Influence of Strontium Doping on The Growth, Structural, Optical, And Nonlinear Optical Properties of Zinc Thiourea Chloride Crystals

Nonlinear optical (NLO) materials are the Pillar of modern photonics and optoelectronics, enabling the manipulation and control of light through nonlinear interactions with electromagnetic waves. These materials find extensive applications in laser technology, optical signal processing, frequency conversion, telecommunications, and data storage [1-3]. The discovery of materials exhibiting second harmonic generation (SHG) efficiency and related nonlinear optical properties has revolutionized the field of photonics, driving the development of advanced devices for various applications [4]. Among these, thiourea-based materials have garnered significant attention owing to their desirable properties, including high optical transparency, good mechanical strength, low dielectric constant, and substantial nonlinear optical coefficients. Zinc thiourea chloride (ZTC), a well-established member of this family, has emerged as a prominent candidate for NLO applications. ZTC exhibits remarkable SHG efficiency, surpassing that of urea, and offers high optical transparency in the visible spectrum, which is crucial for photonic applications [5-8]. Despite its promising features, ZTC is not without limitations, particularly in its moderate thermal stability and optical transparency in specific wavelength regions [9]. Consequently, researchers have turned to doping strategies to enhance the intrinsic properties of ZTC crystals, expanding their utility in high-performance photonic devices.  Doping, the deliberate introduction of foreign ions into a crystal lattice, has been widely explored to tailor the structural, optical, and thermal properties of NLO materials. This process induces structural modifications that alter electronic transitions, improve optical properties, and enhance thermal stability, thereby optimizing the material for specific applications [14-18]. Among the various dopants studied, alkaline earth metals have shown great potential for enhancing the performance of NLO crystals. Strontium (Sr), a divalent alkaline earth metal, is particularly notable for its ability to improve crystalline, modify optical behaviour, and enhance nonlinear properties of host materials. However, research on Sr-doped ZTC crystals remains sparse, leaving significant gaps in understanding the influence of Sr on the structural, optical, and nonlinear optical properties of ZTC. Addressing this gap is critical for advancing the development of thiourea-based materials and their applications in photonic technologies [19].  In this study, pure ZTC and Sr-doped ZTC crystals were grown using the slow evaporation solution growth method at a constant temperature of 35°C. The Sr doping concentration was maintained at 2 mol%, aimed at systematically exploring its impact on the properties of ZTC crystals. Structural analysis was performed using powder X-ray diffraction (PXRD) to determine the lattice parameters, crystallite size, and any structural modifications induced by doping. Optical transparency and band gap analysis were conducted using UV-Visible spectroscopy, while Fourier Transform Infrared (FT-IR) spectroscopy was employed to identify functional groups and analyze bonding characteristics. The nonlinear optical performance was evaluated using the Kurtz and Perry powder technique, with SHG efficiency measured using an ND-YAG laser operating at a wavelength of 1064 nm in Q-switched mode. The results provide valuable insights into the structural, optical, and nonlinear properties of Sr-doped ZTC crystals, contributing to the growing body of knowledge on advanced NLO materials for photonic and optoelectronic applications. 

1. Experimental

Zinc thiourea chloride (ZTC) salt was prepared by combining zinc chloride (ZnCl?) and thiourea [CS(NH?) ?] in a 1:2 stoichiometric ratio. Precisely measured quantities of zinc chloride and thiourea were dissolved in deionized water to create a pure ZTC solution. To this solution, 0.1 M strontium chloride (SrCl?) was added at a concentration of 2 mol%. The mixture was continuously stirred for around 2 hours using a magnetic stirrer at a constant speed to ensure uniform mixing and complete dissolution of the dopant. The resulting solution was filtered through a 0.45 µm membrane filter to remove any impurities. The filtered solution was poured into clean, pre-rinsed glass beakers and placed in a stable, temperature-controlled bath maintained at 35°C. The crystallization process was carried out using the slow evaporation technique under controlled conditions. Over three weeks, the solution gradually evaporated, leading to the formation of single crystals of both pure and Sr-doped ZTC. The crystals were further purified through repeated crystallization to enhance their quality.

Figure 1. Photograph of Sr-doped ZTC single crystal

The grown single crystals displayed excellent transparency and sizes suitable for further analysis. This carefully executed process ensured the synthesis of high-quality pure and Sr-doped ZTC crystals, enabling comprehensive structural, optical, and nonlinear optical studies. An image of the obtained single crystal is presented in Figure 1.

RESULTS AND DISCUSSION

3.1 Structural Analysis (XRD)

The structural characteristics of Sr-doped zinc thiourea chloride (ZTC) crystals were analyzed using powder X-ray diffraction (XRD), as illustrated in Fig. 2. The diffraction patterns of Sr-doped ZTC crystals exhibited sharper and more intense peaks compared to those of pure ZTC crystals, indicating improved crystalline resulting from the integration of Sr ions into the crystal lattice. The distinct peaks at specific 2θ values verified the high-quality crystalline structure of the grown crystals, with Sr doping enhancing structural regularity.

Figure 2. PXRD pattern of Sr-doped ZTC single crystal

XRD analysis confirmed that both pure and Sr-doped ZTC crystals adopted an orthorhombic structure with P-lattice symmetry. The unit cell dimensions for pure ZTC crystals were determined to be a = 13.014 Å, b = 12.772 Å, c = 5.893 Å, while for Sr-doped ZTC crystals, the dimensions showed slight alterations to a = 13.017 Å, b = 12.764 Å, c = 5.898 Å. These subtle changes in lattice parameters affirmed the successful incorporation of Sr ions into the ZTC lattice without causing significant structural distortion. The enhanced crystallinity and minor lattice modifications observed in Sr-doped ZTC crystals underscore the positive impact of Sr doping. This structural improvement makes Sr-doped ZTC crystals highly suitable for advanced photonic and nonlinear optical applications [20].

3.2 Functional Group Study (FTIR)

The FT-IR spectra of pure and Sr-doped zinc thiourea chloride (ZTC) crystals, recorded Fig. 3 in the range of 500–4500 cm?¹, reveal significant vibrational modes and functional group characteristics. Key peaks observed at 3873 cm?¹ and 3537 cm?¹ correspond to N–H stretching vibrations, indicating hydrogen bonding, with shifts suggesting structural changes upon Sr doping. Peaks at 3309 cm?¹ and 3171 cm?¹ represent O–H stretching vibrations, showing alterations due to Sr incorporation. The band at 1874 cm?¹ is associated with C=S stretching, and the shift observed in Sr-doped crystals suggests modifications in sulfur coordination.

Figure 3. FTIR spectrum of Sr-doped ZTC single crystal

The 1369 cm?¹ peak corresponds to C–N stretching, reflecting interactions between the thiourea group and Sr ions, while the 1102 cm?¹ band is attributed to Zn–O stretching, with Sr doping causing shifts and new vibrational modes [21]. Peaks at 863 cm?¹ and 695 cm?¹ are linked to C=S bending and skeletal deformations, respectively, further indicating structural changes. These results confirm that Sr doping influences the bonding and structural integrity of ZTC crystals, enhancing their potential for photonic applications.

3.3 Optical Study (UV-Vis-NIR)

Optical analysis is essential for understanding the energy band structure and bonding characteristics of materials. Internal transmittance refers to energy loss due to absorption, while total transmittance includes absorption, scattering, and reflection. The optical spectrum helps in examining electronic transitions, where specific energy states or functional chromophores absorb optical energy. The transparency of the material, a key optical property, was studied in the range of 200–1000 nm.

Figure 4. UV-Vis-NIR spectrum of Pure and Sr-doped ZTC single crystal

The results showed that the 2 mol% Sr-doped ZTC crystal exhibited higher transparency than the pure ZTC crystal, indicating that Sr doping enhances optical clarity and may influence the crystal's energy band structure [22]. This improvement in transparency is attributed to changes in electronic transitions and reduced absorption at specific wavelengths due to Sr incorporation. For pure ZTC, transparency was 38% at a cutoff wavelength of 230 nm, and it decreased as the wavelength increased. In contrast, Sr-doped ZTC exhibited 40% transparency at the cutoff wavelength and also showed a decrease in transparency at higher wavelengths, as shown in Fig. 4. This decline in transparency with increasing wavelength is due to the increased absorption at longer wavelengths.

3.4 Second Harmonic Generation (SHG) Study

The second-order optical properties, specifically second harmonic generation (SHG), of the synthesized crystals were assessed using the powder SHG technique introduced by Kurtz and Perry [23]. A Q-switched ND-YAG laser with a wavelength of 1064 nm was employed to excite the crystals. The crystal was powdered and tightly packed into a sample holder with a quartz cavity, followed by irradiation with the ND-YAG laser [24]. The SHG output was collected via an optical fiber connected to a spectrophotometer for analysis.

Figure 5. Intensity Dependent SHG Response of Pure and Sr-doped ZTC single crystal