SrCdGeS₄ and SrCdGeSe₄: Promising Infrared Nonlinear Optical Materials with Congruent-Melting Behavior

Abstract

The quaternary chalcogenides SrCdGeS4 and SrCdGeSe4 were prepared by high-temperature reactions in the form of single crystals (at 1223 and 1123 K, respectively) or microcrystalline samples (at 1023 K). They adopt non-centrosymmetric structures (orthorhombic, space group Ama2, Z = 4; a = 10.3352(14) Å, b = 10.2335(14) Å, c = 6.4408(9) Å for SrCdGeS4; a = 10.8245(8) Å, b = 10.6912(8) Å, c = 6.6792(5) Å for SrCdGeSe4) consisting of anionic [CdGeCh4]2– layers separated by Sr2+ cations. Although the structure is closely related to the BaZnSiSe4-type, the substitution of Cd for Zn is accompanied by a significant displacement away from an ideal tetrahedral site toward a trigonal pyramidal geometry. Relationships to other similar structures are also elucidated. Large optical band gaps of 2.6 eV for SrCdGeS4 and 1.9 eV for SrCdGeSe4 were deduced from the absorption edges, consistent with the light yellow and red colors, respectively, of crystals of these compounds. Nonlinear optical measurements on powder samples indicated that these compounds show intense second harmonic generation signals (2× AgGaS2 for SrCdGeS4 and 5× AgGaS2 for SrCdGeSe4) and type-I phase-matching behavior using 2090 nm as the fundamental laser wavelength. They also melt congruently at relatively low temperatures (958 °C for SrCdGeS4 and 851 °C for SrCdGeSe4).