Synthesis, crystal structures and nonlinear optical properties of polymorphism: α- and β-RbHgI3·H2O

Journal of Alloys and Compounds 771 (2019) 547e554

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Synthesis, crystal structures and nonlinear optical properties of polymorphism: a- and b-RbHgI3$H2O Ling Huang a, Qian Wang a, Fangfang He a, Xiaoyan Liu a, Ziwei Chen a, Wen He a, Xueying Luo a, Daojiang Gao a, Jian Bi a, Guohong Zou b, * a b

College of Chemistry and Materials Science, Sichuan Normal University, Chengdu, 610068, PR China College of Chemistry, Sichuan University, Chengdu, 610064, PR China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 26 May 2018 Received in revised form 31 August 2018 Accepted 1 September 2018 Available online 4 September 2018

New noncentrosymmetric polymorphs, namely, a- and b-RbHgI3$H2O, have been synthesized by conventional solution reaction. The two titled compounds crystallize in the monoclinic space groups Cc (no. 9) and Pc (no. 7), respectively. In their crystal structures, [HgI4] tetrahedrons connect with each other via sharing corners into one-dimensional (1-D) [HgI3] chains, and then all the 1-D chains connect with [Rb(H2O)]þ complexes to form the three-dimensional (3-D) frameworks. The consistent packing style results in a strong macroscopic nonlinear optical effect due to the superimposition of the microscopic second-order susceptibilities of [HgI4] tetrahedra. Powder second-harmonic generation (SHG) measurements show that both a- and b-RbHgI3$H2O are phase-matchable (type I), and the measured SHG coefficients are about 15.0 and 14.0 times that of KH2PO4 (KDP). Both the compounds exhibit wide transparency from near UV to mid and far-infrared region, suggesting that a- and b-RbHgI3$H2O are potential infrared NLO crystals. Band structure and optical property were also calculated by DFT methods. © 2018 Elsevier B.V. All rights reserved.

Keywords: Polymorphism Nonlinear optical material SHG Phase-matchable DFT

1. Introduction Second-order nonlinear optical (NLO) materials [1e14] have become increasingly important due to their promising applications in laser science and technology, such as laser frequency conversion, optical communication, optical parameter oscillators, photolithography, micro-machining and advanced instrument development. To search for new second-order NLO crystals has attracted intense attention over the past decades, which promoted the development of several commercial ultraviolet (UV) and visible-near infrared (Vis-NIR) NLO crystals, for example, KTiOPO4 (KTP) [15], LiNbO3 (LN) [16], b-BaB2O4 (BBO) [17] and LiB3O5 (LBO) [18]. These crystals have already meet practical application requirements in the UV and Vis-NIR regions. However, in deep-ultraviolet (DUV) and mid and far-infrared (MFIR) regions, the reported materials could not meet market requirements. Therefore, searching for suitable NLO materials in these spectrum regions is still a particularly difficult challenge, and it is also a hot topic in inorganic solid-state chemistry

* Corresponding author. E-mail address: [email protected] (G. Zou). https://doi.org/10.1016/j.jallcom.2018.09.005 0925-8388/© 2018 Elsevier B.V. All rights reserved.

field. A promising MFIR NLO material [19e22] commonly should meet the following five criteria: (1) wide IR transparency range; (2) larger efficient second harmonic generation (SHG) coefficient (dij) than 10  d36 of KDP; (3) high laser damage threshold (LDT); (4) a moderate birefringence (0.03e0.1) for phase matching; (5) easiness crystal growth and stable chemical and mechanical properties. Because of difficulty in crystal growth and low laser damage threshold, many known IR NLO crystals, such as AgGaS2 [23,24], AgGaSe2 [25], and ZnGeP2 [26], are less suitable for practical application. In the past decades, most of studies on searching for new IR NLO materials have focused on chalcogenide compounds possessing large NLO coefficients and wide IR transparent windows [27e31]. However, the low LDT corresponding to the narrow energy band gap seriously limits the application for harmonic generation. In order to get IR NLO crystals with wide band gap, attention has been transferred to halide-based materials in recent years. And a series of halide-based NLO crystals, including CsGeX3 (X ¼ Cl, Br, I) [32e34], Tl4HgI6 [35], Tl3PbBr5 [36], SbF3 [37], NaSb3F10 [38], and RbIO2F2 [39] have been discovered which possess wide IR transmission, relatively high LDT and suitable SHG coefficients. Qin's

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group reported a lot of excellent work in mercury halogenated compounds, for example in Hg-X (X ¼ Cl, Br, I) system, HgBr2 [40], Hg2BrI3 [41] and Hg2Br3I [42]; and Cs-Hg-X (X ¼ Cl, Br, I) system, Cs2Hg3I8 [43], CsHgBr3 [44], Cs2HgCl2I2 [45] and Cs2Hg2Br2I4$H2O [46] have been successfully synthesized, which have been proven to be the promising materials for mid-IR applications. Rubidiumcontaining NLO compounds usually exhibit excellent optical performances such as large nonlinear coefficients due to high asymmetric coordination in RbPbCO3F [8] and Rb3VO(O2)2CO3 [10] etc.. However mercury halogenated compounds containing rubidium as IR NLO materials have rarely been reported [47]. Hence, systematic investigations have been made in the Rb-Hg-X (X ¼ Cl, Br, I) system to explore new NLO materials in this work. As a result, Polymorphs of RbHgI3$H2O (a-presents the low temperature phase and b-presents the high temperature phase) have been obtained with wide IR transmission (0.47e25 mm) and strong SHG coefficients (15.0 and 14.0  KDP), indicating that these two compounds are potential candidates for IR NLO materials. Herein, synthesis, crystal structures, properties for a- and b-RbHgI3$H2O have been reported. In addition, theoretical calculation analysis for band structures and optical properties has been performed. 2. Experimental section 2.1. Synthesis All the starting materials HgI2, RbI and acetone were bought from commercial sources and used without further purifying. Crystals of a- and b-RbHgI3$H2O were synthesized via the conventional solution reaction. RbI (0.212 g, 1 mmol) and HgI2 (0.454 g, 1 mmol) were dissolved in a mixture composed of 5 mL distilled water and 30 ml acetone. At room temperature the mixture was stirred for 10 h, and then the light yellow solution was evaporated in air. Ten days later, several rod yellow transparent crystals of aRbHgI3$H2O and block shaped yellow transparent crystals of bRbHgI3$H2O shown in Fig. S1 in Electronic Supporting Information were obtained in yields of about 28% and 40% (on the basis of Hg). After being exposed to air for about five months, the colour of yellow crystals is still maintained and the crystals are not deliquescent. 2.2. Experimental instruments Single crystal X-ray diffraction data of the two compounds were collected on a Bruker SMART BREEZE diffractometer equipped with a 1 K CCD area detector using graphite monochromated Mo Ka radiation at room temperature. The crystal structures were solved by the direct methods and refined by SHELX-97 [48]. The structures were further verified using the program PLATON [49], and no higher symmetries can be found. Relevant crystallographic data, details of the measurement conditions, atomic coordinates and isotropic displacement coefficients, and selected bond lengths and angles for a- and b-RbHgI3$H2O are summarized in Table 1, Tables S1eS4. X-ray diffraction patterns of polycrystalline materials were obtained on a Rigaku Smartlab powder X-ray diffractometer by using Cu Ka radiation (l ¼ 1.5418 Å) in the range of 5e70 , and with the scan step width of 0.05 and a fixed time of 0.2s. Differential thermal analysis (DTA) and thermogravimetric analysis (TGA) was conducted on a Netzsch STA 449C instrument. Crystal samples (10 mg) were enclosed in Al2O3 crucibles and heated from room temperature to 600  C with the heating rate of 10  C/min under a constant flow of nitrogen gas. IR spectra were recorded on a Vertex 70 Fourier transform infrared (FT-IR) spectrometer in the range of 4000e400 cm1, the samples (1 mg) were mixed thoroughly with oven-dried KBr (100 mg). The UVevis diffuse reflection

Table 1 Crystal Data and Structure Refinement for RbHgI3$H2O.a Formula

a-RbHgI3$H2O

b-RbHgI3$H2O

Formula Mass (amu) Crystal System Space Group a (Å) b (Å) c (Å) b ( ) V(Å3) Z r(calcd) (g/cm3) Temperature (K) l(Å) F(000) m (mm1) Final R indices (I > 2s (I))a R1/wR2 GOF on F2 Absolute Structure Parameter

684.78 Monoclinic Cc 11.0385(11) 9.8457(11) 8.8229(10) 90.324 (4) 1140.64 (3) 4 4.743 296(2) 0.71073 1144 30.67 0.047/0.130 1.03 0.049(15)

684.78 Monoclinic Pc 8.7046(3) 7.3028(3) 14.5385(6) 90.074(3) 924.18(6) 4 4.922 296(2) 0.71073 1144 31.822 0.0639/0.1402 1.072 0.067(16)

a

2

2 1=2

R1 ðFÞ ¼ SjjFo j  jFcjj=SjFo j$ wR2 ðF2o Þ ¼ ½SwðF2o  F2c Þ =SwðF2o Þ 

.

data were recorded on a PerkinElmer Lamda-900 UV/vis/NIR spectrophotometer using BaSO4 as the standard (100% reflectance) and scanned in the range of 200e2500 nm, and Kubelka-Munk function were used in converting the reflectance spectra to absorbance [50,51]. Using 1064 nm radiation generated by a Q-switched Nd:YAG solid state laser, powder second-harmonic generation (SHG) signals were measured, and the method is adapted from Kurtz and Perry [52]. Crystal samples of a- and b-RbHgI3$H2O were ground and sieved into the particle size in the range of 25e45, 45e62, 62e75, 75e109, 109e150 and 150e212 mm. Crystalline KDP and KTP were also ground and sieved into the same particle size and used as the references. The energy of each pulse was measured to be about 300 mJ. An optical concave lens was used to adjust the diameter of the laser beam to obtain different intensities. The samples endured gradually enhanced radiation until their appearance changed under a microscope after the irradiation. 2.3. Computational details By using the plane-wave basis pseudopotential method as implemented in the CASTEP, the band structure and density of states (DOS) calculations were carried out [53]. Perdew-BurkeErnzerhof (PBE) was used to consider the exchange and correlation effects in the generalized gradient approximation (GGA) [54]. The interactions between the ionic cores and the valence electrons are described by choosing Norm-conserving pseudopotentials [55]. In the containing atoms, H 1s1, O 2s22p4, I 4d105s25p5, Rb 3d104s24p65s1 and Hg 5s25p65d106s2 were regarded as valence electrons. A kinetic-energy cutoff of 450 eV was set for the selfconsistent-field convergence of the total electronic energy. Monkhorst-Pack k-point meshes with a density of 3  3  3 points in the Brillouin zone of the a-RbHgI3$H2O unit cell are chosen. The default values of the CASTEP code was used to decide the other parameters and convergence criteria. To obtain the linear optical properties, the complex dielectric function ε(u) ¼ ε1(u) þ iε2(u) has been determined in the random phase approximation from the PBE wavefunctions. The imaginary part of the dielectric function due to direct inter-band transitions is given by the expression,

ε2 ðZuÞ ¼

 2  2e2 pX c jk ju,rjjvk  d Ekc  Ekv  E ; Uε0 k;v;c

where U, u, u, n and c are the unit-cell volume, photon frequencies,

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the vector defining the polarization of the incident electric field, valence and conduction bands, respectively. The real part of the dielectric function is obtained from ε2 by a Kramers-Kronig transformation,

ε1 ðuÞ ¼ 1 þ

  Zþ∞ 2

p

0

d u0

u0 2 ε2 ðuÞ ; u0 2  u2

In calculation of the static c(2) coefficients, the so-called lengthgauge formalism derived by Aversa and Sipe and modified by Rashkeev et al. [56] is adopted, which has been proved to be successful in calculating the second order susceptibility for semiconductors and insulators. In the static case, the imaginary part of the static second-order optical susceptibility can be expressed as:

cabc

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structure is shown in Fig. 2. There are two independent Rb atoms, two independent Hg atoms, six independent iodine atoms and two independent O atoms in the asymmetric unit. In b-RbHgI3$H2O, Hg1 is four coordinated with four iodine atoms (one I1, one I2, one I3 and one I4), Hg2 is also four coordinated with four iodine atoms (one I2, one I4, one I5 and one I6), forming the distorted [HgI4] tetrahedron (Fig. 2a). Among the six iodine atoms, I2 and I4 are playing the role as bridge atoms, and the rest are dangling atoms. Hg1 and Hg2 connect with each other by I2 and I4 to form a chain A, and the adjacent chain A0 is rotating about 180  C compared with A. While only one kind of chain exists in a-phase (Fig. 2b). The discrepancy of the distance between the [HgI3] chains in aand b-phases results in the different locations of Rbþ ions. In aphase, the distance is 7.39 Å (Fig. 1b) in ab plane and in b-phase it's 8.69 Å (Fig. 2b) in bc plane. So in a-phase all the Rbþ ions locate at



a b c c b e3 X rnm rml rln þ rml rln ¼ 2 ½un fml þ um fln þ ul fnm  2unm uml uln Z U nml;k

3







i X fnm h a b c b a c c a b rnm rmn;c þ rnm rmn;c þ rnm rmn;b ; þ rmn;b þ rmn;a þ rmn;a 2

ie þ 2 4Z U nm;kumn

where r is the position operator, ħunm ¼ ħun - ħum is the energy difference for the bands m and n, fmn ¼ fm - fn is the difference of the Fermi distribution functions, subscripts a, b, and c are Cartesian indices, and rbmn;a is the so-called generalized derivative of the coordinate operator in k space, b rnm;a ¼

a Db þ r b Da rnm nm mn mn

unm

þ

i

unm



X



a b b a ulm rnl rlm  unl rnl rlm ;

l

where Danm ¼ (pann - pamm)/m is the difference between the electronic velocities at the bands n and m.

the middle of triangle composed of three [HgI3] chains, and all the Rbþ ions coordinate with water molecules and arrange in a line along the c-axis (Fig. 1d). And in b-phase, all the Rbþ ions locate in the cavity between two [HgI3] chains and meanwhile the different directions of A and A0 chains result in the different directions of the adjacent [Rb(H2O)]þ chains along the b axis (Fig. 2d). The structures discrepancy cause the distortion directions of all the tetrahedra are almost antiparallel to the direction of the a axis in a-phase and c axis in b-phase (Figs. 1b and 2b). This consistent packing style is beneficial in the superimposition of the microscopic second-order susceptibilities of all tetrahedra, resulting in the strong macroscopic NLO effect for the two titled compounds. The slight difference of the Hg-I bonds and angles cause the slight difference of the SHG responses.

3. Results and discussion 3.2. Powder X-ray diffraction 3.1. Crystal structure description

a-RbHgI3$H2O crystallizes into the asymmetrical monoclinic crystal space group of Cc (no. 9), and the structure of a-phase is shown in Fig. 1. There are one independent Rb atom, one independent Hg atom, three independent iodine atoms and one independent O atom in the asymmetric unit. Each Hg atom is surrounded by four iodine atoms (one I1, two I2 and one I3), forming a distorted [HgI4] tetrahedron (Fig. 1a). Then [HgI4] tetrahedrons connect with each other via sharing corners into 1-D [HgI3] chains along c-axis (Fig. 1b), where I2 atoms play the role of bridging and I1, I3 atoms are dangling. All the dangling bonds HgeI1 and HgeI3 (2.724 Å and 2.716 Å) are shorter than the bridging bonds HgeI2 (2.842 Å and 2.899 Å). Rbþ ions reside in the interlayer space and connect with water molecules through covalent bond, and then bridge with I through weak ionic bond to construct the 3-D framework of a-RbHgI3$H2O (Fig. 1c and d). Rbþ ions make an overall charge balance. Be similar with a-phase, b-RbHgI3$H2O crystallizes into the asymmetrical monoclinic crystal space group of Pc (no. 7), and the

The powder XRD patterns of a- and b-RbHgI3$H2O confirm that the two titled compounds are pure, for that experimental patterns match well with the calculated patterns fitted from single-crystal X-ray diffraction (Fig. 3). The most intense peak of XRD for aRbHgI3$H2O is (1 1 0) with 2q at 12.036 , (0 1 1) with 2q at 13.558 for b-RbHgI3$H2O. 3.3. Phase transition and thermal analysis

a- and b-RbHgI3$H2O crystallize in noncentrosymmetric space groups Cc for a-RbHgI3$H2O, Pc for b-RbHgI3$H2O. Furthermore, the phase transitions of RbHgI3$H2O polymorphs and thermal behaviours were investigated using differential thermal analysis (DTA), thermogravimetric analysis (TGA) and powder XRD at different temperatures for a-RbHgI3$H2O. As is shown in Fig. 4, the DTA diagram of a-RbHgI3∙H2O exhibited one endothermic peak at 100  C and no weight loss can be observed, which corresponded to the phase transition from a-phase to the b-phase. And then the phase transition was confirmed by the XRD patterns at different

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Fig. 1. (a) Ball-and-stick representation of the coordination mode of HgI4 tetrahedron (Rb, O, H atoms are omitted for clarity) in a-RbHgI3$H2O, each Hg atom is bonded to four iodine atoms with two short and two long bonds; (b) The HgI3 chain along the c axis; (c) View of the structure of a-RbHgI3$H2O down the b axis, (d) View of the structure of aRbHgI3$H2O down the c axis. All the longer Hg-I bonds are always located above the Hg atom in each HgI4 tetrahedron, giving rise to a net dipole moment parallel to the a axis (indicated by the black arrow).

temperatures shown in Fig. S2. It is clear that the a-RbHgI3$H2O phase was still the pure phase at 90  C and at a higher temperature, 100  C, b-RbHgI3$H2O generated and coexisted with a phase, then the pure phase of b-RbHgI3$H2O appeared accompanied by disappearance of a-RbHgI3$H2O at 110  C, which suggested that the temperature for the a to b phase transformation was around 100  C. RbHgI3$H2O can stabilize to 127  C and the TGA curve shows that the weight loss undergoes two steps in the range of 140e430  C under a nitrogen atmosphere, resulting in a total weight loss of about 69.25% (calculated value 68.98%). In the range of 140e210  C, the first step showed a weight loss of about 2.86% (calculated value 2.63%) which can be assigned to the removal of one water molecules in the structure. The second step, with the weight loss about 66.50% (calculated value 66.39%). In the range of 210e430  C, corresponded to the decomposition of RbHgI3 (Fig. S3). 3.4. Optical properties The IR spectra of a- and b-RbHgI3$H2O are presented in Fig. S4. Both of them exhibit wide infrared transmission ranges from 4000 to 400 cm1 (2.5e25 mm) except two absorption peaks around 3500 and 1600 cm1, which can be ascribed to the O-H vibrations in the water molecules. The UVevis diffuse reflectance spectra were collected for the two phases of RbHgI3$H2O and shown in Fig. 5, and the following Kubelka-Munk function was used to calculate the absorption (K/S) data:

FðRÞ ¼

ð1  RÞ2 K ¼ S 2R

In this function R refers to the reflectance, K is the absorption,

and S is the scattering [50,51]. In the (K/S)-versus-E plots, extrapolating the linear part of the rising curve to zero provides the onset of absorption. Both a- and b-RbHgI3$H2O are light yellow, and the spectra show that the absorption edges near the UV side are at about 476 and 495 nm, respectively, indicating that the optical band gaps of the titled compounds are about 2.6 eV and 2.45 eV. 3.5. NLO properties and LDT measurements The SHG signal measured with a 1064 nm laser as the fundamental wave is shown in Fig. 6 as a function of the particle diameters of the ground a- and b-RbHgI3$H2O crystals. For both the compounds, KDP and KTP samples were used as the references. Initially, as the sample size increased, the intensity of the SHG signal gradually increased. And then they tended to be constant from 150 mm. It is clear that the SHG responses of the two titled compounds are phase-matchable (type І). The second-harmonic signals are found to be 15.0  KDP and 1.1  KTP for aRbHgI3$H2O, 14.0  KDP and 1  KTP for b-RbHgI3$H2O. With the consideration that the NLO coefficient of KTP is about 14 times that of KDP, the measured relative magnitude of SHG coefficients are in accordance with each other. A compound with the same chemical formula RbHgI3$H2O has been reported in Ref. [57] which is not phase-matchable with a weak SHG intensity (4.2  a-SiO2). Compared with a- and bRbHgI3$H2O, it just reported the chemical formula instead of giving the crystal structure data which may be another phase of RbHgI3$H2O. In our work, a- and b-RbHgI3$H2O have been proved to be phase-matchable and exhibit strong SHG responds which are in accordance with the theoretical calculation results hereinafter. RbHgI3 reported by Li etc. in Ref. [47] possessed a similar crystal

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Fig. 2. (a) Ball-and-stick representation of the coordination mode of HgI4 tetrahedron (Rb, O, H atoms are omitted for clarity) in b-RbHgI3$H2O each Hg atom is bonded to four iodine atoms with two short and two long bonds; (b) The HgI3 chain along the a axis; (c) View of the structure of b-RbHgI3$H2O down the b axis, (d) View of the structure of bRbHgI3$H2O down the a axis. All the longer Hg-I bonds are always located above the Hg atom in each HgI4 tetrahedron, giving rise to a net dipole moment parallel to the c axis (indicated by the black arrow).

Fig. 3. Experimental and calculated XRD patterns for a- and b-RbHgI3$H2O. The black curves are the calculated ones, the red are the patterns of experimental sample. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

structure data with a-RbHgI3$H2O in our work. Both of them exhibited 1-D [HgI3] chains composed of [HgI4] tetrahedrons which made the major contribution to the strong SHG coefficients. While the difference between them is that there are water molecules in the crystal structure of a-RbHgI3$H2O which coordinate with Rbþ ions to influence the SHG effect. Since the SHG coefficient directly influence the conversion efficiency of the fundamental laser, the advantage of RbHgI3$H2O is with stronger SHG responses in compare to RbHgI3. The preliminary examinations of the LDT measurement have

been carried out on the crystalline samples using a Q-switched laser source. The samples showed a damage threshold of about 30.3 MW/cm2 (1064 nm, 10 ns) for a-RbHgI3$H2O and 32.3 MW/ cm2 for b-RbHgI3$H2O, which are approximately equal to that of AgGaS2 (30 MW/cm2, 1064 nm) [57e64]. 3.6. Theoretical calculations Based on DFT methods, theoretical calculations were performed to gain further insight into optical properties and NLO properties of

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Fig. 4. DTA and TGA curves for a-RbHgI3$H2O.

RbHgI3$H2O. The band structures are presented in Fig. S5. The results of band structure calculations indicate that compound aRbHgI3$H2O is a semiconductor with an indirect band gap of 1.87 eV, b-RbHgI3$H2O with a direct band gap of 1.82 eV. We can see that the calculated value is smaller than the experimental value of UVevis diffuse reflectance spectra, which can be ascribed to the common underestimation of the band gap by the DFT method. Fig. 7 presents the total and partial densities of states (DOS and PDOS) for a-RbHgI3$H2O and b-RbHgI3$H2O. In a-RbHgI3$H2O, the valence bands (VB) spanning over 25.0 to 15.0 eV predominately derive from O-2s and H-1s, to a lesser extent, from O 2p states. VBs ranging from 15.0 to 5.0 eV are mainly formed by the hybridization of O-2p, I-5s, Hg-5d and Rb-4p states with small amounts of O-2s and H-1s states, while VBs from 5.0 eV to the Fermi level (EF) are mostly contributed by O-2p, and I-5p states with small amount of Hg-5s6s states. I-5p and Hg-5s6s states mixed with a small amount of I-4d states make up the conduction bands (CB) between EF up to 5.0 eV. The DOS and PDOS for b-RbHgI3$H2O are similar with a-RbHgI3$H2O. In oxides, the principal contributions for SHG is the phonons including anharmonic ones. Since linear and nonlinear optical properties are mainly determined by the states close to the forbidden band mainly consisting of the p orbital of the [HgI4]2 groups and the electron transition is provided by inside

Fig. 6. Phase-matching curves for a- and b-RbHgI3$H2O. The inset represents an oscilloscope trace showing SHG intensity for a- and b-RbHgI3$H2O. The SHG intensities for KDP and KTP are also plotted for comparisons.

excitation of the [HgI4]2 group, which indicate that HgI4 polyhedra make a great contribution to the SHG coefficients. That is, the NLO features of the two titled compounds are determined by the dipole moments of intra- and inter-molecular bonds. Based on the electronic structure, the linear and nonlinear optical properties of a- and b-RbHgI3$H2O were also calculated. The curves of the birefringence indices versus wavelength are shown in Fig. S6. Both of the titled compounds are negative biaxial crystals with moderate birefringence, 0.05@1064 nm for a-RbHgI3$H2O and 0.055@1064 nm for b-RbHgI3$H2O, which are guaranteed to achieve the phase matching condition in the IR region. Since the space groups of a- and b-RbHgI3$H2O belong to class m, under the restriction of Kleinman's symmetry, there are six independent SHG tensor components (d11, d12, d13, d15, d24 and d33). The calculated frequency-dependent SHG tensor components of a- and bRbHgI3$H2O are plotted in Fig. S7. The value of d12 at the wavelength of 1064 nm (1.165 eV) is 30 pm/V for a-RbHgI3$H2O, and 29 pm/V for b-RbHgI3$H2O. The results are in accordance with the

Fig. 5. UVevis optical diffuse reflectance spectra of a- and b-RbHgI3$H2O.

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Fig. 7. Total and partial densities of states for (a) a-, and (b) b-RbHgI3$H2O.

experimental values that are approximately related to the effective SHG coefficients. 4. Conclusions In summary, two new noncentrosymmetric polymorphs, namely, a- and b-RbHgI3$H2O, were synthesized by reaction of RbI and HgI2 (1:1) in acetone solution. Both of the titled compounds exhibit the 3-D frameworks composed of 1-D [HgI3] chains and [Rb(H2O)]þ complexes. Both a- and b-RbHgI3$H2O show strong phase-matchable SHG intensities of about 15.0 and 14.0 times that of KDP which originate from superimposition of the microscopic second-order susceptibilities of distorted [HgI4] tetrahedra in consistent packing style. Theoretical calculations further confirmed the sources. a- and b-RbHgI3$H2O are transparent from near UV to mid and far-infrared region suggesting that the two titled compounds are potential candidates for NLO materials in the IR region. The growth of large crystals for further physical properties studies is ongoing. Acknowledgements This research was supported by the National Natural Science Foundation of China (Nos. 21501161 and 21875146). Appendix A. Supplementary data Supplementary data related to this article can be found at https://doi.org/10.1016/j.jallcom.2018.09.005. References [1] S.P. Guo, Y. Chi, G.C. Guo, Recent achievements on middle and far-infrared second order nonlinear optical materials, Coord. Chem. Rev. 335 (2017) 44e57. [2] I. Chung, M.G. Kanatzidis, Metal chalcogenides: a rich source of nonlinear optical materials, Chem. Mater. 26 (2014) 849e869. [3] K.M. Ok, E.O. Chi, P.S. Halasyamani, Bulk characterization methods for noncentrosymmetric materials: second-harmonic generation, piezoelectricity, pyroelectricity, and ferroelectricity, Chem. Soc. Rev. 35 (2006) 710e717. [4] B.B. Zhang, G.P. Han, Y. Wang, X.L. Chen, Z.H. Yang, S.L. Pan, Expanding frontiers of ultraviolet nonlinear optical materials with fluorophosphates, Chem. Mater. 30 (2018) 5397e5403. [5] M. Luo, F. Liang, Y.X. Song, D. Zhao, N. Ye, Z.S. Lin, Rational design of the first lead/tin fluorooxoborates MB2O3F2(M ¼ Pb, Sn), containing flexible twodimensional [B6O12F6]∞ single layers with widely divergent second harmonic generation effects, J. Am. Chem. Soc. 140 (2018) 6814e6817. [6] Y. Wang, B.B. Zhang, Z.H. Yang, S.L. Pan, Cation-tuned synthesis of fluorooxoborates: towards optimal deep-ultraviolet nonlinear optical materials, Angew. Chem. Int. Ed. 57 (2018) 2150e2154.

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