Structure Tuning, Strong Second Harmonic Generation Response, and High Optical Stability of the Polar Semiconductors Na1-xKxAsQ2

The mixed cation compounds Na1-xKxAsSe2 (x = 0.8, 0.65, 0.5) and Na0.1K0.9AsS2 crystallize in the polar noncentrosymmetric space group Cc. The AAsQ(2) (A = alkali metals, Q = S, Se) family features one-dimensional (1D) 1/infinity[AQ(2)(-)] chains comprising corner-sharing pyramidal AQ(3) units in which the packing of these chains is dependent on the alkali metals. The parallel 1/infinity[AQ(2)(-)] chains interact via short As center dot center dot center dot Se contacts, which increase in length when the fraction of K atoms is increased. The increase in the As center dot center dot center dot Se interchain distance increases the band gap from 1.75 eV in gamma-NaAsSe2 to 2.01 eV in Na0.35K0.65AsSe2, 2.07 eV in Na0.2K0.8AsSe2, and 2.18 eV in Na0.1K0.9AsS2. The Na1-xKxAsSe2 (x = 0.8, 0.65) compounds melt congruently at approximately 316 degrees C. Wavelength-dependent second harmonic generation (SHG) measurements on powder samples of Na1-xKxAsSe2 (x = 0.8, 0.65, 0.5) and Na0.1K0.9AsS2 suggest that Na0.2K0.8AsSe2 and K-Na0.1(0.9AsS2) have the highest SHG response and exhibit significantly higher laser-induced damage thresholds (LIDTs). Theoretical SHG calculations on Na0.5K0.5AsSe2 confirm its SHG response with the highest value of d(33) = 22.5 pm/V (chi((2))(333) = 45.0 pm/V). The effective nonlinearity for a randomly oriented powder is calculated to be d(eff) = 18.9 pm/V (chi((2))(eff) = 37.8 pm/V), which is consistent with the experimentally obtained value of d(eff) = 16.5 pm/V (chi((2))(eff) = 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 Na0.2K0.8AsSe2 exhibiting the highest stability.

Automated summary

The study investigated the crystal structures and optical properties of mixed cation compounds Na1-xKxAsSe2 (x = 0.8, 0.65, 0.5) and Na0.1K0.9AsS2, revealing that the interaction between different ratios of K atoms can affect the properties and band gap of the compounds.