Deep ultraviolet (deep-UV) nonlinear optical(NLO) crystals are key crystalline materials for deep-UV laser output by frequency doubling effect, and have important applications in the field of optoelectronics. Traditional deep-UV NLO materials are mainly limited to p-conjugated systems such as borates.  

In a recent study published in J. Am. Chem. Soc., a research team led by Prof. LUO Junhua and Prof. ZHAO Sangen from Fujian Institute of Research on the Structure of Matter (FJIRSM), Chinese Academy of Sciences, obtained two anhydrous sulfates deep-UV NLO materials, namely NH₄NaLi₂(SO₄)₂ (Ι) and (NH₄)₂Na₃Li₉(SO₄)₇ (II).  

By a facile evaporation method, researchers successfully obtained a bulk Ι crystal with sizes of 25 × 3 × 2 mm³ by a facile evaporation method. As a result, the sulfate was confirmed to be transparent in the deep-UV region by the direct measurement on a single crystal.  

Besides, they found that Ι and II show a distinct gap with phase-matching second-harmonic generation (SHG) responses of 1.1 and 0.5 times that of the benchmark KH₂PO₄, respectively.  

The co-authors Prof. LIN Zheshuai et al from Technical Institute of Physics and Chemistry, Chinese Academy of Sciences performed theoretical calculations based on the first principles, and found that the distinct SHG gap between I and II is mainly aroused by the nonbonding O 2p orbitals of different orientations in the S1O₄ groups for I and II.  

Supposing the highly “NLO-active” [SO₄]^2- anions should be only about one third that of the overall ones in I, it is expected that there is still much room to boost the SHG responses for sulfates.  

This study indicates that non-p-conjugated sulfates represent an innovative source that is distinct from the traditional p-conjugated deep-UV NLO materials.