Phase stability and magnetocaloric properties of Gd₂OS₂ and γ-Gd₂S₃: a combined experimental and theoretical study

Modifying the crystal structure of materials through thermal treatment and sulphurisation enables the formation of materials that are chemically challenging to stabilize using classical syntheses. This research investigates the effect of synthesis conditions during thermal treatment starting from the sulphide α-Gd₂S₃ and examines the structural, magnetic, and magnetocaloric properties of γ-Gd₂S₃ phase and Gd₂OS₂-rich sample, obtained in a partially sulphurised atmosphere. Both compounds are known to be chemically unstable, prompting us to calculate their phase diagrams using density functional theory (DFT) to better understand their chemical stability. For the first time, the Néel temperature (T_N) of antiferromagnetic (AFM) ordering, the effective magnetic moment (μ_eff), the magnetocaloric effect (MCE), and the magnetic transition temperature (T_r) corresponding to the maximum MCE are reported for the Gd₂OS₂ oxysulphide and compared with those of α-Gd₂S₃ and its sub-phases. DFT investigation of the magnetic exchange interactions J_ij was conducted to better understand the antiferromagnetic ordering in these systems. The magnetic phase transition was analyzed using the phenomenological universal curve of magnetic entropy change, revealing a first-order magnetic transition for all samples. The structural change from α-Gd₂S₃ (Pnma space group) to Gd₂OS₂ (P2₁/c space group) lowered T_N from 10.0 K to 6.9 K and enhanced the MCE by ∼40%, reaching 79 mJ cm^-3 K^-1 at 6 K for µ₀ΔH = 5 T.