Ba5Eu8Zn4O21 crystallizes in the tetragonal I4/m space group. There are two inequivalent Ba sites. In the first Ba site, Ba(1) is bonded in a 10-coordinate geometry to two equivalent O(2), four equivalent O(1), and four equivalent O(3) atoms. In the second Ba site, Ba(2) is bonded in a distorted q6 geometry to two equivalent O(4) and eight equivalent O(2) atoms. There are two inequivalent Eu sites. In the first Eu site, Eu(1) is bonded to one O(4), two equivalent O(1), and four equivalent O(2) atoms to form distorted EuO7 pentagonal bipyramids that share  a cornercorner with one Eu(1)O7 pentagonal bipyramid, corners with two equivalent Zn(1)O5 trigonal bipyramids, edges with two equivalent Zn(1)O5 trigonal bipyramids, and faces with two equivalent Eu(1)O7 pentagonal bipyramids. In the second Eu site, Eu(2) is bonded in a 7-coordinate geometry to one O(3), two equivalent O(2), and four equivalent O(1) atoms. Zn(1) is bonded to one O(3), two equivalent O(1), and two equivalent O(2) atoms to form distorted ZnO5 trigonal bipyramids that share corners with two equivalent Eu(1)O7 pentagonal bipyramids and edges with two equivalent Eu(1)O7 pentagonal bipyramids. There are four inequivalent O sites. In the first O site, O(1) is bonded in a 4-coordinate geometry to two equivalent Ba(1), one Eu(1), two equivalent Eu(2), and one Zn(1) atom. In the second O site, O(2) is bonded to one Ba(1), one Ba(2), one Eu(2), two equivalent Eu(1), and one Zn(1) atom to form distorted OBa2Eu3Zn octahedra that share  a cornercorner with one O(4)Ba2Eu4 octahedra, corners with three equivalent O(3)Ba4EuZn octahedra, corners with six equivalent O(2)Ba2Eu3Zn octahedra, edges with two equivalent O(2)Ba2Eu3Zn octahedra,  a faceface with one O(4)Ba2Eu4 octahedra,  a faceface with one O(3)Ba4EuZn octahedra, and faces with two equivalent O(2)Ba2Eu3Zn octahedra. The corner-sharing octahedral tilt angles range from 0-77°. In the third O site, O(3) is bonded to four equivalent Ba(1), one Eu(2), and one Zn(1) atom to form distorted OBa4EuZn octahedra that share corners with two equivalent O(3)Ba4EuZn octahedra, corners with six equivalent O(2)Ba2Eu3Zn octahedra, edges with five equivalent O(3)Ba4EuZn octahedra, and faces with two equivalent O(2)Ba2Eu3Zn octahedra. The corner-sharing octahedral tilt angles range from 7-58°. In the fourth O site, O(4) is bonded to two equivalent Ba(2) and four equivalent Eu(1) atoms to form OBa2Eu4 octahedra that share corners with two equivalent O(4)Ba2Eu4 octahedra, corners with eight equivalent O(2)Ba2Eu3Zn octahedra, and faces with eight equivalent O(2)Ba2Eu3Zn octahedra. The corner-sharing octahedral tilt angles range from 0-61°.
Ba5Eu8Zn4O21 crystallizes in the tetragonal I4/m space group. There are two inequivalent Ba sites. In the first Ba site, Ba(1) is bonded in a 10-coordinate geometry to two equivalent O(2), four equivalent O(1), and four equivalent O(3) atoms. Both Ba(1)-O(2) bond lengths are 2.98 Å. All Ba(1)-O(1) bond lengths are 3.26 Å. There are a spread of Ba(1)-O(3) bond distances ranging from 2.69-2.92 Å. In the second Ba site, Ba(2) is bonded in a distorted q6 geometry to two equivalent O(4) and eight equivalent O(2) atoms. Both Ba(2)-O(4) bond lengths are 2.92 Å. All Ba(2)-O(2) bond lengths are 2.99 Å. There are two inequivalent Eu sites. In the first Eu site, Eu(1) is bonded to one O(4), two equivalent O(1), and four equivalent O(2) atoms to form distorted EuO7 pentagonal bipyramids that share  a cornercorner with one Eu(1)O7 pentagonal bipyramid, corners with two equivalent Zn(1)O5 trigonal bipyramids, edges with two equivalent Zn(1)O5 trigonal bipyramids, and faces with two equivalent Eu(1)O7 pentagonal bipyramids. The Eu(1)-O(4) bond length is 2.38 Å. Both Eu(1)-O(1) bond lengths are 2.39 Å. There are two shorter (2.39 Å) and two longer (2.47 Å) Eu(1)-O(2) bond lengths. In the second Eu site, Eu(2) is bonded in a 7-coordinate geometry to one O(3), two equivalent O(2), and four equivalent O(1) atoms. The Eu(2)-O(3) bond length is 2.60 Å. Both Eu(2)-O(2) bond lengths are 2.45 Å. There are two shorter (2.34 Å) and two longer (2.45 Å) Eu(2)-O(1) bond lengths. Zn(1) is bonded to one O(3), two equivalent O(1), and two equivalent O(2) atoms to form distorted ZnO5 trigonal bipyramids that share corners with two equivalent Eu(1)O7 pentagonal bipyramids and edges with two equivalent Eu(1)O7 pentagonal bipyramids. The Zn(1)-O(3) bond length is 2.02 Å. Both Zn(1)-O(1) bond lengths are 2.05 Å. Both Zn(1)-O(2) bond lengths are 2.15 Å. There are four inequivalent O sites. In the first O site, O(1) is bonded in a 4-coordinate geometry to two equivalent Ba(1), one Eu(1), two equivalent Eu(2), and one Zn(1) atom. In the second O site, O(2) is bonded to one Ba(1), one Ba(2), one Eu(2), two equivalent Eu(1), and one Zn(1) atom to form distorted OBa2Eu3Zn octahedra that share  a cornercorner with one O(4)Ba2Eu4 octahedra, corners with three equivalent O(3)Ba4EuZn octahedra, corners with six equivalent O(2)Ba2Eu3Zn octahedra, edges with two equivalent O(2)Ba2Eu3Zn octahedra,  a faceface with one O(4)Ba2Eu4 octahedra,  a faceface with one O(3)Ba4EuZn octahedra, and faces with two equivalent O(2)Ba2Eu3Zn octahedra. The corner-sharing octahedral tilt angles range from 0-77°. In the third O site, O(3) is bonded to four equivalent Ba(1), one Eu(2), and one Zn(1) atom to form distorted OBa4EuZn octahedra that share corners with two equivalent O(3)Ba4EuZn octahedra, corners with six equivalent O(2)Ba2Eu3Zn octahedra, edges with five equivalent O(3)Ba4EuZn octahedra, and faces with two equivalent O(2)Ba2Eu3Zn octahedra. The corner-sharing octahedral tilt angles range from 7-58°. In the fourth O site, O(4) is bonded to two equivalent Ba(2) and four equivalent Eu(1) atoms to form OBa2Eu4 octahedra that share corners with two equivalent O(4)Ba2Eu4 octahedra, corners with eight equivalent O(2)Ba2Eu3Zn octahedra, and faces with eight equivalent O(2)Ba2Eu3Zn octahedra. The corner-sharing octahedral tilt angles range from 0-61°.