KErP2O9 crystallizes in the monoclinic P2_1/c space group. K(1) is bonded in a 9-coordinate geometry to one O(1), one O(2), one O(3), one O(4), one O(5), one O(6), one O(7), one O(8), and one O(9) atom. Er(1) is bonded to one O(1), one O(2), one O(3), one O(5), one O(6), one O(7), and one O(9) atom to form distorted ErO7 pentagonal bipyramids that share corners with three equivalent P(1)O4 tetrahedra and corners with three 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(3), and one O(4) atom to form PO4 tetrahedra that share corners with three equivalent Er(1)O7 pentagonal bipyramids and  a cornercorner with one P(2)O4 tetrahedra. In the second P site, P(2) is bonded to one O(4), one O(5), one O(6), and one O(7) atom to form PO4 tetrahedra that share corners with three equivalent Er(1)O7 pentagonal bipyramids and  a cornercorner with one P(1)O4 tetrahedra. There are nine inequivalent O sites. In the first O site, O(5) is bonded in a 2-coordinate geometry to one K(1), one Er(1), and one P(2) atom. In the second O site, O(6) is bonded in a 3-coordinate geometry to one K(1), one Er(1), and one P(2) atom. In the third O site, O(7) is bonded in a 3-coordinate geometry to one K(1), one Er(1), and one P(2) atom. In the fourth O site, O(8) is bonded in a 2-coordinate geometry to one K(1) and one O(9) atom. In the fifth O site, O(1) is bonded in a 2-coordinate geometry to one K(1), one Er(1), and one P(1) atom. In the sixth O site, O(9) is bonded in a 3-coordinate geometry to one K(1), one Er(1), and one O(8) atom. In the seventh O site, O(2) is bonded in a 3-coordinate geometry to one K(1), one Er(1), and one P(1) atom. In the eighth O site, O(3) is bonded in a 2-coordinate geometry to one K(1), one Er(1), and one P(1) atom. In the ninth O site, O(4) is bonded in a distorted bent 120 degrees geometry to one K(1), one P(1), and one P(2) atom.
KErP2O9 crystallizes in the monoclinic P2_1/c space group. K(1) is bonded in a 9-coordinate geometry to one O(1), one O(2), one O(3), one O(4), one O(5), one O(6), one O(7), one O(8), and one O(9) atom. The K(1)-O(1) bond length is 3.13 Å. The K(1)-O(2) bond length is 2.71 Å. The K(1)-O(3) bond length is 2.99 Å. The K(1)-O(4) bond length is 2.98 Å. The K(1)-O(5) bond length is 3.07 Å. The K(1)-O(6) bond length is 2.80 Å. The K(1)-O(7) bond length is 2.76 Å. The K(1)-O(8) bond length is 3.18 Å. The K(1)-O(9) bond length is 3.22 Å. Er(1) is bonded to one O(1), one O(2), one O(3), one O(5), one O(6), one O(7), and one O(9) atom to form distorted ErO7 pentagonal bipyramids that share corners with three equivalent P(1)O4 tetrahedra and corners with three equivalent P(2)O4 tetrahedra. The Er(1)-O(1) bond length is 2.30 Å. The Er(1)-O(2) bond length is 2.25 Å. The Er(1)-O(3) bond length is 2.27 Å. The Er(1)-O(5) bond length is 2.25 Å. The Er(1)-O(6) bond length is 2.26 Å. The Er(1)-O(7) bond length is 2.26 Å. The Er(1)-O(9) bond length is 2.56 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(3), and one O(4) atom to form PO4 tetrahedra that share corners with three equivalent Er(1)O7 pentagonal bipyramids and  a cornercorner with one P(2)O4 tetrahedra. The P(1)-O(1) bond length is 1.53 Å. The P(1)-O(2) bond length is 1.52 Å. The P(1)-O(3) bond length is 1.54 Å. The P(1)-O(4) bond length is 1.64 Å. In the second P site, P(2) is bonded to one O(4), one O(5), one O(6), and one O(7) atom to form PO4 tetrahedra that share corners with three equivalent Er(1)O7 pentagonal bipyramids and  a cornercorner with one P(1)O4 tetrahedra. The P(2)-O(4) bond length is 1.65 Å. The P(2)-O(5) bond length is 1.53 Å. The P(2)-O(6) bond length is 1.53 Å. The P(2)-O(7) bond length is 1.53 Å. There are nine inequivalent O sites. In the first O site, O(5) is bonded in a 2-coordinate geometry to one K(1), one Er(1), and one P(2) atom. In the second O site, O(6) is bonded in a 3-coordinate geometry to one K(1), one Er(1), and one P(2) atom. In the third O site, O(7) is bonded in a 3-coordinate geometry to one K(1), one Er(1), and one P(2) atom. In the fourth O site, O(8) is bonded in a 2-coordinate geometry to one K(1) and one O(9) atom. The O(8)-O(9) bond length is 2.02 Å. In the fifth O site, O(1) is bonded in a 2-coordinate geometry to one K(1), one Er(1), and one P(1) atom. In the sixth O site, O(9) is bonded in a 3-coordinate geometry to one K(1), one Er(1), and one O(8) atom. In the seventh O site, O(2) is bonded in a 3-coordinate geometry to one K(1), one Er(1), and one P(1) atom. In the eighth O site, O(3) is bonded in a 2-coordinate geometry to one K(1), one Er(1), and one P(1) atom. In the ninth O site, O(4) is bonded in a distorted bent 120 degrees geometry to one K(1), one P(1), and one P(2) atom.