Elsevier

Optical Materials

Volume 114, April 2021, 110983
Optical Materials

Impacts of phosphoric acid on zinc tris (thiourea) sulfate (ZTS) solution stability and single crystal growth

https://doi.org/10.1016/j.optmat.2021.110983Get rights and content

Highlights

  • ZTS solution stability was investigated by measuring metastable zone width and induction period.

  • ZTS crystals were grown using slow-cooling method in aqueous solution.

  • Impacts of H3PO4 on ZTS solution stability, crystal growth and crystal property.

Abstract

The metastable zone width and induction period of ZTS solution have been measured to investigate the influence of H3PO4 on solution stability, and it could be improved by adding phosphoric acid. Pure and H3PO4 doped ZTS crystals have been successfully grown using the slow-cooling method in aqueous solution. Several crystal samples were characterized by XRD, FTIR, TG, DTA, DTG, UV-VIS and SHG. The results show that some changes have taken place on external morphology and internal structure after H3PO4-doping. Thermostability, light transmittance and SHG efficiency of ZTS crystals are also affected by H3PO4, and all of them become better than undoped crystal, which are meaningful for practical applications of crystals.

Introduction

Zinc tris (thiourea) sulfate (ZTS) crystal is a kind of well-known non-linear optical material and its chemical formula is [Zn(CS(NH2)2)3SO4]. The ZTS crystal structure belongs to the orthorhombic system and has lattice parameters of a = 1.113 nm, b = 0.777 nm, c = 1.549 nm, V = 1.339 nm3, Z = 4, space group of Pca21, and point group of C2V-mm2 [[1], [2], [3], [4]]. And ZTS crystal possesses superior nonlinear optical coefficient, high laser damage, well birefringence and stable physico-chemical property, which benefit from its typical metal-organic complex and noncentrosymmetric structure [[5], [6], [7], [8], [9]].

For a long time, it is difficult to gain ZTS crystals with large-size and high-quality on account of low temperature coefficient of solubility, narrow metastable zone width, short induction period, and so on. Slow-cooling method in aqueous solution can be used to grow ZTS single crystals, but the suitable technology needs continuous exploration. In addition, solution stability is of great significance for crystal growth because that ZTS solution stability is easily influenced by additive or dopant. From literature reports in recent years, reducing the pH value is beneficial to ZTS solution stability and crystal growth, and different acidic additive makes a great difference [[10], [11], [12], [13], [14], [15]].

In this study, phosphoric acid (H3PO4) is chosen as the acidity regulator and doping agent. Firstly, H3PO4 can not only change the pH value of the solution but also enter into Zn(CS(NH2)2)3SO4 and replace SO42− during crystal growth [[16], [17], [18]]. In the work, we firstly measured the metastable zone width and induction period of ZTS solution under different acidic conditions, which can elaborate the influence of H3PO4 on solution stability. Then, pure and H3PO4 doped ZTS crystals were grown utilizing the slow-cooling technique in aqueous solution. And the impacts of H3PO4 on crystal growth and crystal properties were investigated.

Section snippets

Experimental

On the basis of the ZTS solubility curve obtained in our previous experiments, moderate ZTS raw materials were dissolved in deionized water to prepare saturated solution at a certain temperature, and we abbreviated saturated temperature as Ts [19]. Then, we overheated the solution for 12 h and cooled to the temperature of 5 °C higher than Ts. Under continuous stirring, we set temperature decrease-rate as 3.0 °C/h until small grains were formed by spontaneous crystallization, and recorded this

Impacts of H3PO4 on solution stability

The solution status is usually divided into stable zone, metastable zone and unstable zone. The metastable zone is an area where the seed crystal can grow but the solution can not nucleate spontaneously. Thus, only in the metastable zone, it is possible to grow crystals well and steadily using the slow-cooling method in aqueous solution. After the careful measurements, we draw the solubility and spontaneous crystallization curves of ZTS solutions respectively, which are shown in Fig. 1. The

Conclusions

After adding H3PO4 into ZTS solution, the metastable zone width is broadened and induction period is prolonged, so its solution stability is improved and appropriate phosphoric acid is beneficial. Pure and H3PO4 doped ZTS crystals are obtained by the slow-cooling method under same growth process. External morphology and lattice parameters of H3PO4 doped crystal change a little, but its internal defects reduce significantly. Melting point of ZTS crystal is gained and it is raised from 232.8 °C

CRediT authorship contribution statement

Hua-Jian Yu: Writing – original draft, preparation. Xu-Ping Wang: Methodology. Yuan-Yuan Zhang: Investigation. Xian-Shun Lv: Validation. Lei Wei: Data curation. Jing Li: Visualization. Yan-Yan Hu: Conceptualization. Cheng-Cheng Qiu: Project administration. Hua-Di Zhang: Software. Bing Liu: Writing- Editing.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This work was supported by National Natural Science Foundation of China (51972179, 51902168 and 52072189); Science and Technology Program for Young Innovation Team in Colleges and Universities of Shandong Province, China (2019KJA003); Studio for the Leader of Scientific Research in Jinan (2019GXRC059).

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