Sr2Er2Al3Si5N11O3 crystallizes in the trigonal P3m1 space group. There are two inequivalent Sr sites. In the first Sr site, Sr(1) is bonded to three equivalent N(1) and three equivalent O(1) atoms to form SrN3O3 octahedra that share corners with three equivalent Al(1)N4 tetrahedra, corners with three equivalent Si(2)N4 tetrahedra, and corners with six equivalent Si(3)N2O2 tetrahedra. In the second Sr site, Sr(2) is bonded to three equivalent N(4) and three equivalent N(5) atoms to form SrN6 octahedra that share corners with three equivalent Si(3)N2O2 tetrahedra, corners with three equivalent Si(1)N4 tetrahedra, and corners with six equivalent Al(1)N4 tetrahedra. There are two inequivalent Er sites. In the first Er site, Er(1) is bonded in a 3-coordinate geometry to three equivalent N(5) atoms. In the second Er site, Er(2) is bonded in a 3-coordinate geometry to three equivalent N(5) atoms. Al(1) is bonded to one N(1), one N(2), and two equivalent N(5) atoms to form AlN4 tetrahedra that share  a cornercorner with one Sr(1)N3O3 octahedra, corners with two equivalent Sr(2)N6 octahedra,  a cornercorner with one Si(1)N4 tetrahedra,  a cornercorner with one Si(2)N4 tetrahedra, and corners with four equivalent Al(1)N4 tetrahedra. The corner-sharing octahedral tilt angles range from 58-60°. There are three inequivalent Si sites. In the first Si site, Si(1) is bonded to one N(2) and three equivalent N(4) atoms to form SiN4 tetrahedra that share corners with three equivalent Sr(2)N6 octahedra, corners with three equivalent Al(1)N4 tetrahedra, and corners with three equivalent Si(3)N2O2 tetrahedra. The corner-sharing octahedral tilt angles are 58°. In the second Si site, Si(2) is bonded to one N(3) and three equivalent N(1) atoms to form SiN4 tetrahedra that share corners with three equivalent Sr(1)N3O3 octahedra, corners with three equivalent Al(1)N4 tetrahedra, and corners with three equivalent Si(3)N2O2 tetrahedra. The corner-sharing octahedral tilt angles are 52°. In the third Si site, Si(3) is bonded to one N(3), one N(4), and two equivalent O(1) atoms to form SiN2O2 tetrahedra that share  a cornercorner with one Sr(2)N6 octahedra, corners with two equivalent Sr(1)N3O3 octahedra,  a cornercorner with one Si(1)N4 tetrahedra,  a cornercorner with one Si(2)N4 tetrahedra, and corners with four equivalent Si(3)N2O2 tetrahedra. The corner-sharing octahedral tilt angles range from 62-65°. There are five inequivalent N sites. In the first N site, N(1) is bonded in a distorted trigonal planar geometry to one Sr(1), one Al(1), and one Si(2) atom. In the second N site, N(2) is bonded to three equivalent Al(1) and one Si(1) atom to form distorted corner-sharing NAl3Si trigonal pyramids. In the third N site, N(3) is bonded in a tetrahedral geometry to one Si(2) and three equivalent Si(3) atoms. In the fourth N site, N(4) is bonded in a trigonal planar geometry to one Sr(2), one Si(1), and one Si(3) atom. In the fifth N site, N(5) is bonded to one Sr(2), one Er(1), one Er(2), and two equivalent Al(1) atoms to form NSrEr2Al2 trigonal bipyramids that share corners with two equivalent N(2)Al3Si trigonal pyramids and edges with four equivalent N(5)SrEr2Al2 trigonal bipyramids. O(1) is bonded in a trigonal planar geometry to one Sr(1) and two equivalent Si(3) atoms.
Sr2Er2Al3Si5N11O3 crystallizes in the trigonal P3m1 space group. There are two inequivalent Sr sites. In the first Sr site, Sr(1) is bonded to three equivalent N(1) and three equivalent O(1) atoms to form SrN3O3 octahedra that share corners with three equivalent Al(1)N4 tetrahedra, corners with three equivalent Si(2)N4 tetrahedra, and corners with six equivalent Si(3)N2O2 tetrahedra. All Sr(1)-N(1) bond lengths are 2.38 Å. All Sr(1)-O(1) bond lengths are 2.60 Å. In the second Sr site, Sr(2) is bonded to three equivalent N(4) and three equivalent N(5) atoms to form SrN6 octahedra that share corners with three equivalent Si(3)N2O2 tetrahedra, corners with three equivalent Si(1)N4 tetrahedra, and corners with six equivalent Al(1)N4 tetrahedra. All Sr(2)-N(4) bond lengths are 2.40 Å. All Sr(2)-N(5) bond lengths are 2.48 Å. There are two inequivalent Er sites. In the first Er site, Er(1) is bonded in a 3-coordinate geometry to three equivalent N(5) atoms. All Er(1)-N(5) bond lengths are 2.28 Å. In the second Er site, Er(2) is bonded in a 3-coordinate geometry to three equivalent N(5) atoms. All Er(2)-N(5) bond lengths are 2.42 Å. Al(1) is bonded to one N(1), one N(2), and two equivalent N(5) atoms to form AlN4 tetrahedra that share  a cornercorner with one Sr(1)N3O3 octahedra, corners with two equivalent Sr(2)N6 octahedra,  a cornercorner with one Si(1)N4 tetrahedra,  a cornercorner with one Si(2)N4 tetrahedra, and corners with four equivalent Al(1)N4 tetrahedra. The corner-sharing octahedral tilt angles range from 58-60°. The Al(1)-N(1) bond length is 1.82 Å. The Al(1)-N(2) bond length is 2.01 Å. Both Al(1)-N(5) bond lengths are 1.88 Å. There are three inequivalent Si sites. In the first Si site, Si(1) is bonded to one N(2) and three equivalent N(4) atoms to form SiN4 tetrahedra that share corners with three equivalent Sr(2)N6 octahedra, corners with three equivalent Al(1)N4 tetrahedra, and corners with three equivalent Si(3)N2O2 tetrahedra. The corner-sharing octahedral tilt angles are 58°. The Si(1)-N(2) bond length is 1.82 Å. All Si(1)-N(4) bond lengths are 1.73 Å. In the second Si site, Si(2) is bonded to one N(3) and three equivalent N(1) atoms to form SiN4 tetrahedra that share corners with three equivalent Sr(1)N3O3 octahedra, corners with three equivalent Al(1)N4 tetrahedra, and corners with three equivalent Si(3)N2O2 tetrahedra. The corner-sharing octahedral tilt angles are 52°. The Si(2)-N(3) bond length is 2.06 Å. All Si(2)-N(1) bond lengths are 1.69 Å. In the third Si site, Si(3) is bonded to one N(3), one N(4), and two equivalent O(1) atoms to form SiN2O2 tetrahedra that share  a cornercorner with one Sr(2)N6 octahedra, corners with two equivalent Sr(1)N3O3 octahedra,  a cornercorner with one Si(1)N4 tetrahedra,  a cornercorner with one Si(2)N4 tetrahedra, and corners with four equivalent Si(3)N2O2 tetrahedra. The corner-sharing octahedral tilt angles range from 62-65°. The Si(3)-N(3) bond length is 2.02 Å. The Si(3)-N(4) bond length is 1.65 Å. Both Si(3)-O(1) bond lengths are 1.72 Å. There are five inequivalent N sites. In the first N site, N(1) is bonded in a distorted trigonal planar geometry to one Sr(1), one Al(1), and one Si(2) atom. In the second N site, N(2) is bonded to three equivalent Al(1) and one Si(1) atom to form distorted corner-sharing NAl3Si trigonal pyramids. In the third N site, N(3) is bonded in a tetrahedral geometry to one Si(2) and three equivalent Si(3) atoms. In the fourth N site, N(4) is bonded in a trigonal planar geometry to one Sr(2), one Si(1), and one Si(3) atom. In the fifth N site, N(5) is bonded to one Sr(2), one Er(1), one Er(2), and two equivalent Al(1) atoms to form NSrEr2Al2 trigonal bipyramids that share corners with two equivalent N(2)Al3Si trigonal pyramids and edges with four equivalent N(5)SrEr2Al2 trigonal bipyramids. O(1) is bonded in a trigonal planar geometry to one Sr(1) and two equivalent Si(3) atoms.