KCs2 [Pb2 Br5 (HCOO)2 ]: A Polar 3D Lead-Bromide Framework Exhibiting Strong Second-Harmonic Generation Response.

The discovery of new nonlinear optical (NLO) crystals with excellent properties is in urgently demand because of their ability to generate coherent light. Herein, we report an unique NLO lead bromide formate, KCs2 [Pb2 Br5 (HCOO)2 ], which has been synthesized by a mix-solvothermal method. KCs2 [Pb2 Br5 (HCOO)2 ] exhibits strong phase-matching second-harmonic generation (SHG) response (6.5×KDP), large birefringence (0.16@ 1064 nm), and a wide transparent window in most visible light and mid-IR region. Interestingly, KCs2 [Pb2 Br5 (HCOO)2 ] features a polar 3D lead-bromide framework in which adjacent Pb-Br layers containing coplanar Pb6 Br6 rings are not only parallel to each other, but also orient in the same direction. These oriented arrangements are responsible for the strong SHG response and large birefringence that are elucidated by both local dipole moment and theoretical calculations. This research provides a new strategy to explore subsequent NLO crystals.

Two Lead Halides with Strong SHG Response Obtained by the Isovalent Substitutions of Alkali Metal Cation and Halogen Anion.

The substitution of alkali metal cation or halogen anion based on nonlinear crystals is an effective strategy to exploit new optical materials. The strategy has been successfully expanded to discover two new lead halides, Rb3Pb2(CH3COO)2X5 (X = Br, Cl). The substitution of the Cs+ cation with a Rb+ cation can not only increase the local dipole moment of the distorted [PbBr4O2] polyhedron but also reduce the cell unit, resulting in a large net macroscopic polarization. Therefore, Rb3Pb2(CH3COO)2Br5 possesses a strong second-harmonic generation (SHG) response (6 × KDP) and a large birefringence (0.18@1064 nm). Furthermore, by the substitution of the Br- anion with a Cl- anion, Rb3Pb2(CH3COO)2Cl5 exhibits a high laser damage threshold (LDT, 84 × AgGaS2) and a short UV cutoff edge of 287 nm, as well as moderate SHG response (3 × KDP) and birefringence (0.11@1064 nm). Detailed theory calculations elucidate the origin of the linear and nonlinear optical properties of these compounds.

Titanium Iodates with a Second-Harmonic-Generation Response.

Nonlinear-optical (NLO) crystal is an important photoelectric functional material. In this work, a new NLO titanium iodate, (H3O)2Ti(IO3)6, along with Ti(IO3)4 has been synthesized under facile conditions. The space group of (H3O)2Ti(IO3)6 is the chiral noncentrosymmetric group R3 (No. 146), with an interesting three-dimensional framework, while that of Ti(IO3)4 is the centrosymmetric space group P1̅ (No. 2) containing one-dimensional chains. Thermogravimetric analysis shows that (H3O)2Ti(IO3)6 and Ti(IO3)4 have no weight loss below 220 and 390 °C, respectively. In addition, (H3O)2Ti(IO3)6 not only is thermally stable up to 200 °C in an air atmosphere but also is stable in water. (H3O)2Ti(IO3)6 has a moderate-intensity second-harmonic-generation (SHG) response (1.4×KDP), a large laser-induced damage threshold (46×AgGaS2), and high transmittance in the wavelength ranges of 0.5-1.4 and 2.5-10 µm. Both local dipole moment and systematic theoretical calculations reveal that the SHG response of (H3O)2Ti(IO3)6 is mainly because of the combined effect of [TiO6] octahedra and IO3 and IO4 units. In a word, (H3O)2Ti(IO3)6 exhibits good NLO performances, as well as water resistance and facile growth of a single crystal with high quality, indicating its potential application as NLO materials in the visible and mid-IR regions, especially the visible region.

Cs3 Pb2 (CH3 COO)2 X5 (X=I, Br): Halides with Strong Second-Harmonic Generation Response Induced by Acetate Groups.

Nonlinear optical (NLO) crystals are the key component in solid-state lasers. New second-harmonic generation (SHG) effective halides, Cs3 Pb2 (CH3 COO)2 X5 (X=I, Br), have been rationally synthesized by introducing acetate groups into lead halide perovskites under moderate solvothermal conditions. Cs3 Pb2 (CH3 COO)2 X5 (X=I, Br) feature highly oriented anionic chains constructed by distorted [PbX4 O2 ] (X=I, Br) polyhedrons. Both the theoretical studies and the dipole moment calculations uncover that their SHG effects are mainly originated from the distorted [PbX4 O2 ] (X=I, Br) polyhedrons. Remarkably, Cs3 Pb2 (CH3 COO)2 I5 exhibits strong phase-matching SHG response (8×KDP) and large birefringence (0.26@ 1064 nm). Moreover, Cs3 Pb2 (CH3 COO)2 X5 (X=I, Br) also possess high laser damage threshold (LDT) which are almost 27 and 41 times that of AgGaS2 , respectively.

Boosting Zn-Ion Energy Storage Capability of Hierarchically Porous Carbon by Promoting Chemical Adsorption.

The construction of advanced Zn-ion hybrid supercapacitors (ZHSCs) with high energy density is promising but still challenging, especially at high current densities. In this work, a high-energy and ultrastable aqueous ZHSC is demonstrated by introducing N dopants into a hierarchically porous carbon cathode for the purpose of enhancing its chemical adsorption of Zn ions. Experimental results and theoretical simulations reveal that N doping not only significantly facilitates the chemical adsorption process of Zn ions, but also greatly increases its conductivity, surface wettability, and active sites. Consequently, the as-fabricated aqueous ZHSC based on this N-doped porous carbon cathode displays an exceptionally high energy density of 107.3 Wh kg-1 at a high current density of 4.2 A g-1 , a superb power density of 24.9 kW kg-1 , and an ultralong-term lifespan (99.7% retention after 20 000 cycles), substantially superior to state-of-the-art ZHSCs. Particularly, such a cathode also leads to a quasi-solid-state device with satisfactory energy storage performance, delivering a remarkable energy density of 91.8 Wh kg-1 . The boosted energy storage strategy by tuning the chemical adsorption capability is also applicable to other carbon materials.