BaFe2Si2O7 crystallizes in the monoclinic P2_1/c space group. Ba(1) is bonded in a 7-coordinate geometry to one O(1), one O(4), one O(6), two equivalent O(3), and two equivalent O(5) atoms. There are two inequivalent Fe sites. In the first Fe site, Fe(1) is bonded to one O(1), one O(2), one O(7), and two equivalent O(4) atoms to form distorted FeO5 trigonal bipyramids that share  a cornercorner with one Si(1)O4 tetrahedra, corners with two equivalent Si(2)O4 tetrahedra, corners with two equivalent Fe(2)O4 trigonal pyramids,  an edgeedge with one Si(1)O4 tetrahedra, and  an edgeedge with one Fe(1)O5 trigonal bipyramid. In the second Fe site, Fe(2) is bonded to one O(2), one O(3), one O(5), and one O(7) atom to form distorted FeO4 trigonal pyramids that share corners with two equivalent Si(1)O4 tetrahedra, corners with two equivalent Si(2)O4 tetrahedra, and corners with two equivalent Fe(1)O5 trigonal bipyramids. There are two inequivalent Si sites. In the first Si site, Si(1) is bonded to one O(2), one O(4), one O(5), and one O(6) atom to form SiO4 tetrahedra that share  a cornercorner with one Si(2)O4 tetrahedra,  a cornercorner with one Fe(1)O5 trigonal bipyramid, corners with two equivalent Fe(2)O4 trigonal pyramids, and  an edgeedge with one Fe(1)O5 trigonal bipyramid. In the second Si site, Si(2) is bonded to one O(1), one O(3), one O(6), and one O(7) atom to form SiO4 tetrahedra that share  a cornercorner with one Si(1)O4 tetrahedra, corners with two equivalent Fe(1)O5 trigonal bipyramids, and corners with two equivalent Fe(2)O4 trigonal pyramids. There are seven inequivalent O sites. In the first O site, O(4) is bonded in a 4-coordinate geometry to one Ba(1), two equivalent Fe(1), and one Si(1) atom. In the second O site, O(5) is bonded in a 2-coordinate geometry to two equivalent Ba(1), one Fe(2), and one Si(1) atom. In the third O site, O(6) is bonded in a distorted bent 120 degrees geometry to one Ba(1), one Si(1), and one Si(2) atom. In the fourth O site, O(7) is bonded in a trigonal planar geometry to one Fe(1), one Fe(2), and one Si(2) atom. In the fifth O site, O(1) is bonded in a distorted trigonal planar geometry to one Ba(1), one Fe(1), and one Si(2) atom. In the sixth O site, O(2) is bonded in a 3-coordinate geometry to one Fe(1), one Fe(2), and one Si(1) atom. In the seventh O site, O(3) is bonded in a 4-coordinate geometry to two equivalent Ba(1), one Fe(2), and one Si(2) atom.
BaFe2Si2O7 crystallizes in the monoclinic P2_1/c space group. Ba(1) is bonded in a 7-coordinate geometry to one O(1), one O(4), one O(6), two equivalent O(3), and two equivalent O(5) atoms. The Ba(1)-O(1) bond length is 2.68 Å. The Ba(1)-O(4) bond length is 2.78 Å. The Ba(1)-O(6) bond length is 2.92 Å. There is one shorter (2.86 Å) and one longer (3.04 Å) Ba(1)-O(3) bond length. There is one shorter (3.01 Å) and one longer (3.03 Å) Ba(1)-O(5) bond length. There are two inequivalent Fe sites. In the first Fe site, Fe(1) is bonded to one O(1), one O(2), one O(7), and two equivalent O(4) atoms to form distorted FeO5 trigonal bipyramids that share  a cornercorner with one Si(1)O4 tetrahedra, corners with two equivalent Si(2)O4 tetrahedra, corners with two equivalent Fe(2)O4 trigonal pyramids,  an edgeedge with one Si(1)O4 tetrahedra, and  an edgeedge with one Fe(1)O5 trigonal bipyramid. The Fe(1)-O(1) bond length is 2.07 Å. The Fe(1)-O(2) bond length is 2.12 Å. The Fe(1)-O(7) bond length is 2.04 Å. There is one shorter (2.06 Å) and one longer (2.46 Å) Fe(1)-O(4) bond length. In the second Fe site, Fe(2) is bonded to one O(2), one O(3), one O(5), and one O(7) atom to form distorted FeO4 trigonal pyramids that share corners with two equivalent Si(1)O4 tetrahedra, corners with two equivalent Si(2)O4 tetrahedra, and corners with two equivalent Fe(1)O5 trigonal bipyramids. The Fe(2)-O(2) bond length is 2.04 Å. The Fe(2)-O(3) bond length is 2.01 Å. The Fe(2)-O(5) bond length is 2.01 Å. The Fe(2)-O(7) bond length is 2.03 Å. There are two inequivalent Si sites. In the first Si site, Si(1) is bonded to one O(2), one O(4), one O(5), and one O(6) atom to form SiO4 tetrahedra that share  a cornercorner with one Si(2)O4 tetrahedra,  a cornercorner with one Fe(1)O5 trigonal bipyramid, corners with two equivalent Fe(2)O4 trigonal pyramids, and  an edgeedge with one Fe(1)O5 trigonal bipyramid. The Si(1)-O(2) bond length is 1.65 Å. The Si(1)-O(4) bond length is 1.64 Å. The Si(1)-O(5) bond length is 1.64 Å. The Si(1)-O(6) bond length is 1.68 Å. In the second Si site, Si(2) is bonded to one O(1), one O(3), one O(6), and one O(7) atom to form SiO4 tetrahedra that share  a cornercorner with one Si(1)O4 tetrahedra, corners with two equivalent Fe(1)O5 trigonal bipyramids, and corners with two equivalent Fe(2)O4 trigonal pyramids. The Si(2)-O(1) bond length is 1.63 Å. The Si(2)-O(3) bond length is 1.65 Å. The Si(2)-O(6) bond length is 1.69 Å. The Si(2)-O(7) bond length is 1.64 Å. There are seven inequivalent O sites. In the first O site, O(4) is bonded in a 4-coordinate geometry to one Ba(1), two equivalent Fe(1), and one Si(1) atom. In the second O site, O(5) is bonded in a 2-coordinate geometry to two equivalent Ba(1), one Fe(2), and one Si(1) atom. In the third O site, O(6) is bonded in a distorted bent 120 degrees geometry to one Ba(1), one Si(1), and one Si(2) atom. In the fourth O site, O(7) is bonded in a trigonal planar geometry to one Fe(1), one Fe(2), and one Si(2) atom. In the fifth O site, O(1) is bonded in a distorted trigonal planar geometry to one Ba(1), one Fe(1), and one Si(2) atom. In the sixth O site, O(2) is bonded in a 3-coordinate geometry to one Fe(1), one Fe(2), and one Si(1) atom. In the seventh O site, O(3) is bonded in a 4-coordinate geometry to two equivalent Ba(1), one Fe(2), and one Si(2) atom.