Na4SiSe4 crystallizes in the orthorhombic Pnma space group. There are three inequivalent Na sites. In the first Na site, Na(1) is bonded in a distorted square co-planar geometry to two equivalent Se(1) and two equivalent Se(3) atoms. In the second Na site, Na(2) is bonded in a distorted pentagonal planar geometry to one Se(2) and four equivalent Se(1) atoms. In the third Na site, Na(3) is bonded in a 6-coordinate geometry to one Si(1), one Se(3), two equivalent Se(1), and two equivalent Se(2) atoms. Si(1) is bonded to two equivalent Na(3), one Se(2), one Se(3), and two equivalent Se(1) atoms to form SiNa2Se4 tetrahedra that share corners with four equivalent Se(1)Na5Si octahedra and corners with two equivalent Se(3)Na4Si trigonal bipyramids. The corner-sharing octahedral tilt angles range from 64-137°. There are three inequivalent Se sites. In the first Se site, Se(3) is bonded to two equivalent Na(1), two equivalent Na(3), and one Si(1) atom to form distorted SeNa4Si trigonal bipyramids that share corners with six equivalent Se(1)Na5Si octahedra, corners with two equivalent Si(1)Na2Se4 tetrahedra, corners with two equivalent Se(3)Na4Si trigonal bipyramids, and edges with two equivalent Se(1)Na5Si octahedra. The corner-sharing octahedral tilt angles range from 76-80°. In the second Se site, Se(1) is bonded to one Na(1), two equivalent Na(2), two equivalent Na(3), and one Si(1) atom to form distorted SeNa5Si octahedra that share corners with six equivalent Se(1)Na5Si octahedra, corners with two equivalent Si(1)Na2Se4 tetrahedra, corners with three equivalent Se(3)Na4Si trigonal bipyramids, edges with two equivalent Se(1)Na5Si octahedra, and  an edgeedge with one Se(3)Na4Si trigonal bipyramid. The corner-sharing octahedral tilt angles range from 0-54°. In the third Se site, Se(2) is bonded in a 6-coordinate geometry to one Na(2), four equivalent Na(3), and one Si(1) atom.
Na4SiSe4 crystallizes in the orthorhombic Pnma space group. There are three inequivalent Na sites. In the first Na site, Na(1) is bonded in a distorted square co-planar geometry to two equivalent Se(1) and two equivalent Se(3) atoms. Both Na(1)-Se(1) bond lengths are 2.85 Å. Both Na(1)-Se(3) bond lengths are 3.12 Å. In the second Na site, Na(2) is bonded in a distorted pentagonal planar geometry to one Se(2) and four equivalent Se(1) atoms. The Na(2)-Se(2) bond length is 2.97 Å. There are two shorter (2.95 Å) and two longer (3.03 Å) Na(2)-Se(1) bond lengths. In the third Na site, Na(3) is bonded in a 6-coordinate geometry to one Si(1), one Se(3), two equivalent Se(1), and two equivalent Se(2) atoms. The Na(3)-Si(1) bond length is 3.16 Å. The Na(3)-Se(3) bond length is 2.96 Å. There is one shorter (2.88 Å) and one longer (2.98 Å) Na(3)-Se(1) bond length. There is one shorter (3.12 Å) and one longer (3.16 Å) Na(3)-Se(2) bond length. Si(1) is bonded to two equivalent Na(3), one Se(2), one Se(3), and two equivalent Se(1) atoms to form SiNa2Se4 tetrahedra that share corners with four equivalent Se(1)Na5Si octahedra and corners with two equivalent Se(3)Na4Si trigonal bipyramids. The corner-sharing octahedral tilt angles range from 64-137°. The Si(1)-Se(2) bond length is 2.29 Å. The Si(1)-Se(3) bond length is 2.28 Å. Both Si(1)-Se(1) bond lengths are 2.27 Å. There are three inequivalent Se sites. In the first Se site, Se(3) is bonded to two equivalent Na(1), two equivalent Na(3), and one Si(1) atom to form distorted SeNa4Si trigonal bipyramids that share corners with six equivalent Se(1)Na5Si octahedra, corners with two equivalent Si(1)Na2Se4 tetrahedra, corners with two equivalent Se(3)Na4Si trigonal bipyramids, and edges with two equivalent Se(1)Na5Si octahedra. The corner-sharing octahedral tilt angles range from 76-80°. In the second Se site, Se(1) is bonded to one Na(1), two equivalent Na(2), two equivalent Na(3), and one Si(1) atom to form distorted SeNa5Si octahedra that share corners with six equivalent Se(1)Na5Si octahedra, corners with two equivalent Si(1)Na2Se4 tetrahedra, corners with three equivalent Se(3)Na4Si trigonal bipyramids, edges with two equivalent Se(1)Na5Si octahedra, and  an edgeedge with one Se(3)Na4Si trigonal bipyramid. The corner-sharing octahedral tilt angles range from 0-54°. In the third Se site, Se(2) is bonded in a 6-coordinate geometry to one Na(2), four equivalent Na(3), and one Si(1) atom.