RbGd2CuSe4 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. Gd(1) is bonded to one Se(1), two equivalent Se(3), and three equivalent Se(2) atoms to form GdSe6 octahedra that share corners with three equivalent Gd(1)Se6 octahedra,  a cornercorner with one Cu(1)Se4 tetrahedra, edges with five equivalent Gd(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 Gd(1)Se6 octahedra, corners with two equivalent Cu(1)Se4 tetrahedra, and edges with four equivalent Gd(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 Gd(1), and two equivalent Cu(1) atoms. In the second Se site, Se(2) is bonded to two equivalent Rb(1), three equivalent Gd(1), and one Cu(1) atom to form distorted SeRb2Gd3Cu octahedra that share corners with two equivalent Se(2)Rb2Gd3Cu octahedra, corners with four equivalent Se(3)Rb2Gd4 octahedra, edges with three equivalent Se(3)Rb2Gd4 octahedra, edges with four equivalent Se(2)Rb2Gd3Cu octahedra, and  a faceface with one Se(2)Rb2Gd3Cu octahedra. The corner-sharing octahedral tilt angles range from 4-65°. In the third Se site, Se(3) is bonded to two equivalent Rb(1) and four equivalent Gd(1) atoms to form distorted SeRb2Gd4 octahedra that share corners with two equivalent Se(3)Rb2Gd4 octahedra, corners with eight equivalent Se(2)Rb2Gd3Cu octahedra, edges with two equivalent Se(3)Rb2Gd4 octahedra, and edges with six equivalent Se(2)Rb2Gd3Cu octahedra. The corner-sharing octahedral tilt angles range from 4-47°.
RbGd2CuSe4 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 3.35 Å. Both Rb(1)-Se(3) bond lengths are 3.96 Å. All Rb(1)-Se(2) bond lengths are 3.45 Å. Gd(1) is bonded to one Se(1), two equivalent Se(3), and three equivalent Se(2) atoms to form GdSe6 octahedra that share corners with three equivalent Gd(1)Se6 octahedra,  a cornercorner with one Cu(1)Se4 tetrahedra, edges with five equivalent Gd(1)Se6 octahedra, and edges with two equivalent Cu(1)Se4 tetrahedra. The corner-sharing octahedral tilt angles range from 0-37°. The Gd(1)-Se(1) bond length is 2.86 Å. Both Gd(1)-Se(3) bond lengths are 2.99 Å. There are two shorter (2.87 Å) and one longer (2.93 Å) Gd(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 Gd(1)Se6 octahedra, corners with two equivalent Cu(1)Se4 tetrahedra, and edges with four equivalent Gd(1)Se6 octahedra. The corner-sharing octahedral tilt angles are 4°. Both Cu(1)-Se(1) bond lengths are 2.47 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 Gd(1), and two equivalent Cu(1) atoms. In the second Se site, Se(2) is bonded to two equivalent Rb(1), three equivalent Gd(1), and one Cu(1) atom to form distorted SeRb2Gd3Cu octahedra that share corners with two equivalent Se(2)Rb2Gd3Cu octahedra, corners with four equivalent Se(3)Rb2Gd4 octahedra, edges with three equivalent Se(3)Rb2Gd4 octahedra, edges with four equivalent Se(2)Rb2Gd3Cu octahedra, and  a faceface with one Se(2)Rb2Gd3Cu octahedra. The corner-sharing octahedral tilt angles range from 4-65°. In the third Se site, Se(3) is bonded to two equivalent Rb(1) and four equivalent Gd(1) atoms to form distorted SeRb2Gd4 octahedra that share corners with two equivalent Se(3)Rb2Gd4 octahedra, corners with eight equivalent Se(2)Rb2Gd3Cu octahedra, edges with two equivalent Se(3)Rb2Gd4 octahedra, and edges with six equivalent Se(2)Rb2Gd3Cu octahedra. The corner-sharing octahedral tilt angles range from 4-47°.