MgTa4(CuO6)2 crystallizes in the tetragonal P-4 space group. Mg(1) is bonded to four equivalent O(1) atoms to form distorted MgO4 trigonal pyramids that share corners with four equivalent Cu(1)O6 octahedra and edges with four equivalent Ta(1)O6 octahedra. The corner-sharing octahedral tilt angles are 73°. There are two inequivalent Ta sites. In the first Ta site, Ta(1) is bonded to two equivalent O(3) and four equivalent O(1) atoms to form TaO6 octahedra that share corners with four equivalent Ta(1)O6 octahedra, corners with four equivalent Cu(1)O6 octahedra,  an edgeedge with one Ta(2)O6 octahedra,  an edgeedge with one Cu(1)O6 octahedra, and edges with two equivalent Mg(1)O4 trigonal pyramids. The corner-sharing octahedral tilt angles range from 33-57°. In the second Ta site, Ta(2) is bonded to two equivalent O(3) and four equivalent O(2) atoms to form TaO6 octahedra that share corners with four equivalent Ta(2)O6 octahedra, corners with four equivalent Cu(1)O6 octahedra,  an edgeedge with one Ta(1)O6 octahedra, and  an edgeedge with one Cu(1)O6 octahedra. The corner-sharing octahedral tilt angles range from 38-53°. Cu(1) 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 Ta(1)O6 octahedra, corners with four equivalent Ta(2)O6 octahedra, corners with two equivalent Mg(1)O4 trigonal pyramids,  an edgeedge with one Ta(1)O6 octahedra, and  an edgeedge with one Ta(2)O6 octahedra. The corner-sharing octahedral tilt angles range from 45-57°. There are three inequivalent O sites. In the first O site, O(1) is bonded in a distorted see-saw-like geometry to one Mg(1), two equivalent Ta(1), and one Cu(1) atom. In the second O site, O(2) is bonded in a 3-coordinate geometry to two equivalent Ta(2) and one Cu(1) atom. In the third O site, O(3) is bonded in a distorted trigonal planar geometry to one Ta(1), one Ta(2), and one Cu(1) atom.
MgTa4(CuO6)2 crystallizes in the tetragonal P-4 space group. Mg(1) is bonded to four equivalent O(1) atoms to form distorted MgO4 trigonal pyramids that share corners with four equivalent Cu(1)O6 octahedra and edges with four equivalent Ta(1)O6 octahedra. The corner-sharing octahedral tilt angles are 73°. All Mg(1)-O(1) bond lengths are 1.90 Å. There are two inequivalent Ta sites. In the first Ta site, Ta(1) is bonded to two equivalent O(3) and four equivalent O(1) atoms to form TaO6 octahedra that share corners with four equivalent Ta(1)O6 octahedra, corners with four equivalent Cu(1)O6 octahedra,  an edgeedge with one Ta(2)O6 octahedra,  an edgeedge with one Cu(1)O6 octahedra, and edges with two equivalent Mg(1)O4 trigonal pyramids. The corner-sharing octahedral tilt angles range from 33-57°. Both Ta(1)-O(3) bond lengths are 1.91 Å. There are two shorter (2.02 Å) and two longer (2.17 Å) Ta(1)-O(1) bond lengths. In the second Ta site, Ta(2) is bonded to two equivalent O(3) and four equivalent O(2) atoms to form TaO6 octahedra that share corners with four equivalent Ta(2)O6 octahedra, corners with four equivalent Cu(1)O6 octahedra,  an edgeedge with one Ta(1)O6 octahedra, and  an edgeedge with one Cu(1)O6 octahedra. The corner-sharing octahedral tilt angles range from 38-53°. Both Ta(2)-O(3) bond lengths are 2.11 Å. There are two shorter (1.95 Å) and two longer (2.04 Å) Ta(2)-O(2) bond lengths. Cu(1) 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 Ta(1)O6 octahedra, corners with four equivalent Ta(2)O6 octahedra, corners with two equivalent Mg(1)O4 trigonal pyramids,  an edgeedge with one Ta(1)O6 octahedra, and  an edgeedge with one Ta(2)O6 octahedra. The corner-sharing octahedral tilt angles range from 45-57°. Both Cu(1)-O(1) bond lengths are 2.37 Å. Both Cu(1)-O(2) bond lengths are 2.24 Å. Both Cu(1)-O(3) bond lengths are 2.30 Å. There are three inequivalent O sites. In the first O site, O(1) is bonded in a distorted see-saw-like geometry to one Mg(1), two equivalent Ta(1), and one Cu(1) atom. In the second O site, O(2) is bonded in a 3-coordinate geometry to two equivalent Ta(2) and one Cu(1) atom. In the third O site, O(3) is bonded in a distorted trigonal planar geometry to one Ta(1), one Ta(2), and one Cu(1) atom.