A Challenge toward Novel Quaternary Sulfides SrLnCuS (Ln = La, Nd, Tm): Unraveling Synthetic Pathways, Structures and Properties.

We report on the novel heterometallic quaternary sulfides SrLnCuS (Ln = La, Nd, Tm), obtained as both single crystals and powdered samples. The structures of both the single crystal and powdered samples of SrLaCuS and SrNdCuS belong to the orthorhombic space group but are of different structural types, while both samples of SrTmCuS crystallize in the orthorhombic space group with the structural type KZrCuS. Three-dimensional crystal structures of SrLaCuS and SrNdCuS are formed from the (Sr/Ln)S capped trigonal prisms and CuS tetrahedra. In SrLaCuS, alternating 2D layers are stacked, while the main backbone of the structure of SrNdCuS is a polymeric 3D framework [(Sr/Ln)S], strengthened by 1D polymeric chains (CuS) with 1D channels, filled by the other Sr/Ln cations, which, in turn, form 1D dimeric ribbons. A 3D crystal structure of SrTmCuS is constructed from the SrS trigonal prisms, TmS octahedra and CuS tetrahedra. The latter two polyhedra are packed together into 2D layers, which are separated by 1D chains (SrS) and 1D free channels. In both crystal structures of SrLaCuS obtained in this work, the crystallographic positions of strontium and lanthanum were partially mixed, while only in the structure of SrNdCuS, solved from the powder X-ray diffraction data, were the crystallographic positions of strontium and neodymium partially mixed. Band gaps of SrLnCuS (Ln = La, Nd, Tm) were found to be 1.86, 1.94 and 2.57 eV, respectively. Both SrNdCuS and SrTmCuS were found to be paramagnetic at 20-300 K, with the experimental magnetic characteristics being in good agreement with the corresponding calculated parameters.