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 Li1 xNaxAsSe2 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.
ASJC Scopus subject areas
- Colloid and Surface Chemistry