Tb3CrSe6 crystallizes in the orthorhombic Pnnm space group. There are three inequivalent Tb sites. In the first Tb site, Tb(1) is bonded to one Se(2), one Se(3), one Se(6), two equivalent Se(1), and two equivalent Se(4) atoms to form distorted TbSe7 pentagonal bipyramids that share  a cornercorner with one Cr(2)Se6 octahedra, edges with two equivalent Cr(1)Se6 octahedra, and edges with two equivalent Tb(1)Se7 pentagonal bipyramids. The corner-sharing octahedral tilt angles are 34°. In the second Tb site, Tb(2) is bonded in a 8-coordinate geometry to one Se(5), one Se(6), two equivalent Se(1), two equivalent Se(2), and two equivalent Se(3) atoms. In the third Tb site, Tb(3) is bonded in a 8-coordinate geometry to one Se(1), one Se(6), two equivalent Se(3), two equivalent Se(4), and two equivalent Se(5) atoms. There are two inequivalent Cr sites. In the first Cr site, Cr(1) is bonded to two equivalent Se(4) and four equivalent Se(6) atoms to form CrSe6 octahedra that share edges with two equivalent Cr(1)Se6 octahedra and edges with four equivalent Tb(1)Se7 pentagonal bipyramids. In the second Cr site, Cr(2) is bonded to two equivalent Se(2) and four equivalent Se(5) atoms to form CrSe6 octahedra that share corners with two equivalent Tb(1)Se7 pentagonal bipyramids and edges with two equivalent Cr(2)Se6 octahedra. There are six inequivalent Se sites. In the first Se site, Se(5) is bonded to one Tb(2), two equivalent Tb(3), and two equivalent Cr(2) atoms to form distorted SeTb3Cr2 trigonal bipyramids that share corners with two equivalent Se(5)Tb3Cr2 trigonal bipyramids, corners with two equivalent Se(4)Tb4Cr trigonal bipyramids, corners with three equivalent Se(6)Tb3Cr2 trigonal bipyramids, corners with four equivalent Se(3)Tb5 trigonal bipyramids, corners with two equivalent Se(2)Tb3Cr trigonal pyramids,  an edgeedge with one Se(4)Tb4Cr trigonal bipyramid,  an edgeedge with one Se(3)Tb5 trigonal bipyramid, edges with three equivalent Se(5)Tb3Cr2 trigonal bipyramids, and edges with two equivalent Se(2)Tb3Cr trigonal pyramids. In the second Se site, Se(6) is bonded to one Tb(1), one Tb(2), one Tb(3), and two equivalent Cr(1) atoms to form distorted SeTb3Cr2 trigonal bipyramids that share  a cornercorner with one Se(3)Tb5 trigonal bipyramid, corners with three equivalent Se(5)Tb3Cr2 trigonal bipyramids, corners with four equivalent Se(6)Tb3Cr2 trigonal bipyramids, corners with three equivalent Se(2)Tb3Cr trigonal pyramids,  an edgeedge with one Se(6)Tb3Cr2 trigonal bipyramid, edges with two equivalent Se(3)Tb5 trigonal bipyramids, and edges with four equivalent Se(4)Tb4Cr trigonal bipyramids. In the third Se site, Se(1) is bonded in a 5-coordinate geometry to one Tb(3), two equivalent Tb(1), and two equivalent Tb(2) atoms. In the fourth Se site, Se(2) is bonded to one Tb(1), two equivalent Tb(2), and one Cr(2) atom to form distorted SeTb3Cr trigonal pyramids that share corners with two equivalent Se(5)Tb3Cr2 trigonal bipyramids, corners with two equivalent Se(4)Tb4Cr trigonal bipyramids, corners with two equivalent Se(3)Tb5 trigonal bipyramids, corners with three equivalent Se(6)Tb3Cr2 trigonal bipyramids, corners with three equivalent Se(2)Tb3Cr trigonal pyramids, edges with two equivalent Se(5)Tb3Cr2 trigonal bipyramids, and  a faceface with one Se(3)Tb5 trigonal bipyramid. In the fifth Se site, Se(3) is bonded to one Tb(1), two equivalent Tb(2), and two equivalent Tb(3) atoms to form distorted SeTb5 trigonal bipyramids that share  a cornercorner with one Se(6)Tb3Cr2 trigonal bipyramid, corners with four equivalent Se(5)Tb3Cr2 trigonal bipyramids, corners with four equivalent Se(4)Tb4Cr trigonal bipyramids, corners with two equivalent Se(2)Tb3Cr trigonal pyramids,  an edgeedge with one Se(5)Tb3Cr2 trigonal bipyramid,  an edgeedge with one Se(4)Tb4Cr trigonal bipyramid, edges with two equivalent Se(6)Tb3Cr2 trigonal bipyramids, edges with two equivalent Se(3)Tb5 trigonal bipyramids, and  a faceface with one Se(2)Tb3Cr trigonal pyramid. In the sixth Se site, Se(4) is bonded to two equivalent Tb(1), two equivalent Tb(3), and one Cr(1) atom to form distorted SeTb4Cr trigonal bipyramids that share  a cornercorner with one Se(4)Tb4Cr trigonal bipyramid, corners with two equivalent Se(5)Tb3Cr2 trigonal bipyramids, corners with four equivalent Se(3)Tb5 trigonal bipyramids, corners with two equivalent Se(2)Tb3Cr trigonal pyramids,  an edgeedge with one Se(5)Tb3Cr2 trigonal bipyramid,  an edgeedge with one Se(3)Tb5 trigonal bipyramid, edges with two equivalent Se(4)Tb4Cr trigonal bipyramids, and edges with four equivalent Se(6)Tb3Cr2 trigonal bipyramids.
Tb3CrSe6 crystallizes in the orthorhombic Pnnm space group. There are three inequivalent Tb sites. In the first Tb site, Tb(1) is bonded to one Se(2), one Se(3), one Se(6), two equivalent Se(1), and two equivalent Se(4) atoms to form distorted TbSe7 pentagonal bipyramids that share  a cornercorner with one Cr(2)Se6 octahedra, edges with two equivalent Cr(1)Se6 octahedra, and edges with two equivalent Tb(1)Se7 pentagonal bipyramids. The corner-sharing octahedral tilt angles are 34°. The Tb(1)-Se(2) bond length is 2.85 Å. The Tb(1)-Se(3) bond length is 2.99 Å. The Tb(1)-Se(6) bond length is 2.96 Å. Both Tb(1)-Se(1) bond lengths are 2.87 Å. Both Tb(1)-Se(4) bond lengths are 2.91 Å. In the second Tb site, Tb(2) is bonded in a 8-coordinate geometry to one Se(5), one Se(6), two equivalent Se(1), two equivalent Se(2), and two equivalent Se(3) atoms. The Tb(2)-Se(5) bond length is 3.07 Å. The Tb(2)-Se(6) bond length is 3.05 Å. Both Tb(2)-Se(1) bond lengths are 2.94 Å. Both Tb(2)-Se(2) bond lengths are 2.90 Å. Both Tb(2)-Se(3) bond lengths are 2.99 Å. In the third Tb site, Tb(3) is bonded in a 8-coordinate geometry to one Se(1), one Se(6), two equivalent Se(3), two equivalent Se(4), and two equivalent Se(5) atoms. The Tb(3)-Se(1) bond length is 3.17 Å. The Tb(3)-Se(6) bond length is 3.27 Å. Both Tb(3)-Se(3) bond lengths are 2.89 Å. Both Tb(3)-Se(4) bond lengths are 2.98 Å. Both Tb(3)-Se(5) bond lengths are 3.01 Å. There are two inequivalent Cr sites. In the first Cr site, Cr(1) is bonded to two equivalent Se(4) and four equivalent Se(6) atoms to form CrSe6 octahedra that share edges with two equivalent Cr(1)Se6 octahedra and edges with four equivalent Tb(1)Se7 pentagonal bipyramids. Both Cr(1)-Se(4) bond lengths are 2.55 Å. All Cr(1)-Se(6) bond lengths are 2.58 Å. In the second Cr site, Cr(2) is bonded to two equivalent Se(2) and four equivalent Se(5) atoms to form CrSe6 octahedra that share corners with two equivalent Tb(1)Se7 pentagonal bipyramids and edges with two equivalent Cr(2)Se6 octahedra. Both Cr(2)-Se(2) bond lengths are 2.49 Å. All Cr(2)-Se(5) bond lengths are 2.61 Å. There are six inequivalent Se sites. In the first Se site, Se(5) is bonded to one Tb(2), two equivalent Tb(3), and two equivalent Cr(2) atoms to form distorted SeTb3Cr2 trigonal bipyramids that share corners with two equivalent Se(5)Tb3Cr2 trigonal bipyramids, corners with two equivalent Se(4)Tb4Cr trigonal bipyramids, corners with three equivalent Se(6)Tb3Cr2 trigonal bipyramids, corners with four equivalent Se(3)Tb5 trigonal bipyramids, corners with two equivalent Se(2)Tb3Cr trigonal pyramids,  an edgeedge with one Se(4)Tb4Cr trigonal bipyramid,  an edgeedge with one Se(3)Tb5 trigonal bipyramid, edges with three equivalent Se(5)Tb3Cr2 trigonal bipyramids, and edges with two equivalent Se(2)Tb3Cr trigonal pyramids. In the second Se site, Se(6) is bonded to one Tb(1), one Tb(2), one Tb(3), and two equivalent Cr(1) atoms to form distorted SeTb3Cr2 trigonal bipyramids that share  a cornercorner with one Se(3)Tb5 trigonal bipyramid, corners with three equivalent Se(5)Tb3Cr2 trigonal bipyramids, corners with four equivalent Se(6)Tb3Cr2 trigonal bipyramids, corners with three equivalent Se(2)Tb3Cr trigonal pyramids,  an edgeedge with one Se(6)Tb3Cr2 trigonal bipyramid, edges with two equivalent Se(3)Tb5 trigonal bipyramids, and edges with four equivalent Se(4)Tb4Cr trigonal bipyramids. In the third Se site, Se(1) is bonded in a 5-coordinate geometry to one Tb(3), two equivalent Tb(1), and two equivalent Tb(2) atoms. In the fourth Se site, Se(2) is bonded to one Tb(1), two equivalent Tb(2), and one Cr(2) atom to form distorted SeTb3Cr trigonal pyramids that share corners with two equivalent Se(5)Tb3Cr2 trigonal bipyramids, corners with two equivalent Se(4)Tb4Cr trigonal bipyramids, corners with two equivalent Se(3)Tb5 trigonal bipyramids, corners with three equivalent Se(6)Tb3Cr2 trigonal bipyramids, corners with three equivalent Se(2)Tb3Cr trigonal pyramids, edges with two equivalent Se(5)Tb3Cr2 trigonal bipyramids, and  a faceface with one Se(3)Tb5 trigonal bipyramid. In the fifth Se site, Se(3) is bonded to one Tb(1), two equivalent Tb(2), and two equivalent Tb(3) atoms to form distorted SeTb5 trigonal bipyramids that share  a cornercorner with one Se(6)Tb3Cr2 trigonal bipyramid, corners with four equivalent Se(5)Tb3Cr2 trigonal bipyramids, corners with four equivalent Se(4)Tb4Cr trigonal bipyramids, corners with two equivalent Se(2)Tb3Cr trigonal pyramids,  an edgeedge with one Se(5)Tb3Cr2 trigonal bipyramid,  an edgeedge with one Se(4)Tb4Cr trigonal bipyramid, edges with two equivalent Se(6)Tb3Cr2 trigonal bipyramids, edges with two equivalent Se(3)Tb5 trigonal bipyramids, and  a faceface with one Se(2)Tb3Cr trigonal pyramid. In the sixth Se site, Se(4) is bonded to two equivalent Tb(1), two equivalent Tb(3), and one Cr(1) atom to form distorted SeTb4Cr trigonal bipyramids that share  a cornercorner with one Se(4)Tb4Cr trigonal bipyramid, corners with two equivalent Se(5)Tb3Cr2 trigonal bipyramids, corners with four equivalent Se(3)Tb5 trigonal bipyramids, corners with two equivalent Se(2)Tb3Cr trigonal pyramids,  an edgeedge with one Se(5)Tb3Cr2 trigonal bipyramid,  an edgeedge with one Se(3)Tb5 trigonal bipyramid, edges with two equivalent Se(4)Tb4Cr trigonal bipyramids, and edges with four equivalent Se(6)Tb3Cr2 trigonal bipyramids.