High-pressure synthesis, spin-glass behaviour, and magnetocaloric effects in FexTi2S4 heideite sulphides

Intercalation compounds based on layered TiS2 sulphides are gaining much attention, since the incorporation of transition metals often dramatically change the physical properties and unlocks new intriguing phenomena. Here, we report a rapid high-pressure preparation method under 3.5 GPa at moderate temperatures for the synthesis of FexTi2S4 polycrystalline materials, starting from TiS2 and Fe metals. Three different compositions with x = 0.24, 0.32, and 0.42 have been stabilized at decreasing temperatures in the range of 800-900 degrees C; at room temperature, the crystallographic features have been probed by a neutron powder diffraction (NPD) experiment for the x = 0.42 sample. All the compounds crystallize in a Heideite-type phase with space group C12/m1; the structure consists of layers of [TiS6] octahedra sharing edges with Fe atoms located in between the layers, also in octahedral coordination. The NPD study unveils a discrete Fe/Ti inversion (<6%) at the TiS2 layers. The surface chemistry from XPS at Fe 2p and Ti 2p core levels revealed the presence of Fe2+ in all samples, whereas the Ti main contribution mainly arises from the Ti3+ state, with a smaller contribution of Ti2+ and Ti4+ states. The magnetic properties stemming from Fe2+ and Ti3+ spins offer a complex scenario with antiferromagnetic interactions, characterized by a strongly negative Weiss constant (e.g. theta(W) = -398 K for x = 0.42), predominant for the Fe-rich phase Fe0.42Ti2S4, combined with ferromagnetic-like interactions as x decreases (e.g. theta(W) = 204 K for x = 0.24), leading to spin-glass or cluster-glass behaviours. The study of the magnetocaloric effect yields relative cooling power (RCP) values at a 7 T of 135.3, 124.5, and 96.0 J kg(-1) for the x = 0.24, 0.32 and 0.42 samples, respectively, better than other transition-metal sulphides already reported in the literature, with a temperature stability that is desirable for an ideal Ericson refrigeration cycle.

Automated summary

In this study, FexTi2S4 polycrystalline materials were prepared using a rapid high-pressure method, and their crystal structure and magnetic properties were investigated. The results revealed a Fe/Ti inversion in the TiS2 layers and different magnetic behaviors depending on the Fe content. Furthermore, the material exhibited better relative cooling power and temperature stability compared to other transition metal sulfides reported in the literature.