KNb(PO4)2 crystallizes in the monoclinic P2_1/c space group. K(1) is bonded in a 9-coordinate geometry to one O(2), one O(3), one O(5), one O(8), two equivalent O(7), and three equivalent O(4) atoms. Nb(1) is bonded to one O(1), one O(3), one O(5), one O(6), one O(7), and one O(8) atom to form distorted NbO6 octahedra that share corners with two equivalent P(2)O4 tetrahedra and corners with three equivalent P(1)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(6) atom to form PO4 tetrahedra that share corners with three equivalent Nb(1)O6 octahedra and  a cornercorner with one P(2)O4 tetrahedra. The corner-sharing octahedral tilt angles range from 23-30°. In the second P site, P(2) is bonded to one O(2), one O(3), one O(4), and one O(8) atom to form PO4 tetrahedra that share corners with two equivalent Nb(1)O6 octahedra and  a cornercorner with one P(1)O4 tetrahedra. The corner-sharing octahedral tilt angles range from 29-31°. There are eight inequivalent O sites. In the first O site, O(5) is bonded in a 1-coordinate geometry to one K(1), one Nb(1), and one P(1) atom. In the second O site, O(6) is bonded in a bent 150 degrees geometry to one Nb(1) and one P(1) atom. In the third O site, O(7) is bonded in a distorted single-bond geometry to two equivalent K(1) and one Nb(1) atom. In the fourth O site, O(8) is bonded in a distorted bent 150 degrees geometry to one K(1), one Nb(1), and one P(2) atom. In the fifth O site, O(1) is bonded in a bent 150 degrees geometry to one Nb(1) and one P(1) atom. In the sixth O site, O(2) is bonded in a 2-coordinate geometry to one K(1), one P(1), and one P(2) atom. In the seventh O site, O(3) is bonded in a distorted bent 150 degrees geometry to one K(1), one Nb(1), and one P(2) atom. In the eighth O site, O(4) is bonded in a 1-coordinate geometry to three equivalent K(1) and one P(2) atom.
KNb(PO4)2 crystallizes in the monoclinic P2_1/c space group. K(1) is bonded in a 9-coordinate geometry to one O(2), one O(3), one O(5), one O(8), two equivalent O(7), and three equivalent O(4) atoms. The K(1)-O(2) bond length is 3.04 Å. The K(1)-O(3) bond length is 3.10 Å. The K(1)-O(5) bond length is 3.12 Å. The K(1)-O(8) bond length is 2.97 Å. There is one shorter (2.92 Å) and one longer (3.27 Å) K(1)-O(7) bond length. There are a spread of K(1)-O(4) bond distances ranging from 2.65-3.39 Å. Nb(1) is bonded to one O(1), one O(3), one O(5), one O(6), one O(7), and one O(8) atom to form distorted NbO6 octahedra that share corners with two equivalent P(2)O4 tetrahedra and corners with three equivalent P(1)O4 tetrahedra. The Nb(1)-O(1) bond length is 2.04 Å. The Nb(1)-O(3) bond length is 2.03 Å. The Nb(1)-O(5) bond length is 2.30 Å. The Nb(1)-O(6) bond length is 2.03 Å. The Nb(1)-O(7) bond length is 1.76 Å. The Nb(1)-O(8) bond length is 2.06 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(6) atom to form PO4 tetrahedra that share corners with three equivalent Nb(1)O6 octahedra and  a cornercorner with one P(2)O4 tetrahedra. The corner-sharing octahedral tilt angles range from 23-30°. The P(1)-O(1) bond length is 1.54 Å. The P(1)-O(2) bond length is 1.62 Å. The P(1)-O(5) bond length is 1.50 Å. The P(1)-O(6) bond length is 1.54 Å. In the second P site, P(2) is bonded to one O(2), one O(3), one O(4), and one O(8) atom to form PO4 tetrahedra that share corners with two equivalent Nb(1)O6 octahedra and  a cornercorner with one P(1)O4 tetrahedra. The corner-sharing octahedral tilt angles range from 29-31°. The P(2)-O(2) bond length is 1.63 Å. The P(2)-O(3) bond length is 1.55 Å. The P(2)-O(4) bond length is 1.49 Å. The P(2)-O(8) bond length is 1.54 Å. There are eight inequivalent O sites. In the first O site, O(5) is bonded in a 1-coordinate geometry to one K(1), one Nb(1), and one P(1) atom. In the second O site, O(6) is bonded in a bent 150 degrees geometry to one Nb(1) and one P(1) atom. In the third O site, O(7) is bonded in a distorted single-bond geometry to two equivalent K(1) and one Nb(1) atom. In the fourth O site, O(8) is bonded in a distorted bent 150 degrees geometry to one K(1), one Nb(1), and one P(2) atom. In the fifth O site, O(1) is bonded in a bent 150 degrees geometry to one Nb(1) and one P(1) atom. In the sixth O site, O(2) is bonded in a 2-coordinate geometry to one K(1), one P(1), and one P(2) atom. In the seventh O site, O(3) is bonded in a distorted bent 150 degrees geometry to one K(1), one Nb(1), and one P(2) atom. In the eighth O site, O(4) is bonded in a 1-coordinate geometry to three equivalent K(1) and one P(2) atom.