Two Non-π-Conjugated Deep-UV Nonlinear Optical Sulfates

The non-π-conjugated sulfate system has long been overlooked as potential deep-UV nonlinear optical (NLO) materials. Here we report two asymmetric anhydrous sulfates, namely, NH₄NaLi₂(SO₄)₂ (Ι) and (NH₄)₂Na₃Li₉(SO₄)₇ (Π), which consist of non-π-conjugated [SO₄]²⁻ anions. Their single crystals can be readily grown by a facile evaporation method from water solution. Both sulfates are transparent down to the deep-UV region. Interestingly, there is a large NLO gap between I and Π, with phase-matching NLO responses of 1.1 and 0.5 times that of the benchmark KH₂PO₄, respectively. The first-principles studies reveal that the non-π-conjugated [SO₄]²⁻ anions are the dominate NLO-active groups, and the large NLO gap between I and Π can be ascribed to the nonbonding O 2 p orbitals of different orientations in the crystallographically independent S1O₄ groups. This work provides an innovative non-π-conjugated source that is distinct from the traditional π-conjugated ones for deep-UV NLO materials.