Ca2Ta2FeO8 crystallizes in the monoclinic C2 space group. There are two inequivalent Ca sites. In the first Ca site, Ca(1) is bonded in a 5-coordinate geometry to one O(7), two equivalent O(5), two equivalent O(6), and two equivalent O(8) atoms. In the second Ca site, Ca(2) is bonded in a 5-coordinate geometry to one O(8), two equivalent O(5), two equivalent O(6), and two equivalent O(7) atoms. There are two inequivalent Ta sites. In the first Ta site, Ta(1) is bonded to one O(1), one O(2), one O(3), one O(5), one O(6), and one O(8) atom to form distorted TaO6 octahedra that share corners with two equivalent Fe(1)O6 octahedra, corners with two equivalent Fe(2)O6 octahedra, and edges with two equivalent Ta(2)O6 octahedra. The corner-sharing octahedral tilt angles range from 45-54°. In the second Ta site, Ta(2) is bonded to one O(1), one O(2), one O(4), one O(5), one O(6), and one O(7) atom to form distorted TaO6 octahedra that share corners with two equivalent Fe(1)O6 octahedra, corners with two equivalent Fe(2)O6 octahedra, and edges with two equivalent Ta(1)O6 octahedra. The corner-sharing octahedral tilt angles range from 45-55°. There are two inequivalent Fe sites. In the first Fe site, Fe(1) is bonded to two equivalent O(1), two equivalent O(3), and two equivalent O(4) atoms to form FeO6 octahedra that share corners with four equivalent Ta(1)O6 octahedra, corners with four equivalent Ta(2)O6 octahedra, and edges with two equivalent Fe(2)O6 octahedra. The corner-sharing octahedral tilt angles range from 45-54°. In the second Fe site, Fe(2) is bonded to two equivalent O(2), two equivalent O(3), and two equivalent O(4) atoms to form FeO6 octahedra that share corners with four equivalent Ta(1)O6 octahedra, corners with four equivalent Ta(2)O6 octahedra, and edges with two equivalent Fe(1)O6 octahedra. The corner-sharing octahedral tilt angles range from 45-55°. There are eight inequivalent O sites. In the first O site, O(7) is bonded to one Ca(1), two equivalent Ca(2), and one Ta(2) atom to form a mixture of distorted corner and edge-sharing OCa3Ta trigonal pyramids. In the second O site, O(8) is bonded to one Ca(2), two equivalent Ca(1), and one Ta(1) atom to form a mixture of distorted corner and edge-sharing OCa3Ta trigonal pyramids. In the third O site, O(1) is bonded in a distorted trigonal planar geometry to one Ta(1), one Ta(2), and one Fe(1) atom. In the fourth O site, O(2) is bonded in a distorted trigonal planar geometry to one Ta(1), one Ta(2), and one Fe(2) atom. In the fifth O site, O(3) is bonded in a 3-coordinate geometry to one Ta(1), one Fe(1), and one Fe(2) atom. In the sixth O site, O(4) is bonded in a 3-coordinate geometry to one Ta(2), one Fe(1), and one Fe(2) atom. In the seventh O site, O(5) is bonded in a 6-coordinate geometry to two equivalent Ca(1), two equivalent Ca(2), one Ta(1), and one Ta(2) atom. In the eighth O site, O(6) is bonded in a 6-coordinate geometry to two equivalent Ca(1), two equivalent Ca(2), one Ta(1), and one Ta(2) atom.
Ca2Ta2FeO8 crystallizes in the monoclinic C2 space group. There are two inequivalent Ca sites. In the first Ca site, Ca(1) is bonded in a 5-coordinate geometry to one O(7), two equivalent O(5), two equivalent O(6), and two equivalent O(8) atoms. The Ca(1)-O(7) bond length is 2.24 Å. There is one shorter (2.58 Å) and one longer (2.96 Å) Ca(1)-O(5) bond length. There is one shorter (2.24 Å) and one longer (2.87 Å) Ca(1)-O(6) bond length. There is one shorter (2.22 Å) and one longer (2.36 Å) Ca(1)-O(8) bond length. In the second Ca site, Ca(2) is bonded in a 5-coordinate geometry to one O(8), two equivalent O(5), two equivalent O(6), and two equivalent O(7) atoms. The Ca(2)-O(8) bond length is 2.24 Å. There is one shorter (2.24 Å) and one longer (2.87 Å) Ca(2)-O(5) bond length. There is one shorter (2.58 Å) and one longer (2.97 Å) Ca(2)-O(6) bond length. There is one shorter (2.22 Å) and one longer (2.35 Å) Ca(2)-O(7) bond length. There are two inequivalent Ta sites. In the first Ta site, Ta(1) is bonded to one O(1), one O(2), one O(3), one O(5), one O(6), and one O(8) atom to form distorted TaO6 octahedra that share corners with two equivalent Fe(1)O6 octahedra, corners with two equivalent Fe(2)O6 octahedra, and edges with two equivalent Ta(2)O6 octahedra. The corner-sharing octahedral tilt angles range from 45-54°. The Ta(1)-O(1) bond length is 2.01 Å. The Ta(1)-O(2) bond length is 2.09 Å. The Ta(1)-O(3) bond length is 1.89 Å. The Ta(1)-O(5) bond length is 2.04 Å. The Ta(1)-O(6) bond length is 2.28 Å. The Ta(1)-O(8) bond length is 1.93 Å. In the second Ta site, Ta(2) is bonded to one O(1), one O(2), one O(4), one O(5), one O(6), and one O(7) atom to form distorted TaO6 octahedra that share corners with two equivalent Fe(1)O6 octahedra, corners with two equivalent Fe(2)O6 octahedra, and edges with two equivalent Ta(1)O6 octahedra. The corner-sharing octahedral tilt angles range from 45-55°. The Ta(2)-O(1) bond length is 2.09 Å. The Ta(2)-O(2) bond length is 2.01 Å. The Ta(2)-O(4) bond length is 1.89 Å. The Ta(2)-O(5) bond length is 2.27 Å. The Ta(2)-O(6) bond length is 2.04 Å. The Ta(2)-O(7) bond length is 1.94 Å. There are two inequivalent Fe sites. In the first Fe site, Fe(1) is bonded to two equivalent O(1), two equivalent O(3), and two equivalent O(4) atoms to form FeO6 octahedra that share corners with four equivalent Ta(1)O6 octahedra, corners with four equivalent Ta(2)O6 octahedra, and edges with two equivalent Fe(2)O6 octahedra. The corner-sharing octahedral tilt angles range from 45-54°. Both Fe(1)-O(1) bond lengths are 2.28 Å. Both Fe(1)-O(3) bond lengths are 2.06 Å. Both Fe(1)-O(4) bond lengths are 2.44 Å. In the second Fe site, Fe(2) is bonded to two equivalent O(2), two equivalent O(3), and two equivalent O(4) atoms to form FeO6 octahedra that share corners with four equivalent Ta(1)O6 octahedra, corners with four equivalent Ta(2)O6 octahedra, and edges with two equivalent Fe(1)O6 octahedra. The corner-sharing octahedral tilt angles range from 45-55°. Both Fe(2)-O(2) bond lengths are 2.28 Å. Both Fe(2)-O(3) bond lengths are 2.43 Å. Both Fe(2)-O(4) bond lengths are 2.06 Å. There are eight inequivalent O sites. In the first O site, O(7) is bonded to one Ca(1), two equivalent Ca(2), and one Ta(2) atom to form a mixture of distorted corner and edge-sharing OCa3Ta trigonal pyramids. In the second O site, O(8) is bonded to one Ca(2), two equivalent Ca(1), and one Ta(1) atom to form a mixture of distorted corner and edge-sharing OCa3Ta trigonal pyramids. In the third O site, O(1) is bonded in a distorted trigonal planar geometry to one Ta(1), one Ta(2), and one Fe(1) atom. In the fourth O site, O(2) is bonded in a distorted trigonal planar geometry to one Ta(1), one Ta(2), and one Fe(2) atom. In the fifth O site, O(3) is bonded in a 3-coordinate geometry to one Ta(1), one Fe(1), and one Fe(2) atom. In the sixth O site, O(4) is bonded in a 3-coordinate geometry to one Ta(2), one Fe(1), and one Fe(2) atom. In the seventh O site, O(5) is bonded in a 6-coordinate geometry to two equivalent Ca(1), two equivalent Ca(2), one Ta(1), and one Ta(2) atom. In the eighth O site, O(6) is bonded in a 6-coordinate geometry to two equivalent Ca(1), two equivalent Ca(2), one Ta(1), and one Ta(2) atom.