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

The non-pi-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) (I) and (NH4)(2)Na3Li9(SO4)(7) (Pi), which consist of non-pi-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 Pi, 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-pi-conjugated [SO4](2-) anions are the dominate NLO-active groups, and the large NLO gap between I and Pi can be ascribed to the nonbonding O 2p orbitals of different orientations in the crystallographically independent S1O(4) groups. This work provides an innovative non-pi-conjugated source that is distinct from the traditional pi-conjugated ones for deep-UV NLO materials.