MgTa2(CuO4)2 crystallizes in the monoclinic P2_1/c space group. Mg(1) is bonded to one O(1), one O(4), one O(7), one O(8), and two equivalent O(3) atoms to form MgO6 octahedra that share corners with three equivalent Ta(1)O4 tetrahedra, corners with three equivalent Ta(2)O4 tetrahedra,  an edgeedge with one Mg(1)O6 octahedra, and edges with two equivalent Cu(1)O5 trigonal bipyramids. There are two inequivalent Ta sites. In the first Ta site, Ta(1) is bonded to one O(1), one O(2), one O(4), and one O(7) atom to form TaO4 tetrahedra that share corners with three equivalent Mg(1)O6 octahedra and corners with two equivalent Cu(1)O5 trigonal bipyramids. The corner-sharing octahedral tilt angles range from 30-50°. In the second Ta site, Ta(2) is bonded to one O(3), one O(5), one O(6), and one O(8) atom to form TaO4 tetrahedra that share corners with three equivalent Mg(1)O6 octahedra and corners with three equivalent Cu(1)O5 trigonal bipyramids. The corner-sharing octahedral tilt angles range from 53-60°. There are two inequivalent Cu sites. In the first Cu site, Cu(1) is bonded to one O(1), one O(3), one O(5), one O(6), and one O(7) atom to form CuO5 trigonal bipyramids that share corners with two equivalent Ta(1)O4 tetrahedra, corners with three equivalent Ta(2)O4 tetrahedra, and edges with two equivalent Mg(1)O6 octahedra. In the second Cu site, Cu(2) is bonded in a rectangular see-saw-like geometry to one O(2), one O(4), one O(6), and one O(8) atom. There are eight inequivalent O sites. In the first O site, O(7) is bonded in a 3-coordinate geometry to one Mg(1), one Ta(1), and one Cu(1) atom. In the second O site, O(8) is bonded in a distorted trigonal planar geometry to one Mg(1), one Ta(2), and one Cu(2) atom. In the third O site, O(1) is bonded in a distorted trigonal planar geometry to one Mg(1), one Ta(1), and one Cu(1) atom. In the fourth O site, O(2) is bonded in a bent 150 degrees geometry to one Ta(1) and one Cu(2) atom. In the fifth O site, O(3) is bonded in a distorted rectangular see-saw-like geometry to two equivalent Mg(1), one Ta(2), and one Cu(1) atom. In the sixth O site, O(4) is bonded in a distorted trigonal planar geometry to one Mg(1), one Ta(1), and one Cu(2) atom. In the seventh O site, O(5) is bonded in a distorted bent 150 degrees geometry to one Ta(2) and one Cu(1) atom. In the eighth O site, O(6) is bonded in a 3-coordinate geometry to one Ta(2), one Cu(1), and one Cu(2) atom.
MgTa2(CuO4)2 crystallizes in the monoclinic P2_1/c space group. Mg(1) is bonded to one O(1), one O(4), one O(7), one O(8), and two equivalent O(3) atoms to form MgO6 octahedra that share corners with three equivalent Ta(1)O4 tetrahedra, corners with three equivalent Ta(2)O4 tetrahedra,  an edgeedge with one Mg(1)O6 octahedra, and edges with two equivalent Cu(1)O5 trigonal bipyramids. The Mg(1)-O(1) bond length is 2.07 Å. The Mg(1)-O(4) bond length is 2.11 Å. The Mg(1)-O(7) bond length is 2.03 Å. The Mg(1)-O(8) bond length is 2.09 Å. There is one shorter (2.12 Å) and one longer (2.31 Å) Mg(1)-O(3) bond length. There are two inequivalent Ta sites. In the first Ta site, Ta(1) is bonded to one O(1), one O(2), one O(4), and one O(7) atom to form TaO4 tetrahedra that share corners with three equivalent Mg(1)O6 octahedra and corners with two equivalent Cu(1)O5 trigonal bipyramids. The corner-sharing octahedral tilt angles range from 30-50°. The Ta(1)-O(1) bond length is 1.90 Å. The Ta(1)-O(2) bond length is 1.85 Å. The Ta(1)-O(4) bond length is 1.90 Å. The Ta(1)-O(7) bond length is 1.84 Å. In the second Ta site, Ta(2) is bonded to one O(3), one O(5), one O(6), and one O(8) atom to form TaO4 tetrahedra that share corners with three equivalent Mg(1)O6 octahedra and corners with three equivalent Cu(1)O5 trigonal bipyramids. The corner-sharing octahedral tilt angles range from 53-60°. The Ta(2)-O(3) bond length is 1.91 Å. The Ta(2)-O(5) bond length is 1.82 Å. The Ta(2)-O(6) bond length is 1.93 Å. The Ta(2)-O(8) bond length is 1.86 Å. There are two inequivalent Cu sites. In the first Cu site, Cu(1) is bonded to one O(1), one O(3), one O(5), one O(6), and one O(7) atom to form CuO5 trigonal bipyramids that share corners with two equivalent Ta(1)O4 tetrahedra, corners with three equivalent Ta(2)O4 tetrahedra, and edges with two equivalent Mg(1)O6 octahedra. The Cu(1)-O(1) bond length is 1.93 Å. The Cu(1)-O(3) bond length is 2.09 Å. The Cu(1)-O(5) bond length is 1.91 Å. The Cu(1)-O(6) bond length is 2.03 Å. The Cu(1)-O(7) bond length is 2.31 Å. In the second Cu site, Cu(2) is bonded in a rectangular see-saw-like geometry to one O(2), one O(4), one O(6), and one O(8) atom. The Cu(2)-O(2) bond length is 1.92 Å. The Cu(2)-O(4) bond length is 1.96 Å. The Cu(2)-O(6) bond length is 2.05 Å. The Cu(2)-O(8) bond length is 1.96 Å. There are eight inequivalent O sites. In the first O site, O(7) is bonded in a 3-coordinate geometry to one Mg(1), one Ta(1), and one Cu(1) atom. In the second O site, O(8) is bonded in a distorted trigonal planar geometry to one Mg(1), one Ta(2), and one Cu(2) atom. In the third O site, O(1) is bonded in a distorted trigonal planar geometry to one Mg(1), one Ta(1), and one Cu(1) atom. In the fourth O site, O(2) is bonded in a bent 150 degrees geometry to one Ta(1) and one Cu(2) atom. In the fifth O site, O(3) is bonded in a distorted rectangular see-saw-like geometry to two equivalent Mg(1), one Ta(2), and one Cu(1) atom. In the sixth O site, O(4) is bonded in a distorted trigonal planar geometry to one Mg(1), one Ta(1), and one Cu(2) atom. In the seventh O site, O(5) is bonded in a distorted bent 150 degrees geometry to one Ta(2) and one Cu(1) atom. In the eighth O site, O(6) is bonded in a 3-coordinate geometry to one Ta(2), one Cu(1), and one Cu(2) atom.