**6. Halides**

A halide has a binary form in which one region is a halogen and the other region is a component or radical that is less electronegative/electropositive than the halogen [65]. For preparing NLO halides, the physical, chemical, and crystallographic aspects are important. In halides, hyperpolarizabilities were used to find out the NLO susceptibilities. Moreover, these crystals have high LDT and good mechanical properties. HgBr2 crystal was grown using a lowering temperature technique with a size of about 15 × 15 × 1.5 mm3 [66]. Tl3PbBr5, Tl4PbI6, Tl4HgI6, and Tl3PbI5 were grown by the vertical Bridgman method [5]. The 3:1:5 ratio showed orthorhombic symmetry and the 4:1:6 ratio indicated tetragonal symmetry. In this method, they maintained the temperature gradients in the range from 20 K/cm to 30 K/cm and cooling rates were in the range from 5 to 10 K/h, which showed a growth of 1 cm/day and 3–5 cm/day. Tl3PbCl5 [67] and Tl3PbBr5 [68] single crystals were grown by Bridgman-Stockbarger technique. Tl3PbI5 is colorless and Tl3PbBr5 (**Figure 6**) crystal is a yellow color and transparent. Tl4HgI6 crystal is grown using Bridgman-Stockbarger method [69, 70] and they melt consistently at 396°C. The crystal was red and when the iodine concentration increases in the stoichiometric ratio, the crystal becomes changed to black. It belongs to the point symmetry group C4v. BaMgF4 crystal was grown using the Czochralski technique [71]. It belongs to the pyroelectric fluoride group BaMF4 (M = Mg, Co, Ni, Zn) and it has space group Cmc21. SrAlF5 crystal belongs to the class of uniaxial ferroelectric [72] and is grown using Czochralski technique [71]. Tl3PbCl5 and Tl3PbBr5 are nonhygroscopic. In Tl3PbBr5, the phase transition is observed at ~237°C, and for Tl3PbCl5, phase transition is noted at 171°C. Tl2HgI4 compound has a melting point temperature of 318°C.

HgBr2 crystal showed good phase matchable SHG efficiency, which is 10 times greater than that of KDP and has transparency between 2.5 and 25 mm. It covers the whole mid-IR range. Tl3PbCl5 and Tl3PbBr5 have transparency of 0.5–20 mm and 0.65–24 mm, respectively. The Tl4HgI6 crystal is optically positive and transparent vfrom 1.2 to 40 mm. BaMgF4 is a ferroelectric fluoride due to its wide transparency between 125 nm and 13 mm, and it can be used for UV and mid-IR optical applications [71, 72]. In the UV region, the shortest band is noted at 368 nm, which represents the potential behavior of BaMgF4 as a nonlinear material. It can be used for the production of all solid-state lasers and mid-IR wavelength areas. BaMgF4 and SrAlF5 crystals are promising crystals for solid-state lasers. The LDT value of the HgBr2 crystal is 0.3 GW/cm2 .

**Figure 6.** *Crystal of Tl3PbBr5.*
