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 (Eg) of 3.13 eV, which achieves a desirable balance for a promising infrared nonlinear optical (IR NLO) candidate. Theoretical calculations reveal that both the Eg 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 Eg and SHG performances.