K3ScSi2O7 crystallizes in the hexagonal P6_3/mmc space group. There are two inequivalent K sites. In the first K site, K(1) is bonded in a 9-coordinate geometry to nine equivalent O(1) atoms. In the second K site, K(2) is bonded in a 6-coordinate geometry to six equivalent O(1) atoms. Sc(1) is bonded to six equivalent O(1) atoms to form ScO6 octahedra that share corners with six equivalent Si(1)O4 tetrahedra. Si(1) is bonded to one O(2) and three equivalent O(1) atoms to form SiO4 tetrahedra that share corners with three equivalent Sc(1)O6 octahedra and  a cornercorner with one Si(1)O4 tetrahedra. The corner-sharing octahedral tilt angles are 19°. There are two inequivalent O sites. In the first O site, O(1) is bonded in a 2-coordinate geometry to one K(2), three equivalent K(1), one Sc(1), and one Si(1) atom. In the second O site, O(2) is bonded in a linear geometry to two equivalent Si(1) atoms.
K3ScSi2O7 crystallizes in the hexagonal P6_3/mmc space group. There are two inequivalent K sites. In the first K site, K(1) is bonded in a 9-coordinate geometry to nine equivalent O(1) atoms. There are six shorter (2.84 Å) and three longer (3.00 Å) K(1)-O(1) bond lengths. In the second K site, K(2) is bonded in a 6-coordinate geometry to six equivalent O(1) atoms. All K(2)-O(1) bond lengths are 2.78 Å. Sc(1) is bonded to six equivalent O(1) atoms to form ScO6 octahedra that share corners with six equivalent Si(1)O4 tetrahedra. All Sc(1)-O(1) bond lengths are 2.15 Å. Si(1) is bonded to one O(2) and three equivalent O(1) atoms to form SiO4 tetrahedra that share corners with three equivalent Sc(1)O6 octahedra and  a cornercorner with one Si(1)O4 tetrahedra. The corner-sharing octahedral tilt angles are 19°. The Si(1)-O(2) bond length is 1.67 Å. All Si(1)-O(1) bond lengths are 1.64 Å. There are two inequivalent O sites. In the first O site, O(1) is bonded in a 2-coordinate geometry to one K(2), three equivalent K(1), one Sc(1), and one Si(1) atom. In the second O site, O(2) is bonded in a linear geometry to two equivalent Si(1) atoms.