Magnetic Properties and Large Second-Harmonic Generation Response of a Chiral Ternary Chalcogenide: Eu₂SiSe₄

Eu-(II)-containing chalcogenides are an emerging class of materials that are of great interest due to their high optical activity and intriguing magnetism. Here, we synthesized Eu₂SiSe₄ as red-colored single crystals and characterized its structure with single-crystal X-ray diffraction, confirming the reported chiral monoclinic P2₁ symmetry at room temperature. The crystal structure of Eu₂SiSe₄ comprises distorted SiSe₄ tetrahedral units and charge-balancing Eu-(II) cations. Here, we develop a two-step solid-state synthesis method for Eu₂SiSe₄ and compare it to the known boron chalcogenide method. We find the second-harmonic generation (SHG) activity of polycrystalline Eu₂SiSe₄ to be ∼7 × AgGaS₂, placing it among the highest-known SHG-active chalcogenides. No symmetry lowering is observed down to 100 K in single-crystal X-ray diffraction, although an anomalous expansion in the b-axis lattice parameter occurs and may be correlated to lattice modes of the SiSe₄ tetrahedra. We investigate the physical properties of Eu₂SiSe₄ using magnetometry and heat capacity measurements and find a transition to an antiferromagnetic ground state at T _N ≈ 5.5 K. The low-temperature transition releases less entropy than expected, which may be due to the complex crystal electric field effects of Eu-(II).