From CdPb₈(SeO₃)₄Br₁₀ to Pb₃(TeO₃)Br₄: the first tellurite bromide exhibiting an SHG response and mid-IR transparency

Abstract

Herein, we report two new selenite/tellurite bromide compounds, namely, CdPb₈(SeO₃)₄Br₁₀ and Pb₃(TeO₃)Br₄. The former crystallized in a centrosymmetric space group, while the latter is a non-centrosymmetric compound. Specifically, CdPb₈(SeO₃)₄Br₁₀ demonstrates high thermal stability (425 °C) and a wide optical transparency window (0.33–6.5 μm), while Pb₃(TeO₃)Br₄ exhibits an second harmonic generation response approximately equivalent to that of KDP, appropriate birefringence (0.095@532 nm and 0.066@1064 nm), wide optical transparency window (0.33–6.5 μm) and high thermal stability (522 °C). These results indicated that lead tellurite bromides could be promising nonlinear optical material systems in the mid-IR wavelength.

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Introduction

Nonlinear optical (NLO) crystals are widely used in various fields such as frequency conversion, optical modulation, optical switching and limiting, optical sensing, remote sensing, environmental monitoring, and free-space communications.^1–6 Second-order nonlinear optical materials require compounds to crystallize in non-centrosymmetric (NCS) space groups.^7–10 According to the ICSD database, the probability of obtaining a non-centrosymmetric structure is less than 20%.^11–13 So, the rational design of inorganic compounds with non-centrosymmetric symmetry remains a challenging but beneficial task for researchers in chemical and materials science.

Selenite and tellurite compounds are important systems for exploring NLO materials.¹⁴ The coordination environment of Se⁴⁺ and Te⁴⁺ is inherently polar due to the presence of stereo-chemically active lone pairs (SALP) of electrons, with oxygen ligands being located on one side of the cations.^15,16 Researchers have made significant efforts to enhance the probability of obtaining NCS selenite and tellurite crystals. In this endeavor, one effective strategy employed is to combine different nonlinear active groups within a single structure.¹⁷ Known nonlinear active units include, but are not limited to, d⁰ transition metal octahedra (TiO₆, MoO₆, WO₆, VO₆, NbO₆, etc.)^11,18 and their derivatives (GaO₆, GaO₃F₃, etc.),^19–21 d¹⁰ transition metals with large polar displacement (Zn²⁺, Cd²⁺, Hg²⁺),^22,23 and SALP cations (Pb²⁺, Bi³⁺, Sn²⁺, Sb³⁺, etc.).^24,25 Some high-performance NLO materials have been explored, such as Pb₂(SeO₃)(NO₃)₂ (2 × KDP),²⁶ NaNbO(SeO₃)₂ (7.8 × KDP),¹⁸ LiNbTeO₅ (17 × KDP),²⁷ Cd₂Nb₂Te₄O₁₅ (31 × KDP),²⁸ β-BaTeW₂O₉ (1.5 × KTP)²⁹ and TlSb₃Te₂O₁₂ (37.2 × KDP).³⁰ Furthermore, replacing the oxygen atoms in the coordination with VIIA anions is another effective approach for obtaining NCS compounds.^31,32 For example, the first UV NLO selenite, Y₃F(SeO₃)₄ (5.5 × KDP), was created through a fluorination control strategy.³³ Ba(MoO₂F)₂(TeO₃)₂, containing partially fluorinated MoO₆ octahedra, can demonstrate a large second harmonic generation (SHG) intensity (7.8 × KDP).³⁴ Additionally, some halogenated selenite/tellurite compounds, such as Pb₂GaF₂(SeO₃)₂Cl (4.5 × KDP),³⁵ Pb₂Bi(SeO₃)₂Cl₃ (13.5 × KDP),³⁶ Cs(TiOF)₃(SeO₃)₂ (5 × KDP),³⁷ RbGa₃F₆(SeO₃)₂ (5.6 × KDP),³⁸ BaF₂TeF₂(OH)₂ (3 × KDP)³⁹ and RbTeMo₂O₈F (27 × KDP),⁴⁰ also exhibit excellent SHG effects. Based on literature research, it can be observed that most of the research on halogenated selenite/tellurite NLO materials focuses on F⁻ and Cl⁻, while there is relatively less research on Br⁻ and I⁻. Only the following selenites, Pb₂NbO₂(SeO₃)₂Br (1.4 × KDP),⁴¹ Pb₂GaF₂(SeO₃)₂Br (4.5 × KDP),⁴¹ Pb₂Cd(SeO₃)₂Br₂ (1.4 × KDP),¹⁷ and Pb₃(SeO₃)Br₄ (1 × KDP),⁴² have been reported to exhibit SHG effects.

Generally, Br⁻ and I⁻ anions have higher polarizability and lower electronegativity compared to F⁻ and Cl⁻ anions.⁴³ This leads to more favorable effects of Br⁻ and I⁻ anions on the SHG efficency.^42,44,45 Additionally, heavier elements such as Br⁻ and I⁻ are more helpful for transparency in the mid-infrared (MIR) range.⁴⁶ Based on this, we conducted research on halide selenite/tellurite compounds. To achieve high transparency in the MIR spectrum and increase the probability of obtaining NCS compounds, we have chosen the heavy element Pb(II) and d¹⁰ TM Cd(II) as counter cations. After numerous attempts, we have synthesized a new compound of CdPb₈(SeO₃)₄Br₁₀ through mild hydrothermal reactions. Unfortunately, CdPb₈(SeO₃)₄Br₁₀ crystallized in a centrosymmetric (CS) space group. Through structural analysis, we found that the cadmium is surrounded by four SeO₃ groups and the polarity of the SALP Se⁴⁺ was almost cancelled out in the [Cd(SeO₃)₄]⁶⁻ unit. In order to achieve the structural transformation from CS to NCS, such an unfavourable arrangement should be changed.⁴⁷ We attempted to remove the CdO₆ groups from the structure, and introduce larger Te⁴⁺ cations to support the framework. Ultimately, we successfully obtained the compound Pb₃(TeO₃)Br₄, which crystallized in an NCS space group. It is worth noting that Pb₃(TeO₃)Br₄ is the first reported NLO material with SHG activity and exhibits good transparency in the MIR region. Here, we provide a detailed description of their synthesis, crystal structures, thermal stability, and optical properties.

Results and discussion

CdPb₈(SeO₃)₄Br₁₀ and Pb₃(TeO₃)Br₄ were synthesized using a mild hydrothermal synthesis method. Details of the synthesis can be found in the ESI (ESI Experimental section†). Additionally, their powder X-ray diffraction (PXRD) patterns were recorded and found to perfectly match those of simulated data, indicating that the obtained samples are produced in pure phases (Fig. S1†). The crystallographic data of CdPb₈(SeO₃)₄Br₁₀ and Pb₃(TeO₃)Br₄ are listed in ESI Table S1.†

CdPb₈(SeO₃)₄Br₁₀ crystallizes in the C2/c (No. 15) space group with a CS structure. Its asymmetric unit contains one Cd, four Pb, two Se, five Br, and six O atoms, totalling eighteen atoms. Only Cd(1) occupies a special position with an occupancy of 0.5. In the structure, Cd(1) is coordinated with six O atoms forming a CdO₆ octahedron with Cd–O bond lengths in the range of 2.263–2.617 Å. The Se atoms are connected with three O atoms forming SeO₃ trigonal pyramids with Se–O bond lengths ranging from 1.692 to 1.736 Å. The Pb atoms are connected with O and Br atoms, with Pb–Br and Pb–O bond lengths in the range of 3.022–3.193 Å and 2.435–2.734 Å, respectively. Bond valence calculations revealed that Cd(1), Pb(1)–Pb(4), and Se(1)–Se(2) exhibit bond valences of 1.744, 1.492–2.039, and 3.922–4.036, respectively. The deviation of Pb(1), Pb(2) and Pb(4) from their ideal oxidation states can be attributed to their fewer primary coordination bonds and longer secondary coordination bonds that are often overlooked in calculations. This phenomenon is a common occurrence in lead-containing compounds, as evidenced by some previously reported compounds.⁴⁸ If longer Pb⋯Br distances were considered, the BVS of Pb(1)–Pb(4) can be raised to 1.885–2.039.

CdPb₈(SeO₃)₄Br₁₀ features a novel three-dimensional (3D) network structure composed of a lead oxybromide framework decorated with CdO₆ octahedra and SeO₃ trigonal pyramids (Fig. 1). One CdO₆ octahedron is connected with four SeO₃ groups via edge- and corner-sharing to form a [Cd(SeO₃)₄]⁶⁻ unit (Fig. S2a†). Two Pb(1)O₅Br₁ and two Pb(2)O₅Br₁ units are connected via oxygen atoms to form Pb₄O₁₂Br₄ tetramers (Fig. S2b†). The Pb(3)O₂Br₆ and Pb(4)O₂Br₃ units share bromine atoms to form a 3D network with four-membered polyhedral ring (4-MR) tunnels (Fig. S2c†). The [Cd(SeO₃)₄]⁶⁻ units and Pb₄O₁₂Br₄ tetramers are interconnected into one-dimensional (1D) chains along the b-axis (Fig. 1b and S2d†), which are located in the center of the 4-MR tunnels to support the framework.

Fig. 1 The 3D network structure (a) and the 1D chain component (b) of CdPb₈(SeO₃)₄Br₁₀.

Pb₃(TeO₃)Br₄ crystallizes in the NCS space group Pna2₁ (No. 33), isostructural with Pb₃(TeO₃)Cl₄ ⁴⁹ and Pb₃(SeO₃)Br₄.⁴² The asymmetric unit of the compound consists of a total of 11 atoms, which include three Pb, one Te, four Br, and three O atoms. All these atoms are located in general positions. The Te(1) atom adopts a TeO₃ trigonal pyramid coordination mode with Te–O bond lengths of 1.860–1.891 Å. The Pb atoms are coordinated with Br and O atoms with Pb–O and Pb–Br bond lengths in the range of 2.373–2.581 Å and 2.938–3.230 Å, respectively. Based on valence bond analysis, the oxidation states of lead and tellurium atoms are +2 and +4, respectively. The calculated values for Pb(1), Pb(2), Pb(3), and Te(1) are 1.797, 1.844, 1.640, and 3.984, respectively. Similarly, due to the neglect of longer secondary bonds involving Pb²⁺ cations, the calculated oxidation states tend to be underestimated.⁵⁰ If longer Pb⋯Br and Pb⋯O distances were to be considered, the BVS of Pb(1)–Pb(3) can be raised to 1.943–1.967.

Pb₃(TeO₃)Br₄ exhibits a 3D framework composed of Pb–O–Br skeletons modified by TeO₃ groups (Fig. 2). The Pb(1)O₂Br₃, Pb(2)O₁Br₅, and Pb(3)O₃Br₃ polyhedra are connected via shared Br(1), Br(3), O(2) and O(3) atoms to form Pb₃O₄Br₉ trimers. These Pb₃O₄Br₉ trimers are interconnected through Pb–Br bonds to form a 1D chain with 4-MR tunnels along the c-axis. The TeO₃ groups are embedded in the center of the tunnels and connected to Pb atoms through bridging O atoms. The 1D chains are further interconnected via Pb–Br bonds to form the 3D framework structure (Fig. S3†).

Fig. 2 The 3D network structure (a) and the 1D chain component (b) of Pb₃(TeO₃)Br₄.

To figure out the relationships between the macroscopic symmetries and the arrangements of building blocks, the local dipole moments of CdPb₈(SeO₃)₄Br₁₀ and Pb₃(TeO₃)Br₄ have been calculated. As shown in Table S3,† the polarity of the four selenite groups around the CdO₆ octahedron is almost cancelled out. Three quarters of the polarity of Pb(3)₂Pb(4)₂ 4-MR has been counteracted by the CdO₆ octahedron and Pb₄O₁₂Br₄ tetramer. Although the 1D chain component of CdPb₈(SeO₃)₄Br₁₀ is polar, the polarities of two neighbouring chains have been cancelled out completely. As for Pb₃(TeO₃)Br₄, the polarities of TeO₃ groups can be superimposed at z-components. Although the polarities of Te and Pb polyhedra are opposite, the 1D chain component is also polar. Furthermore, the polarities of the neighboring chains have been superimposed too, which leads to the NCS and polar space group of Pb₃(TeO₃)Br₄.

The thermal stability of CdPb₈(SeO₃)₄Br₁₀ and Pb₃(TeO₃)Br₄ was investigated through thermogravimetric analysis (TGA) in the temperature range of 25–1200 °C under an N₂ atmosphere. As shown in Fig. S4,† CdPb₈(SeO₃)₄Br₁₀ and Pb₃(TeO₃)Br₄ can remain stable up to 425 °C and 522 °C, respectively. After reaching these temperatures, both compounds began to experience weight loss. At 1200 °C, CdPb₈(SeO₃)₄Br₁₀ exhibited a total weight loss of 88.95%, corresponding to the loss of all SeO₂ and bromides of Cd and Pb, and partial loss of Pb oxides. Pb₃(TeO₃)Br₄ showed a weight loss of 80.13% (calculated value: 80.02%), equivalent to the loss of one molecule of TeO₂ and two molecules of PbBr₂.

The UV-vis-NIR diffuse-reflectance spectra reveal that CdPb₈(SeO₃)₄Br₁₀ and Pb₃(TeO₃)Br₄ are nearly transparent between 500 and 2000 nm. The UV absorption cutoff edges for these two compounds are 326 nm and 328 nm, respectively. Their band gaps were determined to be 3.32 eV and 3.31 eV, respectively (Fig. S5†), which are comparable to previously reported compounds, such as Pb₂NbO₂(SeO₃)₂Br (3.17 eV),⁴¹ Ba(MoO₂F)₂(SeO₃)₂ (3.23 eV),³⁴ Lu₃F(SeO₃)₄ (3.57 eV),⁵¹ Ag₂(TeO₂F₂) (3.22 eV)⁵² and Hg₃(Te₃O₈)(SO₄) (3.36 eV).²²

The infrared spectra (IR) demonstrate that CdPb₈(SeO₃)₄Br₁₀ and Pb₃(TeO₃)Br₄ exhibit good transparency in the range of 4000–775 cm⁻¹ (Fig. S6†). In particular, the absorption peaks at 400–455 cm⁻¹ can be attributed to the vibration of the Pb–O and Cd–O bonds. The prominent absorption peaks at 605–775 cm⁻¹ correspond to the bending and stretching vibrations of the Se–O and Te–O bonds, which are consistent with previous literature reports.^17,50 In summary, CdPb₈(SeO₃)₄Br₁₀ and Pb₃(TeO₃)Br₄ exhibit a wide transparency range of 0.33–12.96 μm and 0.33–12.95 μm, respectively. Considering the deviation of the IR cutoff edge for powder and crystal measurements due to multiphonon absorption, 50% eliminations were made.^53,54 So, the transparency range of CdPb₈(SeO₃)₄Br₁₀ and Pb₃(TeO₃)Br₄ should be 0.33–6.5 μm, covering an important atmospheric transparency window (3–5 μm) in the MIR region^28,55 (Fig. 3). This indicates that lead tellurite bromides could be potential NLO material systems in the MIR wavelength.

Fig. 3 UV-vis-NIR diffuse reflectance and IR transmittance spectra of CdPb₈(SeO₃)₄Br₁₀ (a) and Pb₃(TeO₃)Br₄ (b).

Pb₃(TeO₃)Br₄ possesses a large bandgap (3.31 eV), which tends to result in a high laser-induced damage threshold (LIDT). We measured the LIDT of Pb₃(TeO₃)Br₄ using the reported powder method. The LIDT of Pb₃(TeO₃)Br₄ is 21.5 MW cm⁻², which is significantly higher than that of AgGaS₂ (2.6 MW cm⁻²).

Since the structure of Pb₃(TeO₃)Br₄ crystallizes in an NCS space group and contains two different nonlinear active units, it is necessary to detect its second-order nonlinear-optical response. A Q-switched Nd:YAG 1064 nm laser was chosen as the fundamental radiation, and the SHG signal was measured using the Kurtz–Perry method (ESI Experimental section†).⁵⁶ The SHG measurement indicates that Pb₃(TeO₃)Br₄ exhibits a frequency-doubling efficiency comparable to that of the commercial KDP (Fig. 4a). Compared with the SHG-inactive isomorphic compound Pb₃(TeO₃)Cl₄,⁴⁹ Pb₃(TeO₃)Br₄ shows a stronger SHG response, primarily attributed to the more polarizable property of the Br⁻ anion compared with the Cl⁻ anion.⁴² In order to better understand the difference in the SHG effects between Pb₃(TeO₃)Cl₄ and Pb₃(TeO₃)Br₄, we compared the local dipole moments of the TeO₃ groups and PbO_nX_m (X = Cl, Br) polyhedra, as well as the net dipole moments of these two compounds in their unit cells (Table S4†). The net dipole moment in the unit cell of Pb₃(TeO₃)Cl₄ is 8.68 D, while the net dipole moment in the unit cell of Pb₃(TeO₃)Br₄ is 10.062 D. The larger net dipole moment in Pb₃(TeO₃)Br₄ results in a larger SHG response compared to Pb₃(TeO₃)Cl₄. It is worth noting that Pb₃(TeO₃)Br₄ is the first example of tellurite bromide that has been detected to exhibit the SHG effect.

Fig. 4 The oscilloscope traces of the SHG signals for the powder crystals (150–210 μm) of Pb₃(TeO₃)Br₄ and KDP under laser irradiation at 1064 nm (a). The total and partial density of states (b), the calculated refractive indices and birefringence (c) and the electron density difference (EDD) maps (d) of Pb₃(TeO₃)Br₄.

To investigate the electronic structures and optical properties of CdPb₈(SeO₃)₄Br₁₀ and Pb₃(TeO₃)Br₄, theoretical calculations based on the density functional theory (DFT) method were performed. The state energies of the lowest conduction band and the highest valence band are listed in Table S4.† As shown in Fig. S7,† CdPb₈(SeO₃)₄Br₁₀ is an indirect bandgap compound with a calculated bandgap of 2.955 eV, while Pb₃(TeO₃)Br₄ is a direct bandgap compound with a calculated bandgap of 3.134 eV. Due to the limitations of the DFT-GGA-PBE (GGA = generalized gradient approximation and PBE = Perdew−Burke−Ernzerhof) exchange–correlation functional, the calculated bandgaps are underestimated compared to the experimental values. Therefore, in the subsequent calculations, we employed a scissor operator with energy offsets of 0.365 eV and 0.176 eV, respectively.

The linear optical response characteristics of compounds CdPb₈(SeO₃)₄Br₁₀ and Pb₃(TeO₃)Br₄ were calculated based on the complex dielectric function ε(ω) = ε₁(ω) + iε₂(ω). Both compounds belong to biaxial crystals, where the three principal permittivity coefficients are unequal, ε₁ ≠ ε₂ ≠ ε₃, and the refractive indices are also different, n₁ ≠ n₂ ≠ n₃. For CdPb₈(SeO₃)₄Br₁₀, the refractive index order is n₀₀₁ > n₁₀₀ > n₀₁₀. The calculated birefringence (Δn) values are 0.028@532 nm and 0.014@1064 nm (Fig. S8†), which are similar to those previously reported for compounds La₂Hg₃(SeO₃)₄(SO₄)₂(H₂O)₂ (0.013@532 nm and 0.008@1064 nm) and Ag₂Cd(Se₂O₅)(Se_0.3S_0.7O₄) (0.026@532 nm and 0.022@1064 nm) by our group.⁵⁷ For Pb₃(TeO₃)Br₄, the refractive index order is n₁₀₀ > n₀₁₀ > n₀₀₁. The calculated birefringence (Δn) values are 0.095@532 nm and 0.066@1064 nm (Fig. 4b). It is worth noting that Pb₃(TeO₃)Br₄ exhibits a similar birefringence to those of previously reported Te oxides, such as Y₃(TeO₃)₂(SO₄)₂(OH)(H₂O) (0.092@532 nm),¹⁵ AgAl(Te₄O₁₀) (0.104@532 nm),¹⁹ BaF₂TeF₂(OH)₂ (∼0.078@300–700 nm),³⁹ Rb[Te₂O₄(OH)₅] (0.0545@1064 nm),⁵⁸ Li₂ZrTeO₆ (0.064@1064 nm)⁵⁹ and AgTeO₂F (0.078@1064 nm).⁵²

Partial density of states (PDOS) and total density of states (TDOS) calculations were performed for CdPb₈(SeO₃)₄Br₁₀ and Pb₃(TeO₃)Br₄. As shown in Fig. 4c, the lowest conduction bands of Pb₃(TeO₃)Br₄ mainly originate from the unoccupied Pb-6p orbitals and Te-5p orbitals, while the highest valence bands are mainly contributed by the Br-4p and O-2p non-bonding states. As for CdPb₈(SeO₃)₄Br₁₀, the lowest conduction bands are mainly from the unoccupied Pb-6p orbitals while the highest valence bands are mainly contributed by the Br-4p non-bonding states (Fig. S9†). Therefore, the band gap of Pb₃(TeO₃)Br₄ is mainly determined by the Pb and Te polyhedra, while the band gap of CdPb₈(SeO₃)₄Br₁₀ is mainly determined by the Pb–Br bonds.

It is worth noting that the states near the Fermi level have a significant impact on the linear and nonlinear properties of Pb₃(TeO₃)Br₄. The main influencing factors are Pb-6p, Te-5p, O-2p, and Br-4p orbitals in this compound. This indicates that the optical properties of Pb₃(TeO₃)Br₄ primarily originate from the synergistic interaction between the TeO₃ groups and the lead oxybromide polyhedra. To further support this inference, we have studied the electron density difference (EDD) map of Pb₃(TeO₃)Br₄. Fig. 4d shows that Te⁴⁺ possesses stereo-chemically active lone pair electrons and there is charge transfer from Pb atoms to O/Br atoms.

Conclusions

In summary, two new bromine-containing selenite/tellurite compounds, namely, CdPb₈(SeO₃)₄Br₁₀ and Pb₃(TeO₃)Br₄, were obtained by a mild hydrothermal method in the Pb²⁺-Se⁴⁺O₃/Te⁴⁺O₃-Br⁻ system. Interestingly, due to the difference in the arrangement of the polar building blocks, these two compounds exhibit two different lead oxybromide 3D structures. CdPb₈(SeO₃)₄Br₁₀ is crystallized in a CS space group of C2/c while Pb₃(TeO₃)Br₄ is crystallized in an NCS space group of Pna2₁. Importantly, Pb₃(TeO₃)Br₄ is the first reported tellurite bromide compound that exhibits SHG activity. It demonstrates an SHG intensity comparable to that of KDP, high thermal stability (522 °C), wide optical transparency window (0.33–6.5 μm) and appropriate birefringence (0.095@532 nm and 0.066@1064 nm). This work proved that lead tellurite bromides can be promising NLO material systems in the mid-IR wavelength.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (Grant No. 22031009, 22375201, 21921001 and 91963105) and the NSF of Fujian Province (Grant No. 2023J01216).

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Footnote

† Electronic supplementary information (ESI) available: Detailed synthesis, PXRD patterns, crystal data, TG curves, UV-vis-NIR diffuse-reflectance spectra, IR spectra, and computational method. CCDC 2278543 for CdPb₈(SeO₃)₄Br₁₀ and 2278544 for Pb₃(TeO₃)Br₄]. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d3qi01937c

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