MgTa4(CuO6)2 crystallizes in the monoclinic P2 space group. Mg(1) is bonded to two equivalent O(3), two equivalent O(4), and two equivalent O(5) atoms to form distorted MgO6 octahedra that share corners with two equivalent Ta(1)O6 octahedra, corners with two equivalent Ta(4)O6 octahedra, edges with two equivalent Ta(2)O6 octahedra, edges with two equivalent Ta(3)O6 octahedra, and faces with two equivalent Cu(2)O6 octahedra. The corner-sharing octahedral tilt angles range from 51-52°. There are four inequivalent Ta sites. In the first Ta site, Ta(1) is bonded to two equivalent O(1), two equivalent O(4), and two equivalent O(6) atoms to form TaO6 octahedra that share corners with two equivalent Mg(1)O6 octahedra, corners with four equivalent Ta(3)O6 octahedra, corners with four equivalent Cu(1)O6 octahedra,  an edgeedge with one Ta(4)O6 octahedra, and  an edgeedge with one Cu(2)O6 octahedra. The corner-sharing octahedral tilt angles range from 35-51°. In the second Ta site, Ta(2) is bonded to two equivalent O(2), two equivalent O(3), and two equivalent O(5) atoms to form TaO6 octahedra that share corners with four equivalent Ta(4)O6 octahedra, corners with four equivalent Cu(2)O6 octahedra,  an edgeedge with one Ta(3)O6 octahedra,  an edgeedge with one Cu(1)O6 octahedra, and edges with two equivalent Mg(1)O6 octahedra. The corner-sharing octahedral tilt angles range from 35-52°. In the third Ta site, Ta(3) is bonded to two equivalent O(1), two equivalent O(4), and two equivalent O(5) atoms to form TaO6 octahedra that share corners with four equivalent Ta(1)O6 octahedra, corners with four equivalent Cu(2)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)O6 octahedra. The corner-sharing octahedral tilt angles range from 35-54°. In the fourth Ta site, Ta(4) is bonded to two equivalent O(2), two equivalent O(3), and two equivalent O(6) atoms to form TaO6 octahedra that share corners with two equivalent Mg(1)O6 octahedra, 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(2)O6 octahedra. The corner-sharing octahedral tilt angles range from 35-52°. There are two inequivalent Cu sites. In the first Cu site, Cu(1) is bonded to two equivalent O(1), two equivalent O(2), and two equivalent O(6) atoms to form CuO6 octahedra that share corners with four equivalent Ta(1)O6 octahedra, corners with four equivalent Ta(4)O6 octahedra,  an edgeedge with one Ta(2)O6 octahedra, and  an edgeedge with one Ta(3)O6 octahedra. The corner-sharing octahedral tilt angles range from 50-51°. In the second Cu site, Cu(2) is bonded to two equivalent O(3), two equivalent O(4), and two equivalent O(5) atoms to form distorted CuO6 octahedra that share corners with four equivalent Ta(2)O6 octahedra, corners with four equivalent Ta(3)O6 octahedra,  an edgeedge with one Ta(1)O6 octahedra,  an edgeedge with one Ta(4)O6 octahedra, and faces with two equivalent Mg(1)O6 octahedra. The corner-sharing octahedral tilt angles range from 45-54°. There are six inequivalent O sites. In the first O site, O(1) is bonded in a 3-coordinate geometry to one Ta(1), one Ta(3), and one Cu(1) atom. In the second O site, O(2) is bonded in a 3-coordinate geometry to one Ta(2), one Ta(4), and one Cu(1) atom. In the third O site, O(3) is bonded in a rectangular see-saw-like geometry to one Mg(1), one Ta(2), one Ta(4), and one Cu(2) atom. In the fourth O site, O(4) is bonded in a rectangular see-saw-like geometry to one Mg(1), one Ta(1), one Ta(3), and one Cu(2) atom. In the fifth O site, O(5) is bonded to one Mg(1), one Ta(2), one Ta(3), and one Cu(2) atom to form a mixture of distorted edge and corner-sharing OMgTa2Cu trigonal pyramids. In the sixth O site, O(6) is bonded in a distorted trigonal planar geometry to one Ta(1), one Ta(4), and one Cu(1) atom.
MgTa4(CuO6)2 crystallizes in the monoclinic P2 space group. Mg(1) is bonded to two equivalent O(3), two equivalent O(4), and two equivalent O(5) atoms to form distorted MgO6 octahedra that share corners with two equivalent Ta(1)O6 octahedra, corners with two equivalent Ta(4)O6 octahedra, edges with two equivalent Ta(2)O6 octahedra, edges with two equivalent Ta(3)O6 octahedra, and faces with two equivalent Cu(2)O6 octahedra. The corner-sharing octahedral tilt angles range from 51-52°. Both Mg(1)-O(3) bond lengths are 2.18 Å. Both Mg(1)-O(4) bond lengths are 2.20 Å. Both Mg(1)-O(5) bond lengths are 1.95 Å. There are four inequivalent Ta sites. In the first Ta site, Ta(1) is bonded to two equivalent O(1), two equivalent O(4), and two equivalent O(6) atoms to form TaO6 octahedra that share corners with two equivalent Mg(1)O6 octahedra, corners with four equivalent Ta(3)O6 octahedra, corners with four equivalent Cu(1)O6 octahedra,  an edgeedge with one Ta(4)O6 octahedra, and  an edgeedge with one Cu(2)O6 octahedra. The corner-sharing octahedral tilt angles range from 35-51°. Both Ta(1)-O(1) bond lengths are 2.01 Å. Both Ta(1)-O(4) bond lengths are 2.07 Å. Both Ta(1)-O(6) bond lengths are 1.98 Å. In the second Ta site, Ta(2) is bonded to two equivalent O(2), two equivalent O(3), and two equivalent O(5) atoms to form TaO6 octahedra that share corners with four equivalent Ta(4)O6 octahedra, corners with four equivalent Cu(2)O6 octahedra,  an edgeedge with one Ta(3)O6 octahedra,  an edgeedge with one Cu(1)O6 octahedra, and edges with two equivalent Mg(1)O6 octahedra. The corner-sharing octahedral tilt angles range from 35-52°. Both Ta(2)-O(2) bond lengths are 1.94 Å. Both Ta(2)-O(3) bond lengths are 2.04 Å. Both Ta(2)-O(5) bond lengths are 2.07 Å. In the third Ta site, Ta(3) is bonded to two equivalent O(1), two equivalent O(4), and two equivalent O(5) atoms to form TaO6 octahedra that share corners with four equivalent Ta(1)O6 octahedra, corners with four equivalent Cu(2)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)O6 octahedra. The corner-sharing octahedral tilt angles range from 35-54°. Both Ta(3)-O(1) bond lengths are 1.94 Å. Both Ta(3)-O(4) bond lengths are 2.04 Å. Both Ta(3)-O(5) bond lengths are 2.07 Å. In the fourth Ta site, Ta(4) is bonded to two equivalent O(2), two equivalent O(3), and two equivalent O(6) atoms to form TaO6 octahedra that share corners with two equivalent Mg(1)O6 octahedra, 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(2)O6 octahedra. The corner-sharing octahedral tilt angles range from 35-52°. Both Ta(4)-O(2) bond lengths are 2.01 Å. Both Ta(4)-O(3) bond lengths are 2.07 Å. Both Ta(4)-O(6) bond lengths are 1.99 Å. There are two inequivalent Cu sites. In the first Cu site, Cu(1) is bonded to two equivalent O(1), two equivalent O(2), and two equivalent O(6) atoms to form CuO6 octahedra that share corners with four equivalent Ta(1)O6 octahedra, corners with four equivalent Ta(4)O6 octahedra,  an edgeedge with one Ta(2)O6 octahedra, and  an edgeedge with one Ta(3)O6 octahedra. The corner-sharing octahedral tilt angles range from 50-51°. Both Cu(1)-O(1) bond lengths are 2.26 Å. Both Cu(1)-O(2) bond lengths are 2.27 Å. Both Cu(1)-O(6) bond lengths are 2.28 Å. In the second Cu site, Cu(2) is bonded to two equivalent O(3), two equivalent O(4), and two equivalent O(5) atoms to form distorted CuO6 octahedra that share corners with four equivalent Ta(2)O6 octahedra, corners with four equivalent Ta(3)O6 octahedra,  an edgeedge with one Ta(1)O6 octahedra,  an edgeedge with one Ta(4)O6 octahedra, and faces with two equivalent Mg(1)O6 octahedra. The corner-sharing octahedral tilt angles range from 45-54°. Both Cu(2)-O(3) bond lengths are 2.22 Å. Both Cu(2)-O(4) bond lengths are 2.17 Å. Both Cu(2)-O(5) bond lengths are 2.30 Å. There are six inequivalent O sites. In the first O site, O(1) is bonded in a 3-coordinate geometry to one Ta(1), one Ta(3), and one Cu(1) atom. In the second O site, O(2) is bonded in a 3-coordinate geometry to one Ta(2), one Ta(4), and one Cu(1) atom. In the third O site, O(3) is bonded in a rectangular see-saw-like geometry to one Mg(1), one Ta(2), one Ta(4), and one Cu(2) atom. In the fourth O site, O(4) is bonded in a rectangular see-saw-like geometry to one Mg(1), one Ta(1), one Ta(3), and one Cu(2) atom. In the fifth O site, O(5) is bonded to one Mg(1), one Ta(2), one Ta(3), and one Cu(2) atom to form a mixture of distorted edge and corner-sharing OMgTa2Cu trigonal pyramids. In the sixth O site, O(6) is bonded in a distorted trigonal planar geometry to one Ta(1), one Ta(4), and one Cu(1) atom.