MgCu2O4 crystallizes in the orthorhombic Pmmn space group. There are two inequivalent Mg sites. In the first Mg site, Mg(1) is bonded to two equivalent O(1) and four equivalent O(3) atoms to form distorted MgO6 pentagonal pyramids that share corners with six equivalent Cu(1)O6 octahedra, edges with two equivalent Cu(1)O6 octahedra, edges with four equivalent Cu(2)O6 octahedra, and edges with two equivalent Mg(2)O6 pentagonal pyramids. The corner-sharing octahedral tilt angles range from 4-17°. In the second Mg site, Mg(2) is bonded to two equivalent O(2) and four equivalent O(3) atoms to form distorted MgO6 pentagonal pyramids that share corners with four equivalent Cu(1)O6 octahedra, corners with eight equivalent Cu(2)O6 octahedra, edges with two equivalent Mg(1)O6 pentagonal pyramids, and faces with two equivalent Cu(1)O6 octahedra. The corner-sharing octahedral tilt angles range from 45-53°. There are two inequivalent Cu sites. In the first Cu site, Cu(1) is bonded to one O(1), one O(2), and four equivalent O(3) atoms to form CuO6 octahedra that share corners with two equivalent Mg(2)O6 pentagonal pyramids, corners with three equivalent Mg(1)O6 pentagonal pyramids, edges with two equivalent Cu(1)O6 octahedra, edges with four equivalent Cu(2)O6 octahedra,  an edgeedge with one Mg(1)O6 pentagonal pyramid, and  a faceface with one Mg(2)O6 pentagonal pyramid. In the second Cu site, Cu(2) is bonded to two equivalent O(1), two equivalent O(2), and two equivalent O(3) atoms to form CuO6 octahedra that share corners with four equivalent Mg(2)O6 pentagonal pyramids, edges with two equivalent Cu(2)O6 octahedra, edges with four equivalent Cu(1)O6 octahedra, and edges with two equivalent Mg(1)O6 pentagonal pyramids. There are three inequivalent O sites. In the first O site, O(1) is bonded in a rectangular see-saw-like geometry to one Mg(1), one Cu(1), and two equivalent Cu(2) atoms. In the second O site, O(2) is bonded to one Mg(2), one Cu(1), and two equivalent Cu(2) atoms to form OMgCu3 trigonal pyramids that share corners with four equivalent O(3)Mg2Cu3 trigonal bipyramids, corners with three equivalent O(2)MgCu3 trigonal pyramids, and edges with four equivalent O(3)Mg2Cu3 trigonal bipyramids. In the third O site, O(3) is bonded to one Mg(1), one Mg(2), one Cu(2), and two equivalent Cu(1) atoms to form OMg2Cu3 trigonal bipyramids that share corners with five equivalent O(3)Mg2Cu3 trigonal bipyramids, corners with two equivalent O(2)MgCu3 trigonal pyramids, edges with four equivalent O(3)Mg2Cu3 trigonal bipyramids, and edges with two equivalent O(2)MgCu3 trigonal pyramids.
MgCu2O4 crystallizes in the orthorhombic Pmmn space group. There are two inequivalent Mg sites. In the first Mg site, Mg(1) is bonded to two equivalent O(1) and four equivalent O(3) atoms to form distorted MgO6 pentagonal pyramids that share corners with six equivalent Cu(1)O6 octahedra, edges with two equivalent Cu(1)O6 octahedra, edges with four equivalent Cu(2)O6 octahedra, and edges with two equivalent Mg(2)O6 pentagonal pyramids. The corner-sharing octahedral tilt angles range from 4-17°. Both Mg(1)-O(1) bond lengths are 2.09 Å. All Mg(1)-O(3) bond lengths are 2.17 Å. In the second Mg site, Mg(2) is bonded to two equivalent O(2) and four equivalent O(3) atoms to form distorted MgO6 pentagonal pyramids that share corners with four equivalent Cu(1)O6 octahedra, corners with eight equivalent Cu(2)O6 octahedra, edges with two equivalent Mg(1)O6 pentagonal pyramids, and faces with two equivalent Cu(1)O6 octahedra. The corner-sharing octahedral tilt angles range from 45-53°. Both Mg(2)-O(2) bond lengths are 2.12 Å. All Mg(2)-O(3) bond lengths are 2.15 Å. There are two inequivalent Cu sites. In the first Cu site, Cu(1) is bonded to one O(1), one O(2), and four equivalent O(3) atoms to form CuO6 octahedra that share corners with two equivalent Mg(2)O6 pentagonal pyramids, corners with three equivalent Mg(1)O6 pentagonal pyramids, edges with two equivalent Cu(1)O6 octahedra, edges with four equivalent Cu(2)O6 octahedra,  an edgeedge with one Mg(1)O6 pentagonal pyramid, and  a faceface with one Mg(2)O6 pentagonal pyramid. The Cu(1)-O(1) bond length is 1.96 Å. The Cu(1)-O(2) bond length is 1.96 Å. There are two shorter (2.00 Å) and two longer (2.11 Å) Cu(1)-O(3) bond lengths. In the second Cu site, Cu(2) is bonded to two equivalent O(1), two equivalent O(2), and two equivalent O(3) atoms to form CuO6 octahedra that share corners with four equivalent Mg(2)O6 pentagonal pyramids, edges with two equivalent Cu(2)O6 octahedra, edges with four equivalent Cu(1)O6 octahedra, and edges with two equivalent Mg(1)O6 pentagonal pyramids. Both Cu(2)-O(1) bond lengths are 1.92 Å. Both Cu(2)-O(2) bond lengths are 2.00 Å. Both Cu(2)-O(3) bond lengths are 2.07 Å. There are three inequivalent O sites. In the first O site, O(1) is bonded in a rectangular see-saw-like geometry to one Mg(1), one Cu(1), and two equivalent Cu(2) atoms. In the second O site, O(2) is bonded to one Mg(2), one Cu(1), and two equivalent Cu(2) atoms to form OMgCu3 trigonal pyramids that share corners with four equivalent O(3)Mg2Cu3 trigonal bipyramids, corners with three equivalent O(2)MgCu3 trigonal pyramids, and edges with four equivalent O(3)Mg2Cu3 trigonal bipyramids. In the third O site, O(3) is bonded to one Mg(1), one Mg(2), one Cu(2), and two equivalent Cu(1) atoms to form OMg2Cu3 trigonal bipyramids that share corners with five equivalent O(3)Mg2Cu3 trigonal bipyramids, corners with two equivalent O(2)MgCu3 trigonal pyramids, edges with four equivalent O(3)Mg2Cu3 trigonal bipyramids, and edges with two equivalent O(2)MgCu3 trigonal pyramids.