Synthesis, Crystal Structure and Nonlinear Optical Property of RbHgI₃

Abstract

Searching for new nonlinear optical (NLO) crystals to be used in the infrared (IR) region is still a challenge. This paper presents the synthesis, crystal structure and properties of a new halide, RbHgI₃. Its non-centrosymmetric single crystal can be grown in solution. In its crystal structure, all the polar [HgI₄]²⁻ groups align in such a way that brings a favorable net polarization. The measurement by Kurtz–Perry powder technique indicates that RbHgI₃ shows a phase-matchable second harmonic generation (SHG) property seven times stronger than that of KH₂PO₄ (KDP). RbHgI₃ displays excellent transparency in the range of 0.48–25 μm with relatively good thermal stability. The UV absorption implies that this yellow compound’s band gap is about 2.56 eV, close to that of AgGaS₂. A preliminary measurement indicates that the laser-induced damage threshold of the crystal is about 28.3 MW/cm². These preliminary experimental data reveal that RbHgI₃ is a new candidate as nonlinear optical material in the infrared region.

1. Introduction

Second-order nonlinear optical (NLO) crystals are very important high-tech materials due to their application in laser frequency convention, optical parameter oscillator (OPO), and signal communication [1,2]. According to wavelength range, second-order NLO materials can be classified into three categories, namely ultraviolet NLO crystals, visible NLO crystals and infrared NLO crystals. Great progress has been made in ultraviolet NLO crystals in the last several decades. For instance, some excellent new crystals such as β-BaB₂O₄ (BBO) [3], LiB₃O₅ (LBO) [4], KBe₂BO₃F₂ (KBBF) [5,6,7] have been developed by Chuangtian Chen and his colleagues. In the visible region, there are some excellent NLO crystals such as KH₂PO₄ (KDP) [8], KTiOPO₄ (KTP) [9] and so forth. Nevertheless, numerous NLO crystals in IR regions are not fully satisfied. Some common infrared NLO crystals, such as AgGaS₂ [10], AgGaSe₂ [11] and ZnGeP₂ [12] possess two drawbacks: one is their low laser damage thresholds (LDT), and the other is not easy to grow high-quality crystals. Therefore, searching for new infrared NLO crystals is still a challenge in this field.

However, general knowledge indicates that high LDT of the materials originates from the large gap. Guided by this purpose, thousands of IR NLO crystals have been discovered and many materials exhibit excellent performance such as halides, chalcogenides, iodates and phosphides etc., Two review articles [13,14] about IR NLO materials have been recently published. It has been accepted that the band gap which causes thermal and electronic effects is in accord with laser damage threshold [15,16]. As we know, halides usually exhibit large band gaps, and there are four halogen atoms to choose from. Moreover, halides can be easily dissolved in solvents so that it is easier to grow single crystals. On the other hand, mercury atom is a heavy metal element. This is beneficial for the wide transparence in the IR region. Furthermore, the mercury (II) cation (Hg²⁺) possesses d¹⁰ electronic configuration and does not emerge d-d transition. This will lead to relatively wide band gaps. Therefore, with Chuangtian Chen’s long-term help and collaboration, we have pursued research on exploring new second-order NLO crystals to be used in the mid-IR region from halides, and some new potential mid-IR NLO crystals such as Cs₂HgCl₂I₂ [17], Cs₂Hg₃I₈ [18], HgBrI [19], HgBr₂ [20], Hg₂BrI₃ [21], CsHgBr₃ [22], β-HgBrCl [23], NaSb₃F₁₀ [24] and Rb₂CdBr₂I₂ [25] have been developed by this idea on our group.

In this paper, we report the synthesis, crystal structure, and NLO properties of a new compound RbHgI₃. The compound is pure RbHgI₃ without crystalline water and it differs from the compound of RbHgI₃·H₂O reported in the literature [26]. The powder second harmonic generation (SHG) effect of RbHgI₃·H₂O is very weak and not phase-matchable. The existence of the crystalline water is harmful for thermal stability and transparent region in the IR region. Differently, RbHgI₃ shows a powder second harmonic generation (SHG) property seven times as high as that of KDP. It also exhibits a wide transparent window in the infrared region. It also shows reasonable band gap and thermal stability. To conclude, RbHgI₃ is a potential candidate as new IR NLO material.

2. Results and Discussion

2.1. Synthesis and Analysis

RbHgI₃ was synthesized with HgI₂ and RbI by conventional solution reaction. The phase purity was checked by powder XRD. No impurities were observed, and the diffraction patterns are not consistent with those of HgI₂ [27] or RbI [28] phase, and also not the sum of HgI₂ and RbI phases by the XRD contrast analyses. The measured powder X-ray diffraction patterns are in agreement with the simulated patterns based on the single crystal structure of RbHgI₃ obtained from the solution reaction (See Figure 1).

2.2. Crystal Structure

RbHgI₃ crystallizes in the noncentrosymmetric orthorhombic space group Ama2 (No. 40). Figure 2a shows the packing diagram of RbHgI₃ along bc plane. Each [HgI₄]²⁻ group forms a distorted tetrahedron (Figure 2b), which is connected with the neighbors by sharing an iodine atom to form a one-dimensional (1D) zigzag chain along the a-axis (Figure 2c). The chains are then to form layers along the ab plane, and the planes are further connected into a three-dimensional (3D) framework by the Rb atoms that occupy the empty spaces surrounded by iodine atoms (Figure 2). In the crystal structure, it also exhibits disorder of Rb atom in the symmetry unit with 50% occupancies (Figure 2a). As shown in Figure 2b, in each [HgI₄]²⁻ group there are two types of Hg-I bond lengths, namely, two shorter Hg-I bond length (2.724(3) Å and 2.730(3) Å, respectively), and two longer bond lengths (both 2.868(16) Å). Interestingly, all the shorter Hg-I bonds are always located above the Hg atoms in each [HgI₄] tetrahedron (Figure 2c). This packing style gives rise to a net polarization parallel to the direction of the c axis (indicated by the black arrow in Figure 2b,c). Due to the favorable alignment and high density of the [HgI₄]²⁻ anionic group, the compound can show a relatively strong SHG response.

2.3. Infrared Spectrum and UV-Vis Diffuse Reflectance Spectrum

The attenuated total reflection Fourier-transformed infrared (ATR-FTIR) spectrum of RbHgI₃ crystalline samples is shown in Figure 3. The ATR-FTIR spectrum indicates no absorption in the middle IR region from 4000 to 400 cm⁻¹ (2.5–25 μm). This agrees quite well with the reported compounds such as Tl₄HgI₆ [29] and Cs₂Hg₃I₈ [18,30], and they are transparent between 4000 and 400 cm⁻¹. According to the references, Tl₄HgI₆ exhibits a broad range of transparency from 1.4 to 40 μm, while Cs₂Hg₃I₈ is reported to be transparent between 0.5 and 25 μm. The UV-Vis diffuse reflectance spectrum of RbHgI₃ is shown in Figure 4. The compound is yellow and the spectrum shows that the absorption edge near the UV side is about 484 nm, and this indicates that the optical band gap of the compound is approximately 2.56 eV. Based on these data, the transparent range of RbHgI₃ is 0.48–25 μm.

2.4. NLO Property and LDT Measurement

Powder SHG measurements using 1064 nm laser radiation revealed that RbHgI₃ showed powder SHG efficiencies seven times as strong as that of KDP. The study result of the SHG intensity as a function of particle size (from 20 to 300 μm) is shown in Figure 5. The intensity of SHG signals at first increases gradually with the increase of the sample size and then reaches saturation when the sample size increases further. It is a typical curve for indicating that the SHG effect of RbHgI₃ is phase-matchable [31].

A preliminary examination of the LDT measurement has been carried out on the crystalline sample using a Q-switched laser source. The samples showed a damage threshold of about 28.3 MW/cm² (1064 nm, 10 ns), which is approximately equal to that of AgGaS₂ (30 MW/cm², 1064 nm) [32].

2.5. Thermogravimetric Analysis

The yellow powder crystal of RbHgI₃ is not hygroscopic and does not change its color after being exposed to air for about six months. The thermal behavior of RbHgI₃ was investigated using thermogravimetric analysis (TGA). The TG curve reveals that the compound starts losing its weight when it is heated to above 120 °C (See Figure 6), and then it continues to lose weight and reach a platform at about 450 °C. The thermal stability of RbHgI₃ is not very high and may be due to the existence of the iodine atom.

2.6. Electronic Structure and Optical Properties Calculation

To better understand the relationship of structure and the optical properties of the RbHgI₃, the first-principles density functional theory (DFT) calculations were performed. The calculated band gap is 1.95 eV. The deviations from the experimental values should be attributed to the limitation of DFT [33]. Hence, scissors of 0.61 eV has been added to shift up the conduction band (CB) levels in order to agree with the measured value of the band gap for the following calculations and analyses.

The partial density of states (PDOS) projected on the constitutional atoms in RbHgI₃ are shown in Figure 7. The Rb s, Rb 4p, I 5s and Hg 5d orbitals are strongly localized in the valence band (VB) about −25 eV, −10 eV and −5.8 eV, respectively. The upper of the valence states from −2.5 eV show a large hybridization between Hg 5p (and 5d) and I 5p orbitals, indicating very strong chemical bonds between the Hg and I atoms, but the VB maximum is dominated by I 5p orbitals which are higher than Hg 5d orbitals in the VB. The bottom of CB is mainly composed of the 5s and 5p orbitals of I atom and 6s orbitals of Hg atom, but I 5s orbitals are lower than Hg 5s orbitals in the CB. This means that the iodide anion directly determines the energy band gap of RbHgI₃. This is the reason why the laser damage threshold is not high and it agrees with the experimental value.

On the basis of the above electronic band structure, the refractive indices and second harmonic generation (SHG) coefficients of RbHgI₃ were obtained. The refractive indices are shown in Figure S2 (see the supporting information) and the calculated refractive indices and birefringence at several radiation wavelengths are listed in

Table 1. Calculated Refractive Indices at Selected Wavelengths for RbHgI₃.

Wavelength	nx	ny	nz	Δn (ny − nx)
1000 nm	2.01	2.09	2.07	0.08
1064 nm	2.00	2.08	2.06	0.08
2000 nm	1.97	2.05	2.03	0.08
~∞	1.96	2.04	2.02	0.08

. It is shown that the birefringence Δn is larger than 0.08 as the wavelength is longer than 1000 nm, so RbHgI₃ is phase-matchable for the SHG in the IR region. Furthermore, we theoretically determined the SHG coefficients of RbHgI₃. For the orthorhombic mm2 point group and considering the restriction of Kleinman’s symmetry, the title compound (Ama2) has three (d₁₅, d₂₄ and d₃₃) independent SHG tensors and the static SHG coefficients are as follows: d₁₅ = 28.99 pm/V, d₂₄ = −27.19 pm/V and d₃₃ = 2.95 pm/V. The calculated results agree with the experimental observation that RbHgI₃ exhibits a SHG response seven times that of KDP (d₃₆ = 0.39 pm/V). Therefore, we are confident that RbHgI₃ possess strong SHG effects and is a promising candidate for the nonlinear optical applications.

3. Materials and Methods

3.1. Synthesis and Crystal Growth

All the starting materials are analytically pure from commercial sources and used without further purification. RbHgI₃ was synthesized by conventional solution reaction. HgI₂ and RbI (mole ratio 1:1) are carefully dissolved in ethanol. The mixture was stirred at 80 °C for 5 h. The yellow solution was filtered and slowly cooled, then kept in a flask at room temperature. After a few days, some yellow crystals appeared in the bottom of the flask. The yellow crystals were filtered and carefully washed with cool ethanol. A single phase of RbHgI₃ crystals was obtained with the yield of 65% (based on HgI₂). The RbHgI₃ crystal can be grown from the ethanol solution and a photograph of the RbHgI₃ crystal is presented in Figure S1 (see the supporting information).

3.2. Structure Determination

A single crystal of RbHgI₃ with dimensions of ca. 0.10 × 0.10 × 0.08 mm³ was selected and used for single-crystal diffraction experiment. Data sets were collected using a Bruker SMART APEX diffractometer (Bruker, Karlsruhe, Germany) equipped with a CCD detector (graphite-monochromated Mo-Kα radiation λ = 0.71073 Å) at 298(2) K. Data sets reduction and integration were performed using the software package SAINT PLUS [34]. The crystal structure is solved by direct methods and refined using the SHELXTL 97 software package [35,36]. Single crystal data collection, cell parameters and basic information for RbHgI₃ are summarized in

Table 2. Crystallographic data of RbHgI₃.

Empirical Formula	RbHgI3
Formula weight	666.76
Temperature	296(2)
Wavelength	0.71073
Crystal color	Yellow
Crystal system	Orthorhombic
Space group	Ama2 (No.40)
Crystal size (mm3)	0.10 × 0.10 × 0.08
Unit cell dimensions (Å)	a = 8.838(3), b = 9.824(3), c = 11.050(3)
Z	4
V (Å3)	959.4(5)
Absorption coefficient (mm−1)	30.640
Density (calculated)	4.616
Goodness-of-fit on F2	1.149
Reflections collected	1473
Independent reflection	1337 [R (int) = 0.0293]
R1, wR1 [I > 2σ (I)]	0.0682/0.1845
R2, wR2 (all data)	0.0637/0.1818
Min/max Δρ/e·Å−3	−3.472/3.022

ω = 1/[s² (Fo²) + (0.0702P)² + 51.3875P], where P = (Fo² + 2Fc²)/3.

. Further details of the crystal structure studies may be obtained from the Fachinformationszentrum Karlsruhe, 76344 Eggenstein-Leopoldshafen, Germany (Fax: (49) 7247808666; e-mail: [email protected]), on quoting registry number CSD-432180 for RbHgI₃.

3.3. Powder XRD Measurement

X-ray powder diffraction (XRD) patterns of the polycrystalline material were collected using a Bruker D8 Advanced diffractometer (Bruker, Karlsruhe, Germany) with Cu kα1 radiation (λ = 154186 Å) in the range of 10–80° (2θ) at a scanning rate of 6°/min⁻¹.

3.4. Optical Spectroscopy

The optical transmission in the mid-IR region was recorded on a NICOLET 5700 Fourier-transformed infrared (FT-IR) spectrophotometer (Manufacture, City, Country) in the 4000–400 cm⁻¹ (2.5–25 μm) region using the attenuated total reflection (ATR) technique with a diamond crystal. The crystal sample was loaded on the samples stage and then the ATR-FTIR spectrum was measured. The UV-Vis absorption spectrum was recorded on a Varian Cary 5000 UV-Vis-NIR spectrophotometer (Agilent, Palo Alto, CA, USA) in the region 200–800 nm. A BaSO₄ plate was used as the standard (100% reflectance), on which the finely ground samples from the crystals were coated. The absorption spectrum was calculated from the reflectance spectrum using the Kubelka–Munk [37] function: α/S = (1 − R²)/(2R), where α is the absorption coefficient, S is the scattering coefficient, and R is reflectance.

3.5. Second-Harmonic Generation (SHG) and Laser Damage Threshold (LDT) Measurement

The NLO efficiencies of the samples were investigated using a Kurtz–Perry powder technique [30]. A pulsed Q-switched Nd:YAG laser was utilized to generate fundamental 1064 nm light with a pulse width of 10 ns. Microcrystalline KDP was served as the standard. The particle sizes of the sieved sample and KDP were arranged from 25 to 260 μm for the measurement of size-dependent SHG effect. The energy of each pulse was measured to be about 200 mJ. The sample crystal was fixed on the bracket. An optical concave lens was used to adjust the diameter of the laser beam to obtain different intensities. The samples endured gradually enhanced radiation until their appearance changed under a microscope after the irradiation.

3.6. Thermogravimetric Analysis

The thermogravimetric analysis (TGA) was carried out on a SDTQ 600 simultaneous analyzer instrument (TA Instruments, New Castle, PA, USA). The crystal sample was added into an Al₂O₃ crucible and heated from room temperature to 800 °C at a heating rate of 10 K min⁻¹ under flowing nitrogen gas.

3.7. Theoretical Calculation

The electronic structure and optical properties for the title compound were performed using the first principles plane-wave pseudopotential method implemented in the Cambridge Serial Total Energy Package (CASTEP) [38]. According to the experimental structure, the model was built and Rb atom was located at (0.7500, 0.3831, 0.9670) in the unit cell because of its order. The optimized normal-conserving pseudopotentials [39] in the Kleinman–Bylander form are chosen and the valence electrons are 4s, 4p and 5s for Rb; 5p, 5d and 6s electrons for Hg; and 5s, 5p electrons for I. The local-density approximation (LDA) with a high kinetic energy cutoff of 900 eV is adopted. Monkhorst–Pack [40] k point meshes with a density of 4 × 4 × 4 points in the Brillouin zone of the unit cell are used. The scissor-factors-corrected LDA are used to calculate the second-order susceptibility χ² based on the formula developed by Lin et al. [41].

4. Conclusions

A new ternary RbHgI₃ with an orthorhombic space group Ama2 has been obtained by reaction of HgI₂ and RbI (1:1) in alcohol. The compound shows phase-matchable SHG of about seven times as strong as that of KDP, a wide transparent region (in the range of 0.48–25 μm), and a relatively good stability to the environment. Its crystals can be grown in solution. Owing to these properties, RbHgI₃ appears to be a promising new NLO crystal applicable in the infrared region. Our future efforts will be devoted to growing large and high-quality crystals of RbHgI₃ to further study its optical properties, for instance SHG coefficients, refractive indices and laser damage threshold.