with six equivalent Ca(1)Ca12 cuboctahedra, edges with twelve equivalent Ca(2)Ca3Mg3 cuboctahedra, and faces with six equivalent Ca(1)Ca12 cuboctahedra. All Ca(1)-Ca(1) bond lengths are 3.82 Å. All Ca(1)-Ca(2) bond lengths are 3.92 Å. In the second Ca site, Ca(2) is bonded to three equivalent Ca(1) and three equivalent Mg(1) atoms to form distorted CaCa3Mg3 cuboctahedra that share corners with six equivalent Ca(1)Ca12 cuboctahedra, corners with twelve equivalent Ca(2)Ca3Mg3 cuboctahedra, edges with six equivalent Ca(1)Ca12 cuboctahedra, edges with six equivalent Ca(2)Ca3Mg3 cuboctahedra, and faces with two equivalent Ca(2)Ca3Mg3 cuboctahedra. All Ca(2)-Mg(1) bond lengths are 3.52 Å. Mg(1) is bonded in a 6-coordinate geometry to six equivalent Ca(2) atoms.