RbErP2O7 crystallizes in the monoclinic P2_1/c space group. Rb(1) is bonded in a 10-coordinate geometry to one O(1), one O(2), one O(6), one O(7), two equivalent O(3), two equivalent O(4), and two equivalent O(5) atoms. Er(1) is bonded to one O(1), one O(2), one O(3), one O(4), one O(5), and one O(6) atom to form ErO6 octahedra 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(4), one O(5), and one O(7) atom to form PO4 tetrahedra that share corners with three equivalent Er(1)O6 octahedra and  a cornercorner with one P(2)O4 tetrahedra. The corner-sharing octahedral tilt angles range from 16-47°. In the second P site, P(2) is bonded to one O(2), one O(3), one O(6), and one O(7) atom to form PO4 tetrahedra that share corners with three equivalent Er(1)O6 octahedra and  a cornercorner with one P(1)O4 tetrahedra. The corner-sharing octahedral tilt angles range from 14-50°. There are seven inequivalent O sites. In the first O site, O(5) is bonded in a 2-coordinate geometry to two equivalent Rb(1), one Er(1), and one P(1) atom. In the second O site, O(6) is bonded in a 3-coordinate geometry to one Rb(1), one Er(1), and one P(2) atom. In the third O site, O(7) is bonded in a bent 120 degrees geometry to one Rb(1), one P(1), and one P(2) atom. In the fourth O site, O(1) is bonded in a distorted linear geometry to one Rb(1), one Er(1), and one P(1) atom. In the fifth O site, O(2) is bonded in a distorted linear geometry to one Rb(1), one Er(1), and one P(2) atom. In the sixth O site, O(3) is bonded in a distorted bent 150 degrees geometry to two equivalent Rb(1), one Er(1), and one P(2) atom. In the seventh O site, O(4) is bonded in a 4-coordinate geometry to two equivalent Rb(1), one Er(1), and one P(1) atom.
RbErP2O7 crystallizes in the monoclinic P2_1/c space group. Rb(1) is bonded in a 10-coordinate geometry to one O(1), one O(2), one O(6), one O(7), two equivalent O(3), two equivalent O(4), and two equivalent O(5) atoms. The Rb(1)-O(1) bond length is 3.56 Å. The Rb(1)-O(2) bond length is 3.45 Å. The Rb(1)-O(6) bond length is 2.97 Å. The Rb(1)-O(7) bond length is 3.46 Å. There is one shorter (3.32 Å) and one longer (3.47 Å) Rb(1)-O(3) bond length. There is one shorter (2.99 Å) and one longer (3.17 Å) Rb(1)-O(4) bond length. There is one shorter (3.00 Å) and one longer (3.19 Å) Rb(1)-O(5) bond length. Er(1) is bonded to one O(1), one O(2), one O(3), one O(4), one O(5), and one O(6) atom to form ErO6 octahedra 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 2.19 Å. The Er(1)-O(2) bond length is 2.23 Å. The Er(1)-O(3) bond length is 2.24 Å. The Er(1)-O(4) bond length is 2.27 Å. The Er(1)-O(5) bond length is 2.26 Å. The Er(1)-O(6) bond length is 2.25 Å. There are two inequivalent P sites. In the first P site, P(1) is bonded to one O(1), one O(4), one O(5), and one O(7) atom to form PO4 tetrahedra that share corners with three equivalent Er(1)O6 octahedra and  a cornercorner with one P(2)O4 tetrahedra. The corner-sharing octahedral tilt angles range from 16-47°. The P(1)-O(1) bond length is 1.52 Å. The P(1)-O(4) bond length is 1.54 Å. The P(1)-O(5) bond length is 1.54 Å. The P(1)-O(7) bond length is 1.63 Å. In the second P site, P(2) is bonded to one O(2), one O(3), one O(6), and one O(7) atom to form PO4 tetrahedra that share corners with three equivalent Er(1)O6 octahedra and  a cornercorner with one P(1)O4 tetrahedra. The corner-sharing octahedral tilt angles range from 14-50°. The P(2)-O(2) bond length is 1.53 Å. The P(2)-O(3) 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 1.64 Å. There are seven inequivalent O sites. In the first O site, O(5) is bonded in a 2-coordinate geometry to two equivalent Rb(1), one Er(1), and one P(1) atom. In the second O site, O(6) is bonded in a 3-coordinate geometry to one Rb(1), one Er(1), and one P(2) atom. In the third O site, O(7) is bonded in a bent 120 degrees geometry to one Rb(1), one P(1), and one P(2) atom. In the fourth O site, O(1) is bonded in a distorted linear geometry to one Rb(1), one Er(1), and one P(1) atom. In the fifth O site, O(2) is bonded in a distorted linear geometry to one Rb(1), one Er(1), and one P(2) atom. In the sixth O site, O(3) is bonded in a distorted bent 150 degrees geometry to two equivalent Rb(1), one Er(1), and one P(2) atom. In the seventh O site, O(4) is bonded in a 4-coordinate geometry to two equivalent Rb(1), one Er(1), and one P(1) atom.