Synthesis, Characterization, and Crystal Growth of Cs2Hg3I8: A New Second-Order Nonlinear Optical Material Gang Zhang,† Jingui Qin,*,†,‡ Tao Liu,† Tianxiang Zhu,† Peizhen Fu,§ Yicheng Wu,§ and Chuangtian Chen*,§
CRYSTAL GROWTH & DESIGN 2008 VOL. 8, NO. 8 2946–2949
Department of Chemistry and Center of Nanoscience and Nanotechnology, Wuhan UniVersity, Wuhan 430072, China, and Beijing Center for Crystal Research and DeVelopment, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100080, China ReceiVed January 15, 2008; ReVised Manuscript ReceiVed April 6, 2008
ABSTRACT: This paper reports, for the first time, the second-order nonlinear optical (NLO) property of Cs2Hg3I8, a new potential NLO material in the infrared region. The compound exhibits a powder second harmonic generation (SHG) effect as strong as KTiOPO4 (KTP). It also shows excellent transparency in the range of 0.5-25 µm with relatively high thermal stability. A single crystal up to 25 × 14 × 5 mm3 in size has been grown in acetone by a slow evaporation technique. 1. Introduction Second-order nonlinear optical (NLO) materials have attracted much attention owing to their uses in laser frequency conversion, optical parameter oscillator (OPO), and signal communication. 1,2 This resulted in the development of many important nonlinear optical crystals in UV and visible applications, such as β-BaB2O4 (BBO),3 LiB3O5 (LBO),4 KH2PO4 (KDP),5 KTiOPO4 (KTP),6 and LiNbO3.7 In the infrared (IR) region, on the other hand, although several materials were reported, such as GaSe,8 AgGaS2,8,9 AgGaSe2,10 ZnGeP2,11 Ag3AsS3,12 Tl3AsSe3, 13 CdGeAs2,14 LiMX2 (M ) Al, Ga, In; X ) S, Se, Te),15–17 etc. However, most of them are not good enough for applications18 because their crystals either are difficult to grow or exhibit low optical damage thresholds. So the search for new NLO crystals to be used in the IR region has become one of the greatest challenges in this field. In the last 10 years or so, some other groups and our group have investigated the suitability of halides as such new materials because low-phonon energy complex halides will be beneficial to the improvement of the laser damage threshold because of their large band gaps. Several halides such as CsGeX3 (X ) Cl, Br, I),19–22 Tl3PbBr5,23 Tl4HgI6,24 and CsCdBr325 have been discovered to be potential candidates in this regard. In this paper, we wish to report the NLO property and crystal growth of another complex halide, Cs2Hg3I8, for the first time. It shows a very strong powder second harmonic generation (SHG) property, similar to that of KTiOPO4 (KTP). In comparison with many other IR crystals, Cs2Hg3I8 exhibits a wider transmission in the IR range (up to 25 µm). It also shows relatively high thermal stability. Furthermore, it can be grown to large crystals by the solvent evaporation method, which is much easier for large crystal growth than the commonly used high temperature solid-state method. All these make the crystal a new potential NLO material in the IR region.
Figure 1. Solubility curve of Cs2Hg3I8 in acetone.
2. Experimental Section 2.1. Materials Synthesis. All starting materials were commercially available and used as purchased, and all the synthesis and growth processes were carried out in acetone. * Corresponding author. E-mail:
[email protected] (J.Q.);
[email protected] (C.C.). Phone and Fax: 86-27-68756757 (J.Q.). Phone: 86-10-82543705(C.C.) † Department of Chemistry, Wuhan University. ‡ Center of Nanoscience and Nanotechnology, Wuhan University. § Technical Institute of Physics and Chemistry, Chinese Academy of Sciences.
Figure 2. Photograph of grown Cs2Hg3I8 crystal. The compound Cs2Hg3I8 was obtained by combining the solution of stoichiometric amounts of CsI and HgI2 (molar ratio ) 1:1.5) in acetone followed by slow evaporation of the solution at room temperature. The product was purified by successive recrystallization. 2.2. Solubility and Crystal Growth. The solubility curve of Cs2Hg3I8 in acetone is shown in Figure 1.
10.1021/cg800054x CCC: $40.75 2008 American Chemical Society Published on Web 06/27/2008
Cs2Hg3I8: New Second-Order Nonlinear Optical Material
Crystal Growth & Design, Vol. 8, No. 8, 2008 2947
Figure 3. Ball-and-stick diagrams of Cs2Hg3I8. (a) Ball-and-stick diagrams of Cs2Hg3I8. (b) Ball-and-stick diagrams of Cs2Hg3I8 (Cs atoms are omitted for clarity). (c) HgI4 tetrahedral groups in one unit cell (Each Hg2+ is bonded to four iodine atoms with one shortest (black) and three longer (blue) Hg-I bond distances, Å). The crystal growths were performed in a constant temperature bath of controlled stability ((0.1 °C). Solutions were prepared using acetone as a solvent and saturated at 25-30 °C. After filtering using 0.2 µm porosity Millipore filters, they were preheated to 35-40 °C for 24 h in a crystallizer tube. This procedure was performed to ensure that all ingredients were dissolved. Large single crystals could be obtained utilizing a solvent slow evaporation technique (30 °C). The size of the single crystal has reached 25 × 14 × 5 mm3 (see Figure 2).
3. Instruments for Characterization Single-crystal X-ray diffraction analysis was carried out using a Bruker SMART Diffractometer equipped with a CCD detector (graphite-monochromated Mo KR radiation λ ) 0.71073 Å) at 294 K. The optical transmission spectrum in the region 4000-400 cm-1 with a resolution of 1 cm-1 was recorded on a NICOLET 5700 Fourier-transformed infrared (FT-IR) spec-
trophotometer. The UV-vis-near infrared electronic absorption spectrum was performed on a Varian Cary 5000 UV-vis-NIR spectrophotometer in the region 190-2500 nm. The thermogravimetric analysis (TGA) was carried out on Setaram SETSYS-16 in static nitrogen at a heating rate of 10 K/min. The NLO efficiencies of the crystals were found out using a KurtzPerry powder SHG technique. 26 A pulsed Q-switched Nd:YAG laser was utilized to generate fundamental 1064 nm light with a pulse width of 8 ns. Microcrystalline KTP served as the standard. 4. Results and Discussion 4.1. Synthesis. The synthesis of Cs2Hg3I8 was first reported by Fedorov et al. 27 in aqueous solution. Here, we tried using organic solvent instead of aqueous solution. The advantage of
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Table 1. Crystal Data and Structure Refinement for Cs2Hg3I8 empirical formula fw T (K) wavelength (Å) cryst syst space group a (Å) b (Å) c (Å) R (deg) β (deg) γ (deg) V (Å3) Z density (calcd) (Mg/m3) abs coeff (mm-1) F(000) cryst size (mm3) no. of reflns collected no. of independent reflns GOF on F2 final R indices [I > 2σ(I)] R indices (all data)
Cs2 Hg3 I8 1882.79 294(2) 0.71073 monoclinic Cm (No. 8) 7.4415(6) 21.6629(18) 7.6726(6) 90 108.050(1) 90 1175.99(16) 2 5.317 33.062 1548 0.20 × 0.10 × 0.10 4917 1903 [R(int) ) 0.0542] 1.100 R1 ) 0.0611, wR2 ) 0.1499 R1 ) 0.0650, wR2 ) 0.1554
Figure 6. Thermogravimetric analysis curve for Cs2Hg3I8 crystal.
organic solvent is that the reaction is faster and can be proceeded in room temperature. Three different organic solvents (acetone, acetonitrile, and ethanol) were tested, and acetone was finally chosen as the solvent.
Figure 7. Dependence of SHG intensity on the particle size.
Figure 4. UV-vis-NIR spectrum of Cs2Hg3I8 crystal.
Figure 5. IR spectrum of Cs2Hg3I8 crystal.
4.2. Crystal Structure and X-ray Analysis. Fedorov et al. have done the crystal structure analysis of Cs2Hg3I8. Since we have synthesized this compound in organic solvent instead of aqueous solution which was used by Fedorov et al., we need to verify whether they have the same crystal structure or not. This is crucial for understanding the origin of the second-order nonlinear optical property of our compound. Therefore we have measured the crystal structure of Cs2Hg3I8 again. The results of these two measurements are basically the same, except that our experiments give a better R value (R1 ) 0.065) than the previous one (R1 ) 0.147) reported by Fedorov et al. The structure of Cs2Hg3I8 was solved by direct methods with SHELXTL.28 Single crystal X-ray diffraction data for Cs2Hg3I8 is presented in Table 1. It belongs to monoclinic structure with a noncentrosymmetric space group Cm as reported in literature.27 Its cell parameters are a ) 7.4415(6) Å, b ) 21.6629(18) Å, c ) 7.6726(6) Å, R ) 90°, β ) 108.050(1)°, γ ) 90°, and Z ) 2. Figure 3a shows the ball-and-stick diagrams of Cs2Hg3I8. As shown in structures b and c in Figure 3, each Hg atom is bonded to four iodine atoms with one shortest (black) and three longer (blue) Hg-I bond lengths ranging from 2.655(3) to 2.883(3) Å to form the distorted HgI4 groups. These distorted HgI4 groups are also seen in KHgI3 · H2O29 and Cs2HgI430 crystal structures, where the Hg-I bond lengths in distorted HgI4 groups are
Cs2Hg3I8: New Second-Order Nonlinear Optical Material
ranging from 2.705 to 2.904 Å (for KHgI3 · H2O), or from 2.738 to 2.819 Å (for Cs2HgI4), respectively. From Figure 3c, we can see that there are two independent Hg2+ cations that are marked as Hg(1) and Hg(2). In one unit cell, every two Hg(1)I4 tetrahedral groups are connected with one Hg(2)I4 tetrahedron group through the I(4) atoms. All the shortest bonds of every distorted HgI4 tetrahedral groups are nearly parallel to the c axis (Figure 3c) so that these distorted HgI4 tetrahedral groups are arranged in a noncentrosymmetric manner, leading to substantial SHG responses. 4.3. UV-vis-NIR, FT-IR Spectra, and Thermal Analysis. The UV-vis-NIR spectrum and mid-infrared spectrum of Cs2Hg3I8 are shown in Figures 4 and 5. The UV-visNIR spectrum indicates that the absorption edge is at about 500 nm, and the band gap of the compounds is approximately 2.56 eV. Meanwhile, there is no absorption in the middle IR region. On the basis of these data, we can draw a conclusion that the transparent range of Cs2Hg3I8 is 0.5-25 µm. The thermal behavior of Cs2Hg3I8 was investigated using thermogravimetric analysis (TGA). The compound starts losing weight at about 170 °C (Figure 6). So it is clear that Cs2Hg3I8 is thermally stable up to 170 °C. 4.4. Second Harmonic Generation. The NLO property of Cs2Hg3I8 crystal was confirmed by the Kurtz-Perry powder SHG technique.26 Powder SHG measurements using 1064 nm radiation revealed that Cs2Hg3I8 showed SHG efficiencies approximately as strong as KTP. Study of the SHG intensity as a function of particle size (from 20 to 200 µm) is shown in Figure 7. The intensity of the SHG signals at first increases gradually with the increase of the sample size. And then it reaches a plateau at the maximum value after a certain particle size (of about 140 µm). This is a clear sign for indicating that the SHG of Cs2Hg3I8 is phase-matchable.26 5. Conclusions Cs2Hg3I8 single crystal with the size of 25 × 14 × 5 mm3 has been grown by slow solvent evaporation in acetone at constant temperature. The intensity of second harmonic generation effect is similar to that of KTP and the effect is phasematchable. The compound is transparent in the range of 0.5-25 µm. Its band gap is calculated to be about 2.56 eV. Owing to these properties, Cs2Hg3I8 appears to be a new potential NLO crystal applicable in the infrared region. Further investigation is in process. Acknowledgment. This work was supported by the National Key Fundamental Research (973) Program of China. Supporting Information Available: X-ray crystallographic file in CIF format for the Cs2Hg3I8 single crystal. This material is available free of charge via the Internet at http://pubs.acs.org.
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