Pb10O4(BO3)3I3: a new noncentrosymmetric oxyborate iodide synthesized by the straightforward hydrothermal method

Jinjie Zhou , Hongping Wu , Hongwei Yu *, Zhanggui Hu * and Yicheng Wu
Tianjin Key Laboratory of Functional Crystal Materials, Institute of Functional Crystal, Tianjin University of Technology, Tianjin 300384, China. E-mail: hwyu15@gmail.com

Received 19th June 2019 , Accepted 21st July 2019

First published on 1st August 2019


Borate iodide has interesting functional properties but is challenging for synthesizing. A new non-centrosymmetric borate iodide Pb10O4(BO3)3I3 has been synthesized by a straightforward hydrothermal method. Pb10O4(BO3)3I3 crystallizes in the noncentrosymmetric monoclinic space group Cc. In its structure, the basic structural building units [Pb10O8] connect with BO3 groups to form two-dimensional (2D) [Pb10O8B2O6] layers stacked along the b-axis, which are further linked by the B(1)O3 groups to constitute a 3D framework. Interestingly, in this structure, some oxygen atoms are coordinated by four Pb2+ cations to form [OPb4] groups, rather than directly connecting with B3+ cations. Therefore, this compound belongs to a new type of borate iodide, oxyborate iodide. To the best of our knowledge, this is the first reported oxyborate iodide. In addition, its functional properties including second harmonic generation (SHG) response, UV-vis-NIR diffuse reflectance and IR spectra were also investigated. The origin of the SHG effect was studied by dipole moment calculations and structure comparison.


Introduction

Nonlinear optical (NLO) materials play vital roles in the recent development of laser science and technology.1 Based on the process of second harmonic generation (SHG), NLO materials are able to expand the frequency ranges of lasers, e.g., intense 532, 355, 266 and 177.3 nm laser light has been generated by the SHG of Nd:YAG or Nd:YVO4 lasers.2 Among the useful NLO crystals, borates are most popular owing to their wide transparency range, high resistance against laser damage and a variety of structure types. For instance, β-BaB2O4 (BBO),3,4 LiB3O5 (LBO),5 CsB3O5 (CBO),6 CsLiB6O10 (CLBO)7 and Sr2Be2B2O7 (SBBO)8 have been most widely commercialized. However, the above materials cannot satisfy all the requirements of frequency conservations in the whole wavelength region, especially in the deep-UV and IR regions. So it remains a challenge to design and discover new NLO materials.

Recently, borate compounds containing halogen anions (i.e, F, Cl, Br or I) have been reported with novel compositions and crystal structures which could produce a new generation of NLO materials owing to the following advantages over other borates.9 Firstly, the statistic reported by Pan et al. shows that borate halides have a high non-centrosymmetric (NCS) possibility (∼50%), which is higher than that of other borates (∼36%).10 Secondly, due to the inherent difference between metal–oxygen bonds and metal–halogen bonds, the polyhedra containing metal–oxygen bonds and metal–halogen bonds would have large distortion. This distortion of the polyhedra has been reported to make a positive contribution to the SHG response.11 In addition, halide anions, specifically F, Cl, and Br anions, can also widen the transparency range of the materials owing to their large electronegativity.12 At present, a number of borate halides with excellent NLO performance have been synthesized, including BaAlBO3F2,13 Rb3Al3B3O10F,14 Ca5(BO3)3F,15 NH4Be2BO3F2,16 Ba4B11O20F,12 K3B6O10X (X = Cl, Br),17,18 and Pb2B5O9I.17 Some fluoroborates, such as LiB6O9F2,20 NH4B4O6F,21 M2B10O14F6 (M = Ca2+, Sr2+),22,23 PbB2O3F2[thin space (1/6-em)]24 and AB4O6F (A = K+, Rb+, and Cs+)25 show promise as deep-UV or UV NLO materials.26

Notably, the most reported borate halides are borate fluorides, chlorides or bromides, while borate iodides are rare owing to the difficulty in synthesizing them. It is well known that metal iodides are easy to volatilize and oxidize at high temperature; so they cannot be synthesized by the traditional solid-state method in open air as other borate halides. In addition, according to Wang et al.'s report, borate iodides account for only 7% of the discovered borate halides.10 They include the T3B7O13I (T = Cr2+, Mn2+, Fe2+, Co2+, Ni2+ and Cu2+) series,27 Pb2B5O9I and Pb2BO3I.28 Most of them are synthesized under extreme conditions. For example, the T3B7O13I (T = Cr2+, Mn2+, Fe2+, Co2+, Ni2+ and Cu2+) series was discovered from naturally occurring minerals,27 while Pb2B5O9I was synthesized in a vacuum-sealed fused silica tube.19

Considering the high solubility of iodides in water, we have put forward the case that the straightforward hydrothermal reaction may be an effective method for the syntheses of borate iodides, and a new KBe2BO3F2-like borate iodide, Pb2BO3I, has been synthesized in our previous work. In this study, we also explore other new borate iodides under the hydrothermal conditions in the Pb–B–O–I system. Due to the possible contribution of Pb2+ cations with lone pair electrons towards SHG response, we focus our attention on the Pb-rich part in the Pb–B–O–I system, which leads to the successful synthesis of a new borate iodide, Pb10O4(BO3)3I3. Interestingly, in Pb10O4(BO3)3I3, part oxygen anions do not directly bond with B3+ cations; so this compound is a new type of borate iodide, oxyborate iodide. Herein, we report its synthesis, crystal structure, thermal behavior, UV-vis-IR spectrum and SHG effects.

Experimental

Syntheses

Pb10O4(BO3)3I3 was synthesized by a straightforward hydrothermal reaction with KI as the mineralizer. PbO, H3BO3 and KI with a molar ratio of 7[thin space (1/6-em)]:[thin space (1/6-em)]1[thin space (1/6-em)]:[thin space (1/6-em)]2 were thoroughly mixed and sealed in a Teflon pouch with an impulse sealer. Then the pouch was placed into a 125 ml Parr autoclave with 45 ml of deionized water as backfill. This autoclave was quickly heated to 220 °C, held for 48 h and cooled to room temperature at 3 °C h−1. Thus, the colorless and transparent single crystals of Pb10O4(BO3)3I3 were obtained with filtration in air.

By the solid state reaction, we synthesized Pb10O4(BO3)3I3 polycrystalline samples. PbI2, PbO and H3BO3 with a molar ratio of 3[thin space (1/6-em)]:[thin space (1/6-em)]17[thin space (1/6-em)]:[thin space (1/6-em)]6 were thoroughly mixed and preheated in a platinum crucible at 300 °C for 10 h to eliminate water; the products were cooled to room temperature and ground again. The mixture was then calcined at 460 °C for two days with several intermediate grinding processes until a single-phase powder was obtained.

Powder X-ray diffraction

The purity of the Pb10O4(BO3)3I3 polycrystalline powder was detected using a Bruker D2 PHASER powder X-ray diffractometer with Cu-Kα radiation (λ = 1.5418 Å). During powder diffraction, the angular range and fixed scanning steps remained constant at 2θ = 10°–60°, and 1 s per step, respectively. The XRD pattern is shown in Fig. 1. It indicates that the experimental powder X-ray diffraction pattern of Pb10O4(Bo3)3I3 is in good agreement with the theoretical powder X-ray diffraction pattern deduced using CIF.
image file: c9dt02579k-f1.tif
Fig. 1 Powder X-ray diffraction patterns of Pb10O4(BO3)3I3.

Single-crystal X-ray diffraction

The crystal data of Pb10O4(BO3)3I3 were collected on a Bruker SMART APEX II 4K CCD diffractometer with Mo Kα radiation (λ = 0.71073 Å) at 100(2) K, and the data were integrated with a SAINT program.29 A colorless and transparent Pb10O4(BO3)3I3 crystal with dimensions of 0.074 × 0.053 × 0.027 mm3 was chosen for structure determination. We solved the structure using the SHELXTL system by direct methods.30 A full matrix least-squares technique was employed to refine the atomic positions of the structure. PLATON was used to check the structural symmetry.31 Crystal parameters and structure refinements are given in Table S1. The other structural data including the refined atomic positions, the equivalent isotropic displacement parameters, and the selected bond distances (Å) and angles (°) are given in Tables S2 and S3.

IR spectroscopy

The IR spectrum of Pb10O4(BO3)3I3 was measured at room temperature by using a Shimadzu IR Affinity spectrometer in the range of 400–4000 cm−1. The sample powder (∼4 mg) was mixed thoroughly with dried KBr (∼400 mg).

UV-vis-NIR diffuse reflectance spectroscopy

A Shimadzu SolidSpec-3700DUV spectrophotometer was used to evaluate the optical diffuse reflectance spectrum of Pb10O4(BO3)3I3 in the range of 240–2500 nm at 25 °C.

Thermal analysis

The thermal behaviors of Pb10O4(BO3)3I3 were determined using a NETZSCH STA 449C thermal analysis instrument. Powder samples (∼10 mg) were heated from 100 °C to 900 °C at a rate of 10 °C min−1 with the aid of a flowing nitrogen atmosphere.

Powder SHG test

The SHG property test was performed on the polycrystalline samples of Pb10O4(BO3)3I3 by the Kurtz–Perry method32 with a 1064 nm Nd:YAG laser. The polycrystalline samples of Pb10O4(BO3)3I3 were sieved into distinct size ranges: <20, 20–38, 38–55, 55–88, 88–105, 105–150, and 150–200 microns. The KDP served as the standard and was sieved into the same particle size ranges.

Results and discussion

Crystal structure

The crystal structure was determined by the single-crystal XRD analysis, which shows that Pb10O4(BO3)3I3 crystallizes in an NCS monoclinic space group, Cc (no. 9). In the process of refining the structure, we first tried the centrosymmetric space group C2/c. It implied highly disordered (BO3)3− groups in the structure. These (BO3)3− groups were located at the intersection of the two axes and could be divided into two possible directions, each of which accounted for 50%. This disorder was either a reflection of the true statistical distribution of groups in the structure, or it might be a kind of artificial operation in the selected space group. In order to solve this uncertainty, we tried space groups Cc. We found that the NCS Cc space group perfectly solved this disorder. Therefore, we preferred the NCS space group Cc as the more real space group for Pb10O4(BO3)3I3. Thus, the non-centrosymmetric structure of Pb10O4(BO3)3I3 was mainly due to the [B(2)O3]3− and [B(3)O3]3− groups breaking the possible central symmetry. This means that ∼99% of the total electron density, related to the [Pb10O8]4+ cation pattern, I and [B(1)O3]3− anion clusters, scatters X-rays in a “centrosymmetric” way, whereas only ∼1%, related to the [B(2)O3]3− and [B(3)O3]3− groups, contributes to “non-centrosymmetric” scattering. Considering the weak anomalous scattering by boron and oxygen atoms, one can assume that the absolute structure of Pb10O4(BO3)3I3 cannot be well defined from the X-ray data using conventional Flack statistics. Therefore, a Flack parameter of ∼0.386 was generated during refinement. A similar phenomenon was also observed in LixNa1−xBa12(BO3)7F4.33

The structure is shown in Fig. 2. In an asymmetric unit, there are ten Pb atoms, three B atoms, thirteen O atoms and three I atoms. The B atoms are bonded to three oxygen atoms to form BO3 groups. The Pb2+ cations are bonded to three or four oxygen atoms to form the Pb–O polyhedra. Oxygen ions are distributed on one side of the equatorial surface of the Pb2+ cation. This indicates that the lone pair on each Pb2+ cation is stereochemically active. Furthermore, five distorted Pb–O polyhedra connect with each other by edge-sharing to form crown-like [Pb5O9] groups, two of which are connected by two Pb–O bonds to build [Pb10O8] repeating units (Fig. 2a). The repeating units [Pb10O8] connect with each other through Pb–O bonds and BO3 (B(2)O3 and B(3)O3) triangles to form two-dimensional (2D) [Pb10O8(BO3)2] layers (Fig. 2b), which stack along the b-axis and are further bridged by B(1)O3 groups forming the 3D [Pb10O4(BO3)3] framework with I anions filled in the channels to balance the residual charges (Fig. 2c).


image file: c9dt02579k-f2.tif
Fig. 2 Crystal structure of Pb10O4(BO3)3I3: (a) ten Pb–O polyhedra are constructed into two [Pb5O9] groups. (b) The repeating units [Pb10O8] through BO3 group connection to form 2D [Pb10O8(BO3)2] layers. (c) The [Pb10O8(BO3)2] layers are further connected by B(1)O3 groups to form a 3D [Pb10O4(BO3)3] framework with I anions filled in the channel.

Owing to the stereochemically active lone pairs (SCALP) of Pb2+ cations, Pb–O bonds and Pb–I bonds exhibit wide bond length variations, from 2.193(10) to 2.716(13) Å for Pb–O bonds and from 3.424(12) to 3.714(17) Å for Pb–I bonds. The B–O distances (1.330(2)–1.420(2) Å) in BO3 triangles have an average bond distance of 1.366(2) Å. These values are in agreement with those of other borate compounds reported previously.34,35 Bond valence calculations result in bond valence sums (BVS) of 1.83–2.16 (Pb2+), 2.91–3.03 (B3+), 2.00–2.17 (O2−) and 0.51–0.78 (I) (Table S2). The low BVS of the halide anion is common in Pb2+ containing systems,36,37 owing to the influence of the lone-pair electrons of cations on bond-valence parameters.38

Type characteristics of borate iodides

As described above, borate iodide is difficult to synthesize and only some borate iodides have been reported, including T3B7O13I (T = Cr2+, Mn2+, Fe2+, Co2+, Ni2+ and Cu2+), Pb2B5O9I and Pb2BO3I (Table 1). Structurally, T3B7O13I (T = Cr2+, Mn2+, Fe2+, Co2+, Ni2+ and Cu2+) and Pb2B5O9I are polyborate iodides. In them, the borate groups are BO3 triangles and BO4 tetrahedra, which connect with each other to form a 3D framework and the Pb2+ and I anions fill in the space of the borate framework. According to West et al., these borate iodides can be seen as salt-inclusion solids (SISs).11 With the increase in the number of Pb2+ cations, Pb2BO3I becomes an orthoborate iodide, i.e. the basic building units of the structure are isolated BO3 groups, which are connected with Pb2+ sharing three oxygen atoms to form [Pb2BO3] 2D layers, and these layers are connected by Pb–I bonds to form a 3D framework. Obviously, in orthoborate iodides, Pb2+ and I ions will have more effect on the arrangement of borate groups. However, different from the above borate iodides, Pb10O4(BO3)3I3 is an oxyborate. In its structure, the basic building units contain not only BO3 triangles and I anions but also OPb4 tetrahedra with an O2− anion center. Therefore, the functional properties will depend on not only the borate groups and I anions, but also the OPb4 tetrahedra.
Table 1 A comparison of borate iodides
Compound Space group SHG FBB Compound category
T3B7O13I (T = Cr, Mn, Fe, Co, Ni, Cu) F[4 with combining macron]3c or Pca21 [B7O13]5− Polyborate iodides
Pb2B5O9I Pnm2 13.5 × KDP [B5O9]3− Polyborate iodides
Pb2BO3I P321 10 × KDP [BO3]3− Orthoborate iodides
Pb10O4(BO3)3I3 Cc 0.1 × KDP [BO3]3− isolated O2− Oxyborate iodides


IR spectroscopy

The IR spectrum of Pb10O4(BO3)3I3 is shown in Fig. 3a. The peaks at 1218 and 1142 cm−1 can be attributed to the asymmetric stretching and symmetric stretching vibrations of the BO3 groups, respectively. The peaks at 703, 607, and 564 cm−1 can be assigned to the bending vibrations of the BO3 group. These assignments are consistent with those of other reported borates.39
image file: c9dt02579k-f3.tif
Fig. 3 (a) IR spectrum of Pb10O4(BO3)3I3. (b) Diffuse reflectance spectra of Pb10O4(BO3)3I3.

UV-vis-NIR diffuse reflectance spectroscopy

The absorption spectrum was converted from the reflectance spectrum using the Kubelka–Munk function: F(R) = (1 − R)2/2R = α/S, where R is reflectance, α is absorption and S is scattering.40,41 It is clear that Pb10O4(BO3)3I3 has a wide transparent range. There are no absorption peaks in the range from 400 to 2500 nm and the cutoff edge is close to 361 nm, corresponding to the band gaps of 3.20 eV (Fig. 3b).

Thermal analyses

The thermal behavior was also measured by TG/DTA analysis (Fig. S1a). On the TG curve, there is a continuous weight loss and the weight loss becomes more severe when the sample is heated to 650 °C. At this temperature, one obvious endothermic peak is also observed on the DTA curve. The PXRD analysis shows that the residues after TG/DTA are PbO and PbO(B2O3)2 (Fig. S1b). These indicate that Pb10O4(BO3)3I3 melts incongruently. It will decompose owing to the volatilization of iodine.

Second harmonic generation (SHG) response analysis and the calculation of the dipole moment

Pb10O4(BO3)3I3 crystallizes in the NCS structure. Its SHG response was also measured. It exhibited a weak powder SHG effect, about 0.1 times that of KDP. This SHG response is much weaker than those of other reported borate iodides, such as Pb2B5O9I (∼13.5 × KDP) and Pb2BO3I (∼10 × KDP). This weak SHG response puzzled us. Pb10O4(BO3)3I3 contains multiple SHG-active building units, such as the π-conjugated BO3 group, the Pb2+ cations with SCALP and I and O2− anions with large electronegativity. However, it exhibits a weak SHG response. In order to understand this, we analyzed the dipole moment of the building units of Pb10O4(BO3)3I3 based on the bond valence model.42–46 In the calculations, the lone pairs of the Pb2+ cations are given a charge of −2 and localized 0.86 Å from the Pb2+ cation.47 The calculated results are listed in Table 2. It is clear that the dipole moment of a single [Pb5O9] unit is large. However, the adjacent [Pb5O9] units have opposite orientations, which results in the cancellation of their net dipole moment in the unit cell. In addition, for BO3 triangles, they also exhibit disorganized arrangements (Table S4) (Fig. S3). So their microscopic SHG responses cannot be added. These lead to the weak SHG response of Pb10O4(BO3)3I3.
Table 2 The direction and magnitude (in Debye) of [Pb10O8] for Pb10O4(BO3)3I3
Repeating units Polyhedron X-Axis dipole moment Y-Axis dipole moment Z-Axis dipole moment
[Pb5O9](1) Pb(3)O3I2 −2.43 −2.63 −7.03
Pb(4)O4 −5.03 1.32 4.58
Pb(5)O4I2 −12.01 1.77 −3.09
Pb(7)O4I2 7.31 −6.68 5.01
Pb(10)O3 −5.73 −2.49 −5.86
Total −17.89 −8.71 −6.39
 
[Pb5O9](2) Pb(1)O3I2 2.78 3.13 5.32
Pb(2)O4 6.12 −1.98 −4.42
Pb(6)O4I2 11.60 −2.53 3.04
Pb(8)O4I −3.87 6.39 −1.88
Pb(9)O3I 2.02 2.94 2.87
Total 18.65 7.95 4.93


Conclusions

A new borate iodide Pb10O4(BO3)3I3 was successfully synthesized through a straightforward hydrothermal reaction. It crystallizes in the NCS space group Cc. Structurally, its part O2− anions do not directly connect with B3+ cations. It represents a new type of oxyborate iodide. As for its properties, Pb10O4(BO3)3I3 has a band gap of 3.2 eV and an SHG response 0.1 times that of KDP. The dipole moment analyses suggest that the weak SHG response is mainly because of the opposite orientations of the adjacent [Pb5O4] groups, which results in the cancellation of their microscopic SHG responses. TG-DTA curves indicate that Pb10O4(BO3)3I3 is an incongruent melting compound. The synthesis of the title compound indicates that the straightforward hydrothermal reaction is an effective method for synthesizing new types of borate iodides. The syntheses of other new borate iodides are on the way.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work is supported by the National Natural Science Foundation of China (Grant No. 51802217, 51890864, and 51890865).

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Footnote

Electronic supplementary information (ESI) available: CIF files, atomic coordinates, equivalent isotropic displacement parameters and bond valence sum (BVS), and table of bond lengths and angles for Pb10O4(BO3)3I3. CCDC 1920738. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c9dt02579k

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