FePSe crystallizes in the monoclinic P2_1/c space group. Fe(1) is bonded to three equivalent P(1) and three equivalent Se(1) atoms to form FeP3Se3 octahedra that share corners with eight equivalent Fe(1)P3Se3 octahedra, corners with three equivalent P(1)Fe3Se tetrahedra, corners with three equivalent Se(1)Fe3P tetrahedra, and edges with two equivalent Fe(1)P3Se3 octahedra. The corner-sharing octahedral tilt angles range from 54-59°. P(1) is bonded to three equivalent Fe(1) and one Se(1) atom to form distorted PFe3Se tetrahedra that share corners with three equivalent Fe(1)P3Se3 octahedra, corners with four equivalent P(1)Fe3Se tetrahedra, corners with nine equivalent Se(1)Fe3P tetrahedra, and  an edgeedge with one P(1)Fe3Se tetrahedra. The corner-sharing octahedral tilt angles range from 66-73°. Se(1) is bonded to three equivalent Fe(1) and one P(1) atom to form distorted SeFe3P tetrahedra that share corners with three equivalent Fe(1)P3Se3 octahedra, corners with four equivalent Se(1)Fe3P tetrahedra, corners with nine equivalent P(1)Fe3Se tetrahedra, and  an edgeedge with one Se(1)Fe3P tetrahedra. The corner-sharing octahedral tilt angles range from 74-81°.
FePSe crystallizes in the monoclinic P2_1/c space group. Fe(1) is bonded to three equivalent P(1) and three equivalent Se(1) atoms to form FeP3Se3 octahedra that share corners with eight equivalent Fe(1)P3Se3 octahedra, corners with three equivalent P(1)Fe3Se tetrahedra, corners with three equivalent Se(1)Fe3P tetrahedra, and edges with two equivalent Fe(1)P3Se3 octahedra. The corner-sharing octahedral tilt angles range from 54-59°. There is one shorter (2.29 Å) and two longer (2.30 Å) Fe(1)-P(1) bond lengths. There are a spread of Fe(1)-Se(1) bond distances ranging from 2.32-2.34 Å. P(1) is bonded to three equivalent Fe(1) and one Se(1) atom to form distorted PFe3Se tetrahedra that share corners with three equivalent Fe(1)P3Se3 octahedra, corners with four equivalent P(1)Fe3Se tetrahedra, corners with nine equivalent Se(1)Fe3P tetrahedra, and  an edgeedge with one P(1)Fe3Se tetrahedra. The corner-sharing octahedral tilt angles range from 66-73°. The P(1)-Se(1) bond length is 2.42 Å. Se(1) is bonded to three equivalent Fe(1) and one P(1) atom to form distorted SeFe3P tetrahedra that share corners with three equivalent Fe(1)P3Se3 octahedra, corners with four equivalent Se(1)Fe3P tetrahedra, corners with nine equivalent P(1)Fe3Se tetrahedra, and  an edgeedge with one Se(1)Fe3P tetrahedra. The corner-sharing octahedral tilt angles range from 74-81°.