Simultaneous
regulation and control of non-linear second-harmonic-generation
(SHG) coefficients (d
eff) and linear birefringence
(Δn) of non-centrosymmetric (NCS) crystals
is a crucial aspect of improving the non-linear optical (NLO) performance,
yet it remains a huge challenge in modern laser techniques and science.
Herein, a series of melilite-type oxide-derived NCS oxychalcogenides,
Sr2MGe2OS6 (M = Mn, Zn, and Cd),
are successfully designed and synthesized through a facile partial
isovalent chemical substitution strategy. These isostructural compounds
crystallize in the tetragonal NCS space group P4̅21m (no. 113) and feature unique two-dimensional Cairo pentagonal
tiling layers composed of heteroligand [GeOS3] and tetrahedral
[MS4] asymmetric building units (ABUs). Compared to the
parent melilite-type oxides, the derived title compounds not only
successfully realize the phase matchability transformation [i.e.,
from non-phase-matching (NPM) to phase-matching (PM)] but also greatly
improve the IR-NLO performance (especially, simultaneously boosting d
eff and Δn). Sr2CdGe2OS6 exhibits the best comprehensive performance
among oxychalcogenides, including a wide PM cutoff edge (>525 nm),
a strong d
eff (0.8 × AgGaS2), a giant laser-induced damage threshold (19.2 × AgGaS2), a large band gap (3.62 eV), as well as a broad transmission
cutoff region (0.28–12.0 μm). Furthermore, highly distorted
[GeOS3] ABU in this melilite-type structure is proved to
be the profitable bifunctional NLO-active unit, which can contribute
to the search and design of novel IR-NLO crystals with a large Δn and a strong d
eff based on
the experimental results and theoretical calculations. This work demonstrates
the first examples of PM melilite-type oxychalcogenides and offers
new perspectives on the phase matchability transformation through
the partial isovalent chemical substitution approach, which will play
a constructive role in the future design of high-performance IR-NLO
heteroanionic materials.