Eu2Al4O9 crystallizes in the orthorhombic Pbam space group. Eu(1) is bonded in a 6-coordinate geometry to two equivalent O(4) and four equivalent O(2) atoms. There are two inequivalent Al sites. In the first Al site, Al(1) is bonded to two equivalent O(2), two equivalent O(3), and two equivalent O(4) atoms to form AlO6 octahedra that share corners with four equivalent Al(2)O4 tetrahedra and edges with two equivalent Al(1)O6 octahedra. In the second Al site, Al(2) is bonded to one O(1), one O(3), and two equivalent O(2) atoms to form AlO4 tetrahedra that share corners with four equivalent Al(1)O6 octahedra and  a cornercorner with one Al(2)O4 tetrahedra. The corner-sharing octahedral tilt angles range from 48-56°. There are four inequivalent O sites. In the first O site, O(1) is bonded in a linear geometry to two equivalent Al(2) atoms. In the second O site, O(2) is bonded to two equivalent Eu(1), one Al(1), and one Al(2) atom to form a mixture of distorted edge and corner-sharing OEu2Al2 tetrahedra. In the third O site, O(3) is bonded in a distorted trigonal planar geometry to one Al(2) and two equivalent Al(1) atoms. In the fourth O site, O(4) is bonded to two equivalent Eu(1) and two equivalent Al(1) atoms to form a mixture of distorted edge and corner-sharing OEu2Al2 trigonal pyramids.
Eu2Al4O9 crystallizes in the orthorhombic Pbam space group. Eu(1) is bonded in a 6-coordinate geometry to two equivalent O(4) and four equivalent O(2) atoms. There is one shorter (2.25 Å) and one longer (2.41 Å) Eu(1)-O(4) bond length. There are two shorter (2.34 Å) and two longer (2.54 Å) Eu(1)-O(2) bond lengths. There are two inequivalent Al sites. In the first Al site, Al(1) is bonded to two equivalent O(2), two equivalent O(3), and two equivalent O(4) atoms to form AlO6 octahedra that share corners with four equivalent Al(2)O4 tetrahedra and edges with two equivalent Al(1)O6 octahedra. Both Al(1)-O(2) bond lengths are 1.98 Å. Both Al(1)-O(3) bond lengths are 1.90 Å. Both Al(1)-O(4) bond lengths are 1.94 Å. In the second Al site, Al(2) is bonded to one O(1), one O(3), and two equivalent O(2) atoms to form AlO4 tetrahedra that share corners with four equivalent Al(1)O6 octahedra and  a cornercorner with one Al(2)O4 tetrahedra. The corner-sharing octahedral tilt angles range from 48-56°. The Al(2)-O(1) bond length is 1.73 Å. The Al(2)-O(3) bond length is 1.78 Å. Both Al(2)-O(2) bond lengths are 1.78 Å. There are four inequivalent O sites. In the first O site, O(1) is bonded in a linear geometry to two equivalent Al(2) atoms. In the second O site, O(2) is bonded to two equivalent Eu(1), one Al(1), and one Al(2) atom to form a mixture of distorted edge and corner-sharing OEu2Al2 tetrahedra. In the third O site, O(3) is bonded in a distorted trigonal planar geometry to one Al(2) and two equivalent Al(1) atoms. In the fourth O site, O(4) is bonded to two equivalent Eu(1) and two equivalent Al(1) atoms to form a mixture of distorted edge and corner-sharing OEu2Al2 trigonal pyramids.