EuEr2Se4 crystallizes in the orthorhombic Pnma space group. There are two inequivalent Er sites. In the first Er site, Er(1) is bonded to one Se(1), two equivalent Se(3), and three equivalent Se(2) atoms to form a mixture of edge and corner-sharing ErSe6 octahedra. The corner-sharing octahedral tilt angles range from 47-63°. In the second Er site, Er(2) is bonded to one Se(3), two equivalent Se(1), and three equivalent Se(4) atoms to form a mixture of edge and corner-sharing ErSe6 octahedra. The corner-sharing octahedral tilt angles range from 47-63°. Eu(1) is bonded in a 8-coordinate geometry to two equivalent Se(1), two equivalent Se(2), two equivalent Se(3), and two equivalent Se(4) atoms. There are four inequivalent Se sites. In the first Se site, Se(1) is bonded to one Er(1), two equivalent Er(2), and two equivalent Eu(1) atoms to form a mixture of distorted edge and corner-sharing SeEu2Er3 trigonal bipyramids. In the second Se site, Se(2) is bonded to three equivalent Er(1) and two equivalent Eu(1) atoms to form a mixture of edge and corner-sharing SeEu2Er3 square pyramids. In the third Se site, Se(3) is bonded to one Er(2), two equivalent Er(1), and two equivalent Eu(1) atoms to form a mixture of distorted edge and corner-sharing SeEu2Er3 trigonal bipyramids. In the fourth Se site, Se(4) is bonded to three equivalent Er(2) and two equivalent Eu(1) atoms to form a mixture of distorted edge and corner-sharing SeEu2Er3 trigonal bipyramids.
EuEr2Se4 crystallizes in the orthorhombic Pnma space group. There are two inequivalent Er sites. In the first Er site, Er(1) is bonded to one Se(1), two equivalent Se(3), and three equivalent Se(2) atoms to form a mixture of edge and corner-sharing ErSe6 octahedra. The corner-sharing octahedral tilt angles range from 47-63°. The Er(1)-Se(1) bond length is 2.86 Å. Both Er(1)-Se(3) bond lengths are 2.77 Å. There is one shorter (2.83 Å) and two longer (2.85 Å) Er(1)-Se(2) bond lengths. In the second Er site, Er(2) is bonded to one Se(3), two equivalent Se(1), and three equivalent Se(4) atoms to form a mixture of edge and corner-sharing ErSe6 octahedra. The corner-sharing octahedral tilt angles range from 47-63°. The Er(2)-Se(3) bond length is 2.79 Å. Both Er(2)-Se(1) bond lengths are 2.86 Å. There is one shorter (2.81 Å) and two longer (2.84 Å) Er(2)-Se(4) bond lengths. Eu(1) is bonded in a 8-coordinate geometry to two equivalent Se(1), two equivalent Se(2), two equivalent Se(3), and two equivalent Se(4) atoms. Both Eu(1)-Se(1) bond lengths are 3.18 Å. Both Eu(1)-Se(2) bond lengths are 3.16 Å. There is one shorter (3.38 Å) and one longer (3.42 Å) Eu(1)-Se(3) bond length. Both Eu(1)-Se(4) bond lengths are 3.20 Å. There are four inequivalent Se sites. In the first Se site, Se(1) is bonded to one Er(1), two equivalent Er(2), and two equivalent Eu(1) atoms to form a mixture of distorted edge and corner-sharing SeEu2Er3 trigonal bipyramids. In the second Se site, Se(2) is bonded to three equivalent Er(1) and two equivalent Eu(1) atoms to form a mixture of edge and corner-sharing SeEu2Er3 square pyramids. In the third Se site, Se(3) is bonded to one Er(2), two equivalent Er(1), and two equivalent Eu(1) atoms to form a mixture of distorted edge and corner-sharing SeEu2Er3 trigonal bipyramids. In the fourth Se site, Se(4) is bonded to three equivalent Er(2) and two equivalent Eu(1) atoms to form a mixture of distorted edge and corner-sharing SeEu2Er3 trigonal bipyramids.