MgTa4(AsO5)4 crystallizes in the monoclinic Pm space group. Mg(1) is bonded in a 3-coordinate geometry to one O(3) and two equivalent O(7) atoms. There are four inequivalent Ta sites. In the first Ta site, Ta(1) is bonded to one O(1), one O(12), two equivalent O(13), and two equivalent O(5) atoms to form TaO6 octahedra that share corners with two equivalent Ta(2)O6 octahedra,  a cornercorner with one As(1)O4 tetrahedra, and corners with two equivalent As(2)O4 tetrahedra. The corner-sharing octahedral tilt angles are 4°. In the second Ta site, Ta(2) is bonded to one O(11), one O(2), two equivalent O(13), and two equivalent O(6) atoms to form TaO6 octahedra that share corners with two equivalent Ta(1)O6 octahedra,  a cornercorner with one As(2)O4 tetrahedra,  a cornercorner with one As(3)O4 tetrahedra, and corners with two equivalent As(1)O4 tetrahedra. The corner-sharing octahedral tilt angles are 4°. In the third Ta site, Ta(3) is bonded to one O(10), two equivalent O(14), and two equivalent O(7) atoms to form distorted TaO5 trigonal bipyramids that share corners with two equivalent Ta(4)O6 octahedra and  a cornercorner with one As(3)O4 tetrahedra. The corner-sharing octahedral tilt angles are 11°. In the fourth Ta site, Ta(4) is bonded to one O(4), one O(9), two equivalent O(14), and two equivalent O(8) atoms to form TaO6 octahedra that share  a cornercorner with one As(1)O4 tetrahedra, corners with two equivalent As(3)O4 tetrahedra, and corners with two equivalent Ta(3)O5 trigonal bipyramids. There are four inequivalent As sites. In the first As site, As(1) is bonded to one O(12), one O(4), and two equivalent O(6) atoms to form AsO4 tetrahedra that share  a cornercorner with one Ta(1)O6 octahedra,  a cornercorner with one Ta(4)O6 octahedra, and corners with two equivalent Ta(2)O6 octahedra. The corner-sharing octahedral tilt angles range from 11-36°. In the second As site, As(2) is bonded to one O(11), one O(3), and two equivalent O(5) atoms to form AsO4 tetrahedra that share  a cornercorner with one Ta(2)O6 octahedra and corners with two equivalent Ta(1)O6 octahedra. The corner-sharing octahedral tilt angles range from 15-45°. In the third As site, As(3) is bonded to one O(10), one O(2), and two equivalent O(8) atoms to form AsO4 tetrahedra that share  a cornercorner with one Ta(2)O6 octahedra, corners with two equivalent Ta(4)O6 octahedra, and  a cornercorner with one Ta(3)O5 trigonal bipyramid. The corner-sharing octahedral tilt angles range from 12-26°. In the fourth As site, As(4) is bonded in a trigonal non-coplanar geometry to one O(1) and two equivalent O(7) atoms. There are fourteen inequivalent O sites. In the first O site, O(1) is bonded in a bent 150 degrees geometry to one Ta(1) and one As(4) atom. In the second O site, O(2) is bonded in a bent 150 degrees geometry to one Ta(2) and one As(3) atom. In the third O site, O(3) is bonded in a bent 120 degrees geometry to one Mg(1) and one As(2) atom. In the fourth O site, O(4) is bonded in a bent 150 degrees geometry to one Ta(4) and one As(1) atom. In the fifth O site, O(5) is bonded in a linear geometry to one Ta(1) and one As(2) atom. In the sixth O site, O(6) is bonded in a linear geometry to one Ta(2) and one As(1) atom. In the seventh O site, O(7) is bonded in a trigonal planar geometry to one Mg(1), one Ta(3), and one As(4) atom. In the eighth O site, O(8) is bonded in a linear geometry to one Ta(4) and one As(3) atom. In the ninth O site, O(9) is bonded in a single-bond geometry to one Ta(4) atom. In the tenth O site, O(10) is bonded in a bent 150 degrees geometry to one Ta(3) and one As(3) atom. In the eleventh O site, O(11) is bonded in a distorted bent 150 degrees geometry to one Ta(2) and one As(2) atom. In the twelfth O site, O(12) is bonded in a bent 150 degrees geometry to one Ta(1) and one As(1) atom. In the thirteenth O site, O(13) is bonded in a linear geometry to one Ta(1) and one Ta(2) atom. In the fourteenth O site, O(14) is bonded in a linear geometry to one Ta(3) and one Ta(4) atom.
MgTa4(AsO5)4 crystallizes in the monoclinic Pm space group. Mg(1) is bonded in a 3-coordinate geometry to one O(3) and two equivalent O(7) atoms. The Mg(1)-O(3) bond length is 1.94 Å. Both Mg(1)-O(7) bond lengths are 2.04 Å. There are four inequivalent Ta sites. In the first Ta site, Ta(1) is bonded to one O(1), one O(12), two equivalent O(13), and two equivalent O(5) atoms to form TaO6 octahedra that share corners with two equivalent Ta(2)O6 octahedra,  a cornercorner with one As(1)O4 tetrahedra, and corners with two equivalent As(2)O4 tetrahedra. The corner-sharing octahedral tilt angles are 4°. The Ta(1)-O(1) bond length is 2.00 Å. The Ta(1)-O(12) bond length is 2.01 Å. Both Ta(1)-O(13) bond lengths are 1.92 Å. Both Ta(1)-O(5) bond lengths are 2.02 Å. In the second Ta site, Ta(2) is bonded to one O(11), one O(2), two equivalent O(13), and two equivalent O(6) atoms to form TaO6 octahedra that share corners with two equivalent Ta(1)O6 octahedra,  a cornercorner with one As(2)O4 tetrahedra,  a cornercorner with one As(3)O4 tetrahedra, and corners with two equivalent As(1)O4 tetrahedra. The corner-sharing octahedral tilt angles are 4°. The Ta(2)-O(11) bond length is 2.02 Å. The Ta(2)-O(2) bond length is 2.04 Å. Both Ta(2)-O(13) bond lengths are 1.92 Å. Both Ta(2)-O(6) bond lengths are 2.02 Å. In the third Ta site, Ta(3) is bonded to one O(10), two equivalent O(14), and two equivalent O(7) atoms to form distorted TaO5 trigonal bipyramids that share corners with two equivalent Ta(4)O6 octahedra and  a cornercorner with one As(3)O4 tetrahedra. The corner-sharing octahedral tilt angles are 11°. The Ta(3)-O(10) bond length is 1.90 Å. Both Ta(3)-O(14) bond lengths are 1.85 Å. Both Ta(3)-O(7) bond lengths are 2.07 Å. In the fourth Ta site, Ta(4) is bonded to one O(4), one O(9), two equivalent O(14), and two equivalent O(8) atoms to form TaO6 octahedra that share  a cornercorner with one As(1)O4 tetrahedra, corners with two equivalent As(3)O4 tetrahedra, and corners with two equivalent Ta(3)O5 trigonal bipyramids. The Ta(4)-O(4) bond length is 2.25 Å. The Ta(4)-O(9) bond length is 1.77 Å. Both Ta(4)-O(14) bond lengths are 1.99 Å. Both Ta(4)-O(8) bond lengths are 2.04 Å. There are four inequivalent As sites. In the first As site, As(1) is bonded to one O(12), one O(4), and two equivalent O(6) atoms to form AsO4 tetrahedra that share  a cornercorner with one Ta(1)O6 octahedra,  a cornercorner with one Ta(4)O6 octahedra, and corners with two equivalent Ta(2)O6 octahedra. The corner-sharing octahedral tilt angles range from 11-36°. The As(1)-O(12) bond length is 1.72 Å. The As(1)-O(4) bond length is 1.68 Å. Both As(1)-O(6) bond lengths are 1.73 Å. In the second As site, As(2) is bonded to one O(11), one O(3), and two equivalent O(5) atoms to form AsO4 tetrahedra that share  a cornercorner with one Ta(2)O6 octahedra and corners with two equivalent Ta(1)O6 octahedra. The corner-sharing octahedral tilt angles range from 15-45°. The As(2)-O(11) bond length is 1.72 Å. The As(2)-O(3) bond length is 1.71 Å. Both As(2)-O(5) bond lengths are 1.69 Å. In the third As site, As(3) is bonded to one O(10), one O(2), and two equivalent O(8) atoms to form AsO4 tetrahedra that share  a cornercorner with one Ta(2)O6 octahedra, corners with two equivalent Ta(4)O6 octahedra, and  a cornercorner with one Ta(3)O5 trigonal bipyramid. The corner-sharing octahedral tilt angles range from 12-26°. The As(3)-O(10) bond length is 1.75 Å. The As(3)-O(2) bond length is 1.67 Å. Both As(3)-O(8) bond lengths are 1.68 Å. In the fourth As site, As(4) is bonded in a trigonal non-coplanar geometry to one O(1) and two equivalent O(7) atoms. The As(4)-O(1) bond length is 1.75 Å. Both As(4)-O(7) bond lengths are 1.88 Å. There are fourteen inequivalent O sites. In the first O site, O(1) is bonded in a bent 150 degrees geometry to one Ta(1) and one As(4) atom. In the second O site, O(2) is bonded in a bent 150 degrees geometry to one Ta(2) and one As(3) atom. In the third O site, O(3) is bonded in a bent 120 degrees geometry to one Mg(1) and one As(2) atom. In the fourth O site, O(4) is bonded in a bent 150 degrees geometry to one Ta(4) and one As(1) atom. In the fifth O site, O(5) is bonded in a linear geometry to one Ta(1) and one As(2) atom. In the sixth O site, O(6) is bonded in a linear geometry to one Ta(2) and one As(1) atom. In the seventh O site, O(7) is bonded in a trigonal planar geometry to one Mg(1), one Ta(3), and one As(4) atom. In the eighth O site, O(8) is bonded in a linear geometry to one Ta(4) and one As(3) atom. In the ninth O site, O(9) is bonded in a single-bond geometry to one Ta(4) atom. In the tenth O site, O(10) is bonded in a bent 150 degrees geometry to one Ta(3) and one As(3) atom. In the eleventh O site, O(11) is bonded in a distorted bent 150 degrees geometry to one Ta(2) and one As(2) atom. In the twelfth O site, O(12) is bonded in a bent 150 degrees geometry to one Ta(1) and one As(1) atom. In the thirteenth O site, O(13) is bonded in a linear geometry to one Ta(1) and one Ta(2) atom. In the fourteenth O site, O(14) is bonded in a linear geometry to one Ta(3) and one Ta(4) atom.