Sr2Ta2O7 crystallizes in the orthorhombic Cmc2_1 space group. There are two inequivalent Sr sites. In the first Sr site, Sr(1) is bonded to one O(3), two equivalent O(2), two equivalent O(6), three equivalent O(4), and four equivalent O(5) atoms to form SrO12 cuboctahedra that share corners with six equivalent Sr(1)O12 cuboctahedra, faces with four equivalent Sr(1)O12 cuboctahedra, faces with two equivalent Ta(2)O6 octahedra, and faces with six equivalent Ta(1)O6 octahedra. In the second Sr site, Sr(2) is bonded in a 6-coordinate geometry to three equivalent O(1) and three equivalent O(7) atoms. There are two inequivalent Ta sites. In the first Ta site, Ta(1) is bonded to one O(2), one O(6), two equivalent O(4), and two equivalent O(5) atoms to form TaO6 octahedra that share corners with two equivalent Ta(2)O6 octahedra, corners with four equivalent Ta(1)O6 octahedra, and faces with six equivalent Sr(1)O12 cuboctahedra. The corner-sharing octahedral tilt angles range from 3-15°. In the second Ta site, Ta(2) is bonded to one O(1), one O(2), one O(6), one O(7), and two equivalent O(3) atoms to form TaO6 octahedra that share corners with two equivalent Ta(1)O6 octahedra, corners with two equivalent Ta(2)O6 octahedra, and faces with two equivalent Sr(1)O12 cuboctahedra. The corner-sharing octahedral tilt angles range from 6-16°. There are seven inequivalent O sites. In the first O site, O(1) is bonded to three equivalent Sr(2) and one Ta(2) atom to form a mixture of distorted corner and edge-sharing OSr3Ta tetrahedra. In the second O site, O(2) is bonded in a distorted rectangular see-saw-like geometry to two equivalent Sr(1), one Ta(1), and one Ta(2) atom. In the third O site, O(3) is bonded in a 3-coordinate geometry to one Sr(1) and two equivalent Ta(2) atoms. In the fourth O site, O(4) is bonded to three equivalent Sr(1) and two equivalent Ta(1) atoms to form a mixture of distorted corner and edge-sharing OSr3Ta2 square pyramids. In the fifth O site, O(5) is bonded in a distorted linear geometry to four equivalent Sr(1) and two equivalent Ta(1) atoms. In the sixth O site, O(6) is bonded in a distorted rectangular see-saw-like geometry to two equivalent Sr(1), one Ta(1), and one Ta(2) atom. In the seventh O site, O(7) is bonded to three equivalent Sr(2) and one Ta(2) atom to form a mixture of distorted corner and edge-sharing OSr3Ta tetrahedra.
Sr2Ta2O7 crystallizes in the orthorhombic Cmc2_1 space group. There are two inequivalent Sr sites. In the first Sr site, Sr(1) is bonded to one O(3), two equivalent O(2), two equivalent O(6), three equivalent O(4), and four equivalent O(5) atoms to form SrO12 cuboctahedra that share corners with six equivalent Sr(1)O12 cuboctahedra, faces with four equivalent Sr(1)O12 cuboctahedra, faces with two equivalent Ta(2)O6 octahedra, and faces with six equivalent Ta(1)O6 octahedra. The Sr(1)-O(3) bond length is 2.62 Å. Both Sr(1)-O(2) bond lengths are 2.77 Å. Both Sr(1)-O(6) bond lengths are 2.68 Å. There is one shorter (2.73 Å) and two longer (2.86 Å) Sr(1)-O(4) bond lengths. There are two shorter (2.79 Å) and two longer (2.89 Å) Sr(1)-O(5) bond lengths. In the second Sr site, Sr(2) is bonded in a 6-coordinate geometry to three equivalent O(1) and three equivalent O(7) atoms. There are two shorter (2.48 Å) and one longer (2.55 Å) Sr(2)-O(1) bond length. There are two shorter (2.50 Å) and one longer (2.61 Å) Sr(2)-O(7) bond length. There are two inequivalent Ta sites. In the first Ta site, Ta(1) is bonded to one O(2), one O(6), two equivalent O(4), and two equivalent O(5) atoms to form TaO6 octahedra that share corners with two equivalent Ta(2)O6 octahedra, corners with four equivalent Ta(1)O6 octahedra, and faces with six equivalent Sr(1)O12 cuboctahedra. The corner-sharing octahedral tilt angles range from 3-15°. The Ta(1)-O(2) bond length is 1.90 Å. The Ta(1)-O(6) bond length is 1.92 Å. Both Ta(1)-O(4) bond lengths are 1.99 Å. There is one shorter (2.06 Å) and one longer (2.09 Å) Ta(1)-O(5) bond length. In the second Ta site, Ta(2) is bonded to one O(1), one O(2), one O(6), one O(7), and two equivalent O(3) atoms to form TaO6 octahedra that share corners with two equivalent Ta(1)O6 octahedra, corners with two equivalent Ta(2)O6 octahedra, and faces with two equivalent Sr(1)O12 cuboctahedra. The corner-sharing octahedral tilt angles range from 6-16°. The Ta(2)-O(1) bond length is 1.87 Å. The Ta(2)-O(2) bond length is 2.17 Å. The Ta(2)-O(6) bond length is 2.14 Å. The Ta(2)-O(7) bond length is 1.86 Å. Both Ta(2)-O(3) bond lengths are 2.00 Å. There are seven inequivalent O sites. In the first O site, O(1) is bonded to three equivalent Sr(2) and one Ta(2) atom to form a mixture of distorted corner and edge-sharing OSr3Ta tetrahedra. In the second O site, O(2) is bonded in a distorted rectangular see-saw-like geometry to two equivalent Sr(1), one Ta(1), and one Ta(2) atom. In the third O site, O(3) is bonded in a 3-coordinate geometry to one Sr(1) and two equivalent Ta(2) atoms. In the fourth O site, O(4) is bonded to three equivalent Sr(1) and two equivalent Ta(1) atoms to form a mixture of distorted corner and edge-sharing OSr3Ta2 square pyramids. In the fifth O site, O(5) is bonded in a distorted linear geometry to four equivalent Sr(1) and two equivalent Ta(1) atoms. In the sixth O site, O(6) is bonded in a distorted rectangular see-saw-like geometry to two equivalent Sr(1), one Ta(1), and one Ta(2) atom. In the seventh O site, O(7) is bonded to three equivalent Sr(2) and one Ta(2) atom to form a mixture of distorted corner and edge-sharing OSr3Ta tetrahedra.