Investigating Nonlinear Optical and Thermal Properties in Sr2SiSe4 through Noncentrosymmetric Crystal Structure Comparison with Sr2SnSe4

Abstract

An isovalent anion substitution approach allowed for the discovery of two noncentrosymmetric ternary selenides, Sr₂SiSe₄ and Sr₂SnSe₄. Sr₂SiSe₄ crystallizes in the α-Eu₂GeS₄ type, monoclinic space group P2₁, with lattice parameters a = 6.8866(19) Å, b = 7.0062(19) Å, c = 8.520(2) Å, β = 108.200(6)°, and Z = 2, while Sr₂SnSe₄ adopts the γ-Sr₂GeSe₄ type, orthorhombic space group Ama2, with lattice parameters a = 10.4802(10) Å, b = 10.6965(10) Å, c = 7.5411(7) Å, and Z = 4. Calculated PBE band gaps show that Sr₂SiSe₄ (E_g = 2.31 eV) and Sr₂SnSe₄ (E_g = 2.05 eV) both exhibit a broad transparency window in the infrared region along with moderate band gaps, avoiding two-photon absorption from 2090 nm lasers. The nonlinear optical response of Sr₂SiSe₄ was experimentally determined using AgGaS₂ as a reference, indicating that the material is phase-matchable, producing an intensity approximately 0.5 times that of AgGaS₂ using a 2090 nm laser. In addition to its promising nonlinear optical properties, Sr₂SiSe₄ exhibits ultralow thermal conductivity (0.46 W m^–1 K^–1 at 523 K) despite being composed of rather light elements compared to Bi₂Te₃ and PbTe, making it an interesting─and less toxic─potential candidate for thermoelectric energy conversion applications.