Er4(P2O7)3 crystallizes in the cubic I2_13 space group. There are two inequivalent Er sites. In the first Er site, Er(1) is bonded to three equivalent O(1) and three equivalent O(3) atoms to form distorted ErO6 octahedra that share corners with six equivalent P(1)O4 tetrahedra. All Er(1)-O(1) bond lengths are 2.31 Å. All Er(1)-O(3) bond lengths are 2.20 Å. In the second Er site, Er(2) is bonded in a 6-coordinate geometry to three equivalent O(1) and three equivalent O(2) atoms. All Er(2)-O(1) bond lengths are 2.41 Å. All Er(2)-O(2) bond lengths are 2.20 Å. 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 two equivalent Er(1)O6 octahedra and  a cornercorner with one P(1)O4 tetrahedra. The corner-sharing octahedral tilt angles range from 32-45°. 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(3) bond length is 1.48 Å. The P(1)-O(4) bond length is 1.61 Å. There are four inequivalent O sites. In the first O site, O(1) is bonded in a 3-coordinate geometry to one Er(1), one Er(2), and one P(1) atom. In the second O site, O(2) is bonded in a distorted bent 120 degrees geometry to one Er(2) and one P(1) atom. In the third O site, O(3) is bonded in a bent 150 degrees geometry to one Er(1) and one P(1) atom. In the fourth O site, O(4) is bonded in a bent 120 degrees geometry to two equivalent P(1) atoms. Linkers: 12 [O]P([O])(=O)OP([O])([O])=O. Metal clusters: 16 [Er]. The MOF has largest included sphere 4.33 A, density 3.28 g/cm3, surface area 1417.29 m2/g, accessible volume 0.11 cm3/g