**7. Oxyhalides**

To develop high efficiency in mid-IR-NLO crystals, it is important to have an oxide-based system with an expandable IR transparent range. Many NLO oxides have a range from 3 to 5 μm atmospheric windows. When compared with the AgGaS2 (13 μm), the oxide-based materials, such as La3SnGa5O14 and Pb17O8Cl18, have wide IR transparency between 12 and 13.9 μm. In this lead oxyhalide, NLO crystal Pb17O8Cl18 plays an outstanding overall property. To obtain high LDT, halide and oxide-based crystals with high bandgaps are used. Nowadays, the mixture of heavy metal lone pair cation, Pb2+, and mixed oxyhalides are focused on IR applications. Xinglong Chen et al. discovered the lead mixed oxyhalides, such as Pb13O6Cl9Br5, Pb13O6Cl7Br7, and Pb13O6Cl4Br10 [73], which have broad IR transparency up to 14 μm, high SHG behavior (0.6–0.9 × AgGaS2) and wide bandgaps from 3.05 to 3.21 eV. Pb13O6Cl9Br5 single crystal has the size of 2.9 × 1.3 × 0.5 cm3 , which was grown using the top-seeded solution growth (TSSG) method. It has a wide transparent range from 0.384 to 14.0 *μ*m and a high LDT value (14.6 × AgGaS2). Many crystals, such as APbCO3F (A = Rb, Cs), Pb2BO3Cl, Cs3VO(O2)2CO3, Bi3TeBO9, and BiFSeO3 [74–78], have been reported with outstanding properties and it is a very good material for visible/near-IR nonlinear optical applications. Pb13O6Cl4Br10, Pb13O6Cl7Br7, and Pb13O6Cl9Br5 have orthorhombic crystal structures with space group *Fmm*2 (**Table 8**). These three crystals are isomorphic.

By using Czochralski and flux method, oxide-based crystals can be obtained. The Pb13O6Cl4Br10, Pb13O6Cl7Br7, and Pb13O6Cl9Br5 crystals were grown by flux method and the self-flux method was adopted for PbCl2-PbBr2. The crystals obtained by this method were optically transparent and they showed a good growth rate. Pb13O6Cl9Br5 was used to grow large-size crystals using the TSSG technique. After many technical optimizations, two big size crystals (Dimensions up to 2.9 × 1.3 × 0.5 and 3.7 × 0.4 × 0.7 cm3 ) were grown using the [001] and [100] oriented seeds. They have good transparency and good growth speed. Therefore, to get a better quality crystal the growth parameters (cooling rate, rotation speed, and temperature gradient) are very important in the crystallization process. DSC curves of Pb13O6Cl4Br10, Pb13O6Cl7Br7, and Pb13O6Cl9Br5 show that each of them has one endothermic peak at 501°C, 504°C, and 508°C, respectively, which belongs to the melting point and these crystals have two exothermic peaks, which indicate the decomposing of the compounds. Due to the volatility of the halide materials, there is no weight loss before 490°C in the TGA curves. From the DSC and TGA, it was concluded that these compounds have high thermal stability up to 490°C. Pb13O6Cl9Br5, Pb13O6Cl7Br7, and Pb13O6Cl4Br10 have high reflectance wavelengths in the region between 500 and 2500 nm. When compared with AgGaS2 (2.67 eV, 0.53–13 μm) and ZnGeP2 (1.68 eV, 0.74–12 μm), the crystals of Pb13O6Cl4Br10, Pb13O6Cl7Br7, and Pb13O6Cl9Br5 own greater energy gaps and good transparency in IR region [79, 80].


**Table 8.** *Space group and SHG value of oxyhalides.*

In the application of high-power laser systems, the NLO crystal with laser-induced damage is one of the biggest problems. The crystals which have wider bandgaps are subjected to higher LDTs. Polycrystalline samples are used for the laser-induced damage threshold evaluation, and the polycrystalline material of AgGaS2 was used as reference material. The Pb13O6Cl4Br10, Pb13O6Cl7Br7, and Pb13O6Cl9Br5 have large LDTs values, which are 3.0, 3.2, and 4.0 times higher than that of AgGaS2. The Pb13O6Cl9Br5 crystal has LDT of 439 MW/cm2, this is 14.6 times higher than that of the AgGaS2 crystal (30 MW/cm<sup>2</sup> ). The second harmonic generation intensity is 0.5 times higher than that of AgGaS2. The SHG value of Pb13O6Cl4Br10, Pb13O6Cl7Br7, and Pb13O6Cl9Br5 are found to be 0.6, 0.8, and 0.9, respectively. From this, we concluded that SHG responses of each material have all phase-matchable when it is under the 2090 nm wavelength. Due to the increase in particle size, it showed a positive movement in SHG response.
