Na3Eu2(SiSe3)2 crystallizes in the monoclinic C2/m space group. There are two inequivalent Na sites. In the first Na site, Na(1) is bonded to two equivalent Se(2) and four equivalent Se(1) atoms to form NaSe6 octahedra that share corners with four equivalent Eu(1)Se6 octahedra, corners with two equivalent Si(1)SiSe3 tetrahedra, edges with four equivalent Eu(1)Se6 octahedra, edges with six equivalent Na(2)Se6 octahedra, and edges with two equivalent Si(1)SiSe3 tetrahedra. The corner-sharing octahedral tilt angles are 6°. In the second Na site, Na(2) is bonded to two equivalent Se(2) and four equivalent Se(1) atoms to form NaSe6 octahedra that share corners with four equivalent Eu(1)Se6 octahedra, corners with two equivalent Si(1)SiSe3 tetrahedra, edges with three equivalent Na(1)Se6 octahedra, edges with three equivalent Na(2)Se6 octahedra, edges with four equivalent Eu(1)Se6 octahedra, and edges with two equivalent Si(1)SiSe3 tetrahedra. The corner-sharing octahedral tilt angles range from 6-7°. Eu(1) is bonded to two equivalent Se(2) and four equivalent Se(1) atoms to form EuSe6 octahedra that share corners with two equivalent Na(1)Se6 octahedra, corners with four equivalent Na(2)Se6 octahedra, corners with six equivalent Si(1)SiSe3 tetrahedra, edges with two equivalent Na(1)Se6 octahedra, edges with three equivalent Eu(1)Se6 octahedra, and edges with four equivalent Na(2)Se6 octahedra. The corner-sharing octahedral tilt angles range from 6-7°. Si(1) is bonded to one Si(1), one Se(2), and two equivalent Se(1) atoms to form distorted SiSiSe3 tetrahedra that share  a cornercorner with one Na(1)Se6 octahedra,  a cornercorner with one Se(2)Na3Eu2Si octahedra, corners with two equivalent Na(2)Se6 octahedra, corners with two equivalent Se(1)Na3Eu2Si octahedra, corners with six equivalent Eu(1)Se6 octahedra,  an edgeedge with one Na(1)Se6 octahedra, and edges with two equivalent Na(2)Se6 octahedra. The corner-sharing octahedral tilt angles range from 20-76°. There are two inequivalent Se sites. In the first Se site, Se(1) is bonded to one Na(1), two equivalent Na(2), two equivalent Eu(1), and one Si(1) atom to form SeNa3Eu2Si octahedra that share corners with three equivalent Se(2)Na3Eu2Si octahedra, corners with four equivalent Se(1)Na3Eu2Si octahedra,  a cornercorner with one Si(1)SiSe3 tetrahedra, edges with four equivalent Se(2)Na3Eu2Si octahedra, and edges with six equivalent Se(1)Na3Eu2Si octahedra. The corner-sharing octahedral tilt angles range from 0-91°. In the second Se site, Se(2) is bonded to one Na(1), two equivalent Na(2), two equivalent Eu(1), and one Si(1) atom to form SeNa3Eu2Si octahedra that share  a cornercorner with one Se(2)Na3Eu2Si octahedra, corners with six equivalent Se(1)Na3Eu2Si octahedra,  a cornercorner with one Si(1)SiSe3 tetrahedra, edges with two equivalent Se(2)Na3Eu2Si octahedra, and edges with eight equivalent Se(1)Na3Eu2Si octahedra. The corner-sharing octahedral tilt angles range from 0-91°.
Na3Eu2(SiSe3)2 crystallizes in the monoclinic C2/m space group. There are two inequivalent Na sites. In the first Na site, Na(1) is bonded to two equivalent Se(2) and four equivalent Se(1) atoms to form NaSe6 octahedra that share corners with four equivalent Eu(1)Se6 octahedra, corners with two equivalent Si(1)SiSe3 tetrahedra, edges with four equivalent Eu(1)Se6 octahedra, edges with six equivalent Na(2)Se6 octahedra, and edges with two equivalent Si(1)SiSe3 tetrahedra. The corner-sharing octahedral tilt angles are 6°. Both Na(1)-Se(2) bond lengths are 3.20 Å. All Na(1)-Se(1) bond lengths are 3.03 Å. In the second Na site, Na(2) is bonded to two equivalent Se(2) and four equivalent Se(1) atoms to form NaSe6 octahedra that share corners with four equivalent Eu(1)Se6 octahedra, corners with two equivalent Si(1)SiSe3 tetrahedra, edges with three equivalent Na(1)Se6 octahedra, edges with three equivalent Na(2)Se6 octahedra, edges with four equivalent Eu(1)Se6 octahedra, and edges with two equivalent Si(1)SiSe3 tetrahedra. The corner-sharing octahedral tilt angles range from 6-7°. Both Na(2)-Se(2) bond lengths are 3.08 Å. There are two shorter (3.03 Å) and two longer (3.14 Å) Na(2)-Se(1) bond lengths. Eu(1) is bonded to two equivalent Se(2) and four equivalent Se(1) atoms to form EuSe6 octahedra that share corners with two equivalent Na(1)Se6 octahedra, corners with four equivalent Na(2)Se6 octahedra, corners with six equivalent Si(1)SiSe3 tetrahedra, edges with two equivalent Na(1)Se6 octahedra, edges with three equivalent Eu(1)Se6 octahedra, and edges with four equivalent Na(2)Se6 octahedra. The corner-sharing octahedral tilt angles range from 6-7°. Both Eu(1)-Se(2) bond lengths are 3.09 Å. All Eu(1)-Se(1) bond lengths are 3.10 Å. Si(1) is bonded to one Si(1), one Se(2), and two equivalent Se(1) atoms to form distorted SiSiSe3 tetrahedra that share  a cornercorner with one Na(1)Se6 octahedra,  a cornercorner with one Se(2)Na3Eu2Si octahedra, corners with two equivalent Na(2)Se6 octahedra, corners with two equivalent Se(1)Na3Eu2Si octahedra, corners with six equivalent Eu(1)Se6 octahedra,  an edgeedge with one Na(1)Se6 octahedra, and edges with two equivalent Na(2)Se6 octahedra. The corner-sharing octahedral tilt angles range from 20-76°. The Si(1)-Si(1) bond length is 2.36 Å. The Si(1)-Se(2) bond length is 2.35 Å. Both Si(1)-Se(1) bond lengths are 2.36 Å. There are two inequivalent Se sites. In the first Se site, Se(1) is bonded to one Na(1), two equivalent Na(2), two equivalent Eu(1), and one Si(1) atom to form SeNa3Eu2Si octahedra that share corners with three equivalent Se(2)Na3Eu2Si octahedra, corners with four equivalent Se(1)Na3Eu2Si octahedra,  a cornercorner with one Si(1)SiSe3 tetrahedra, edges with four equivalent Se(2)Na3Eu2Si octahedra, and edges with six equivalent Se(1)Na3Eu2Si octahedra. The corner-sharing octahedral tilt angles range from 0-91°. In the second Se site, Se(2) is bonded to one Na(1), two equivalent Na(2), two equivalent Eu(1), and one Si(1) atom to form SeNa3Eu2Si octahedra that share  a cornercorner with one Se(2)Na3Eu2Si octahedra, corners with six equivalent Se(1)Na3Eu2Si octahedra,  a cornercorner with one Si(1)SiSe3 tetrahedra, edges with two equivalent Se(2)Na3Eu2Si octahedra, and edges with eight equivalent Se(1)Na3Eu2Si octahedra. The corner-sharing octahedral tilt angles range from 0-91°.