ErSe2 crystallizes in the tetragonal P4/nmm space group. Er(1) is bonded in a 9-coordinate geometry to four equivalent Se(1) and five equivalent Se(2) atoms. There are two inequivalent Se sites. In the first Se site, Se(1) is bonded in a 8-coordinate geometry to four equivalent Er(1) and four equivalent Se(1) atoms. In the second Se site, Se(2) is bonded to five equivalent Er(1) atoms to form a mixture of distorted corner and edge-sharing SeEr5 trigonal bipyramids.
ErSe2 crystallizes in the tetragonal P4/nmm space group. Er(1) is bonded in a 9-coordinate geometry to four equivalent Se(1) and five equivalent Se(2) atoms. All Er(1)-Se(1) bond lengths are 3.02 Å. There is one shorter (2.93 Å) and four longer (2.96 Å) Er(1)-Se(2) bond lengths. There are two inequivalent Se sites. In the first Se site, Se(1) is bonded in a 8-coordinate geometry to four equivalent Er(1) and four equivalent Se(1) atoms. All Se(1)-Se(1) bond lengths are 2.85 Å. In the second Se site, Se(2) is bonded to five equivalent Er(1) atoms to form a mixture of distorted corner and edge-sharing SeEr5 trigonal bipyramids.