Soluble semiconductors AAsSe₂ (A = Li, Na) with a direct-band-gap and strong second harmonic generation: A Combined experimental and theoretical study

AAsSe₂ (A = Li, Na) have been identified as a new class of polar direct-band gap semiconductors. These I - V - Vl₂ ternary alkali-metal chalcoarsenates have infinite single chains of (1/∞)[AsQ ₂] derived from corner-sharing pyramidal AsQ₃ units with stereochemically active lone pairs of electrons on arsenic. The conformations and packing of the chains depend on the structure-directing alkali metals. This results in at least four different structural types for the Li_{1 x}Na_xAsSe₂ stoichoimetry (α-LiAsSe ₂, β-LiAsSe₂, γ-NaAsSe₂, and δ-NaAsSe₂). Single-crystal X-ray diffraction studies showed an average cubic NaCI-type structure for (α-LiAsSe₂, which was further demonstrated to be locally distorted by pair distribution function (PDF) analysis. The β and γ forms have polar structures built of different (1/∞)[AsSe₂] chain conformations, whereas the δ form has nonpolar packing. A wide range of direct band gaps are observed, depending on composition: namely, 1.11 eV for (α-LiAsSe₂, 1.60 eV for LiAsS₂, 1.75 eV for γ-NaAsSe₂, 2.23 eV for NaAsS₂. The AAsQ₂ materials are soluble in common solvents such as methanol, which makes them promising candidates for solution processing. Band structure calculations performed with the highly precise screenedexchange sX-LDA FLAPW method confirm the direct-gap nature and agree well with experiment. The polar y-NaAsSe₂ shows very large nonlinear optical (NLO) second harmonic generation (SHG) response in the wavelength range of 600-950 nm. The theoretical studies confirm the experimental results and show that y-NaAsSe₂ has the highest static SHG coefficient known to date, 337.9 pm/V, among materials with band gaps larger than 1.0 eV.