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

Abstract

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 Fe_xTi₂S₄ 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 °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. θ_W = −398 K for x = 0.42), predominant for the Fe-rich phase Fe_0.42Ti₂S₄, combined with ferromagnetic-like interactions as x decreases (e.g. θ_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.