Ba₂MAsQ₅ (Q = S and Se) Family of Polar Structures with Large Second Harmonic Generation and Phase Matchability

Noncentrosymmetric chalcogenides containing stereochemically active lone pair elements, such as As, Sb, and Bi, offer a useful way to tune the band gap and enhance the second harmonic generation (SHG) response. Ba2As2–xSbxQ5, Ba2As2–xBixQ5 (Q = S and Se), and Ba2As2S4.8Se0.2 are new chalcoarsenates that crystallize in the noncentrosymmetric monoclinic space group P21. These five new compounds crystallized with a three-dimensional (3D) Ba2As2Se5-type structure, containing AsQ3 units and [As2Q4]2– dimeric units. The [As2Q4]2– dimeric unit can be tuned by replacing As with Sb or Bi and Se with S. In contrast, Ba2As2S5 crystallizes in the orthorhombic space group Pca21. Differential thermal analysis suggested that Ba2As2S5, Ba2As2Se5, Ba2AsSbSe5, and Ba2As1.25Bi0.75Se5 melt congruently. The [M2Q4]2– unit plays a significant role in tuning the band gap of these compounds, decreasing from 2.02 eV in Ba2As2S5 to 1.37 eV in Ba2As1.25Bi0.75Se5. Powder SHG measurements showed all the compounds are essentially phase-matchable at 3300 nm. Ba2AsSbSe5 exhibits the highest χ(2) of 36 pm/V and a laser-induced damage threshold of 0.10 GW/cm2, comparable to that of AgGaQ2. These materials show significantly improved SHG behavior response to BaGa4Q7 compounds, making them attractive for commercial applications.