KCaEr2CuS5 crystallizes in the orthorhombic Cmcm space group. K(1) is bonded in a 6-coordinate geometry to two equivalent S(3) and four equivalent S(2) atoms. Ca(1) is bonded to two equivalent S(2) and four equivalent S(1) atoms to form CaS6 octahedra that share corners with four equivalent Er(1)S6 octahedra, corners with two equivalent Cu(1)S4 tetrahedra, edges with two equivalent Ca(1)S6 octahedra, and edges with six equivalent Er(1)S6 octahedra. The corner-sharing octahedral tilt angles are 5°. Er(1) is bonded to one S(3), two equivalent S(2), and three equivalent S(1) atoms to form ErS6 octahedra that share  a cornercorner with one Er(1)S6 octahedra, corners with two equivalent Ca(1)S6 octahedra, edges with three equivalent Ca(1)S6 octahedra, edges with four equivalent Er(1)S6 octahedra, and edges with two equivalent Cu(1)S4 tetrahedra. The corner-sharing octahedral tilt angles range from 5-41°. Cu(1) is bonded to two equivalent S(2) and two equivalent S(3) atoms to form CuS4 tetrahedra that share corners with two equivalent Ca(1)S6 octahedra, corners with two equivalent Cu(1)S4 tetrahedra, and edges with four equivalent Er(1)S6 octahedra. The corner-sharing octahedral tilt angles are 4°. There are three inequivalent S sites. In the first S site, S(1) is bonded to two equivalent Ca(1) and three equivalent Er(1) atoms to form SCa2Er3 square pyramids that share corners with four equivalent S(2)K2CaEr2Cu octahedra, corners with two equivalent S(1)Ca2Er3 square pyramids, edges with three equivalent S(2)K2CaEr2Cu octahedra, and edges with five equivalent S(1)Ca2Er3 square pyramids. The corner-sharing octahedral tilt angles range from 7-84°. In the second S site, S(2) is bonded to two equivalent K(1), one Ca(1), two equivalent Er(1), and one Cu(1) atom to form distorted SK2CaEr2Cu octahedra that share corners with three equivalent S(2)K2CaEr2Cu octahedra, corners with four equivalent S(1)Ca2Er3 square pyramids, edges with two equivalent S(2)K2CaEr2Cu octahedra, edges with three equivalent S(1)Ca2Er3 square pyramids, and  a faceface with one S(2)K2CaEr2Cu octahedra. The corner-sharing octahedral tilt angles range from 0-65°. In the third S site, S(3) is bonded in a 6-coordinate geometry to two equivalent K(1), two equivalent Er(1), and two equivalent Cu(1) atoms.
KCaEr2CuS5 crystallizes in the orthorhombic Cmcm space group. K(1) is bonded in a 6-coordinate geometry to two equivalent S(3) and four equivalent S(2) atoms. Both K(1)-S(3) bond lengths are 3.12 Å. All K(1)-S(2) bond lengths are 3.25 Å. Ca(1) is bonded to two equivalent S(2) and four equivalent S(1) atoms to form CaS6 octahedra that share corners with four equivalent Er(1)S6 octahedra, corners with two equivalent Cu(1)S4 tetrahedra, edges with two equivalent Ca(1)S6 octahedra, and edges with six equivalent Er(1)S6 octahedra. The corner-sharing octahedral tilt angles are 5°. Both Ca(1)-S(2) bond lengths are 2.77 Å. All Ca(1)-S(1) bond lengths are 2.89 Å. Er(1) is bonded to one S(3), two equivalent S(2), and three equivalent S(1) atoms to form ErS6 octahedra that share  a cornercorner with one Er(1)S6 octahedra, corners with two equivalent Ca(1)S6 octahedra, edges with three equivalent Ca(1)S6 octahedra, edges with four equivalent Er(1)S6 octahedra, and edges with two equivalent Cu(1)S4 tetrahedra. The corner-sharing octahedral tilt angles range from 5-41°. The Er(1)-S(3) bond length is 2.68 Å. Both Er(1)-S(2) bond lengths are 2.67 Å. There are two shorter (2.79 Å) and one longer (2.81 Å) Er(1)-S(1) bond length. Cu(1) is bonded to two equivalent S(2) and two equivalent S(3) atoms to form CuS4 tetrahedra that share corners with two equivalent Ca(1)S6 octahedra, corners with two equivalent Cu(1)S4 tetrahedra, and edges with four equivalent Er(1)S6 octahedra. The corner-sharing octahedral tilt angles are 4°. Both Cu(1)-S(2) bond lengths are 2.33 Å. Both Cu(1)-S(3) bond lengths are 2.36 Å. There are three inequivalent S sites. In the first S site, S(1) is bonded to two equivalent Ca(1) and three equivalent Er(1) atoms to form SCa2Er3 square pyramids that share corners with four equivalent S(2)K2CaEr2Cu octahedra, corners with two equivalent S(1)Ca2Er3 square pyramids, edges with three equivalent S(2)K2CaEr2Cu octahedra, and edges with five equivalent S(1)Ca2Er3 square pyramids. The corner-sharing octahedral tilt angles range from 7-84°. In the second S site, S(2) is bonded to two equivalent K(1), one Ca(1), two equivalent Er(1), and one Cu(1) atom to form distorted SK2CaEr2Cu octahedra that share corners with three equivalent S(2)K2CaEr2Cu octahedra, corners with four equivalent S(1)Ca2Er3 square pyramids, edges with two equivalent S(2)K2CaEr2Cu octahedra, edges with three equivalent S(1)Ca2Er3 square pyramids, and  a faceface with one S(2)K2CaEr2Cu octahedra. The corner-sharing octahedral tilt angles range from 0-65°. In the third S site, S(3) is bonded in a 6-coordinate geometry to two equivalent K(1), two equivalent Er(1), and two equivalent Cu(1) atoms.