BaMn2P2O9 crystallizes in the monoclinic C2 space group. Ba(1) is bonded in a 10-coordinate geometry to two equivalent O(5), four equivalent O(3), and four equivalent O(4) atoms. Mn(1) is bonded to one O(1), one O(3), one O(4), one O(5), and two equivalent O(2) atoms to form distorted MnO6 pentagonal pyramids that share corners with three equivalent Mn(1)O6 pentagonal pyramids, corners with three equivalent P(1)O4 tetrahedra, and  an edgeedge with one P(1)O4 tetrahedra. P(1) is bonded to one O(2), one O(3), one O(4), and one O(5) atom to form PO4 tetrahedra that share corners with three equivalent Mn(1)O6 pentagonal pyramids and  an edgeedge with one Mn(1)O6 pentagonal pyramid. There are five inequivalent O sites. In the first O site, O(1) is bonded in a bent 120 degrees geometry to two equivalent Mn(1) atoms. In the second O site, O(2) is bonded in a 3-coordinate geometry to two equivalent Mn(1) and one P(1) atom. In the third O site, O(3) is bonded in a 2-coordinate geometry to two equivalent Ba(1), one Mn(1), and one P(1) atom. In the fourth O site, O(4) is bonded in a distorted single-bond geometry to two equivalent Ba(1), one Mn(1), and one P(1) atom. In the fifth O site, O(5) is bonded in a 3-coordinate geometry to one Ba(1), one Mn(1), and one P(1) atom.
BaMn2P2O9 crystallizes in the monoclinic C2 space group. Ba(1) is bonded in a 10-coordinate geometry to two equivalent O(5), four equivalent O(3), and four equivalent O(4) atoms. Both Ba(1)-O(5) bond lengths are 2.84 Å. There are two shorter (2.89 Å) and two longer (3.22 Å) Ba(1)-O(3) bond lengths. There are two shorter (2.87 Å) and two longer (3.05 Å) Ba(1)-O(4) bond lengths. Mn(1) is bonded to one O(1), one O(3), one O(4), one O(5), and two equivalent O(2) atoms to form distorted MnO6 pentagonal pyramids that share corners with three equivalent Mn(1)O6 pentagonal pyramids, corners with three equivalent P(1)O4 tetrahedra, and  an edgeedge with one P(1)O4 tetrahedra. The Mn(1)-O(1) bond length is 1.86 Å. The Mn(1)-O(3) bond length is 2.07 Å. The Mn(1)-O(4) bond length is 2.37 Å. The Mn(1)-O(5) bond length is 2.02 Å. There is one shorter (1.98 Å) and one longer (2.34 Å) Mn(1)-O(2) bond length. P(1) is bonded to one O(2), one O(3), one O(4), and one O(5) atom to form PO4 tetrahedra that share corners with three equivalent Mn(1)O6 pentagonal pyramids and  an edgeedge with one Mn(1)O6 pentagonal pyramid. The P(1)-O(2) bond length is 1.57 Å. The P(1)-O(3) bond length is 1.57 Å. The P(1)-O(4) bond length is 1.54 Å. The P(1)-O(5) bond length is 1.55 Å. There are five inequivalent O sites. In the first O site, O(1) is bonded in a bent 120 degrees geometry to two equivalent Mn(1) atoms. In the second O site, O(2) is bonded in a 3-coordinate geometry to two equivalent Mn(1) and one P(1) atom. In the third O site, O(3) is bonded in a 2-coordinate geometry to two equivalent Ba(1), one Mn(1), and one P(1) atom. In the fourth O site, O(4) is bonded in a distorted single-bond geometry to two equivalent Ba(1), one Mn(1), and one P(1) atom. In the fifth O site, O(5) is bonded in a 3-coordinate geometry to one Ba(1), one Mn(1), and one P(1) atom.