K2CaAs2O7 crystallizes in the monoclinic P2_1/c space group. There are two inequivalent K sites. In the first K site, K(1) is bonded in a 9-coordinate geometry to one O(3), one O(5), two equivalent O(1), two equivalent O(7), and three equivalent O(2) atoms. In the second K site, K(2) is bonded in a 9-coordinate geometry to one O(1), one O(3), one O(5), one O(7), two equivalent O(6), and three equivalent O(4) atoms. Ca(1) is bonded to one O(1), one O(2), one O(3), one O(4), one O(5), and one O(7) atom to form CaO6 octahedra that share corners with three equivalent As(1)O4 tetrahedra and corners with three equivalent As(2)O4 tetrahedra. There are two inequivalent As sites. In the first As site, As(1) is bonded to one O(2), one O(3), one O(5), and one O(6) atom to form AsO4 tetrahedra that share corners with three equivalent Ca(1)O6 octahedra and  a cornercorner with one As(2)O4 tetrahedra. The corner-sharing octahedral tilt angles range from 30-50°. In the second As site, As(2) is bonded to one O(1), one O(4), one O(6), and one O(7) atom to form AsO4 tetrahedra that share corners with three equivalent Ca(1)O6 octahedra and  a cornercorner with one As(1)O4 tetrahedra. The corner-sharing octahedral tilt angles range from 19-51°. There are seven inequivalent O sites. In the first O site, O(1) is bonded in a 5-coordinate geometry to one K(2), two equivalent K(1), one Ca(1), and one As(2) atom. In the second O site, O(2) is bonded in a 2-coordinate geometry to three equivalent K(1), one Ca(1), and one As(1) atom. In the third O site, O(3) is bonded in a 3-coordinate geometry to one K(1), one K(2), one Ca(1), and one As(1) atom. In the fourth O site, O(4) is bonded in a 2-coordinate geometry to three equivalent K(2), one Ca(1), and one As(2) atom. In the fifth O site, O(5) is bonded in a 4-coordinate geometry to one K(1), one K(2), one Ca(1), and one As(1) atom. In the sixth O site, O(6) is bonded in a distorted bent 120 degrees geometry to two equivalent K(2), one As(1), and one As(2) atom. In the seventh O site, O(7) is bonded in a 5-coordinate geometry to one K(2), two equivalent K(1), one Ca(1), and one As(2) atom.
K2CaAs2O7 crystallizes in the monoclinic P2_1/c space group. There are two inequivalent K sites. In the first K site, K(1) is bonded in a 9-coordinate geometry to one O(3), one O(5), two equivalent O(1), two equivalent O(7), and three equivalent O(2) atoms. The K(1)-O(3) bond length is 2.85 Å. The K(1)-O(5) bond length is 3.10 Å. There is one shorter (2.86 Å) and one longer (2.88 Å) K(1)-O(1) bond length. There is one shorter (2.88 Å) and one longer (2.94 Å) K(1)-O(7) bond length. There are a spread of K(1)-O(2) bond distances ranging from 2.96-3.23 Å. In the second K site, K(2) is bonded in a 9-coordinate geometry to one O(1), one O(3), one O(5), one O(7), two equivalent O(6), and three equivalent O(4) atoms. The K(2)-O(1) bond length is 3.03 Å. The K(2)-O(3) bond length is 3.34 Å. The K(2)-O(5) bond length is 2.96 Å. The K(2)-O(7) bond length is 2.79 Å. There is one shorter (2.92 Å) and one longer (3.08 Å) K(2)-O(6) bond length. There are a spread of K(2)-O(4) bond distances ranging from 2.94-3.33 Å. Ca(1) is bonded to one O(1), one O(2), one O(3), one O(4), one O(5), and one O(7) atom to form CaO6 octahedra that share corners with three equivalent As(1)O4 tetrahedra and corners with three equivalent As(2)O4 tetrahedra. The Ca(1)-O(1) bond length is 2.38 Å. The Ca(1)-O(2) bond length is 2.36 Å. The Ca(1)-O(3) bond length is 2.36 Å. The Ca(1)-O(4) bond length is 2.34 Å. The Ca(1)-O(5) bond length is 2.34 Å. The Ca(1)-O(7) bond length is 2.43 Å. There are two inequivalent As sites. In the first As site, As(1) is bonded to one O(2), one O(3), one O(5), and one O(6) atom to form AsO4 tetrahedra that share corners with three equivalent Ca(1)O6 octahedra and  a cornercorner with one As(2)O4 tetrahedra. The corner-sharing octahedral tilt angles range from 30-50°. The As(1)-O(2) bond length is 1.70 Å. The As(1)-O(3) bond length is 1.71 Å. The As(1)-O(5) bond length is 1.70 Å. The As(1)-O(6) bond length is 1.83 Å. In the second As site, As(2) is bonded to one O(1), one O(4), one O(6), and one O(7) atom to form AsO4 tetrahedra that share corners with three equivalent Ca(1)O6 octahedra and  a cornercorner with one As(1)O4 tetrahedra. The corner-sharing octahedral tilt angles range from 19-51°. The As(2)-O(1) bond length is 1.70 Å. The As(2)-O(4) bond length is 1.69 Å. The As(2)-O(6) bond length is 1.83 Å. The As(2)-O(7) bond length is 1.70 Å. There are seven inequivalent O sites. In the first O site, O(1) is bonded in a 5-coordinate geometry to one K(2), two equivalent K(1), one Ca(1), and one As(2) atom. In the second O site, O(2) is bonded in a 2-coordinate geometry to three equivalent K(1), one Ca(1), and one As(1) atom. In the third O site, O(3) is bonded in a 3-coordinate geometry to one K(1), one K(2), one Ca(1), and one As(1) atom. In the fourth O site, O(4) is bonded in a 2-coordinate geometry to three equivalent K(2), one Ca(1), and one As(2) atom. In the fifth O site, O(5) is bonded in a 4-coordinate geometry to one K(1), one K(2), one Ca(1), and one As(1) atom. In the sixth O site, O(6) is bonded in a distorted bent 120 degrees geometry to two equivalent K(2), one As(1), and one As(2) atom. In the seventh O site, O(7) is bonded in a 5-coordinate geometry to one K(2), two equivalent K(1), one Ca(1), and one As(2) atom.