Eu3Si6N4O9 crystallizes in the trigonal P3 space group. There are three inequivalent Eu sites. In the first Eu site, Eu(1) is bonded in a 6-coordinate geometry to three equivalent O(2) and three equivalent O(3) atoms. In the second Eu site, Eu(2) is bonded in a 6-coordinate geometry to three equivalent O(2) and three equivalent O(3) atoms. In the third Eu site, Eu(3) is bonded to three equivalent O(2) and three equivalent O(3) atoms to form distorted EuO6 octahedra that share corners with three equivalent Si(1)N2O2 tetrahedra and corners with three equivalent Si(2)N2O2 tetrahedra. There are two inequivalent Si sites. In the first Si site, Si(1) is bonded to two equivalent N(2), one O(1), and one O(3) atom to form SiN2O2 tetrahedra that share  a cornercorner with one Eu(3)O6 octahedra, corners with two equivalent Si(1)N2O2 tetrahedra, and corners with three equivalent Si(2)N2O2 tetrahedra. The corner-sharing octahedral tilt angles are 72°. In the second Si site, Si(2) is bonded to one N(1), one N(2), one O(1), and one O(2) atom to form SiN2O2 tetrahedra that share  a cornercorner with one Eu(3)O6 octahedra, corners with two equivalent Si(2)N2O2 tetrahedra, and corners with three equivalent Si(1)N2O2 tetrahedra. The corner-sharing octahedral tilt angles are 64°. There are two inequivalent N sites. In the first N site, N(1) is bonded in a trigonal planar geometry to three equivalent Si(2) atoms. In the second N site, N(2) is bonded in a trigonal planar geometry to one Si(2) and two equivalent Si(1) atoms. There are three inequivalent O sites. In the first O site, O(1) is bonded in a bent 120 degrees geometry to one Si(1) and one Si(2) atom. In the second O site, O(2) is bonded to one Eu(1), one Eu(2), one Eu(3), and one Si(2) atom to form distorted corner-sharing OEu3Si tetrahedra. In the third O site, O(3) is bonded in a distorted single-bond geometry to one Eu(1), one Eu(2), one Eu(3), and one Si(1) atom.
Eu3Si6N4O9 crystallizes in the trigonal P3 space group. There are three inequivalent Eu sites. In the first Eu site, Eu(1) is bonded in a 6-coordinate geometry to three equivalent O(2) and three equivalent O(3) atoms. All Eu(1)-O(2) bond lengths are 2.60 Å. All Eu(1)-O(3) bond lengths are 2.75 Å. In the second Eu site, Eu(2) is bonded in a 6-coordinate geometry to three equivalent O(2) and three equivalent O(3) atoms. All Eu(2)-O(2) bond lengths are 2.62 Å. All Eu(2)-O(3) bond lengths are 2.62 Å. In the third Eu site, Eu(3) is bonded to three equivalent O(2) and three equivalent O(3) atoms to form distorted EuO6 octahedra that share corners with three equivalent Si(1)N2O2 tetrahedra and corners with three equivalent Si(2)N2O2 tetrahedra. All Eu(3)-O(2) bond lengths are 2.62 Å. All Eu(3)-O(3) bond lengths are 2.69 Å. There are two inequivalent Si sites. In the first Si site, Si(1) is bonded to two equivalent N(2), one O(1), and one O(3) atom to form SiN2O2 tetrahedra that share  a cornercorner with one Eu(3)O6 octahedra, corners with two equivalent Si(1)N2O2 tetrahedra, and corners with three equivalent Si(2)N2O2 tetrahedra. The corner-sharing octahedral tilt angles are 72°. There is one shorter (1.74 Å) and one longer (1.75 Å) Si(1)-N(2) bond length. The Si(1)-O(1) bond length is 1.66 Å. The Si(1)-O(3) bond length is 1.62 Å. In the second Si site, Si(2) is bonded to one N(1), one N(2), one O(1), and one O(2) atom to form SiN2O2 tetrahedra that share  a cornercorner with one Eu(3)O6 octahedra, corners with two equivalent Si(2)N2O2 tetrahedra, and corners with three equivalent Si(1)N2O2 tetrahedra. The corner-sharing octahedral tilt angles are 64°. The Si(2)-N(1) bond length is 1.75 Å. The Si(2)-N(2) bond length is 1.73 Å. The Si(2)-O(1) bond length is 1.65 Å. The Si(2)-O(2) bond length is 1.62 Å. There are two inequivalent N sites. In the first N site, N(1) is bonded in a trigonal planar geometry to three equivalent Si(2) atoms. In the second N site, N(2) is bonded in a trigonal planar geometry to one Si(2) and two equivalent Si(1) atoms. There are three inequivalent O sites. In the first O site, O(1) is bonded in a bent 120 degrees geometry to one Si(1) and one Si(2) atom. In the second O site, O(2) is bonded to one Eu(1), one Eu(2), one Eu(3), and one Si(2) atom to form distorted corner-sharing OEu3Si tetrahedra. In the third O site, O(3) is bonded in a distorted single-bond geometry to one Eu(1), one Eu(2), one Eu(3), and one Si(1) atom.