Tm2Nb2O7 crystallizes in the cubic Fd-3m space group. Tm(1) is bonded in a body-centered cubic geometry to two equivalent O(2) and six equivalent O(1) atoms. Nb(1) is bonded to six equivalent O(1) atoms to form distorted corner-sharing NbO6 octahedra. The corner-sharing octahedral tilt angles are 58°. There are two inequivalent O sites. In the first O site, O(1) is bonded to two equivalent Tm(1) and two equivalent Nb(1) atoms to form OTm2Nb2 tetrahedra that share corners with two equivalent O(2)Tm4 tetrahedra, corners with twelve equivalent O(1)Tm2Nb2 tetrahedra,  an edgeedge with one O(2)Tm4 tetrahedra, and edges with four equivalent O(1)Tm2Nb2 tetrahedra. In the second O site, O(2) is bonded to four equivalent Tm(1) atoms to form OTm4 tetrahedra that share corners with four equivalent O(2)Tm4 tetrahedra, corners with twelve equivalent O(1)Tm2Nb2 tetrahedra, and edges with six equivalent O(1)Tm2Nb2 tetrahedra.
Tm2Nb2O7 crystallizes in the cubic Fd-3m space group. Tm(1) is bonded in a body-centered cubic geometry to two equivalent O(2) and six equivalent O(1) atoms. Both Tm(1)-O(2) bond lengths are 2.22 Å. All Tm(1)-O(1) bond lengths are 2.39 Å. Nb(1) is bonded to six equivalent O(1) atoms to form distorted corner-sharing NbO6 octahedra. The corner-sharing octahedral tilt angles are 58°. All Nb(1)-O(1) bond lengths are 2.08 Å. There are two inequivalent O sites. In the first O site, O(1) is bonded to two equivalent Tm(1) and two equivalent Nb(1) atoms to form OTm2Nb2 tetrahedra that share corners with two equivalent O(2)Tm4 tetrahedra, corners with twelve equivalent O(1)Tm2Nb2 tetrahedra,  an edgeedge with one O(2)Tm4 tetrahedra, and edges with four equivalent O(1)Tm2Nb2 tetrahedra. In the second O site, O(2) is bonded to four equivalent Tm(1) atoms to form OTm4 tetrahedra that share corners with four equivalent O(2)Tm4 tetrahedra, corners with twelve equivalent O(1)Tm2Nb2 tetrahedra, and edges with six equivalent O(1)Tm2Nb2 tetrahedra.