Mg2Tm2O5 crystallizes in the tetragonal P4/mmm space group. Mg(1) is bonded to one O(2) and four equivalent O(1) atoms to form MgO5 square pyramids that share corners with five equivalent Mg(1)O5 square pyramids and faces with four equivalent Tm(2)O12 cuboctahedra. There are two inequivalent Tm sites. In the first Tm site, Tm(1) is bonded in a body-centered cubic geometry to eight equivalent O(1) atoms. In the second Tm site, Tm(2) is bonded to four equivalent O(2) and eight equivalent O(1) atoms to form TmO12 cuboctahedra that share corners with four equivalent Tm(2)O12 cuboctahedra, faces with four equivalent Tm(2)O12 cuboctahedra, and faces with eight equivalent Mg(1)O5 square pyramids. There are two inequivalent O sites. In the first O site, O(1) is bonded in a 6-coordinate geometry to two equivalent Mg(1), two equivalent Tm(1), and two equivalent Tm(2) atoms. In the second O site, O(2) is bonded in a distorted linear geometry to two equivalent Mg(1) and four equivalent Tm(2) atoms.
Mg2Tm2O5 crystallizes in the tetragonal P4/mmm space group. Mg(1) is bonded to one O(2) and four equivalent O(1) atoms to form MgO5 square pyramids that share corners with five equivalent Mg(1)O5 square pyramids and faces with four equivalent Tm(2)O12 cuboctahedra. The Mg(1)-O(2) bond length is 1.88 Å. All Mg(1)-O(1) bond lengths are 1.94 Å. There are two inequivalent Tm sites. In the first Tm site, Tm(1) is bonded in a body-centered cubic geometry to eight equivalent O(1) atoms. All Tm(1)-O(1) bond lengths are 2.38 Å. In the second Tm site, Tm(2) is bonded to four equivalent O(2) and eight equivalent O(1) atoms to form TmO12 cuboctahedra that share corners with four equivalent Tm(2)O12 cuboctahedra, faces with four equivalent Tm(2)O12 cuboctahedra, and faces with eight equivalent Mg(1)O5 square pyramids. All Tm(2)-O(2) bond lengths are 2.72 Å. All Tm(2)-O(1) bond lengths are 2.90 Å. There are two inequivalent O sites. In the first O site, O(1) is bonded in a 6-coordinate geometry to two equivalent Mg(1), two equivalent Tm(1), and two equivalent Tm(2) atoms. In the second O site, O(2) is bonded in a distorted linear geometry to two equivalent Mg(1) and four equivalent Tm(2) atoms.