From Cd(SCN)₂(CH₄N₂S)₂ to Cd(SCN)₂(C₄H₆N₂)₂: Controlling Sulfur Content in Thiocyanate Systems Significantly Improves the Overall Performance of UV Nonlinear Optical Materials

Abstract

Combining a strong second‐order nonlinear optical (NLO) effect (>1×KH2PO4 (KDP)), a large bandgap (>4.2 eV), and a moderate birefringence in ultraviolet (UV) NLO crystals remains a formidable challenge. Herein, Cd(SCN)2(C4H6N2)2, the first example of a thiocyanate capable of realizing a phase‐matched UV NLO crystal material, is obtained by reducing the sulfur (S) content in the centrosymmetric (CS) structure of Cd(SCN)2(CH4N2S)2. Compared to the "shoulder‐to‐shoulder" one‐dimensional (1D) chain of Cd(SCN)2(CH4N2S)2, Cd(SCN)2(C4H6N2)2 has a different sawtooth 1D chain structure. Cd(SCN)2(CH4N2S)2 has second harmonic generation (SHG) inertia with a bandgap of 3.90 eV and a UV cutoff edge of 342 nm, however, it possesses a large birefringence (0.35@546 nm). In contrast, the symmetry center breaking of Cd(SCN)2(C4H6N2)2 leads to remarkably strong SHG intensity (10 times that of KDP). Furthermore, it has a wide bandgap (4.74 eV), short UV cutoff edge (234 nm), and moderate birefringence capable of phase matching (0.17@546 nm). This research indicates that thiocyanates are a promising class of UV NLO crystal materials, and that modulation of the sulfur content of CS thiocyanates is an effective strategy for the development of UV NLO crystals with excellent overall performances.

From CaBaM2F12 to K2BaM2F12 (M = Zr, Hf): Heterovalent Cation-Substitution-Induced Symmetry Break and Nonlinear-Optical Activity

“…Nonlinear-optical (NLO) materials with short cutoff edges that can convert laser frequency to the ultraviolet (UV) and even deep-UV regions through a second-harmonic-generation (SHG) process play a vital role in laser precision machining, remote sensing, lithography, and other fields, so as to become research hotspot in the NLO realm. − Despite significant efforts being invested in the design and synthesis of short-wavelength NLO materials, further advancement is still needed for optimal performance. Crystallization into a noncentrosymmetric (NCS) space group is the foremost condition, and certain strategies have been implemented to enhance the probability of obtaining NCS structures. − Combining local asymmetric units during synthesis has been proven to be effective in producing new NCS materials. , Nevertheless, asymmetric building units may arrange antiparallel and result in centrosymmetric (CS) structures.…”

(C5H12N2S)Hg(NO3)2: A Mercury Nitrate Nonlinear Optical Crystal Motivated by Heteroatom Coordination Strategy

“…Crystals with second harmonic generation (SHG) effects in the field of nonlinear optics (NLO) hold significant importance, due to their ability to precisely manipulate light and generate coherent radiation at various wavelengths. This characteristic finds extensive applications in high-resolution microscopy imaging, precision manufacturing, lithography, and optical communication. − Consequently, the pursuit of frequency doubling crystals with exceptional performance has become a focal point for the scientific and application community. However, the commercial frequency doubling crystals are limited to minority examples, including AgGaS 2 (AGS), ZnGeP 2 (ZGP), LiB 3 O 5 (LBO), KH 2 PO 4 (KDP), KTiOPO 4 (KTP), KBe 2 BO 3 F 2 (KBBF), and β-BaB 2 O 4 (β-BBO). − Hence, primitive exploration of new frequency doubling crystal materials remains an urgent research task. − …”

“…As previously mentioned, substituting heteroatoms presents an excellent strategy for obtaining high-performance NLO crystals. We have found that replacing O with S atoms having high polarizability is particularly favorable for achieving larger effects in NLO oxysulfide crystals. , Additionally, controlling the sulfur content is also very important, because it contributes to structural transformation and enhances the SHG effect, which can be exemplified by Wu et al’s work (from Cd(SCN) 2 (CH 4 N 2 S) 2 to Cd(SCN) 2 (C 4 H 6 N 2 ) 2 ) . Besides, from the structural perspective, O and S show distinct electronegativity and bonding characteristics; hence, their combination can construct novel functional units, further resulting in interesting crystal structures and optical performances.…”