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 2p 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.