Bi2Ti2O7 crystallizes in the orthorhombic Pna2_1 space group. There are two inequivalent Ti sites. In the first Ti site, Ti(1) is bonded to one O(1), one O(2), one O(3), one O(4), and two equivalent O(6) atoms to form TiO6 octahedra that share corners with two equivalent Ti(1)O6 octahedra, corners with four equivalent Ti(2)O6 octahedra,  a cornercorner with one Bi(1)O6 pentagonal pyramid, and  an edgeedge with one Bi(1)O6 pentagonal pyramid. The corner-sharing octahedral tilt angles range from 40-49°. In the second Ti site, Ti(2) is bonded to one O(1), one O(2), one O(3), one O(4), and two equivalent O(5) atoms to form TiO6 octahedra that share corners with two equivalent Ti(2)O6 octahedra, corners with four equivalent Ti(1)O6 octahedra,  a cornercorner with one Bi(1)O6 pentagonal pyramid, and edges with two equivalent Bi(1)O6 pentagonal pyramids. The corner-sharing octahedral tilt angles range from 40-49°. There are two inequivalent Bi sites. In the first Bi site, Bi(1) is bonded to one O(1), one O(2), one O(3), one O(5), and two equivalent O(7) atoms to form distorted BiO6 pentagonal pyramids that share  a cornercorner with one Ti(1)O6 octahedra,  a cornercorner with one Ti(2)O6 octahedra, corners with two equivalent Bi(1)O6 pentagonal pyramids,  an edgeedge with one Ti(1)O6 octahedra, and edges with two equivalent Ti(2)O6 octahedra. The corner-sharing octahedral tilt angles range from 62-67°. In the second Bi site, Bi(2) is bonded in a 5-coordinate geometry to one O(3), one O(4), one O(6), and two equivalent O(7) atoms. There are seven inequivalent O sites. In the first O site, O(5) is bonded in a distorted trigonal non-coplanar geometry to two equivalent Ti(2) and one Bi(1) atom. In the second O site, O(6) is bonded in a 3-coordinate geometry to two equivalent Ti(1) and one Bi(2) atom. In the third O site, O(7) is bonded to two equivalent Bi(1) and two equivalent Bi(2) atoms to form corner-sharing OBi4 tetrahedra. In the fourth O site, O(1) is bonded in a 3-coordinate geometry to one Ti(1), one Ti(2), and one Bi(1) atom. In the fifth O site, O(2) is bonded in a 3-coordinate geometry to one Ti(1), one Ti(2), and one Bi(1) atom. In the sixth O site, O(3) is bonded in a 4-coordinate geometry to one Ti(1), one Ti(2), one Bi(1), and one Bi(2) atom. In the seventh O site, O(4) is bonded in a distorted trigonal non-coplanar geometry to one Ti(1), one Ti(2), and one Bi(2) atom.
Bi2Ti2O7 crystallizes in the orthorhombic Pna2_1 space group. There are two inequivalent Ti sites. In the first Ti site, Ti(1) is bonded to one O(1), one O(2), one O(3), one O(4), and two equivalent O(6) atoms to form TiO6 octahedra that share corners with two equivalent Ti(1)O6 octahedra, corners with four equivalent Ti(2)O6 octahedra,  a cornercorner with one Bi(1)O6 pentagonal pyramid, and  an edgeedge with one Bi(1)O6 pentagonal pyramid. The corner-sharing octahedral tilt angles range from 40-49°. The Ti(1)-O(1) bond length is 1.95 Å. The Ti(1)-O(2) bond length is 1.92 Å. The Ti(1)-O(3) bond length is 2.11 Å. The Ti(1)-O(4) bond length is 1.98 Å. There is one shorter (1.88 Å) and one longer (2.13 Å) Ti(1)-O(6) bond length. In the second Ti site, Ti(2) is bonded to one O(1), one O(2), one O(3), one O(4), and two equivalent O(5) atoms to form TiO6 octahedra that share corners with two equivalent Ti(2)O6 octahedra, corners with four equivalent Ti(1)O6 octahedra,  a cornercorner with one Bi(1)O6 pentagonal pyramid, and edges with two equivalent Bi(1)O6 pentagonal pyramids. The corner-sharing octahedral tilt angles range from 40-49°. The Ti(2)-O(1) bond length is 2.02 Å. The Ti(2)-O(2) bond length is 1.93 Å. The Ti(2)-O(3) bond length is 1.95 Å. The Ti(2)-O(4) bond length is 2.01 Å. There is one shorter (1.96 Å) and one longer (1.98 Å) Ti(2)-O(5) bond length. There are two inequivalent Bi sites. In the first Bi site, Bi(1) is bonded to one O(1), one O(2), one O(3), one O(5), and two equivalent O(7) atoms to form distorted BiO6 pentagonal pyramids that share  a cornercorner with one Ti(1)O6 octahedra,  a cornercorner with one Ti(2)O6 octahedra, corners with two equivalent Bi(1)O6 pentagonal pyramids,  an edgeedge with one Ti(1)O6 octahedra, and edges with two equivalent Ti(2)O6 octahedra. The corner-sharing octahedral tilt angles range from 62-67°. The Bi(1)-O(1) bond length is 2.28 Å. The Bi(1)-O(2) bond length is 2.56 Å. The Bi(1)-O(3) bond length is 2.61 Å. The Bi(1)-O(5) bond length is 2.25 Å. There is one shorter (2.24 Å) and one longer (2.36 Å) Bi(1)-O(7) bond length. In the second Bi site, Bi(2) is bonded in a 5-coordinate geometry to one O(3), one O(4), one O(6), and two equivalent O(7) atoms. The Bi(2)-O(3) bond length is 2.39 Å. The Bi(2)-O(4) bond length is 2.21 Å. The Bi(2)-O(6) bond length is 2.39 Å. There is one shorter (2.28 Å) and one longer (2.30 Å) Bi(2)-O(7) bond length. There are seven inequivalent O sites. In the first O site, O(5) is bonded in a distorted trigonal non-coplanar geometry to two equivalent Ti(2) and one Bi(1) atom. In the second O site, O(6) is bonded in a 3-coordinate geometry to two equivalent Ti(1) and one Bi(2) atom. In the third O site, O(7) is bonded to two equivalent Bi(1) and two equivalent Bi(2) atoms to form corner-sharing OBi4 tetrahedra. In the fourth O site, O(1) is bonded in a 3-coordinate geometry to one Ti(1), one Ti(2), and one Bi(1) atom. In the fifth O site, O(2) is bonded in a 3-coordinate geometry to one Ti(1), one Ti(2), and one Bi(1) atom. In the sixth O site, O(3) is bonded in a 4-coordinate geometry to one Ti(1), one Ti(2), one Bi(1), and one Bi(2) atom. In the seventh O site, O(4) is bonded in a distorted trigonal non-coplanar geometry to one Ti(1), one Ti(2), and one Bi(2) atom.