*3.1.2 Ternary chalcogenides*

When selecting a crystal for laser energy conversion in the IR, it is essential to have an ideal mixture of various considerations like birefringence value not less than 0.03 and LDT value of around 100 Mcm−2, energy gap value of more than 3.3 eV and the NLO coefficient should be more than 4 pm V−1 [20]. The Li- and Ba-having chalcogenides meet these desires. The ZnGeP2, AgGaSe2, CdSiP2, and AgGaTe2 crystals have high NLO susceptibility, but their forbidden energy band is too low. The most commonly used nonlinear crystals for the MIR are AgGaS2 AgGaSe2, and ZnGeP2. However, they all own serious disadvantages [6, 21, 22]. In recent times, consideration was given to chalcogenide crystals, such as alkali and alkali-earth metals (Li and Ba) (**Table 1**). These crystals permit one to resolve some difficulties in the MIR region. The birefringence value in Li-comprising crystals is significantly larger. LiBC2 (B = Ga; C = S,Se) crystals can be applied for SHG applications wavelengths between 1.4 and 12 μm [23, 24]. Telluride crystals also have MIR properties, especially LiGaTe2 has phase-matching in the entire transparency region. The SHG conversion efficiency of LiGaTe2 is 10.6 μm, which is higher than that of AGSe [25]. To enhance the energy gap, Ag cation is to be substituted with alkali/alkaline earth metal (Li, Ba). Adding


#### **Table 1.**

*Point group, transparency range, band gap, and LDT value of ternary chalcogenides.*

*Recent Advances in Infrared Nonlinear Optical Crystal DOI: http://dx.doi.org/10.5772/intechopen.108173*

**Figure 1.** *Single crystals of BaGa4S7.*

these metal, we can get LiBC2 (B = In, Ga; C = S, Se, Te) and BaGa4C7 (C = S, Se) group crystals. These crystals own a high bandgap value [26–28]. A little mass of Li is the reason for high thermal conductivity and high vibrational frequencies. The thermal conductivity of Li mixtures is around five times higher than that of AGS (Se) [29] and four to eight times more than that of BaGa4S7 (Se) (**Figure 1**) [6, 28]. The laser damage threshold for LGS is 3.5 Jcm−2, which is five times larger than LISe [30]. A similar result has been obtained when the Ag ion is substituted with Ba. BaGa4S7 crystal has high LDT and NLO susceptibility. Though, Ba cation slightly drops the band gap value. The point group is mm2 and m for BaGa4S7 (**Figure 1**) and BaGa4Se7, respectively. Wide transparency regions of 0.35–12 and 0.47–15.0 μm, and energy gaps of 3.54 and 2.64 eV were found for both BaGa4S7 and BaGa4Se7 crystals, respectively. Both crystals have strong absorption peak at 15 μm. The BaGa4Se7 showed high nonlinear susceptibility of d11 = 18.2 pm V−1 [6, 31, 32].

## *3.1.3 Quaternary chalcogenides*

Quaternary chalcogenides have four kinds of ions together with a chalcogen anion. These kinds of materials have different applications, such as MIR-NLO, solar cell absorbers, photocatalysts, and so on. Quaternary materials adopt different kinds of elements, which permits comparatively complex structural, electronic, and optical properties [6, 33]. Using the quaternary crystals, the IR-NLO parameters can be enhanced with a high content of NLO-active parts [6]. A compact organization of the microscopic NLO-active parts increases high macroscopic NLO outcomes [6]. The birefringence value of AgGaSe2 is 0.05 and this value for AgGaGe3Se8 is 0.11. The enhanced LDT value of AgGaGeS4 shows that it is a potential alternative crystal to the generally used AgGaS2 for IR-NLO applications. Li2In2SiSe6, Li2In2GeSe6, Li2Ga2GeS6, and LiGaGe2Se6 are the Li-having quaternary chalcogenides crystals (**Table 2**) [6, 34–37]. All these crystals have non-centrosymmetric crystal structures. Li2Ga2GeS6 (**Figure 2**), LiGaGe2Se6 (**Figure 3**) crystals have orthorhombic crystal systems with space group Fdd2 and Li2In2GeSe6, Li2In2SiSe6 crystals own monoclinic crystal systems with space group Cc. Ba-having quaternary BaGa2GeS6, BaGa2GeSe6 structures, which are promising crystals for NLO applications [37]. The NLO susceptibilities of Li2Ga2GeS6 are 16 pm/V, which is significantly higher than LiGaS2 (5.8 pm/V). A similar result was noted for LiGaGe2Se6. BaGa2GeS6 and BaGa2GeSe6 crystals also have improved NLO parameters. The SHG experiments showed that both materials have phase-matched behavior. The calculated SHG coefficient is ∼2.1 and

#### *Crystal Growth and Chirality - Technologies and Applications*


**Table 2.**

*Point group, transparency range, band gap, and LDT value of quaternary chalcogenides.*

**Figure 2.** *Crystal of LiGaGe2Se6.*

**Figure 3.** *Crystal structure of LiGaGe2Se6.*

*Recent Advances in Infrared Nonlinear Optical Crystal DOI: http://dx.doi.org/10.5772/intechopen.108173*

∼3.5 times higher than that of AgGaS2. The nonlinear susceptibilities are 26.3 pm/V and 43.7 pm/V for BaGa2GeS6 and BaGa2GeSe6, respectively [37]. The transparency region in Li2Ga2GeS6 and LiGaGe2Se6 is 0.35–12 and 0.37–14 μm, respectively. For BaGa2GeS6 and BaGa2GeSe6, the transparency regions are 0.380–13.7 μm and 0.44–18 μm, respectively. Band gaps of BaGa2GeS6 and BaGa2GeSe6 are 3.26 and 2.81 eV, respectively. Li2In2GeSe6 and Li2In2SiSe6 crystals have the energy gap values of 2.30 and 3.61 eV, respectively [6].
