Layered and Cubic Semiconductors <i>A</i>Ga<i>M</i>′<i>Q</i>₄ (<i>A</i>⁺ = K⁺, Rb⁺, Cs⁺, Tl⁺; <i>M</i>′⁴⁺ = Ge⁴⁺, Sn⁴⁺; <i>Q</i>^2– = S^2–, Se^2–) and High Third-Harmonic Generation

Abstract

Eighteen new quaternary chalcogenides <i>A</i>Ga<i>M</i>'<i>Q</i>₄ (<i>A</i>⁺ = K⁺, Rb⁺, Cs⁺, Tl⁺; <i>M</i>'⁴⁺ = Ge⁴⁺, Sn⁴⁺; <i>Q</i>^2- = S^2-, Se^2-) 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 < <i>E</i>_g < 2.3 eV) and the mostly white sulfides absorbing light in the blue-ultraviolet range (2.5 eV < <i>E</i>_g < 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 <i>A</i>Ga<i>M</i>'<i>Q</i>₄ 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.