**4. Chalcohalides**

Chalcohalides could be considered potential materials for mid-IR applications. The chalcohalides contain a combination of sulfur and halogen. Chalcohalides have non-centrosymmetric coordinated surroundings that can stimulate mid-IR-NLO efficiency. Based on the chemical compounds present in the chalcohalides, it can be divided into [4]:


#### **4.1 Alkali/alkaline-earth metal chalcohalides**

The Ba4Ge3S9Cl2 chalcohalide has excellent mid-IR-NLO properties [38]. It has a space group P63. The bandgap energy of Ba4Ge3S9Cl2 was 2.91 eV. The SHG response of Ba4Ge3S9Cl2 was 2.4 times higher than that of AGS [38]. The bandgap of Ba4Ge3S9Cl2 is 2.67 eV, which was calculated by the DFT method. NLO coefficients are found to be d15 = d24 = 7.61 and d33 = 13.81 pm/V. The bandgap of NaBa4Ge3S10Cl was 3.49 eV and SHG efficiency was 0.3 times that of AgGaS2 (AGS) [39]. The theoretically calculated bandgap of this crystal is 2.94 eV. Then, the NLO coefficients are calculated to be d15 = d24 = 3.89 and d33 = 9.32 pm/V. The mid-IR-NLO crystals [A3X] [Ga3PS8] (A = K, Rb; X = Cl, Br) were synthesized and reported by B.W. Liu et al. in the year 2016 [40]. Two Cl-crystals [K3Cl][Ga3PS8] and [Rb3Cl][Ga3PS8] are having isostructural with space group Pmn21, while the other two Br-crystals [Rb3Br][Ga3PS8] and [K3Br][Ga3PS8] belong to Pm space group. These four compounds showed outstanding mid-IR-NLO behavior and the energy gaps of [K3Cl][Ga3PS8], [Rb3Cl][Ga3PS8], [K3Br][Ga3PS8], and [Rb3Br][Ga3-PS8] are 3.60, 3.65, 3.85, and 3.50 eV, respectively. All four compounds showed large SHG responses of 4.0, 5.0, 7.0, and 9.0 times that of AgGaS2 (AGS) at 1064 nm [39]. Moreover, these four compounds have higher laser threshold damage (LDT) of 37, 35, 31, and 29 times than that of AGS (**Table 3**).


#### **Table 3.**

*Space group, band gap, and LDT value of alkali/alkaline-earth metal chalcohalides.*


#### **Table 4.**

*Space group, band gap, SHG, and transparency range value of adduct-type chalcohalides.*

#### **4.2 Adduct-type chalcohalides**

The (SbI3) (S8)3 and (SnI4)-(S8)2 are the adduct-type of chalcohalides. The (TI3). (S8)3 (T = As, Sb) [4, 41–44] adduct-type chalcohalides have isostructural with R3m space group (**Table 4**). The (SbI3). (S8)3 and (AsI3). (S8)3 chalcohalides have reasonable energy gap value of 2.52 and 2.31 eV and their theoretical energy gaps are 2.69 and 2.21 eV, respectively. SHG responses of these two chalcohalides have 1.0 and 0.8 times that of AgGaS2. They also have wide IR transparent wavelengths around 2.5–25.0 μm and 0.4–25.0 μm [41, 42]. The calculated NLO coefficients are d15 = 9.21, d22 = 9.22, d33 = 6.91 pm/V for (SbI3).(S8)3 and d15 = 3.40, d22 = 6.21, d33 = 0.73 pm/V for (AsI3).(S8)3. The chalcohalides (SnI4). (S8)2 has space group Fdd2 [44] and a wide IR window in the range of 2.5–25.0 μm. The energy gap of (SnI4). (S8)2 is 2.17 eV. The SHG response of (SnI4). (S8)2 is 0.5 times stronger than that of AgGaS2 at 2.1 μm.

#### **4.3 Lewis acid adduct chalcohalides**

Lewis acid adduct chalcohalides have sulfur-nitrogen rings with a variety of structures. These chalcohalides have moderate band gaps and IR-NLO properties (**Table 5**). A new chalcohalide (NSF)4 [45] has a space group of P-421c. The calculated energy gaps are 4.57 eV (HSE06 method) and 3.58 eV (GGA method). For a high LDT, the large bandgap is more advantageous. The birefringence for (NSF)4 is 0.220 at 1064 nm and the calculated NLO coefficient for (NSF)4 is d14 = 3.20 pm/V. Increased bandgap occurs due to the large electronegativity of F atoms in the *Recent Advances in Infrared Nonlinear Optical Crystal DOI: http://dx.doi.org/10.5772/intechopen.108173*


**Table 5.**

*Space group and band gap, and value of Lewis acid adduct chalcohalides.*

compound. S3N5PF2 crystal [46] has an R3m space group. It has a wide bandgap of 3.49 eV (HSE06). The estimated birefringence is around 0.110 at 1064 nm and NLO coefficients are d15 = 1.71, d21 = 0.19 and d33 = 3.69 pm/V. The d33 coefficient is around 9.2 times stronger than that of KDP.

#### **4.4 Main group element clusters chalcohalides**

The chalcohalides (Bi4S4)(AlCl4)4 belong to the space group I-4 (**Figure 4**) [47] and it has two classes of main group component clusters that are in AlCl4 tetrahedron and Bi4S4 cube. The energy gap of (Bi4S4)(AlCl4)4 is calculated to be 3.59 eV. (Bi4S4) (AlCl4)4 is the largest main group element cluster chalcohalide, which has an NLO coefficient value of d14 = 1.52 pm/V and it is 3.8 times stronger than that of KDP. The DFT method showed that the main group component cluster chalcohalides has insignificant NLO coefficients.

#### **4.5 Other main groups of metal chalcohalides**

Both In5S5Cl [48] and In5S5Br [49] have isostructural properties, which are having the space groups Pmn21. The In5S5Cl and In5S5Br materials have band gap of 1.76 and 1.84 eV, respectively (**Table 6**). The estimated NLO coefficients are found to be d15 = 0.36, d24 = 2.83, d33 = 13.38 pm/V for In5S5Cl and d15 = 2.07, d24 = 2.21, d33 = 7.38 pm/V for In5S5Br. When compared with the NLO coefficients of AGS, the main group metal chalcohalides In5S5Cl and In5S5Br have d33=1.0 and 0.5 times than that of AGS. The (CS3N2Br)Br3 main group metal chalcohalides [50] crystallizes in space group Pna21. The estimated energy gap for (CS3N2Br)Br3 is 2.21 eV (HSE06), and the NLO coefficients of (CS3N2Br)Br3 are d15 = 8.90, d24 = 5.40, d33 = 1.00 pm/V. From the NLO coefficient result, d15 has a good NLO coefficient, which is about 0.6 times that of AGS.

**Figure 4.** *Crystal structure of (Bi4S4)(AlCl4)4.*


**Table 6.**

*Space group, band gap, and SHG value of other main groups of metal chalcohalides.*

#### **4.6 Transition metal chalcohalides**

Asymmetric distribution of electron clouds is generally caused by polyhedron with the d10 transition metals because of the dp orbital hybridization and group distortion, so it produces high SHG behavior. Due to the dp hybridization, the compound has a red-shifted absorption edge, this might reduce the bandgap of IR-NLO materials. To enhance the chalcogenides energy gap, halogen elements are introduced, that is, by combining cations with d10 configuration, which led to the equilibrium among energy gaps and SHG behavior in the IR-NLO crystals. So, they separated the asymmetric chalcohalides with a d10 electronic configuration. The transition metal chalcohalide Ag2HgSI2 has the space group Cmc21 [51]. The bandgap of Ag2HgSI2 is 2.65 eV and it is compared with AGS of (2.70 eV). The SHG of Ag2HgSI2 is 4.2 times stronger than that of KDP. The theoretical birefringence of Ag2HgSI2 is 0.210 at 1064 nm. The crystal (P4S3)3(CuCl)7 belongs to the space group P31c [52]. The bandgap of (P4S3)3(CuCl)7 is 2.77 eV (HSE06 method) and NLO coefficients of (P4S3)3(CuCl)7 are d33 = 3.34 and d15 = d24 = 1.81 pm/V. In the NLO coefficients, the d33 value is about 8.5 times that of KDP and has a birefringence value of 0.150 at 1064 nm.
