CaSc4(Si2O7)2 crystallizes in the trigonal R3m space group. Ca(1) is bonded to three equivalent O(3) and three equivalent O(4) atoms to form distorted CaO6 octahedra that share corners with three equivalent Sc(2)O7 hexagonal pyramids, corners with three equivalent Si(2)O6 octahedra, edges with three equivalent Sc(1)O6 octahedra, and edges with three equivalent Si(1)O5 trigonal bipyramids. The corner-sharing octahedral tilt angles are 8°. There are two inequivalent Sc sites. In the first Sc site, Sc(1) is bonded to one O(5), one O(6), two equivalent O(2), and two equivalent O(3) atoms to form distorted ScO6 octahedra that share  a cornercorner with one Sc(2)O7 hexagonal pyramid, corners with two equivalent Si(1)O5 trigonal bipyramids,  an edgeedge with one Ca(1)O6 octahedra, edges with two equivalent Si(2)O6 octahedra, and edges with four equivalent Sc(1)O6 octahedra. In the second Sc site, Sc(2) is bonded to one O(5), three equivalent O(1), and three equivalent O(4) atoms to form ScO7 hexagonal pyramids that share corners with three equivalent Ca(1)O6 octahedra, corners with three equivalent Sc(1)O6 octahedra, and edges with six equivalent Si(1)O5 trigonal bipyramids. The corner-sharing octahedral tilt angles range from 47-62°. There are two inequivalent Si sites. In the first Si site, Si(1) is bonded to one O(2), two equivalent O(1), and two equivalent O(4) atoms to form distorted SiO5 trigonal bipyramids that share  a cornercorner with one Si(2)O6 octahedra, corners with two equivalent Sc(1)O6 octahedra, corners with four equivalent Si(1)O5 trigonal bipyramids, edges with two equivalent Sc(2)O7 hexagonal pyramids, and  an edgeedge with one Ca(1)O6 octahedra. The corner-sharing octahedral tilt angles range from 40-71°. In the second Si site, Si(2) is bonded to three equivalent O(2) and three equivalent O(3) atoms to form SiO6 octahedra that share corners with three equivalent Ca(1)O6 octahedra, corners with three equivalent Si(1)O5 trigonal bipyramids, and edges with six equivalent Sc(1)O6 octahedra. The corner-sharing octahedral tilt angles are 8°. There are six inequivalent O sites. In the first O site, O(4) is bonded to one Ca(1), one Sc(2), and two equivalent Si(1) atoms to form distorted OCaScSi2 tetrahedra that share  a cornercorner with one O(5)Sc4 tetrahedra, corners with two equivalent O(4)CaScSi2 tetrahedra, corners with two equivalent O(2)Sc2Si2 tetrahedra, and edges with two equivalent O(4)CaScSi2 tetrahedra. In the second O site, O(5) is bonded to one Sc(2) and three equivalent Sc(1) atoms to form distorted OSc4 tetrahedra that share corners with three equivalent O(4)CaScSi2 tetrahedra and corners with six equivalent O(2)Sc2Si2 tetrahedra. In the third O site, O(6) is bonded in a distorted T-shaped geometry to three equivalent Sc(1) atoms. In the fourth O site, O(1) is bonded in a 3-coordinate geometry to one Sc(2) and two equivalent Si(1) atoms. In the fifth O site, O(2) is bonded to two equivalent Sc(1), one Si(1), and one Si(2) atom to form distorted OSc2Si2 tetrahedra that share corners with two equivalent O(4)CaScSi2 tetrahedra, corners with two equivalent O(5)Sc4 tetrahedra, and corners with four equivalent O(2)Sc2Si2 tetrahedra. In the sixth O site, O(3) is bonded in a distorted see-saw-like geometry to one Ca(1), two equivalent Sc(1), and one Si(2) atom.
CaSc4(Si2O7)2 crystallizes in the trigonal R3m space group. Ca(1) is bonded to three equivalent O(3) and three equivalent O(4) atoms to form distorted CaO6 octahedra that share corners with three equivalent Sc(2)O7 hexagonal pyramids, corners with three equivalent Si(2)O6 octahedra, edges with three equivalent Sc(1)O6 octahedra, and edges with three equivalent Si(1)O5 trigonal bipyramids. The corner-sharing octahedral tilt angles are 8°. All Ca(1)-O(3) bond lengths are 2.43 Å. All Ca(1)-O(4) bond lengths are 2.37 Å. There are two inequivalent Sc sites. In the first Sc site, Sc(1) is bonded to one O(5), one O(6), two equivalent O(2), and two equivalent O(3) atoms to form distorted ScO6 octahedra that share  a cornercorner with one Sc(2)O7 hexagonal pyramid, corners with two equivalent Si(1)O5 trigonal bipyramids,  an edgeedge with one Ca(1)O6 octahedra, edges with two equivalent Si(2)O6 octahedra, and edges with four equivalent Sc(1)O6 octahedra. The Sc(1)-O(5) bond length is 2.23 Å. The Sc(1)-O(6) bond length is 2.08 Å. Both Sc(1)-O(2) bond lengths are 2.21 Å. Both Sc(1)-O(3) bond lengths are 2.26 Å. In the second Sc site, Sc(2) is bonded to one O(5), three equivalent O(1), and three equivalent O(4) atoms to form ScO7 hexagonal pyramids that share corners with three equivalent Ca(1)O6 octahedra, corners with three equivalent Sc(1)O6 octahedra, and edges with six equivalent Si(1)O5 trigonal bipyramids. The corner-sharing octahedral tilt angles range from 47-62°. The Sc(2)-O(5) bond length is 1.96 Å. All Sc(2)-O(1) bond lengths are 2.25 Å. All Sc(2)-O(4) bond lengths are 2.28 Å. There are two inequivalent Si sites. In the first Si site, Si(1) is bonded to one O(2), two equivalent O(1), and two equivalent O(4) atoms to form distorted SiO5 trigonal bipyramids that share  a cornercorner with one Si(2)O6 octahedra, corners with two equivalent Sc(1)O6 octahedra, corners with four equivalent Si(1)O5 trigonal bipyramids, edges with two equivalent Sc(2)O7 hexagonal pyramids, and  an edgeedge with one Ca(1)O6 octahedra. The corner-sharing octahedral tilt angles range from 40-71°. The Si(1)-O(2) bond length is 1.67 Å. Both Si(1)-O(1) bond lengths are 1.71 Å. Both Si(1)-O(4) bond lengths are 1.77 Å. In the second Si site, Si(2) is bonded to three equivalent O(2) and three equivalent O(3) atoms to form SiO6 octahedra that share corners with three equivalent Ca(1)O6 octahedra, corners with three equivalent Si(1)O5 trigonal bipyramids, and edges with six equivalent Sc(1)O6 octahedra. The corner-sharing octahedral tilt angles are 8°. All Si(2)-O(2) bond lengths are 1.98 Å. All Si(2)-O(3) bond lengths are 1.77 Å. There are six inequivalent O sites. In the first O site, O(4) is bonded to one Ca(1), one Sc(2), and two equivalent Si(1) atoms to form distorted OCaScSi2 tetrahedra that share  a cornercorner with one O(5)Sc4 tetrahedra, corners with two equivalent O(4)CaScSi2 tetrahedra, corners with two equivalent O(2)Sc2Si2 tetrahedra, and edges with two equivalent O(4)CaScSi2 tetrahedra. In the second O site, O(5) is bonded to one Sc(2) and three equivalent Sc(1) atoms to form distorted OSc4 tetrahedra that share corners with three equivalent O(4)CaScSi2 tetrahedra and corners with six equivalent O(2)Sc2Si2 tetrahedra. In the third O site, O(6) is bonded in a distorted T-shaped geometry to three equivalent Sc(1) atoms. In the fourth O site, O(1) is bonded in a 3-coordinate geometry to one Sc(2) and two equivalent Si(1) atoms. In the fifth O site, O(2) is bonded to two equivalent Sc(1), one Si(1), and one Si(2) atom to form distorted OSc2Si2 tetrahedra that share corners with two equivalent O(4)CaScSi2 tetrahedra, corners with two equivalent O(5)Sc4 tetrahedra, and corners with four equivalent O(2)Sc2Si2 tetrahedra. In the sixth O site, O(3) is bonded in a distorted see-saw-like geometry to one Ca(1), two equivalent Sc(1), and one Si(2) atom.