Ba2CuP2O9 crystallizes in the monoclinic P2_1/m space group. There are two inequivalent Ba sites. In the first Ba site, Ba(1) is bonded in a 10-coordinate geometry to one O(3), one O(7), two equivalent O(1), two equivalent O(2), and four equivalent O(4) atoms. In the second Ba site, Ba(2) is bonded in a 11-coordinate geometry to one O(2), one O(5), two equivalent O(1), two equivalent O(3), two equivalent O(4), and three equivalent O(6) atoms. Cu(1) is bonded to two equivalent O(1), two equivalent O(5), and two equivalent O(7) atoms to form distorted CuO6 octahedra that share corners with two equivalent P(1)O4 tetrahedra, corners with two equivalent P(2)O4 tetrahedra, and edges with two equivalent Cu(1)O6 octahedra. There are two inequivalent P sites. In the first P site, P(1) is bonded to one O(2), one O(3), and two equivalent O(1) atoms to form PO4 tetrahedra that share corners with two equivalent Cu(1)O6 octahedra. The corner-sharing octahedral tilt angles are 52°. In the second P site, P(2) is bonded to one O(5), one O(6), and two equivalent O(4) atoms to form PO4 tetrahedra that share corners with two equivalent Cu(1)O6 octahedra. The corner-sharing octahedral tilt angles are 44°. There are seven inequivalent O sites. In the first O site, O(1) is bonded in a distorted bent 120 degrees geometry to one Ba(1), one Ba(2), one Cu(1), and one P(1) atom. In the second O site, O(2) is bonded in a distorted single-bond geometry to one Ba(2), two equivalent Ba(1), and one P(1) atom. In the third O site, O(3) is bonded in a distorted single-bond geometry to one Ba(1), two equivalent Ba(2), and one P(1) atom. In the fourth O site, O(4) is bonded in a distorted single-bond geometry to one Ba(2), two equivalent Ba(1), and one P(2) atom. In the fifth O site, O(5) is bonded in a distorted single-bond geometry to one Ba(2), two equivalent Cu(1), and one P(2) atom. In the sixth O site, O(6) is bonded in a distorted single-bond geometry to three equivalent Ba(2) and one P(2) atom. In the seventh O site, O(7) is bonded in a distorted water-like geometry to one Ba(1) and two equivalent Cu(1) atoms.
Ba2CuP2O9 crystallizes in the monoclinic P2_1/m space group. There are two inequivalent Ba sites. In the first Ba site, Ba(1) is bonded in a 10-coordinate geometry to one O(3), one O(7), two equivalent O(1), two equivalent O(2), and four equivalent O(4) atoms. The Ba(1)-O(3) bond length is 2.78 Å. The Ba(1)-O(7) bond length is 2.87 Å. Both Ba(1)-O(1) bond lengths are 2.95 Å. Both Ba(1)-O(2) bond lengths are 3.13 Å. There are two shorter (2.70 Å) and two longer (2.89 Å) Ba(1)-O(4) bond lengths. In the second Ba site, Ba(2) is bonded in a 11-coordinate geometry to one O(2), one O(5), two equivalent O(1), two equivalent O(3), two equivalent O(4), and three equivalent O(6) atoms. The Ba(2)-O(2) bond length is 2.77 Å. The Ba(2)-O(5) bond length is 2.76 Å. Both Ba(2)-O(1) bond lengths are 3.02 Å. Both Ba(2)-O(3) bond lengths are 3.29 Å. Both Ba(2)-O(4) bond lengths are 2.79 Å. There is one shorter (2.85 Å) and two longer (3.18 Å) Ba(2)-O(6) bond lengths. Cu(1) is bonded to two equivalent O(1), two equivalent O(5), and two equivalent O(7) atoms to form distorted CuO6 octahedra that share corners with two equivalent P(1)O4 tetrahedra, corners with two equivalent P(2)O4 tetrahedra, and edges with two equivalent Cu(1)O6 octahedra. Both Cu(1)-O(1) bond lengths are 1.92 Å. Both Cu(1)-O(5) bond lengths are 2.51 Å. Both Cu(1)-O(7) bond lengths are 1.80 Å. There are two inequivalent P sites. In the first P site, P(1) is bonded to one O(2), one O(3), and two equivalent O(1) atoms to form PO4 tetrahedra that share corners with two equivalent Cu(1)O6 octahedra. The corner-sharing octahedral tilt angles are 52°. The P(1)-O(2) bond length is 1.53 Å. The P(1)-O(3) bond length is 1.53 Å. Both P(1)-O(1) bond lengths are 1.59 Å. In the second P site, P(2) is bonded to one O(5), one O(6), and two equivalent O(4) atoms to form PO4 tetrahedra that share corners with two equivalent Cu(1)O6 octahedra. The corner-sharing octahedral tilt angles are 44°. The P(2)-O(5) bond length is 1.56 Å. The P(2)-O(6) bond length is 1.56 Å. Both P(2)-O(4) bond lengths are 1.55 Å. There are seven inequivalent O sites. In the first O site, O(1) is bonded in a distorted bent 120 degrees geometry to one Ba(1), one Ba(2), one Cu(1), and one P(1) atom. In the second O site, O(2) is bonded in a distorted single-bond geometry to one Ba(2), two equivalent Ba(1), and one P(1) atom. In the third O site, O(3) is bonded in a distorted single-bond geometry to one Ba(1), two equivalent Ba(2), and one P(1) atom. In the fourth O site, O(4) is bonded in a distorted single-bond geometry to one Ba(2), two equivalent Ba(1), and one P(2) atom. In the fifth O site, O(5) is bonded in a distorted single-bond geometry to one Ba(2), two equivalent Cu(1), and one P(2) atom. In the sixth O site, O(6) is bonded in a distorted single-bond geometry to three equivalent Ba(2) and one P(2) atom. In the seventh O site, O(7) is bonded in a distorted water-like geometry to one Ba(1) and two equivalent Cu(1) atoms.