Polymorphism and antiferromagnetic behavior in the quaternary chalcogenide Ag2FeGeS4: Structural insights into the stannite-type phase

The quaternary chalcogenide Ag 2 FeGeS 4 was synthesized by the melt and annealing through direct reaction of the constituent elements. Differential thermal analysis reveals a peritectic melting point at 717 °C, and establishes the phase sequence α → α + β → β → β 1 + L → L, where α denotes the stannite-type phase and β, β 1 are high-temperature zinc-blende modifications. The crystal structure, determined by powder X-ray diffraction and the Rietveld method, confirms crystallization in the stannite-type structure, in the non-centrosymmetric tetragonal space group I 4̅2 m (No. 121), Z = 2, with unit cell parameters a = 5.6990(2) Å, c = 10.5315(4) Å, V = 342.05(2) Å 3 . The structure consists of a three-dimensional close-packed arrangement of slightly distorted AgS 4 , FeS 4 , GeS 4 , and S(Ag 2 ,Fe,Ge) tetrahedra interconnected through shared corners. Bond valence sum calculations confirm the expected formal oxidation states Ag 1+ , Fe 2+ , Ge 4+ , and S 2- . A comparative analysis with the kesterite-type polymorph (space group I 4̅) reveals significantly higher distortion in the latter, particularly in the AgS 4 tetrahedra, highlighting the influence of cation distribution and symmetry on local coordination environments. Notably, the formation of these polymorphs appears to depend on the synthetic route: mechanochemical conditions favor the kesterite-type structure, whereas high-temperature fusion followed by annealing stabilizes the stannite-type phase, indicating a strong influence of kinetic and thermal factors on the structural outcome. Hirshfeld surface and fingerprint plot analyses show that the crystal packing is dominated by M···S contacts, while void surface analysis reveals a compact structure without continuous void channels. Magnetic measurements show Curie-Weiss paramagnetism at high temperatures and antiferromagnetic ordering below T N ≈ 20 K. The negative Weiss temperature (θ ≈ −50 K) confirms dominant antiferromagnetic interactions, while the low-temperature behavior suggests a possible weak ferromagnetic component. These results highlight the multifunctional potential of Ag 2 FeGeS 4 as a candidate material for antiferromagnetic spintronics, nonlinear optical devices (NLO), and thermoelectric applications.