Heteroanionic Melilite Oxysulfide: A Promising Infrared Nonlinear Optical Candidate with a Strong Second-Harmonic Generation Response, Sufficient Birefringence, and Wide Bandgap

The
achievement of balanced performance with a strong second-harmonic
generation (SHG) response, proper birefringence, and wide band gap
concurrently is a crucial but challenging task in infrared nonlinear
optical (IR-NLO) crystals. Here, we theoretically confirmed that the
heteroanionic oxysulfide tetrahedron would produce improved polarizability
anisotropy and quadratic hyperpolarizability compared with the monoanionic
oxide or sulfide tetrahedra. When this anion-mixing strategy was applied,
melilite oxysulfide with the four representative members A2GeGa2OS6 (A = Ca, Sr) and Sr2MGe2OS6 (M = Zn, Cd) was successfully discovered as
a promising IR-NLO material system. Remarkably, compared with the
monoanionic melilite oxides, these compounds exhibited unbiased performances
of proper birefringences (0.106–0.143), strong SHG responses
(>10× melilite oxide Ba2CdGe2O7, 1.3–2.1× AgGaS2 (AGS) @1570 nm) with phase-matchable
ability, wide band gaps (2.95–3.15 eV), and large laser-induced
damage thresholds (LIDTs, 5.6–13.4× AGS). The high structure
tolerance of melilite offers a great possibility to improve the SHG
response via tuning the orbital composition and hybridization near
band edges. This work provides an effective approach for the design
of high-performance IR-NLO materials.