Sr2Al2SiO7 crystallizes in the monoclinic P2_1 space group. There are two inequivalent Sr sites. In the first Sr site, Sr(1) is bonded in a 8-coordinate geometry to one O(1), one O(3), one O(6), one O(7), two equivalent O(2), and two equivalent O(4) atoms. In the second Sr site, Sr(2) is bonded in a 8-coordinate geometry to one O(1), one O(2), one O(6), one O(7), two equivalent O(3), and two equivalent O(5) atoms. There are two inequivalent Al sites. In the first Al site, Al(1) is bonded to one O(4), one O(5), one O(6), and one O(7) atom to form AlO4 tetrahedra that share corners with two equivalent Al(2)O4 tetrahedra and corners with two equivalent Si(1)O4 tetrahedra. In the second Al site, Al(2) is bonded to one O(1), one O(2), one O(4), and one O(7) atom to form AlO4 tetrahedra that share  a cornercorner with one Si(1)O4 tetrahedra and corners with two equivalent Al(1)O4 tetrahedra. Si(1) is bonded to one O(1), one O(3), one O(5), and one O(6) atom to form SiO4 tetrahedra that share  a cornercorner with one Al(2)O4 tetrahedra and corners with two equivalent Al(1)O4 tetrahedra. There are seven inequivalent O sites. In the first O site, O(1) is bonded in a 4-coordinate geometry to one Sr(1), one Sr(2), one Al(2), and one Si(1) atom. In the second O site, O(2) is bonded to one Sr(2), two equivalent Sr(1), and one Al(2) atom to form distorted corner-sharing OSr3Al tetrahedra. In the third O site, O(3) is bonded in a 4-coordinate geometry to one Sr(1), two equivalent Sr(2), and one Si(1) atom. In the fourth O site, O(4) is bonded in a 4-coordinate geometry to two equivalent Sr(1), one Al(1), and one Al(2) atom. In the fifth O site, O(5) is bonded in a distorted bent 120 degrees geometry to two equivalent Sr(2), one Al(1), and one Si(1) atom. In the sixth O site, O(6) is bonded in a distorted bent 120 degrees geometry to one Sr(1), one Sr(2), one Al(1), and one Si(1) atom. In the seventh O site, O(7) is bonded in a 4-coordinate geometry to one Sr(1), one Sr(2), one Al(1), and one Al(2) atom.
Sr2Al2SiO7 crystallizes in the monoclinic P2_1 space group. There are two inequivalent Sr sites. In the first Sr site, Sr(1) is bonded in a 8-coordinate geometry to one O(1), one O(3), one O(6), one O(7), two equivalent O(2), and two equivalent O(4) atoms. The Sr(1)-O(1) bond length is 2.63 Å. The Sr(1)-O(3) bond length is 2.79 Å. The Sr(1)-O(6) bond length is 3.06 Å. The Sr(1)-O(7) bond length is 2.55 Å. There is one shorter (2.49 Å) and one longer (2.62 Å) Sr(1)-O(2) bond length. There is one shorter (2.60 Å) and one longer (2.90 Å) Sr(1)-O(4) bond length. In the second Sr site, Sr(2) is bonded in a 8-coordinate geometry to one O(1), one O(2), one O(6), one O(7), two equivalent O(3), and two equivalent O(5) atoms. The Sr(2)-O(1) bond length is 2.58 Å. The Sr(2)-O(2) bond length is 2.53 Å. The Sr(2)-O(6) bond length is 2.65 Å. The Sr(2)-O(7) bond length is 2.83 Å. There is one shorter (2.58 Å) and one longer (2.71 Å) Sr(2)-O(3) bond length. There is one shorter (2.60 Å) and one longer (3.00 Å) Sr(2)-O(5) bond length. There are two inequivalent Al sites. In the first Al site, Al(1) is bonded to one O(4), one O(5), one O(6), and one O(7) atom to form AlO4 tetrahedra that share corners with two equivalent Al(2)O4 tetrahedra and corners with two equivalent Si(1)O4 tetrahedra. The Al(1)-O(4) bond length is 1.77 Å. The Al(1)-O(5) bond length is 1.81 Å. The Al(1)-O(6) bond length is 1.80 Å. The Al(1)-O(7) bond length is 1.77 Å. In the second Al site, Al(2) is bonded to one O(1), one O(2), one O(4), and one O(7) atom to form AlO4 tetrahedra that share  a cornercorner with one Si(1)O4 tetrahedra and corners with two equivalent Al(1)O4 tetrahedra. The Al(2)-O(1) bond length is 1.80 Å. The Al(2)-O(2) bond length is 1.73 Å. The Al(2)-O(4) bond length is 1.79 Å. The Al(2)-O(7) bond length is 1.79 Å. Si(1) is bonded to one O(1), one O(3), one O(5), and one O(6) atom to form SiO4 tetrahedra that share  a cornercorner with one Al(2)O4 tetrahedra and corners with two equivalent Al(1)O4 tetrahedra. The Si(1)-O(1) bond length is 1.65 Å. The Si(1)-O(3) bond length is 1.62 Å. The Si(1)-O(5) bond length is 1.67 Å. The Si(1)-O(6) bond length is 1.67 Å. There are seven inequivalent O sites. In the first O site, O(1) is bonded in a 4-coordinate geometry to one Sr(1), one Sr(2), one Al(2), and one Si(1) atom. In the second O site, O(2) is bonded to one Sr(2), two equivalent Sr(1), and one Al(2) atom to form distorted corner-sharing OSr3Al tetrahedra. In the third O site, O(3) is bonded in a 4-coordinate geometry to one Sr(1), two equivalent Sr(2), and one Si(1) atom. In the fourth O site, O(4) is bonded in a 4-coordinate geometry to two equivalent Sr(1), one Al(1), and one Al(2) atom. In the fifth O site, O(5) is bonded in a distorted bent 120 degrees geometry to two equivalent Sr(2), one Al(1), and one Si(1) atom. In the sixth O site, O(6) is bonded in a distorted bent 120 degrees geometry to one Sr(1), one Sr(2), one Al(1), and one Si(1) atom. In the seventh O site, O(7) is bonded in a 4-coordinate geometry to one Sr(1), one Sr(2), one Al(1), and one Al(2) atom.