Structure Tuning, Strong Second Harmonic Generation Response, and High Optical Stability of the Polar Semiconductors NaKAs.

The mixed cation compounds NaKAsSe ( = 0.8, 0.65, 0.5) and NaKAsS crystallize in the polar noncentrosymmetric space group . The As ( = alkali metals, = S, Se) family features one-dimensional (1D) /[] chains comprising corner-sharing pyramidal units in which the packing of these chains is dependent on the alkali metals. The parallel /[] chains interact via short As···Se contacts, which increase in length when the fraction of K atoms is increased. The increase in the As···Se interchain distance increases the band gap from 1.75 eV in γ-NaAsSe to 2.01 eV in NaKAsSe, 2.07 eV in NaKAsSe, and 2.18 eV in NaKAsS. The NaKAsSe ( = 0.8, 0.65) compounds melt congruently at approximately 316 °C. Wavelength-dependent second harmonic generation (SHG) measurements on powder samples of NaKAsSe ( = 0.8, 0.65, 0.5) and NaKAsS suggest that NaKAsSe and NaKAsS have the highest SHG response and exhibit significantly higher laser-induced damage thresholds (LIDTs). Theoretical SHG calculations on NaKAsSe confirm its SHG response with the highest value of = 22.5 pm/V (χ = 45.0 pm/V). The effective nonlinearity for a randomly oriented powder is calculated to be = 18.9 pm/V (χ = 37.8 pm/V), which is consistent with the experimentally obtained value of = 16.5 pm/V (χ = 33.0 pm/V). Three-photon absorption is the dominant mechanism for the optical breakdown of the compounds under intense excitation at 1580 nm, with NaKAsSe exhibiting the highest stability.