Ba3Er2MoO9 crystallizes in the monoclinic C2/c space group. There are two inequivalent Ba sites. In the first Ba site, Ba(1) is bonded to one O(5), two equivalent O(1), three equivalent O(2), three equivalent O(3), and three equivalent O(4) atoms to form distorted BaO12 cuboctahedra that share corners with three equivalent Ba(2)O12 cuboctahedra, corners with nine equivalent Ba(1)O12 cuboctahedra, faces with three equivalent Ba(1)O12 cuboctahedra, faces with three equivalent Ba(2)O12 cuboctahedra, faces with three equivalent Mo(1)O6 octahedra, and faces with five equivalent Er(1)O6 octahedra. In the second Ba site, Ba(2) is bonded to two equivalent O(2), two equivalent O(3), two equivalent O(4), two equivalent O(5), and four equivalent O(1) atoms to form BaO12 cuboctahedra that share corners with six equivalent Ba(1)O12 cuboctahedra, corners with six equivalent Ba(2)O12 cuboctahedra, faces with six equivalent Ba(1)O12 cuboctahedra, faces with two equivalent Mo(1)O6 octahedra, and faces with six equivalent Er(1)O6 octahedra. Er(1) is bonded to one O(2), one O(3), one O(4), one O(5), and two equivalent O(1) atoms to form ErO6 octahedra that share corners with three equivalent Er(1)O6 octahedra, corners with three equivalent Mo(1)O6 octahedra, faces with three equivalent Ba(2)O12 cuboctahedra, and faces with five equivalent Ba(1)O12 cuboctahedra. The corner-sharing octahedral tilt angles range from 0-14°. Mo(1) is bonded to two equivalent O(2), two equivalent O(3), and two equivalent O(4) atoms to form MoO6 octahedra that share corners with six equivalent Er(1)O6 octahedra, faces with two equivalent Ba(2)O12 cuboctahedra, and faces with six equivalent Ba(1)O12 cuboctahedra. The corner-sharing octahedral tilt angles range from 1-12°. There are five inequivalent O sites. In the first O site, O(4) is bonded in a distorted linear geometry to one Ba(2), three equivalent Ba(1), one Er(1), and one Mo(1) atom. In the second O site, O(5) is bonded in a distorted linear geometry to two equivalent Ba(1), two equivalent Ba(2), and two equivalent Er(1) atoms. In the third O site, O(1) is bonded in a 2-coordinate geometry to two equivalent Ba(1), two equivalent Ba(2), and two equivalent Er(1) atoms. In the fourth O site, O(2) is bonded in a 2-coordinate geometry to one Ba(2), three equivalent Ba(1), one Er(1), and one Mo(1) atom. In the fifth O site, O(3) is bonded in a 2-coordinate geometry to one Ba(2), three equivalent Ba(1), one Er(1), and one Mo(1) atom.
Ba3Er2MoO9 crystallizes in the monoclinic C2/c space group. There are two inequivalent Ba sites. In the first Ba site, Ba(1) is bonded to one O(5), two equivalent O(1), three equivalent O(2), three equivalent O(3), and three equivalent O(4) atoms to form distorted BaO12 cuboctahedra that share corners with three equivalent Ba(2)O12 cuboctahedra, corners with nine equivalent Ba(1)O12 cuboctahedra, faces with three equivalent Ba(1)O12 cuboctahedra, faces with three equivalent Ba(2)O12 cuboctahedra, faces with three equivalent Mo(1)O6 octahedra, and faces with five equivalent Er(1)O6 octahedra. The Ba(1)-O(5) bond length is 2.97 Å. There is one shorter (2.80 Å) and one longer (3.18 Å) Ba(1)-O(1) bond length. There are a spread of Ba(1)-O(2) bond distances ranging from 2.89-3.22 Å. There are a spread of Ba(1)-O(3) bond distances ranging from 2.91-3.19 Å. There are a spread of Ba(1)-O(4) bond distances ranging from 3.03-3.06 Å. In the second Ba site, Ba(2) is bonded to two equivalent O(2), two equivalent O(3), two equivalent O(4), two equivalent O(5), and four equivalent O(1) atoms to form BaO12 cuboctahedra that share corners with six equivalent Ba(1)O12 cuboctahedra, corners with six equivalent Ba(2)O12 cuboctahedra, faces with six equivalent Ba(1)O12 cuboctahedra, faces with two equivalent Mo(1)O6 octahedra, and faces with six equivalent Er(1)O6 octahedra. Both Ba(2)-O(2) bond lengths are 2.92 Å. Both Ba(2)-O(3) bond lengths are 3.21 Å. Both Ba(2)-O(4) bond lengths are 3.05 Å. There is one shorter (3.02 Å) and one longer (3.03 Å) Ba(2)-O(5) bond length. There are two shorter (2.84 Å) and two longer (3.23 Å) Ba(2)-O(1) bond lengths. Er(1) is bonded to one O(2), one O(3), one O(4), one O(5), and two equivalent O(1) atoms to form ErO6 octahedra that share corners with three equivalent Er(1)O6 octahedra, corners with three equivalent Mo(1)O6 octahedra, faces with three equivalent Ba(2)O12 cuboctahedra, and faces with five equivalent Ba(1)O12 cuboctahedra. The corner-sharing octahedral tilt angles range from 0-14°. The Er(1)-O(2) bond length is 2.34 Å. The Er(1)-O(3) bond length is 2.34 Å. The Er(1)-O(4) bond length is 2.34 Å. The Er(1)-O(5) bond length is 2.15 Å. Both Er(1)-O(1) bond lengths are 2.15 Å. Mo(1) is bonded to two equivalent O(2), two equivalent O(3), and two equivalent O(4) atoms to form MoO6 octahedra that share corners with six equivalent Er(1)O6 octahedra, faces with two equivalent Ba(2)O12 cuboctahedra, and faces with six equivalent Ba(1)O12 cuboctahedra. The corner-sharing octahedral tilt angles range from 1-12°. Both Mo(1)-O(2) bond lengths are 1.96 Å. Both Mo(1)-O(3) bond lengths are 1.96 Å. Both Mo(1)-O(4) bond lengths are 1.97 Å. There are five inequivalent O sites. In the first O site, O(4) is bonded in a distorted linear geometry to one Ba(2), three equivalent Ba(1), one Er(1), and one Mo(1) atom. In the second O site, O(5) is bonded in a distorted linear geometry to two equivalent Ba(1), two equivalent Ba(2), and two equivalent Er(1) atoms. In the third O site, O(1) is bonded in a 2-coordinate geometry to two equivalent Ba(1), two equivalent Ba(2), and two equivalent Er(1) atoms. In the fourth O site, O(2) is bonded in a 2-coordinate geometry to one Ba(2), three equivalent Ba(1), one Er(1), and one Mo(1) atom. In the fifth O site, O(3) is bonded in a 2-coordinate geometry to one Ba(2), three equivalent Ba(1), one Er(1), and one Mo(1) atom.