RbDy2CuSe4 crystallizes in the orthorhombic Cmcm space group. Rb(1) is bonded in a 8-coordinate geometry to two equivalent Se(1), two equivalent Se(3), and four equivalent Se(2) atoms. Dy(1) is bonded to one Se(1), two equivalent Se(3), and three equivalent Se(2) atoms to form DySe6 octahedra that share corners with three equivalent Dy(1)Se6 octahedra,  a cornercorner with one Cu(1)Se4 tetrahedra, edges with five equivalent Dy(1)Se6 octahedra, and edges with two equivalent Cu(1)Se4 tetrahedra. The corner-sharing octahedral tilt angles range from 0-37°. Cu(1) is bonded to two equivalent Se(1) and two equivalent Se(2) atoms to form CuSe4 tetrahedra that share corners with two equivalent Dy(1)Se6 octahedra, corners with two equivalent Cu(1)Se4 tetrahedra, and edges with four equivalent Dy(1)Se6 octahedra. The corner-sharing octahedral tilt angles are 4°. There are three inequivalent Se sites. In the first Se site, Se(1) is bonded in a 6-coordinate geometry to two equivalent Rb(1), two equivalent Dy(1), and two equivalent Cu(1) atoms. In the second Se site, Se(2) is bonded to two equivalent Rb(1), three equivalent Dy(1), and one Cu(1) atom to form distorted SeRb2Dy3Cu octahedra that share corners with two equivalent Se(2)Rb2Dy3Cu octahedra, corners with four equivalent Se(3)Rb2Dy4 octahedra, edges with three equivalent Se(3)Rb2Dy4 octahedra, edges with four equivalent Se(2)Rb2Dy3Cu octahedra, and  a faceface with one Se(2)Rb2Dy3Cu octahedra. The corner-sharing octahedral tilt angles range from 5-65°. In the third Se site, Se(3) is bonded to two equivalent Rb(1) and four equivalent Dy(1) atoms to form distorted SeRb2Dy4 octahedra that share corners with two equivalent Se(3)Rb2Dy4 octahedra, corners with eight equivalent Se(2)Rb2Dy3Cu octahedra, edges with two equivalent Se(3)Rb2Dy4 octahedra, and edges with six equivalent Se(2)Rb2Dy3Cu octahedra. The corner-sharing octahedral tilt angles range from 5-47°.
RbDy2CuSe4 crystallizes in the orthorhombic Cmcm space group. Rb(1) is bonded in a 8-coordinate geometry to two equivalent Se(1), two equivalent Se(3), and four equivalent Se(2) atoms. Both Rb(1)-Se(1) bond lengths are 3.32 Å. Both Rb(1)-Se(3) bond lengths are 3.94 Å. All Rb(1)-Se(2) bond lengths are 3.43 Å. Dy(1) is bonded to one Se(1), two equivalent Se(3), and three equivalent Se(2) atoms to form DySe6 octahedra that share corners with three equivalent Dy(1)Se6 octahedra,  a cornercorner with one Cu(1)Se4 tetrahedra, edges with five equivalent Dy(1)Se6 octahedra, and edges with two equivalent Cu(1)Se4 tetrahedra. The corner-sharing octahedral tilt angles range from 0-37°. The Dy(1)-Se(1) bond length is 2.84 Å. Both Dy(1)-Se(3) bond lengths are 2.97 Å. There are two shorter (2.84 Å) and one longer (2.91 Å) Dy(1)-Se(2) bond length. Cu(1) is bonded to two equivalent Se(1) and two equivalent Se(2) atoms to form CuSe4 tetrahedra that share corners with two equivalent Dy(1)Se6 octahedra, corners with two equivalent Cu(1)Se4 tetrahedra, and edges with four equivalent Dy(1)Se6 octahedra. The corner-sharing octahedral tilt angles are 4°. Both Cu(1)-Se(1) bond lengths are 2.46 Å. Both Cu(1)-Se(2) bond lengths are 2.48 Å. There are three inequivalent Se sites. In the first Se site, Se(1) is bonded in a 6-coordinate geometry to two equivalent Rb(1), two equivalent Dy(1), and two equivalent Cu(1) atoms. In the second Se site, Se(2) is bonded to two equivalent Rb(1), three equivalent Dy(1), and one Cu(1) atom to form distorted SeRb2Dy3Cu octahedra that share corners with two equivalent Se(2)Rb2Dy3Cu octahedra, corners with four equivalent Se(3)Rb2Dy4 octahedra, edges with three equivalent Se(3)Rb2Dy4 octahedra, edges with four equivalent Se(2)Rb2Dy3Cu octahedra, and  a faceface with one Se(2)Rb2Dy3Cu octahedra. The corner-sharing octahedral tilt angles range from 5-65°. In the third Se site, Se(3) is bonded to two equivalent Rb(1) and four equivalent Dy(1) atoms to form distorted SeRb2Dy4 octahedra that share corners with two equivalent Se(3)Rb2Dy4 octahedra, corners with eight equivalent Se(2)Rb2Dy3Cu octahedra, edges with two equivalent Se(3)Rb2Dy4 octahedra, and edges with six equivalent Se(2)Rb2Dy3Cu octahedra. The corner-sharing octahedral tilt angles range from 5-47°.