KFe(H2P2O7)2 crystallizes in the triclinic P-1 space group. K(1) is bonded in a 6-coordinate geometry to two equivalent O(1), two equivalent O(3), and two equivalent O(7) atoms. 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 two equivalent P(1)O4 tetrahedra and corners with four equivalent P(2)O4 tetrahedra. There are two inequivalent P sites. In the first P site, P(1) is bonded to one O(1), one O(2), one O(5), and one O(7) atom to form PO4 tetrahedra that share  a cornercorner with one Fe(1)O6 octahedra and  a cornercorner with one P(2)O4 tetrahedra. The corner-sharing octahedral tilt angles are 45°. In the second P site, P(2) is bonded to one O(3), one O(4), one O(5), and one O(6) atom to form PO4 tetrahedra that share corners with two equivalent Fe(1)O6 octahedra and  a cornercorner with one P(1)O4 tetrahedra. The corner-sharing octahedral tilt angles range from 41-45°. There are two inequivalent H sites. In the first H site, H(1) is bonded in a linear geometry to one O(2) and one O(6) atom. In the second H site, H(2) is bonded in a linear geometry to one O(2) and one O(7) atom. There are seven inequivalent O sites. In the first O site, O(1) is bonded in a 2-coordinate geometry to one K(1), one Fe(1), and one P(1) atom. In the second O site, O(2) is bonded in a distorted trigonal planar geometry to one P(1), one H(1), and one H(2) atom. In the third O site, O(3) is bonded in a 3-coordinate geometry to one K(1), one Fe(1), and one P(2) atom. In the fourth O site, O(4) is bonded in a bent 150 degrees geometry to one Fe(1) and one P(2) atom. In the fifth O site, O(5) is bonded in a distorted bent 120 degrees geometry to one P(1) and one P(2) atom. In the sixth O site, O(6) is bonded in a bent 120 degrees geometry to one P(2) and one H(1) atom. In the seventh O site, O(7) is bonded in a distorted bent 120 degrees geometry to one K(1), one P(1), and one H(2) atom.
KFe(H2P2O7)2 crystallizes in the triclinic P-1 space group. K(1) is bonded in a 6-coordinate geometry to two equivalent O(1), two equivalent O(3), and two equivalent O(7) atoms. Both K(1)-O(1) bond lengths are 2.96 Å. Both K(1)-O(3) bond lengths are 2.73 Å. Both K(1)-O(7) bond lengths are 2.85 Å. 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 two equivalent P(1)O4 tetrahedra and corners with four equivalent P(2)O4 tetrahedra. Both Fe(1)-O(1) bond lengths are 2.05 Å. Both Fe(1)-O(3) bond lengths are 2.01 Å. Both Fe(1)-O(4) bond lengths are 2.01 Å. There are two inequivalent P sites. In the first P site, P(1) is bonded to one O(1), one O(2), one O(5), and one O(7) atom to form PO4 tetrahedra that share  a cornercorner with one Fe(1)O6 octahedra and  a cornercorner with one P(2)O4 tetrahedra. The corner-sharing octahedral tilt angles are 45°. The P(1)-O(1) bond length is 1.52 Å. The P(1)-O(2) bond length is 1.51 Å. The P(1)-O(5) bond length is 1.62 Å. The P(1)-O(7) bond length is 1.58 Å. In the second P site, P(2) is bonded to one O(3), one O(4), one O(5), and one O(6) atom to form PO4 tetrahedra that share corners with two equivalent Fe(1)O6 octahedra and  a cornercorner with one P(1)O4 tetrahedra. The corner-sharing octahedral tilt angles range from 41-45°. The P(2)-O(3) bond length is 1.53 Å. The P(2)-O(4) bond length is 1.51 Å. The P(2)-O(5) bond length is 1.61 Å. The P(2)-O(6) bond length is 1.56 Å. There are two inequivalent H sites. In the first H site, H(1) is bonded in a linear geometry to one O(2) and one O(6) atom. The H(1)-O(2) bond length is 1.52 Å. The H(1)-O(6) bond length is 1.03 Å. In the second H site, H(2) is bonded in a linear geometry to one O(2) and one O(7) atom. The H(2)-O(2) bond length is 1.58 Å. The H(2)-O(7) bond length is 1.03 Å. There are seven inequivalent O sites. In the first O site, O(1) is bonded in a 2-coordinate geometry to one K(1), one Fe(1), and one P(1) atom. In the second O site, O(2) is bonded in a distorted trigonal planar geometry to one P(1), one H(1), and one H(2) atom. In the third O site, O(3) is bonded in a 3-coordinate geometry to one K(1), one Fe(1), and one P(2) atom. In the fourth O site, O(4) is bonded in a bent 150 degrees geometry to one Fe(1) and one P(2) atom. In the fifth O site, O(5) is bonded in a distorted bent 120 degrees geometry to one P(1) and one P(2) atom. In the sixth O site, O(6) is bonded in a bent 120 degrees geometry to one P(2) and one H(1) atom. In the seventh O site, O(7) is bonded in a distorted bent 120 degrees geometry to one K(1), one P(1), and one H(2) atom.