Regresar

(Cu)tet(Cr2- xSnx)octS4- ySey Spinels: Crystal Structure, Density Functional Theory Calculations, and Magnetic Behavior

Abstract:

A new series of (Cu)<inf>tet</inf>[Cr<inf>2-x</inf>Sn<inf>x</inf>]<inf>oct</inf>S<inf>4-y</inf>Se<inf>y</inf> compounds was prepared by solid-state reaction at high temperature. Determination of the crystal structures by single-crystal X-ray diffraction revealed that CuCr<inf>1.0</inf>Sn<inf>1.0</inf>S<inf>2.1</inf>Se<inf>1.9</inf>, CuCr<inf>1.2</inf>Sn<inf>0.8</inf>S<inf>2.1</inf>Se<inf>1.9</inf>, CuCr<inf>1.3</inf>Sn<inf>0.7</inf>S<inf>2.2</inf>Se<inf>1.8</inf>, and CuCr<inf>1.5</inf>Sn<inf>0.5</inf>S<inf>2.2</inf>Se<inf>1.8</inf> crystallize in a normal spinel-type structure (cubic Fd3m space group). The powder X-ray diffraction patterns and Rietveld refinements of nominal CuCr<inf>2-x</inf>Sn<inf>x</inf>S<inf>2</inf>Se<inf>2</inf> (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 A<inf>1g</inf>, E<inf>g</inf>, and three F<inf>2g</inf> vibrational modes were observed in the spectra. The signal at ∼382 cm-1, corresponding to the A<inf>1g</inf> 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 CuCrSnS<inf>2</inf>Se<inf>2</inf> with a Néel temperature (T<inf>N</inf>) of 18.8 K and a θ value of -36.0 K. Density functional theory (DFT) and effective magnetic moments (μ<inf>eff</inf>/μ<inf>theo</inf>) 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<inf>1.0</inf>Sn<inf>1.0</inf>S<inf>2</inf>Se<inf>2</inf> was AF with exchange constants for first- and second-neighbor interactions of J<inf>1</inf> = 56.22 cm-1 and J<inf>2</inf> = -33.88 cm-1. Thus, the AF interactions between ferromagnetic chains in CuCr<inf>1.0</inf>Sn<inf>1.0</inf>S<inf>2</inf>Se<inf>2</inf> originate from the presence of diamagnetic Sn cations.