MgTbSe3 crystallizes in the monoclinic C2/m space group. Mg(1) is bonded in a 4-coordinate geometry to two equivalent Se(1) and two equivalent Se(2) atoms. Tb(1) is bonded in a 6-coordinate geometry to two equivalent Se(2) and four equivalent Se(1) atoms. There are two inequivalent Se sites. In the first Se site, Se(1) is bonded in a 3-coordinate geometry to one Mg(1) and two equivalent Tb(1) atoms. In the second Se site, Se(2) is bonded to two equivalent Mg(1) and two equivalent Tb(1) atoms to form edge-sharing SeTb2Mg2 trigonal pyramids.
MgTbSe3 crystallizes in the monoclinic C2/m space group. Mg(1) is bonded in a 4-coordinate geometry to two equivalent Se(1) and two equivalent Se(2) atoms. Both Mg(1)-Se(1) bond lengths are 2.62 Å. Both Mg(1)-Se(2) bond lengths are 2.64 Å. Tb(1) is bonded in a 6-coordinate geometry to two equivalent Se(2) and four equivalent Se(1) atoms. There is one shorter (2.78 Å) and one longer (2.84 Å) Tb(1)-Se(2) bond length. There are two shorter (2.93 Å) and two longer (2.98 Å) Tb(1)-Se(1) bond lengths. There are two inequivalent Se sites. In the first Se site, Se(1) is bonded in a 3-coordinate geometry to one Mg(1) and two equivalent Tb(1) atoms. In the second Se site, Se(2) is bonded to two equivalent Mg(1) and two equivalent Tb(1) atoms to form edge-sharing SeTb2Mg2 trigonal pyramids.