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 TN ≈ 5.5 K. The low-temperature transition releases less entropy than expected, which may be due to complex crystal electric field effects of Eu(II).