SiPb2Se4 crystallizes in the monoclinic P2_1/c space group. There are two inequivalent Pb sites. In the first Pb site, Pb(1) is bonded in a 3-coordinate geometry to one Se(2), one Se(3), and one Se(4) atom. In the second Pb site, Pb(2) is bonded to one Se(2), one Se(3), one Se(4), and two equivalent Se(1) atoms to form PbSe5 square pyramids that share  a cornercorner with one Si(1)Se4 tetrahedra,  an edgeedge with one Pb(2)Se5 square pyramid, and edges with two equivalent Si(1)Se4 tetrahedra. Si(1) is bonded to one Se(1), one Se(2), one Se(3), and one Se(4) atom to form SiSe4 tetrahedra that share  a cornercorner with one Pb(2)Se5 square pyramid and edges with two equivalent Pb(2)Se5 square pyramids. There are four inequivalent Se sites. In the first Se site, Se(1) is bonded in a distorted single-bond geometry to two equivalent Pb(2) and one Si(1) atom. In the second Se site, Se(2) is bonded in a distorted single-bond geometry to one Pb(1), one Pb(2), and one Si(1) atom. In the third Se site, Se(3) is bonded in a distorted single-bond geometry to one Pb(1), one Pb(2), and one Si(1) atom. In the fourth Se site, Se(4) is bonded in a distorted single-bond geometry to one Pb(1), one Pb(2), and one Si(1) atom.
SiPb2Se4 crystallizes in the monoclinic P2_1/c space group. There are two inequivalent Pb sites. In the first Pb site, Pb(1) is bonded in a 3-coordinate geometry to one Se(2), one Se(3), and one Se(4) atom. The Pb(1)-Se(2) bond length is 2.88 Å. The Pb(1)-Se(3) bond length is 2.92 Å. The Pb(1)-Se(4) bond length is 2.95 Å. In the second Pb site, Pb(2) is bonded to one Se(2), one Se(3), one Se(4), and two equivalent Se(1) atoms to form PbSe5 square pyramids that share  a cornercorner with one Si(1)Se4 tetrahedra,  an edgeedge with one Pb(2)Se5 square pyramid, and edges with two equivalent Si(1)Se4 tetrahedra. The Pb(2)-Se(2) bond length is 3.12 Å. The Pb(2)-Se(3) bond length is 3.19 Å. The Pb(2)-Se(4) bond length is 3.03 Å. There is one shorter (2.94 Å) and one longer (3.09 Å) Pb(2)-Se(1) bond length. Si(1) is bonded to one Se(1), one Se(2), one Se(3), and one Se(4) atom to form SiSe4 tetrahedra that share  a cornercorner with one Pb(2)Se5 square pyramid and edges with two equivalent Pb(2)Se5 square pyramids. The Si(1)-Se(1) bond length is 2.28 Å. The Si(1)-Se(2) bond length is 2.28 Å. The Si(1)-Se(3) bond length is 2.27 Å. The Si(1)-Se(4) bond length is 2.29 Å. There are four inequivalent Se sites. In the first Se site, Se(1) is bonded in a distorted single-bond geometry to two equivalent Pb(2) and one Si(1) atom. In the second Se site, Se(2) is bonded in a distorted single-bond geometry to one Pb(1), one Pb(2), and one Si(1) atom. In the third Se site, Se(3) is bonded in a distorted single-bond geometry to one Pb(1), one Pb(2), and one Si(1) atom. In the fourth Se site, Se(4) is bonded in a distorted single-bond geometry to one Pb(1), one Pb(2), and one Si(1) atom.