KGdSiSe₄ and KGdGeSe₄: two rare earth-based selenides with wide bandgaps and large nonlinear optical responses

Two new rare earth (RE) metal selenides KGdSiSe₄ and KGdGeSe₄ were synthesized by high-temperature solid-state reactions and structurally characterized by single-crystal X-ray diffraction. Both compounds crystallize in the noncentrosymmetric monoclinic P2₁ space group and feature layered anionic frameworks composed of NLO-active [GdSe₇] polyhedra and [Si/GeSe₄] tetrahedra. Optical measurements reveal strong second-harmonic generation (SHG) responses of approximately 1.1 × AGS for KGdGeSe₄ and 0.8 × AgGaS₂ (AGS) for KGdSiSe₄. Notably, KGdSiSe₄ exhibits a wide band gap (E_g) of 3.13 eV, which achieves a desirable balance for a promising infrared nonlinear optical (IR NLO) candidate. Theoretical calculations reveal that both the E_g and SHG responses mainly originate from the synergistic contributions of [Si/GeSe₄] tetrahedra and [GdSe₇] polyhedral units. The results demonstrate that alkali-metal RE metal selenides represent a promising platform for developing advanced IR NLO materials with balanced E_g and SHG performances.