Layered and Cubic Semiconductors AGa M′ Q₄(A⁺= K⁺, Rb⁺, Cs⁺, Tl⁺M′⁴⁺= Ge⁴⁺, Sn⁴⁺Q^2-= S^2-, Se^2-) and High Third-Harmonic Generation

Eighteen new quaternary chalcogenides AGaM′Q4 (A+ = K+, Rb+, Cs+, Tl+ M′4+ = Ge4+, Sn4+ Q2- = S2-, Se2-) have been prepared by solid-state syntheses and structurally characterized using single-crystal X-ray diffraction techniques. These new phases crystallize in a variety of layered structure types. The tin analogues also adopt an extended three-dimensional network structure as polymorphs. The polymorphism and phase-stability in these cases were studied by thermal analysis and high-temperature in situ X-ray powder diffraction. All compounds are semiconductors with the colored selenides absorbing light in the infrared-green region (1.8 eV < Eg < 2.3 eV) and the mostly white sulfides absorbing light in the blue-ultraviolet range (2.5 eV < Eg < 3.6 eV). Based on third-harmonic generation (THG) measurements, the third-order nonlinear optical (NLO) susceptibilities χ(3) of the new and previously reported AGaM′Q4 compounds were determined. These measurements revealed an apparent correlation between the THG response of the sample and its band gap, rather than the crystal structure type. While low-gap materials possess higher nonlinearity in general, we found that layered orthorhombic RbGaGeS4 exhibits an impressive χ(3) value (about four times larger than that of AgGaS2) even with a large band gap and shows stability under ambient conditions with no significant irradiation damage.