A scientifically valid test process to measure polycrystalline birefringence was proposed and confirmed as an effective method for screening anisotropic microcrystals.

Balancing the second-harmonic generation (SHG) coefficient, band gap, and birefringence is a vital but addressable challenge for designing infrared nonlinear optical materials. By applying a “rigidity–flexibility coupling” strategy, a quaternary diamond-like phosphide, Mg2In3Si2P7, with wurtzite-type superstructure was successfully designed and synthesized. Remarkably, it achieved the rare coexistence of giant second-harmonic generation (2 × ZnGeP2 and 7.1 × AgGaS2), suitable band gap (2.21 eV), moderate birefringence (0.107), and wide IR transparent range (0.56–16.4 μm). First-principles calculations revealed that the giant SHG response and large birefringence can be attributed to the synergy of arrangement-aligned [InP4] and [SiP4] tetrahedra. This work not only opens a new avenue for designing advanced infrared nonlinear optical materials but also may spur more explorations on quaternary diamond-like pnictides.

The first alkali-metal nitrate isocyanurates, A­(H3C3N3O3)­(NO3) (A = K, Rb), were synthesized by the tactic of introducing (NO3)− into isocyanurate with a mild hydrothermal technique. They crystallized into the same monoclinic centrosymmetric (CS) space group P21/c, which featured a 2D [(H3C3N3O3)­(NO3)]∞ layered structure separated by K+ and Rb+ cations, respectively. Both compounds exhibited short ultraviolet cutoff edges (λcutoff = 228 and 229 nm) and large birefringences (Δn = 0.253 and 0.224 at 546.1 nm). More importantly, in comparison with most of the isocyanurates and nitrates, they have better thermal stability with decomposition temperatures up to 319.8 and 324.4 °C. In addition, our theoretical calculations reveal that the π-conjugated groups play significant roles in improving the optical anisotropy. Remarkably, introducing a π-conjugated inorganic acid radical (NO3)− into isocyanurate is an extremely meaningful strategy to explore new UV birefringent crystals.

Birefringent materials with large optical anisotropy, which can be used to modulate the polarization of light, play a key role in laser techniques and science. However, the exploration studies of new, superior birefringent materials develop extremely slowly due to the lack of effective guidelines for rational design. Herein, three antimony(III) fluoride oxalates, namely, Na 2 Sb 2 (C 2 O 4 )F 6 , K 2 Sb 2 (C 2 O 4 )F 6 , and Cs 2 Sb 2 -(C 2 O 4 ) 2 F 4 •H 2 O, were successfully synthesized through a rational combination of π-conjugated C 2 O 4 2− anions and Sb 3+ cations with stereochemically active lone pairs. These oxalates feature unique quasi-one-dimensional chain structures that induce large optical anisotropy. Remarkably, Cs 2 Sb 2 (C 2 O 4 ) 2 -F 4 •H 2 O exhibits the largest birefringence (0.325@546 nm) among all reported antimony(III)-based oxysalts. Detailed structural analysis and theoretical calculations confirmed that the optical anisotropy of these oxalates could be tuned through the synergetic interactions of templated cations and anionic functional groups. This work may open the door to efficiently designing excellent birefringent materials and guide the further discovery of other novel structure-driven functional materials.

Two UV NLO phosphatesAMgPO4·6H2O (A = Rb, Cs) were synthesized by a soft hydrothermal method.

A family of noncentrosymmetric alkali-metal bismuth selenite fluorides, A 2 Bi 2 (SeO 3 ) 3 F 2 [A = K(1) and Rb(2)], has been synthesized through a mild hydrothermal method. These isostructural compounds all had onedimensional [Bi 2 O 9 F 2 ] chains, which were further connected with neighbouring chains through bridged selenite groups to construct the 3D framework. Because of the relatively ordered alignment of [BiO x F y ] and SeO 3 groups, they have large second-harmonic generation (SHG) responses of about 15.0(1) and 14.4(2) times that of KH 2 PO 4 (KDP) under 1064 nm laser radiation and 0.40(1) and 0.36(2) times that of AgGaS 2 (AGS) under 2.05 μm laser radiation, respectively. In addition, the SHG response of (1) was the largest among those of all reported metal selenites. In addition, all of them had high thermal stability (∼400 °C), wide energy band gaps (over 3.72 eV), and an excellent IR transparent region, suggesting that they had great potential as high power mid-infrared nonlinear optical crystals.

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The phosphates are employed to design ultraviolet nonlinear optical crystals generally. However, phosphates always suffer from too small birefringence to effectively achieve phase matching. Herein, the Sn2+ cation with stereochemical...