Nd2As4O9 crystallizes in the triclinic P-1 space group. There are two inequivalent Nd sites. In the first Nd site, Nd(1) is bonded in a 9-coordinate geometry to one O(3), one O(6), one O(7), one O(8), one O(9), two equivalent O(1), and two equivalent O(4) atoms. In the second Nd site, Nd(2) is bonded in a 8-coordinate geometry to one O(3), one O(4), one O(6), one O(9), two equivalent O(2), and two equivalent O(7) atoms. There are four inequivalent As sites. In the first As site, As(1) is bonded in a distorted T-shaped geometry to one O(5), one O(8), and one O(9) atom. In the second As site, As(2) is bonded in a distorted trigonal non-coplanar geometry to one O(3), one O(5), and one O(8) atom. In the third As site, As(3) is bonded in a distorted T-shaped geometry to one O(1), one O(2), and one O(7) atom. In the fourth As site, As(4) is bonded in a distorted trigonal non-coplanar geometry to one O(1), one O(4), and one O(6) atom. There are nine inequivalent O sites. In the first O site, O(1) is bonded in a 4-coordinate geometry to two equivalent Nd(1), one As(3), and one As(4) atom. In the second O site, O(2) is bonded in a distorted trigonal non-coplanar geometry to two equivalent Nd(2) and one As(3) atom. In the third O site, O(3) is bonded in a 3-coordinate geometry to one Nd(1), one Nd(2), and one As(2) atom. In the fourth O site, O(4) is bonded to one Nd(2), two equivalent Nd(1), and one As(4) atom to form a mixture of distorted corner and edge-sharing ONd3As tetrahedra. In the fifth O site, O(5) is bonded in a bent 150 degrees geometry to one As(1) and one As(2) atom. In the sixth O site, O(6) is bonded in a 3-coordinate geometry to one Nd(1), one Nd(2), and one As(4) atom. In the seventh O site, O(7) is bonded to one Nd(1), two equivalent Nd(2), and one As(3) atom to form a mixture of distorted corner and edge-sharing ONd3As tetrahedra. In the eighth O site, O(8) is bonded in a distorted trigonal non-coplanar geometry to one Nd(1), one As(1), and one As(2) atom. In the ninth O site, O(9) is bonded in a 3-coordinate geometry to one Nd(1), one Nd(2), and one As(1) atom.
Nd2As4O9 crystallizes in the triclinic P-1 space group. There are two inequivalent Nd sites. In the first Nd site, Nd(1) is bonded in a 9-coordinate geometry to one O(3), one O(6), one O(7), one O(8), one O(9), two equivalent O(1), and two equivalent O(4) atoms. The Nd(1)-O(3) bond length is 2.58 Å. The Nd(1)-O(6) bond length is 2.39 Å. The Nd(1)-O(7) bond length is 2.38 Å. The Nd(1)-O(8) bond length is 2.48 Å. The Nd(1)-O(9) bond length is 2.52 Å. There is one shorter (2.59 Å) and one longer (2.85 Å) Nd(1)-O(1) bond length. There is one shorter (2.46 Å) and one longer (2.63 Å) Nd(1)-O(4) bond length. In the second Nd site, Nd(2) is bonded in a 8-coordinate geometry to one O(3), one O(4), one O(6), one O(9), two equivalent O(2), and two equivalent O(7) atoms. The Nd(2)-O(3) bond length is 2.50 Å. The Nd(2)-O(4) bond length is 2.67 Å. The Nd(2)-O(6) bond length is 2.40 Å. The Nd(2)-O(9) bond length is 2.42 Å. There is one shorter (2.37 Å) and one longer (2.48 Å) Nd(2)-O(2) bond length. There is one shorter (2.42 Å) and one longer (2.58 Å) Nd(2)-O(7) bond length. There are four inequivalent As sites. In the first As site, As(1) is bonded in a distorted T-shaped geometry to one O(5), one O(8), and one O(9) atom. The As(1)-O(5) bond length is 1.80 Å. The As(1)-O(8) bond length is 1.85 Å. The As(1)-O(9) bond length is 1.74 Å. In the second As site, As(2) is bonded in a distorted trigonal non-coplanar geometry to one O(3), one O(5), and one O(8) atom. The As(2)-O(3) bond length is 1.73 Å. The As(2)-O(5) bond length is 1.81 Å. The As(2)-O(8) bond length is 1.85 Å. In the third As site, As(3) is bonded in a distorted T-shaped geometry to one O(1), one O(2), and one O(7) atom. The As(3)-O(1) bond length is 1.90 Å. The As(3)-O(2) bond length is 1.73 Å. The As(3)-O(7) bond length is 1.79 Å. In the fourth As site, As(4) is bonded in a distorted trigonal non-coplanar geometry to one O(1), one O(4), and one O(6) atom. The As(4)-O(1) bond length is 1.88 Å. The As(4)-O(4) bond length is 1.78 Å. The As(4)-O(6) bond length is 1.73 Å. There are nine inequivalent O sites. In the first O site, O(1) is bonded in a 4-coordinate geometry to two equivalent Nd(1), one As(3), and one As(4) atom. In the second O site, O(2) is bonded in a distorted trigonal non-coplanar geometry to two equivalent Nd(2) and one As(3) atom. In the third O site, O(3) is bonded in a 3-coordinate geometry to one Nd(1), one Nd(2), and one As(2) atom. In the fourth O site, O(4) is bonded to one Nd(2), two equivalent Nd(1), and one As(4) atom to form a mixture of distorted corner and edge-sharing ONd3As tetrahedra. In the fifth O site, O(5) is bonded in a bent 150 degrees geometry to one As(1) and one As(2) atom. In the sixth O site, O(6) is bonded in a 3-coordinate geometry to one Nd(1), one Nd(2), and one As(4) atom. In the seventh O site, O(7) is bonded to one Nd(1), two equivalent Nd(2), and one As(3) atom to form a mixture of distorted corner and edge-sharing ONd3As tetrahedra. In the eighth O site, O(8) is bonded in a distorted trigonal non-coplanar geometry to one Nd(1), one As(1), and one As(2) atom. In the ninth O site, O(9) is bonded in a 3-coordinate geometry to one Nd(1), one Nd(2), and one As(1) atom.