Pb₄Zn₂B₁₀O₂₁: a congruently melting lead zinc borate with a novel [B₁₀O₂₄] anionic group and an interesting [Pb₄O₁₂]_∞ chain

Abstract

A new lead zinc borate, Pb₄Zn₂B₁₀O₂₁, has been synthesized and the crystal structure was determined by single crystal X-ray diffraction. It crystallizes in the orthorhombic group Pbcn with a = 14.6062(15) Å, b = 17.4899(16) Å, c = 13.2962(15) Å, Z = 8. Interestingly, in the crystal structure, a new B–O building unit [B₁₀O₂₄] group is observed and the four different coordinated Pb atoms are connected with each other by sharing a corner, edge or face to form an interesting regular [Pb₄O₁₂]_∞ chain. The DSC analysis proves that Pb₄Zn₂B₁₀O₂₁ melts congruently. First-principles electronic structure calculation shows that the calculated band gap of Pb₄Zn₂B₁₀O₂₁ is 3.53 eV, which is in good agreement with the experimental value measured from the UV-vis-NIR absorption spectrum, and the observed absorption peak is assigned as a charge transfer from O 2p states to Pb 6p states.

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Introduction

In recent years, much work has been focused on the synthesis, characterization and physical studies of borate compounds, owing to their rich structures and interesting physical properties.^1–3 Boron is a unique element, it may adopt triangular (BO₃) or tetrahedral oxygen coordination (BO₄). Accordingly, the atomic orbitals are hybridized to planar sp² or three dimensional sp³ structures. Further, such structural units can connect with each other via sharing corners or edges to comprise several different B_xO_y groups, which are responsible for the versatile physical properties of borates.⁴

Over the years, several borates with novel structures or important applications were reported, for example, β-BaB₂O₄ (BBO),⁵ LiB₃O₅ (LBO),⁶ CsB₃O₅ (CBO)⁷ and CsLiB₆O₁₀ (CLBO),⁸ which have been widely used in nonlinear optical (NLO) devices. Sodium borate halides Na₁₁B₂₁O₃₆X₂ (X = Cl, Br) with new graphene-like borate double layers were synthesized by the high temperature solution method by our group.⁹ Other borates with microporous properties have also been found, such as Zn(H₂O)B₂O₄·xH₂O, which contains one-dimensional 8-ring channels formed by ZnO₄ and BO₄ tetrahedra and BO₃ triangles.¹⁰ In addition, some borates with edge-sharing BO₄ tetrahedra, which violate Pauling's rules, have also been synthesized under high-pressure by Huppertz and von der Eltz.¹¹ It is interesting that in Zn-containing borates, the borate with edge-sharing BO₄ tetrahedra can also be synthesized under ambient pressure.^12–14 Obviously the geometric flexibility of boron atoms enrich enormously the topologies of the B–O frameworks.

In this work, we will report a new lead zinc borate, Pb₄Zn₂B₁₀O₂₁. And interestingly, a new [B₁₀O₂₄] group and an interesting regular [Pb₄O₁₂]_∞ chain are observed for the first time in its crystal structure. Also, it is a congruently melting compound, which suggests that it is easy to grow large single crystals.

Experimental section

Synthesis

Polycrystalline Pb₄Zn₂B₁₀O₂₁ was prepared by the high-temperature solid-state reaction. The reactants were PbO (Tianjin henxin Chemical Reagent Co., Ltd, 99.5%), ZnO (Tianjin henxin Chemical Reagent Co., Ltd, 99.5%), H₃BO₃ (Tianjin Baishi Chemical Co., Ltd, 99.5%). Initially, the stoichiometric powder mixture of PbO (8.898 g, 0.04 mol), ZnO (1.627 g, 0.02 mol) and H₃BO₃ (6.183 g, 0.10 mol) was prepared and packed into Pt crucibles, which were heated to 350 °C and held for 10 h to eliminate the water. Then the reaction mixture was calcined at 500 °C for 72 h with intermittent regrinding. Thus, white polycrystalline Pb₄Zn₂B₁₀O₂₁ powder was obtained.

X-ray powder diffraction (XRD) analysis of Pb₄Zn₂B₁₀O₂₁ was performed at room temperature in the angular range of 2θ = 10–70° with a scan step width of 0.02° and a fixed counting time of 1 s per step using an automated Bruker D2 PHASER X-ray diffractometer equipped with a diffracted beam monochromator set for Cu Kα radiation (λ = 1.5418 Å). The experimental XRD pattern of the polycrystalline phase is in good agreement with the calculated one derived from the single-crystal data (Fig. 1).

Fig. 1 Experimental and calculated XRD patterns of Pb₄Zn₂B₁₀O₂₁.

Single-crystal growth

Small single crystals of Pb₄Zn₂B₁₀O₂₁ were grown from a high temperature solution. The stoichiometric Pb₄Zn₂B₁₀O₂₁ was packed into a Pt crucible that was placed into a vertical programmable temperature furnace. The mixture was melted at 750 °C, held at this temperature for 15 hours, quickly cooled down to 670 °C, then a platinum wire was promptly dipped into the melt. The temperature was decreased to 620 °C at a rate of 3 °C h⁻¹, and finally cooled to room temperature at a rate of 20 °C h⁻¹. Thus, some clear, millimeter size, colorless crystals were discovered. Crystals of suitably high quality were selected for single-crystal XRD under an optical microscope.

Structure determination

The crystal structure of Pb₄Zn₂B₁₀O₂₁ was determined by single-crystal XRD on a Bruker SMART APEX II CCD diffractometer using monochromatic Mo Kα radiation (λ = 0.71073 Å) at 296(2) K and integrated with the SAINT program.¹⁵ A colorless and transparent block of crystal with dimensions of 0.21 mm × 0.15 mm × 0.14 mm was chosen for structure determination, unit cell parameters were derived from a least-squares analysis in the range of 1.82 to 27.54°. The data were solved with SHELXS-97 by a direct method, and refined using SHELXL-97.¹⁶ The final difference Fourier synthesis map showed the maximum and minimum peaks at 2.306 (0.80 Å from Pb₃) and −2.167 e Å⁻³ (0.80 Å from Pb₃), respectively. The structures were checked for missing symmetry elements with PLATON.¹⁷ Crystal data and structure refinement information are summarized in Table 1. Final atomic coordinates and equivalent isotropic displacement parameters of the title compound are listed in Table S1 in the ESI.† Selected interatomic distances and angles are given in Table S2 in the ESI.† Inductively coupled plasma optical emission spectrometer (ICP-OES) analysis was also carried out to determine the composition of the crystal. The test was performed on a Varian Vita-Pro CCD simultaneous ICP-OES, which reveals Pb : Zn : B = 4 : 2 : 9.89.

Table 1 Crystal data and structure refinement for Pb₄Zn₂B₁₀O₂₁

Empirical formula	Pb4Zn2B10O21
a R 1 = Σ||Fo| − |Fc||/Σ|Fo| and wR2 = [Σw(Fo^2 − Fc^2)^2/ΣwFo^4]^1/2 for Fo^2 > 2σ(Fo^2).
Formula weight	1403.60
Temperature	296(2) K
Wavelength	0.71073 Å
Crystal system	Orthorhombic
Space group	Pbcn
Unit cell dimensions	a = 14.6062(15) Å, b = 17.4899(16) Å, c = 13.2962(15) Å
Volume, Z	3396.7(6) Å^3, 8
Density (calculated)	5.489 Mg m^−3
Absorption coefficient	42.411 mm^−1
F(000)	4848
Theta range for data collection	1.82 to 27.54°
Limiting indices	−18 ⩽ h ⩽ 18, −22 ⩽ k ⩽ 22, −17 ⩽ l ⩽ 16
Reflections collected/unique	52 989/3918 [R(int) = 0.0848]
Completeness to theta = 27.54	99.7%
Refinement method	Full-matrix least-squares on F^2
Goodness-of-fit on F^2	1.030
Final R indices [Fo^2 > 2σ(Fo^2)]^a	R 1 = 0.0289, wR2 = 0.0623
R indices (all data)^a	R 1 = 0.0488, wR2 = 0.0702
Extinction coefficient	0.000090(6)
Largest diff. peak and hole	2.306 and −2.167 e Å^−3

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Infrared spectroscopy

The infrared spectrum was recorded on a Shimadzu IRAffinity-1 Fourier transform infrared spectrometer in the 400–4000 cm⁻¹ range, the sample was mixed thoroughly with dried KBr.

UV-vis-NIR diffuse reflectance spectrum

The diffuse reflectance spectrum for the Pb₄Zn₂B₁₀O₂₁ crystalline sample was measured from 190 to 2600 nm using a Shimadzu SolidSpec-3700DUV spectrophotometer equipped with an integrating sphere attachment.

Differential thermal analysis

The thermal behaviors of Pb₄Zn₂B₁₀O₂₁ were carried out on a NETZSCH STA 449C simultaneous analyzer under static air. The sample was enclosed in Pt crucibles and heated from room temperature to 800 °C at a rate of 10 °C min⁻¹ under flowing nitrogen gas.

Numerical calculation details

To investigate a deep structure–property relationship, the electronic structures were obtained using density functional theory (DFT) based ab initio calculations implemented in the CASTEP package.¹⁸ The exchange–correlation potential was calculated using the Perdew–Burke–Ernzerhof (PBE) function within the Generalized Gradient Approximation (GGA) scheme.¹⁹ For the purpose of achieving energy convergence, a plane-wave basis set energy cutoff was 340.0 eV within the Norm-conserving pseudopotential. And the Monkhorst–Pack scheme was set at 1 × 1 × 1 in the Brillouin Zone (BZ) of the primitive cell for the total energy calculations. The following orbital electrons were treated as valence electrons, Pb, 5d¹⁰6s²6p²; Zn, 3d¹⁰4s²; B, 2s²2p¹; O, 2s²2p⁴. Other parameters used in the calculations were set by the default values of the CASTEP code.

Results and discussion

Crystal structure

Owing to the high viscosity of melt, the Pb–Zn–B–O system is often considered as glass materials and there are few crystal structures reported, except PbZn₂(BO₃)₂ and Pb₈Zn(BO₃)₆.^20,21 And it is also worth noting that, for PbZn₂(BO₃)₂ and Pb₈Zn(BO₃)₆, their basic B–O building blocks are both isolated BO₃ triangles and their components are both located in the B-poor region, where the viscosity is relatively lower. However, Pb₄Zn₂B₁₀O₂₁ possesses a high B content and its component is located in the B-rich region, where the melt viscosity will be high and it is very difficult to crystallize. Therefore, in this sense, Pb₄Zn₂B₁₀O₂₁ may represent the first example which is crystallized in the B-rich region in the Pb–Zn–B–O system.

Pb₄Zn₂B₁₀O₂₁ crystallizes in the space group Pbcn of the orthorhombic system with a = 14.6062(15) Å, b = 17.4899(16) Å, c = 13.2962(15) Å, and Z = 8. In the asymmetric unit, there are four unique Pb atoms, two unique Zn atoms, ten unique B atoms, and twenty-two unique O atoms. The ten unique B atoms coordinate with three or four O atoms to form BO₃ triangles and BO₄ tetrahedra, which condense into a new fundamental building block (FBB) [B₁₀O₂₄] group, which can be written as 10:2[3:(△+2T)+(1:△)+(1:T)] according to the definition given by Burns et al. (Fig. 2a).²² Other B–O groups containing ten boron atoms, have also been reported such as the [B₁₀O₂₄] groups in [ThB₅O₆(OH)₆][BO(OH)₂]·2.5H₂O,²³ the [B₁₀O₂₁] groups in M₂M′₂B₁₀O₁₇ (M = Cs, Tl; M′ = K, Cs)^24–26 and Pb₆B₁₀O₂₁.²⁷ However, they all exhibit different configurations. In the [B₁₀O₂₄] group of [ThB₅O₆(OH)₆][BO(OH)₂]·2.5H₂O, six BO₄ are connected by four BO₃ triangles to form a crown-like [B₁₀O₂₄] group, which can be written as 10:[10:6△+4T]. The [B₁₀O₂₁] group in M₂M′₂B₁₀O₁₇ (M = Cs, Tl; M′ = K, Cs) can be seen as two [B₅O₁₂] groups sharing one O atom and can be written as 10:2[5:3△+2T], while the [B₁₀O₂₁] group in Pb₆B₁₀O₂₁ is composed of two [B₄O₉] groups bridged by one [B₂O₅] group and can be written as 10:2[4:(2T+2△)+T]. Further, the novel [B₁₀O₂₄] FBB is connected by sharing O atoms to form 2D [B₁₀O₂₁]_∞ layers (Fig. 2b), which stack along the c axis and are connected by the isolated ZnO₄ tetrahedra to form a [Zn₂B₁₀O₂₁]_∞ framework with Pb atoms filling in the space of the framework (Fig. 2c). And in Pb₄Zn₂B₁₀O₂₁, it should be noted that all the O atoms coordinated with Pb atoms fall within the same hemisphere around lead, which indicates that the lone-pairs on all the Pb²⁺ cations are stereo-active. Owing to the repulsion interactions of the lone pair, the lone pairs on the adjacent Pb²⁺ cations are as far apart as possible. Thus when the Pb-centered polyhedra are connected with each other to form [Pb₄O₁₂]_∞ chains, the lone pairs on the adjacent Pb²⁺ cations are pulled to the outside of the [Pb₄O₁₂]_∞ chain. Therefore, the [Pb₄O₁₂]_∞ chain looks reasonably regular (Fig. 2d).

Fig. 2 The crystal structure of Pb₄Zn₂B₁₀O₂₁, (a) the new fundamental building block [B₁₀O₂₄] group; (b) the 2D [B₁₀O₂₁]_∞ layers; (c) the 3D [Zn₂B₁₀O₂₁]_∞ framework with Pb atoms filling in the space; (d) the regular [Pb₄O₁₂]_∞ chain.

In addition, owing to the repulsion interactions of the lone pairs of Pb²⁺ cations, the coordinated environments of the Pb atoms are also complicated. When we only consider the primary coordination sphere, the Pb atoms have three kinds of obviously different coordinated environments. The Pb1 atoms are coordinated by three O atoms with Pb–O distance ranging from 2.293(6) to 2.469(6) Å, the Pb2 and Pb3 atoms are coordinated by four O atoms with Pb–O distance ranging from 2.227(6) to 2.734(6) Å, respectively, and the Pb4 atoms are coordinated by five O atoms with Pb–O distance ranging from 2.231(6) to 2.756(6) Å. The bond valence sums (BVS) of Pb²⁺ cations obtained by the bond valence calculations^28,29 are 1.47, 1.59, 1.36, and 1.75, respectively, which is obviously smaller than the ideal oxidation state of +2. According to Davidovich,³⁰ the coordination environments of Pb atoms are classified as the primary coordination sphere and the secondary coordination sphere. When some longer Pb–O bonds with the lengths ranging from 2.80 to 3.5 Å are considered, the BVS of the Pb atoms are 1.99, 1.94, 1.92, and 1.96, respectively, which are consistent with the oxidation state of Pb²⁺. The flexible coordination environments of Pb atoms are also observed in other Pb-containing borates.^31,32

All the Zn atoms are coordinated by four O atoms and the Zn–O bond lengths range from 1.814(8) to 1.982(6) Å. The B atoms have two coordinated environments, BO₃ triangles and BO₄ tetrahedra. For the BO₃ triangles, the B–O bond lengths range from 1.302(13) to 1.406(11) Å. For BO₄ tetrahedra, the B–O bond lengths range from 1.430(11) to 1.517(11) Å. All the bond lengths are also consistent with those observed in other compounds.^33,34

Pb₄Zn₂B₁₀O₂₁ has similar stoichiometry to Pb₆B₁₀O₂₁, and from the formula, Pb₄Zn₂B₁₀O₂₁ seems to be the compound in which the Zn atoms substitute two Pb atoms of Pb₆B₁₀O₂₁. However, they exhibit obviously different crystal structures. Pb₆B₁₀O₂₁ crystallizes in space group P1̄ of the triclinic system, while Pb₄Zn₂B₁₀O₂₁ crystallizes in space group Pbcn of the orthorhombic system. And as discussed above, in Pb₆B₁₀O₂₁ the FBBs are the isolated [B₁₀O₂₁] groups. However, in Pb₄Zn₂B₁₀O₂₁, the FBBs are the [B₁₀O₂₄] groups and they further condense into 2D [B₁₀O₂₁]_∞ layers. The difference may be easy to understand when the coordination environments of the Pb atoms and Zn atoms are considered. Generally the Zn atoms are coordinated by four or six O atoms to form ZnO₄ tetrahedra or ZnO₆ octahedra. However, owing to Pb possessing the stereo-active lone pairs, the coordination environment of the Pb atoms is more complicated than the Zn atoms. For one thing, the O atoms coordinated with Pb atoms often fall within the same hemisphere around Pb atoms, leading to the Pb-centered polyhedra being extremely distorted. Also, the coordination environments of the Pb atoms are often composed of the primary coordination sphere and the secondary coordination sphere. That is, in Pb-containing compounds, it is often observed that in the coordination environment of the Pb atoms, apart from some normal Pb–O bonds, other longer Pb–O bonds with bond lengths ranging from 2.80 to 3.5 Å must be considered.^20,21,31,32 The obvious differences in the coordination environments of the Pb atoms and the Zn atoms often lead to a substantial change in structure when the Zn atoms partly or totally substitute the Pb atoms.^35,36

Spectroscopic properties

Furthermore, in order to further specify the coordination of boron in the FBBs, the infrared spectrum was measured (Fig. S1 in the ESI†). The strong peaks at 1377, 1298, 1224 and 1124 cm⁻¹ are attributed to the asymmetric stretching of BO₃ and BO₄, respectively.³⁷ The peaks at 943 and 747 cm⁻¹ are attributed to the symmetric stretching of BO₃ and BO₄, respectively. The 693 and 651 cm⁻¹ peaks belong to the out of plane bending of BO₃. The peaks at 587 and 491 cm⁻¹ are attributed to the bending of BO₃ and BO₄. It further confirms the existence of BO₃ triangles and BO₄ tetrahedra, and is consistent with the results obtained from the single-crystal X-ray structural analysis.

The UV-vis-NIR diffuse-reflectance spectrum for Pb₄Zn₂B₁₀O₂₁ in the region 190–2600 nm is displayed in Fig. 3. The reflectance spectrum was converted to absorbance with the Kubelka–Munk function.^38,39 It is clear that Pb₄Zn₂B₁₀O₂₁ has a wide transmission range with a UV absorption edge at 308 nm.

Fig. 3 The UV-vis-NIR diffuse reflectance spectrum of Pb₄Zn₂B₁₀O₂₁.

TG and DSC analysis

The thermal behavior of Pb₄Zn₂B₁₀O₂₁ is measured at a range of 50 to 800 °C. There is one endothermic peak at 658 °C on the DSC curve and there is no weight loss on the TG curve (Fig. 4), which tentatively suggests that Pb₄Zn₂B₁₀O₂₁ melts congruently at 658 °C. In order to further confirm the thermal behavior of Pb₄Zn₂B₁₀O₂₁, the solid samples of Pb₄Zn₂B₁₀O₂₁ are placed into a platinum crucible and heated to 750 °C, then slowly cooled to room temperature. Analysis of the powder XRD pattern of the solidified melt reveals that the solid product exhibits a diffraction pattern identical to that of the initial Pb₄Zn₂B₁₀O₂₁ powder (shown in Fig. 1), further demonstrating that Pb₄Zn₂B₁₀O₂₁ is a congruently melting compound, which indicates that it is easy to grow large single crystals of Pb₄Zn₂B₁₀O₂₁.⁴⁰

Fig. 4 The TG and DSC curves of Pb₄Zn₂B₁₀O₂₁.

Electronic band structures

In order to examine the band structure and to better understand the relationship between electronic structures and optical properties, the electronic structure of Pb₄Zn₂B₁₀O₂₁ was calculated using density functional theory (DFT) based ab initio calculations implemented in the CASTEP package. The calculated band structures along the high symmetry lines in the unit cell are shown in Fig. 5a. It is clear that the state energies (eV) of the lowest conduction band and the highest valence band of the compound both locate at the G point, indicating that Pb₄Zn₂B₁₀O₂₁ is a direct band gap compound with a band gap of 3.53 eV. Furthermore, Fig. 5b shows the partial density of states (DOS) and total DOS of Pb₄Zn₂B₁₀O₂₁. After removing the contribution of kernel electronic orbitals, the energy band is composed of four regions. The lowest region from −21.4 to −17.1 eV mostly originates from the O 2s state, mixed with a small amount of B-2s2p states. The middle part of the valence band (−15.8 to −14.2 eV) is attributed to the isolated Pb 5d state. Around the Fermi level, the top of the valence band is mostly derived from the contributions of O 2p states with a small amount of Pb 6s, Zn 3d, B 2s2p states, and the conduction band is mainly derived from the Pb 6p state, which could contribute to the optical properties of the crystal in the visible and UV spectra.

Fig. 5 (a) Calculated band structure of Pb₄Zn₂B₁₀O₂₁; (b) the density of states of Pb₄Zn₂B₁₀O₂₁.

Conclusions

In conclusion, a new borate, Pb₄Zn₂B₁₀O₂₁, has been synthesized and single crystals have been grown by the high-temperature solution method for the first time. It contains a new FBB [B₁₀O₂₄] group. Owing to the repulsion interactions between the lone pairs, the Pb-centered polyhedra connect with each other to form an interesting and regular [Pb₄O₁₂]_∞ chain. Furthermore, the infrared spectrum confirms the existence of BO₃ and BO₄ units in the basic building block. The UV-vis-NIR diffuse reflectance spectrum shows that the absorption edge of Pb₄Zn₂B₁₀O₂₁ is about 308 nm and TG-DSC measurement shows that Pb₄Zn₂B₁₀O₂₁ melts congruently, which suggests that it is easy to grow large single crystals of Pb₄Zn₂B₁₀O₂₁ for applications. Furthermore, the first principles studies reveal that the title compound is a direct-gap compound with band gaps of 3.53 eV, the charge transfers from O 2p states to Pb 6p states contribute to the observed absorption edge from the UV-vis-NIR absorption spectrum.

Acknowledgements

This work is supported by the National Key Basic Research Program of China (Grant No. 2012CB626803), the “National Natural Science Foundation of China” (Grant No. 21201176, U1129301, 51172277, 21101168, 11104344), Main Direction Program of Knowledge Innovation of CAS (Grant No. KJCX2-EW-H03-03), the “One Hundred Talents Project Foundation Program” of CAS, Major Program of Xinjiang Uygur Autonomous Region of China during the 12th Five-Year Plan Period (Grant No. 201130111), the “High Technology Research and Development Program” of Xinjiang Uygur Autonomous Region of China (Grant No. 201315103, 201116143), and the “Western Light Foundation” Program of CAS (Grant No. XBBS201214).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: The infrared spectrum of Pb₄Zn₂B₁₀O₂₁, atomic coordinates and equivalent isotropic displacement parameters, and selected bond distances and angles tables for Pb₄Zn₂B₁₀O₂₁. CCDC 954072. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c3nj00893b

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