2.1. Synthesis

CuCr_2-xSn_xS₄ compounds were prepared directly by combining high-purity elemental powders (99.99%, Aldrich) in stoichiometric amounts. All manipulations were carried out under argon atmosphere. The reaction mixtures were sealed in evacuated quartz ampoules and placed in a programmable furnace. The ampoules were then slowly heated at a rate of 1 °C/min, from room temperature until 850 °C, and held for 6 days. Then, ampoules were slowly cooled, opened, homogenized, resealed and heated again to the same temperature of 850 °C and maintained for 10 days. Finally, they were again slowly cooled to room temperature at a rate of 1 °C/min, ready for their characterization.

2.2. Powder X-ray Diffraction

Powder X-ray diffraction (PXRD) patterns were collected at room temperature on a Bruker D8 Advance diffractometer equipped with a Cu Kα radiation source (Kα = 1.5406 Å); samples were scanned in the range 5° <2θ <80°.

2.3. SEM-EDS analysis

The chemical compositions of samples were determined by scanning electron microscopy with the aid of energy-dispersive X-ray analysis (SEM-EDS) using a JEOL 5400 system equipped with an Oxford Link ISIS microanalyzer. The working distance was 35 mm and the accelerating voltage was set to 22.5 kV. Samples were mounted into a double-sided carbon tape, which was adhered to an aluminum specimen holder. EDS data were collected for 60 s.

2.4. Raman spectroscopy

The Raman scattering measurements of CuCr_2-xSn_xS₄ were made using a RM1000 Renishaw micro-Raman spectrometer with detectors CCD (Charge-Coupled Device) in combination with a Leica LM/PM microscope using excitation of 532 nm wavelength. The spectrometer was calibrated using a reference single crystal Si sample (Raman peak at 520.7 cm⁻¹). The spectra data was collected at room temperature in back scattering configuration in the spectral range 100 – 450 cm⁻¹, with a laser spot on the sample of about ~1 μm and a laser power ~2mW.

2.5. Magnetic measurements

Magnetic measurements were performed on pelletized powder samples using a Quantum Design MPMS XL5 SQUID susceptometer. The magnetic nature of the material was determined using ZFC/FC (zero-field-cooled/field-cooled) cycles at low fields (typically 500 Oe).

3.1 Powder X-ray diffraction and SEM-EDS

The XRD patterns were fully indexed in the space group Fd3¯m (N°227), indicating that CuCr_2-xSn_xS₄ compounds present spinel-type structures. The experimental XRD patterns were compared with the simulated XRD patterns derived from a single-crystal XRD data. Experimental XRD patterns for CuCr_2-xSn_xS₄ thiospinels are shown in Fig. 1. For these compounds, the cell parameter “a” increased gradually as chromium was replaced by tin (Table 1), obeying Vegard's law. This can be explained by the higher ionic radii of Sn⁴⁺ compared to Cr³⁺ ions. The octahedral ionic radii of Sn⁴⁺ and Cr³⁺ cations published by Shannon are 0.69 Å and 0.615 Å, respectively (for a high-spin configuration) [¹²]. This shows that the increase in ionic radius explains the increase in the size of the unit cell. Chemical compositions of powder samples were examined using EDS analysis (Fig. 2).

Fig. 1 Experimental X-ray powder diffraction patterns for CuCr_2-xSn_xS₄ compared with simulated X-ray pattern of CuCr_1.2Sn_0.8S₄ single crystal [¹³]. 

Fig. 2 Scanning electron microscopy (SEM) micrograph of CuCr_1.6Sn_0.4S₄ (insert) and an example of EDS spectral analysis. 

3.2 Raman spectroscopy

Brüesch and D ‘Ambrogio [¹⁴] have performed a detailed Raman analysis for CdCr₂X₄ (X = S, Se). Through group theory, the authors, assigned the signals to the corresponding symmetrical modes, characteristic of the spinel crystal structure with spatial group. The irreducible representation is Γ= A_1g + E_g + F_1g + 3 F_2g + 2 A_2u + 2 E_u + 5 F_1u + 2 F_2u, where the modes A_1g, E_g and the three F_2g are active in Raman. The Raman spectra for the four studied compositions CuCr_2-xSn_xS₄ (0.4 ≤ × ≤ 1.4) were measured under light excitation with wavelength of 532 nm, in the spectral region between 100 and 450 cm⁻¹ and adjusted by means of Lorentzian functions (Fig. 3).

Fig. 3 Raman Spectra of CuCr_2-xSn_xS₄ (0.4 ≤ × ≤ 1.4) powder samples. 

The spectra were characterized by five signals, showing a good similarity with the normal spinels ACr₂S₄ (A = Cd, Hg) [¹¹], [¹⁴]-[¹⁸]. Table 2 presents a summary of the peaks position and the corresponding symmetry.

Specifically, the signals about 130 cm⁻¹, 300 cm⁻¹ and near to 400 cm⁻¹ reflect the vibrations in the thiospinels tetrahedron, where the mode F_2g (1) at 130 cm⁻¹, is attributed to bending of the CuS₄ unit, while the mode F_2g (2) at 300 cm⁻¹ corresponds to asymmetric stretching of the tetrahedron. For its part, the A_1g mode at ~ 400 cm⁻¹ corresponds to symmetric stretching of the Cu-S bond in the CuS₄ unit [¹⁴], [¹⁵], [¹⁸]. Although, the latest signal reflects a vibration of the tetrahedron, we observe the influence of the substitution at the octahedral site (16d), which is randomly occupied by the chromium and tin cations. The signal A_1g shows a widening and displacement toward higher frequencies, particularly important in the CuCr_0.6Sn_1.4S₄ phase, which correspond to an increase in the force constant of the Cu-S bond in the CuS₄ unit. We suggest that, as the content of the Sn^{4 +} cation ionic radius of 0.69 Å [¹²] increases, it induces an increase in the stress within the tetrahedron.

On the other hand, the E_g and F_2g (3) modes have been assigned to the symmetric and antisymmetric twisting motion of the cation on the Cr/Sn-S link of the BS₆ unit [¹⁴], [¹⁵]. Particularly, the signal corresponding to the E_g mode undergoes a shift toward low energies (Table 2), showing the influence of the substitution of chromium by tin. This result is consistent with the tin content in the samples, since the atomic weight of Sn is greater than the chromium one, leading to red shift of the signals.

This set of results allows us to propose the modes A_1g and E_g as those responsible to qualitatively prove the presence of tin in the phases, confirming new chemical interactions which explain changes in the force constants of the structure.

3.3 Magnetic measurements

The zero-field-cooled (ZFC)/field-cooled (FC) magnetization cycles for CuCr_2-xSn_xS₄ (x = 0.8 and 1.0) were performed under a magnetic field of 500 Oe (Fig. 4 and 5). The insets show the inverse susceptibility in the paramagnetic regime. The inverse susceptibility, 1/χ, was fitted with a classical Curie–Weiss relation, χ = C/(T-θ), in a temperature range that varied depending on the compound.

Fig. 4 ZFC/FC magnetization cycle measured at H_app = 500 Oe for a powder sample of CuCr_1.0Sn_1.0S₄. The insert shows the 1/χ-versus-temperature behavior fitted by a Curie–Weiss law. 

Fig. 5 ZFC/FC magnetization cycle measured at H_app= 500 Oe for a powder sample of CuCr_1.2Sn_0.8S₄. The insert shows the 1/χ-versus-temperature behavior fitted by a Curie–Weiss law. 

In both samples, an antiferromagnetic behavior is observed with a Néel temperature T_N equal to 17.8 K and 24.8 K for CuCr_1.0Sn_1.0S₄ and CuCr_1.2Sn_0.8S₄ respectively. However, the sample with x = 0.8 shows a positive value of θ (+84.2 K, Table 3) which indicates a dominant ferromagnetic character of the exchange interactions (Fig. 5). Both samples (x = 0.8 and 1.0) present an irreversible behavior below the transition temperature that could be associated to a spin glass state. This behavior can be correlated to the increase of the cell parameter “a” (Table 1) that favors an antiferromagnetic interaction when Sn content increased, changing the orientation of the localized magnetic moment and causing that the spin glass state appears. A similar result has been reported in CuCrZrS₄ [⁵]_, and CuCr_2-xSn_xSe₄ [¹⁰] compounds.

The observed magnetic moments for CuCr_1.0Sn_1.0S₄ (µ_eff = 3.79 MB) and CuCr_1.2Sn_0.8S₄ (µ_eff = 3.82 MB) are close to the moments expected for high-spin states Cu¹⁺[Cr_1.0³⁺Cr_(1-x)4+]Sn_x⁴⁺S₄ (µ_theo = 4.07 MB and µ_theo = 3.87 MB for x=1.0 and 0.8) [¹⁹].