(Cu)_tet(Cr_2–<i>x</i>Sn_<i>x</i>)_octS_4–<i>y</i>Se_<i>y</i> Spinels: Crystal\nStructure, Density Functional Theory Calculations, and Magnetic Behavior

Abstract

A new series of (Cu)_tet[Cr_2–<i>x</i>Sn_<i>x</i>]_octS_4–<i>y</i>Se_<i>y</i> compounds was prepared\nby solid-state reaction at high temperature. Determination of the\ncrystal structures by single-crystal X-ray diffraction revealed that\nCuCr_1.0Sn_1.0S_2.1Se_1.9,\nCuCr_1.2Sn_0.8S_2.1Se_1.9,\nCuCr_1.3Sn_0.7S_2.2Se_1.8,\nand CuCr_1.5Sn_0.5S_2.2Se_1.8 crystallize in a normal spinel-type structure (cubic <i>Fd</i>3<i>m</i> space group). The powder\nX-ray diffraction patterns and Rietveld refinements of nominal CuCr_2–<i>x</i>Sn_<i>x</i>S₂Se₂ (<i>x</i> = 0.2, 0.4, 0.6, 0.8, and\n1.0) were consistent with single-crystal X-ray diffraction data. Raman\nscattering analysis revealed that the <i>A</i>_1<i>g</i>, <i>E</i>_<i>g</i>, and three <i>F</i>_2<i>g</i> vibrational\nmodes were observed in the spectra. The signal at ∼382 cm^–1, corresponding to the <i>A</i>_1<i>g</i> mode, is attributed to symmetrical stretching of\nthe chalcogen bond with respect to the tetrahedral metal. The samples\nwith <i>x</i> = 0.2 and 0.4 exhibited ferromagnetic behavior,\ncharacterized by large positive θ values of +261 and +189 K,\nrespectively. In contrast, antiferromagnetic (AF) behavior was observed\nfor CuCrSnS₂Se₂ with a Néel temperature\n(<i>T</i>_N) of 18.8 K and a θ value of\n−36.0 K. Density functional theory (DFT) and effective magnetic\nmoments (μ_eff/μ_theo) experimentally\nmeasured showed that the Sn ion is in oxidation state of 4+, i.e.,\ndiamagnetic behavior. DFT calculations revealed that the most stable\nmagnetic state of CuCr_1.0Sn_1.0S₂Se₂ was AF with exchange constants for first- and second-neighbor\ninteractions of <i>J</i>₁ = 56.22 cm^–1 and <i>J</i>₂ = −33.88 cm^–1. Thus, the AF interactions between ferromagnetic chains in CuCr_1.0Sn_1.0S₂Se₂ originate from\nthe presence of diamagnetic Sn cations.