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

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 (Eg = 2.31 eV) and Sr2SnSe4 (Eg = 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.