AIn₂S₄ (A= Mn-Ni): New synthesis methods and structural characterization

Ternary chalcogenides with the spinel crystal structure have been widely studied for the last 60 years. These compounds are more covalent and allow different interactions between ions when compared to corresponding oxides while maintaining the versatility of the spinel structure. Particularly, spinels with formula AIn₂S₄ (A= Mn-Ni) are found to be semiconductors with a large band gap (~2.0 eV), showing mainly antiferromagnetic interactions at temperatures below 100 K. These properties are dependent on the distribution of cations between octahedral and tetrahedral sites in the structure and also on the synthesis method. Syntheses of these compounds have been extensively reported, involving long heating periods of mixtures of elements or binary sulfides inside evacuated quartz ampoules, lasting for up to two weeks or longer. We report two new, faster synthesis methods for polycrystalline samples of indium thiospinels avoiding evacuated ampoules. Polycrystalline FeIn₂S₄ and MnIn₂S₄ samples were synthesized from stoichiometric mixtures of binary sulfides, heating in a constant gas flow of 1% H₂ in Ar at 1073 K for 6 and 24 hours respectively. Polycrystalline samples of FeIn₂S₄, CoIn₂S₄ and NiIn₂S₄ have also been synthesized in a piston-cylinder press, from stoichiometric mixtures of the binary sulfides, at a pressure of 3.5 GPa at 1173 K for 1 hour. From x-ray powder diffraction data all crystal structures were refined by means of Rietveld analysis. Refined cell parameters are in agreement with previous reports. Different degrees of inversion were obtained for each sample. NiIn₂S₄ and both FeIn₂S₄ samples are in accordance with previous reports, while MnIn₂S₄ and CoIn₂S₄ differ from reported values [1]. FeIn₂S₄ and MnIn₂S₄ synthesized in H₂/Ar flux were characterized by Raman spectroscopy to further assess cation distribution. In addition, their thermal stability in air was tested by Thermogravimetric Analysis (TGA).