(Cu)_tet(Cr_2-Sn)_octS_4-Se Spinels: Crystal Structure, Density Functional Theory Calculations, and Magnetic Behavior

A new series of (Cu)_tet[Cr_2-Sn]_octS_4-Se compounds was prepared by solid-state reaction at high temperature. Determination of the crystal structures by single-crystal X-ray diffraction revealed that CuCr_1.0Sn_1.0S_2.1Se_1.9, CuCr_1.2Sn_0.8S_2.1Se_1.9, CuCr_1.3Sn_0.7S_2.2Se_1.8, and CuCr_1.5Sn_0.5S_2.2Se_1.8 crystallize in a normal spinel-type structure (cubic Fd3m space group). The powder X-ray diffraction patterns and Rietveld refinements of nominal CuCr_2-SnS₂Se₂ (x = 0.2, 0.4, 0.6, 0.8, and 1.0) were consistent with single-crystal X-ray diffraction data. Raman scattering analysis revealed that the A₁, E, and three F₂ vibrational modes were observed in the spectra. The signal at ∼382 cm^-1, corresponding to the A₁ mode, is attributed to symmetrical stretching of the chalcogen bond with respect to the tetrahedral metal. The samples with x = 0.2 and 0.4 exhibited ferromagnetic behavior, characterized by large positive θ values of +261 and +189 K, respectively. In contrast, antiferromagnetic (AF) behavior was observed for CuCrSnS₂Se₂ with a Néel temperature (T_N) of 18.8 K and a θ value of -36.0 K. Density functional theory (DFT) and effective magnetic moments (μ_eff/μ_theo) experimentally measured showed that the Sn ion is in oxidation state of 4+, i.e., diamagnetic behavior. DFT calculations revealed that the most stable magnetic state of CuCr_1.0Sn_1.0S₂Se₂ was AF with exchange constants for first- and second-neighbor interactions of J₁ = 56.22 cm^-1 and J₂ = -33.88 cm^-1. Thus, the AF interactions between ferromagnetic chains in CuCr_1.0Sn_1.0S₂Se₂ originate from the presence of diamagnetic Sn cations.