Pb₃Ba₃Zn₆(BO₃)₈ and α-BaZn₂(BO₃)₂: new members of the zincoborates containing two different dimensional Zn–O units

Abstract

Two new zincoborates, namely, Pb₃Ba₃Zn₆(BO₃)₈ and α-BaZn₂(BO₃)₂, were synthesized by the high-temperature solution method and their structures were determined by single-crystal X-ray diffraction for the first time. The structure of Pb₃Ba₃Zn₆(BO₃)₈ can be described as a three-dimensional framework consisting of [Zn₄B₂O₁₅]_∞ chains and isolated ZnO₄ tetrahedra and BO₃ triangles, which contains three different types of tunnels with Pb²⁺ and Ba²⁺ cations residing among them. α-BaZn₂(BO₃)₂ exhibits a two-dimensional layered structure with a [Zn₂B₂O₆] layer, which is formed by [ZnBO₄] chains and [Zn₂B₂O₈] chains. Interestingly, both Pb₃Ba₃Zn₆(BO₃)₈ and α-BaZn₂(BO₃)₂ both contain two different dimensional Zn–O configurations in their structures, which has not been found in other zincoborates. In addition, thermal property analysis, and UV-vis-NIR diffuse reflectance and infrared spectroscopy were also performed. First-principles theoretical studies were also conducted to aid the understanding of their band structures and densities of states.

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Introduction

Numerous research interests have been directed towards borates due to their abundant structural features and important technical applications as second-order nonlinear optical materials,^1,2 birefringent materials,^3–5 catalysts,^6,7 ion conductors^8,9 and phosphors.^10,11 Generally, a boron atom can be in either triangular or tetrahedral coordination with oxygen atoms to form BO₃ or BO₄ groups, and these two groups can connect with each other through sharing O atoms to construct different degrees of polymerization of the B–O units, such as B₂O₅,¹² B₃O₆,¹³ B₅O₁₀,¹⁴ B₆O₁₂¹⁵ and B₁₈O₃₆.¹⁶ It is remarkable that B₆O₁₂ was the first B–O unit consisting of edge-sharing BO₄ that was synthesized at ambient pressure. Also, a boron atom can connect with fluorine atoms forming [BO_xF_4−x]^(x+1)− units, which can connect with BO₃ or BO₄ anion groups to construct various B–O–F clusters, such as B₂O₅F₂, B₃O₅F₃, B₄O₈F₂, B₅O₉F₃, B₆O₁₁F₂ and B₆O₁₁F₃.^17–28 Such rich B–O or B–O–F groups lead to the abundant structural chemistry of borates. For example, Q₁₈Mg₆(B₅O₁₀)₃(B₇O₁₄)₂F (Q = Rb and Cs)²⁹ and Ba_n+2Zn_n(BO₃)_n(B₂O₅)F_n (n = 1, 2)¹² contain two kinds of B–O groups. However, these plentiful borates cannot completely satisfy the requirements of practical application with the development of science or theoretical research. It is necessary to synthesize a vast number of borates with different topological arrangements of structural configuration to enrich the structure of borates and meet the needs of scientific research.

Recently, many investigations have revealed that introduction of comparatively strong covalent ZnO₄ tetrahedra is a feasible strategy for obtaining new compounds.^30–35 For example, Yu et al. synthesized Cs₃Zn₆B₉O₂₁ and Ba₃ZnB₅O₁₀PO₄, which are new members of the KBe₂BO₃F₂ (KBBF) family and they exhibit excellent NLO properties. Other Zn-containing KBBF-like compounds, such as AZn₂BO₃X₂ (A = K, Rb, NH₄; X = Cl, Br), have also been synthesized by Ye's group and Li's group.^13,36,37 These preserve the structural merits of KBBF and possess large second harmonic generation response. Similarly, the polymorph zincoborates α-, β- and γ-Pb₂Ba₄Zn₄B₁₄O₃₁ have been obtained,³⁸ which all crystallize in the polar space group. Thus, we continued to explore new compounds in the PbO–BaO–ZnO–B₂O₃ system and obtained two new compounds: Pb₃Ba₃Zn₆(BO₃)₈ and α-BaZn₂(BO₃)₂. It is worth noting that Pb₃Ba₃Zn₆(BO₃)₈ and α-BaZn₂(BO₃)₂ both contain two different types of Zn–O units: 0D (zero-dimensional) ZnO₄ tetrahedra and 1D (one-dimensional) Zn₄O₁₂ chains for Pb₃Ba₃Zn₆(BO₃)₈ and 0D Zn₂O₆ dipolymers and 1D Zn₂O₇ chains for α-BaZn₂(BO₃)₂ (named according to the calculated density and β-BaZn₂(BO₃)₂ reported in 1992).³⁹ In this paper, the syntheses, crystal structures, optical properties and theoretical calculations of the two title compounds are reported. In addition, we compared the structure of Pb₃Ba₃Zn₆(BO₃)₈ and α-BaZn₂(BO₃)₂ with other zincoborates containing different types of Zn–O units.

Experimental section

Single-crystal preparation and compound syntheses

A single crystal of Pb₃Ba₃Zn₆(BO₃)₈ was obtained by high-temperature solution reaction with a spontaneous crystallization method using H₃BO₃ and PbF₂ as the flux. The raw materials of BaCO₃, ZnO, H₃BO₃, PbO and PbF₂ in the original ratio of 1 : 4 : 15 : 5 : 5 were mixed homogeneously before being transferred to a platinum crucible. Then, the mixtures were heated to 850 °C and kept at this temperature for 10 h to make the mixture homogeneous. The temperature was then reduced to 400 °C at a rate of 5 °C h⁻¹ before the furnace was switched off. With this procedure, a few transparent, colourless Pb₃Ba₃Zn₆(BO₃)₈ crystals were obtained by mechanical separation from the crucible. A single crystal of α-BaZn₂(BO₃)₂ was also obtained by a spontaneous crystallization method using H₃BO₃ and PbO as the flux.

Polycrystalline powders of Pb₃Ba₃Zn₆(BO₃)₈ were synthesized by solid-state reactions. The stoichiometric ratio of PbO, BaCO₃, ZnO and H₃BO₃ was heated at a temperature of 300 °C for 5 h to release gas and then sintered at 650 °C for 72 h, with several intermediate grindings. In this way, the powder sample of Pb₃Ba₃Zn₆(BO₃)₈ was obtained. As shown in Fig. 1, the experimental powder X-ray diffraction (XRD) pattern agreed well with the calculated one. Unfortunately, we could not obtain the polycrystalline powders of α-BaZn₂(BO₃)₂ though we made many efforts.

Fig. 1 Experimental and calculated XRD patterns of Pb₃Ba₃Zn₆(BO₃)₈.

Structure determination

The single-crystal structures of Pb₃Ba₃Zn₆(BO₃)₈ and α-BaZn₂(BO₃)₂ were determined by single-crystal XRD techniques at room temperature. The colourless crystals with approximate dimensions of 0.11 × 0.09 × 0.08 mm³ for Pb₃Ba₃Zn₆(BO₃)₈ and 0.11 × 0.08 × 0.05 mm³ for α-BaZn₂(BO₃)₂ were selected for the structure analysis on a Bruker SMART APEX II CCD diffractometer, which was used to collect data using monochromatic Mo-Kα radiation (λ = 0.71073 Å) at 296(2) K. The data were integrated with the SAINT program and calculated on the SHELXTL crystallographic software package.⁴⁰ The structures were solved by SHELXS-97 and refined on F² by a full-matrix least-squares method.⁴¹ The structural symmetries were checked with PLATON.⁴² The crystal data and structure refinement information are summarized in Table 1. The atomic coordinates equivalent isotropic displacement parameters, bond valence sum, and selected bond lengths are summarized in Tables S1–S4,† respectively.

Table 1 Crystal data and structure refinement for Pb₃Ba₃Zn₆(BO₃)₈ and α-BaZn₂(BO₃)₂

Empirical formula	Pb3Ba3Zn6(BO3)8	α-BaZn2(BO3)2
a R 1 = ∑||Fo| − |Fc||/∑|Fo| and wR2 = [∑w(Fo^2 − Fc^2)2/∑wFo^4]^1/2 for Fo^2 > 2σ(Fo^2).
Formula weight	1896.29	385.70
Temperature	296(2) K
Wavelength	0.71073 Å
Crystal system	Monoclinic
Space group	C2/c (15)	P21/n(14)
Unit cell dimensions	a = 26.55(3) Å	a = 10.216(3) Å
	b = 4.973(5) Å	b = 4.9556(13) Å
	c = 18.588(19) Å	c = 12.161(3) Å
	β = 106.587(12)°	β = 111.638(13)°
Volume	2352(4) Å^3	573.3(3) Å^3
Z	4	4
Density (g cm^−3)	5.356	4.468
Absorption coefficient (mm^−1)	32.424	15.054
F(000)	3304	696
Theta range for data collection	3.14 to 25.00°	2.24 to 27.50°
Limiting indices	−23 ≤ h ≤ 31, −5 ≤ k ≤ 5, −22 ≤ l ≤ 16	−13 ≤ h ≤ 12, −6 ≤ k ≤ 6, −15 ≤ l ≤ 15
Reflections collected/unique	5525/2044 [R(int) = 0.0575]	7671/1317 [R(int) = 0.0381]
Completeness to theta	99.4%	99.6%
Refinement method	Full-matrix least-squares on F^2	Full-matrix least-squares on F^2
Data/restraints/parameters	2044/6/182	1317/0/101
Goodness-of-fit on F^2	1.025	1.042
Final R indices [Fo^2 > 2σ(Fo^2)]^a	R 1 = 0.0492	R 1 = 0.0222
	wR 2 = 0.1364	wR 2 = 0.0461
R indices (all data)^a	R 1 = 0.0588	R 1 = 0.0312
	wR 2 = 0.1438	wR 2 = 0.0485
Largest diff. peak and hole	4.804 and −3.337 e Å^−3	1.010 and −0.694 e Å^−3

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Thermal properties

The thermal analysis of Pb₃Ba₃Zn₆(BO₃)₈ was carried out on a NETZSCH STA 449C thermal analyzer instrument in an atmosphere of flowing N₂. The measurement temperature ranged from 25 °C to 1200 °C with a heating rate of 5 °C min⁻¹.

Spectroscopic properties

The Fourier transform infrared spectroscopy spectrum of Pb₃Ba₃Zn₆(BO₃)₈ was measured on a Shimadzu IRAffinity-1 Fourier transform IR spectrometer ranging from 400 to 4000 cm⁻¹.

The 3700DUV Shimadzu spectrophotometer was used to measure the diffuse reflectance spectrum of Pb₃Ba₃Zn₆(BO₃)₈ at room temperature over the range from 190 to 2500 nm. The Kubelka–Munk function was applied to transform the reflectance spectrum into absorbance spectrum.

Theoretical calculations

The band structures and density of states (DOS) of Pb₃Ba₃Zn₆(BO₃)₈ and α-BaZn₂(BO₃)₂ were calculated by using the plane-wave pseudo-potential based on the density functional theory with the CASTEP program.⁴³ The generalized gradient-approximation (GGA) with the Perdew–Burke–Ernzerhof (PBE) functional was chosen to describe the exchange–correlation potential. Under the norm-conserving pseudopotential (NCP), the following orbital electrons were treated as valence electrons: Pb 5d¹⁰6s²6p², Ba 5s²5p⁶6s², Zn 3d¹⁰4s², B 2s²2p¹, O 2s²2p⁴.^44–46 The energy cut-off was set at 910 eV for Pb₃Ba₃Zn₆(BO₃)₈ and 830 eV for α-BaZn₂(BO₃)₂. The Monkhorst–Pack k-point sampling was set at 3 × 3 × 1 for Pb₃Ba₃Zn₆(BO₃)₈ and 3 × 5 × 2 for α-BaZn₂(BO₃)₂ in the Brillouin zone. The other calculation parameters and convergent criteria were set by the default values of CASTEP.

Results and discussion

Structure of Pb₃Ba₃Zn₆(BO₃)₈

Pb₃Ba₃Zn₆(BO₃)₈ crystallizes in a monoclinic crystal system with a centrosymmetric space group of C2/c (no. 15). In the asymmetric unit, there are two unique Pb atoms, two unique Ba atoms, three unique Zn atoms, four unique B atoms and twelve unique O atoms. In the structure, the B atoms are three-coordinated, with B–O bond lengths ranging from 1.30(3) to 1.42(3) Å. However, the Zn atoms are four-coordinated, with the Zn–O bond lengths ranging from 1.903(12) to 2.031(12) Å. The bond valence sum (BVS) values of Pb (1.994–2.062), Ba (2.055–2.153), Zn (1.979–2.105), B (2.951–3.093), and O (1.818–2.171) were in good agreement with reasonable values.

In the structure, the ZnO₄ tetrahedra have two different connection modes, the isolated ZnO₄ tetrahedra and the 1D [Zn₄O₁₂]_∞ chain (Fig. 2(a)). The [Zn₄O₁₂]_∞ chains share vertices with isolated BO₃ triangles to form the [Zn₄B₂O₁₅]_∞ chains (Fig. 2(b)), which are further connected by the isolated ZnO₄ tetrahedra and BO₃ triangles to form the three-dimensional (3D) framework (Fig. 2(c)). The Ba and Pb atoms are filled in the space created by the network. Notably in the structure, there are three types of tunnels along the b axis: A-type, B-type and C-type. For the A-tunnel, as shown in Fig. S1(b),† we can notice that it is a super-large tunnel with 40 members, which is very rare in the borate compounds reported so far. The length of this super-tunnel has a large value of 20.607 Å (determined by the distance of Zn(2)–Zn(2)) and the width is 4.467 Å (determined by the distance of B(4)–B(4)). Also, two four-coordinated Pb(1) atoms and two six-coordinated Ba(1) atoms are located in this sort of tunnel (Fig. S2†). For the B-tunnels, as shown in Fig. S1(c),† there are 12 members with the length of 6.795 Å and width of 4.311 Å. Also, one six-coordinated Ba(2) atom dominates this tunnel (Fig. S2†). Apart from these, there is a small C-type tunnel with 8 members in Fig. S1(d);† the cross-sectional dimensions of this tunnel are 4.874 Å × 3.582 Å. Also, one eight-coordinated Pb(2) atom is located in this tunnel (Fig. S2†). The larger A and B tunnels suggest that the Pb²⁺ and Ba²⁺ cations can be replaced by other cations, which provides an idea for the synthesis of new compounds. Besides, the large tunnels in the frameworks could make Pb₃Ba₃Zn₆(BO₃)₈ a promising functional material for application in catalysts, ion-exchangers and molecular sieves.

Fig. 2 Crystal structure of Pb₃Ba₃Zn₆(BO₃)₈: (a) the isolated ZnO₄ tetrahedra and [Zn₄O₁₂]_∞ chain; (b) the [Zn₄B₂O₁₅]_∞ chain; (c) crystal structure viewed along the b-axis. All the Pb–O and Ba–O bonds are removed for clarity.

Structure of α-BaZn₂(BO₃)₂

α-BaZn₂(BO₃)₂ crystallizes in a monoclinic symmetry, with the space group P2₁/n (no. 14). In the asymmetric unit, the Ba, Zn, B and O atoms occupy one, two, two and six crystallographically unique sites, respectively. In the structure, the bond lengths of Ba–O, Zn–O and B–O range from 2.651(3) to 3.328(3) Å, 1.898(3) to 2.060(3) Å and 1.361(6) to 1.395(6) Å, respectively. The calculated total bond valences (Ba, 1.917; Zn, 1.999 and 2.084; B, 2.944 and 2.964; O, 1.908 to 2.068) indicate that the Ba, Zn, B and O atoms are in the oxidation states of +2, +2, +3 and −2, respectively.

The Zn(1)O₄ tetrahedra and the B(1)O₃ triangles alternately connect with each other to form a [ZnBO₄] chain (Fig. 3(a)), and two Zn(2)O₄ tetrahedra connect with each other to form Zn₂O₆ dipolymers, which are further linked by B(2)O₃ triangle to form a [Zn₂B₂O₈] chain (Fig. 3(b)). Then, the [ZnBO₄] chain and [Zn₂B₂O₈] chain alternately link by sharing corner O atoms to form [Zn₂B₂O₆] 2D layers (Fig. 3(c)), and the Ba atoms reside in the space between the two layers (Fig. 3(d)). Similar to Pb₃Ba₃Zn₆(BO₃)₈, the structure of α-BaZn₂(BO₃)₂ contains two different types of Zn–O units: an isolated Zn₂O₆ dipolymer and 1D [Zn₂O₇] chain (Fig. S3†), which has not been reported so far, to the best of our knowledge.

Fig. 3 (a) [ZnBO₄] chain; (b) [Zn₂B₂O₈] chain; (c) the [Zn₂B₂O₆]_∞ layer; (d) crystal structure viewed along the b-axis; all the Ba–O bonds are removed for clarity.

On the contrary, β-BaZn₂(BO₃)₂ possesses a three-dimensional framework consisting of ZnO₄ tetrahedra and BO₃ triangles. As shown in Fig. S4(a),† four ZnO₄ tetrahedra connect with each other by sharing corner O atoms to form Zn₄O₁₃ multimers, which further link with each other to form Zn₄O₁₂ chains. Then, the Zn₄O₁₂ chains connect with the isolated BO₃ triangles to form a 3D framework structure. The Ba atoms are filled in the space created by the network, Fig. S4(c).†

Structure comparison

We summarize the anhydrous zincoborates in Table S5† and note that most of these compounds contain one type of Zn–O units, such as isolated ZnO_n (n = 4, 5, 6) polyhedra, or they connect with each other to construct 0D multimer, 1D chains, 2D layers and 3D networks by sharing corners, edges or faces. Besides, the O atoms in ZnO_n polyhedra can be replaced by other atoms and can form various units, such as the Zn₂O₆F unit in KCdZnBO₃F, Zn₃O₁₂X unit in Zn₃B₇O₁₃X (X = Cl, Br) and Zn₄O₁₂Se unit in Zn₈(BO₂)₁₂Se₂.^47–51 It is worth noting that there are only five compounds possessing two different types of Zn–O units: isolated Zn₂O₇ and Zn₂O₆ in Ba₂KZn₃(B₃O₆)(B₆O₁₃),³³ isolated Zn₂O₇ and Zn₃O₁₀ in Ba₄K₂Zn₅(B₃O₆)₃(B₉O₁₉)³⁴ and isolated ZnO₄ and Zn₂O₆ in α-, β- and γ-Pb₂Ba₄Zn₄B₁₄O₃₁.³⁸ In these five compounds, two types of Zn–O units are all 0D (Fig. S5†). However, for Pb₃Ba₃Zn₆(BO₃)₈ and α-BaZn₂(BO₃)₂, the two types of Zn–O units are 0D and 1D (Fig. S3†), which are reported for the first time in zincoborates.

Thermal properties

The DSC-TG curves for Pb₃Ba₃Zn₆(BO₃)₈ are shown in Fig. 4a. The TG result showed that Pb₃Ba₃Zn₆(BO₃)₈ exhibited no obvious weight loss up to 1000 °C. The DSC curve for Pb₃Ba₃Zn₆(BO₃)₈ exhibited one sharp and one mild endothermic peak at about 678 and 790 °C, respectively. To further confirm the thermal property, firstly, the polycrystalline sample of Pb₃Ba₃Zn₆(BO₃)₈ (5 g) was placed in a platinum crucible and heated to 750 °C and held at this temperature for 3 days. The powder XRD patterns demonstrated that the phase mainly was β-BaZn₂(BO₃)₂ (Fig. S6†). Secondly, the samples of Pb₃Ba₃Zn₆(BO₃)₈ were heated to 1000 °C followed by cooling to room temperature at a rate of 5 °C per hour. Then the residue was mechanically separated from the crucible and some block crystal was obtained. Also, the crystal was determined by single-crystal XRD techniques, and the result showed that it was α-BaZn₂(BO₃)₂. The powder XRD patterns of the residue were in agreement with the calculated pattern of α-BaZn₂(BO₃)₂, as shown in Fig. 4b, which proved the existence of α-BaZn₂(BO₃)₂. The above experiments show that Pb₃Ba₃Zn₆(BO₃)₈ melts incongruently, and decomposes into β-BaZn₂(BO₃)₂ at around 750 °C.

Fig. 4 (a) TG/DSC curves for Pb₃Ba₃Zn₆(BO₃)₈; (b) powder XRD patterns of the residue.

Spectroscopic properties

The IR spectrum is shown in Fig. 5(a). The peak at 1645 cm⁻¹ is a characteristic peak of H–O–H bending motion,⁵² which may be caused by the moisture-absorbing KBr. The peaks at 1440 and 890 cm⁻¹ belong to the BO₃ asymmetric and symmetric stretching vibrations, respectively.⁵² The peaks at 1135 and 450 cm⁻¹ are attributed to the stretching and bending of the Zn₄–O, respectively.³¹ The weak peaks at 730 and 620 cm⁻¹ belong to the out-of-plane bending of BO₃ groups.⁵² Also, the absorption peak at 570 cm⁻¹ corresponds to the bending of BO₃ groups.⁵² These results confirm the existence of BO₃ triangles and ZnO₄ tetrahedra in the structure of Pb₃Ba₃Zn₆(BO₃)₈.

Fig. 5 (a) The infrared spectrum of Pb₃Ba₃Zn₆(BO₃)₈; (b) the UV-vis-NIR diffuse reflectance spectrum of Pb₃Ba₃Zn₆(BO₃)₈.

The UV-vis-NIR diffuse reflectance spectrum of Pb₃Ba₃Zn₆(BO₃)₈ was measured from 190 to 2600 nm. On the basis of the diffuse-reflectance spectrum, it was converted to absorbance based on the Kubelka–Munk function and is shown in Fig. 5(b). The experimental energy gap of Pb₃Ba₃Zn₆(BO₃)₈ was 3.21 eV.

Theoretical calculations

To gain further insight into the electronic structures and optical properties of the title compounds, the band structures and DOS were also calculated. As shown in Fig. 6(a) and (c), Pb₃Ba₃Zn₆(BO₃)₈ and α-BaZn₂(BO₃)₂ are indirect band gap compounds with energy values of 3.43 and 3.39 eV, and for Pb₃Ba₃Zn₆(BO₃)₈ the calculated value is close to the experimental result.

Fig. 6 The calculated band structures and calculated birefrigence (Δn) of Pb₃Ba₃Zn₆(BO₃)₈ (a) and (b), and α-BaZn₂(BO₃)₂ (c) and (d).

The birefringences of Pb₃Ba₃Zn₆(BO₃)₈ and α-BaZn₂(BO₃)₂ were also calculated using CASTEP. As shown in Fig. 6(b) and (d), the birefringences at 1064 nm were 0.07 and 0.06 for Pb₃Ba₃Zn₆(BO₃)₈ and α-BaZn₂(BO₃)₂, respectively.

The DOS of Pb₃Ba₃Zn₆(BO₃)₈ and α-BaZn₂(BO₃)₂ are presented in Fig. 7(a) and (b). For Pb₃Ba₃Zn₆(BO₃)₈, near the Fermi level region, the top of the VBs is mainly occupied by the O-2p, B-2p and Zn-3d orbitals. Also, the bottom of the CBs is mainly occupied by the Pb-6p states, which suggests that the B–O and Zn–O groups make the main contribution to the band gap. While for α-BaZn₂(BO₃)₂, the top of the VBs is mainly contributed by O-2p, B-2p and Zn-3d states and the bottom of the CBs is mainly contributed by Zn-4s and Ba-4d states. This means that the B–O and Zn–O groups control the band gap of α-BaZn₂(BO₃)₂. From Fig. 7, we can see that the d orbital of Zn atoms in Pb₃Ba₃Zn₆(BO₃)₈ is farther from the Fermi level compared with α-BaZn₂(BO₃)₂. It was indicated that the d–p hybridization in Pb₃Ba₃Zn₆(BO₃)₈ has little effect on the band gap.^53,54 This is why the band gap of Pb₃Ba₃Zn₆(BO₃)₈ (3.43 eV) is a little larger than that of α-BaZn₂(BO₃)₂ (3.39 eV).

Fig. 7 The total and partial densities ofstates of Pb₃Ba₃Zn₆(BO₃)₈ (a), and α-BaZn₂(BO₃)₂ (b).

Conclusions

Two new zincoborates, namely, Pb₃Ba₃Zn₆(BO₃)₈ and α-BaZn₂(BO₃)₂, were synthesized and structurally characterized. The structure of Pb₃Ba₃Zn₆(BO₃)₈ features a 3D framework consisting of BO₃ triangles, ZnO₄ tetrahedra and [Zn₄O₁₅]_∞ chains. The structure of α-BaZn₂(BO₃)₂ consists of [Zn₂B₂O₆] layers formed by [ZnBO₄] chains and [Zn₂B₂O₈] chains. Pb₃Ba₃Zn₆(BO₃)₈ and α-BaZn₂(BO₃)₂ both contain 0D and 1D Zn–O units, which is reported for the first time in zincoborates, as far as we know. The TG/DSC curves indicated that Pb₃Ba₃Zn₆(BO₃)₈ melts incongruently. Also, the powder XRD pattern of the residue of Pb₃Ba₃Zn₆(BO₃)₈ proves the existence of α-BaZn₂(BO₃)₂. The infrared spectrum confirms the existence of BO₃ triangles and ZnO₄ tetrahedra in Pb₃Ba₃Zn₆(BO₃)₈. Also, the experimental band gap of Pb₃Ba₃Zn₆(BO₃)₈ was 3.21 eV, which coincides with the calculated value. The first principles studies show that Pb₃Ba₃Zn₆(BO₃)₈ and α-BaZn₂(BO₃)₂ both are indirect band gap compounds with the band gaps of 3.43 and 3.39 eV, respectively. Those are primarily determined by the B–O and Zn–O groups for Pb₃Ba₃Zn₆(BO₃)₈ and α-BaZn₂(BO₃)₂, respectively.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work is supported by National Natural Science Foundation of China (Grant No. 51972230 and 61975096); Research and Free Exploration Project of Shenzhen City (Grant No. JCYJ20180305164316517); Natural Science Foundation of Shandong Province (Grant No. ZR2017MF031).

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Footnote

† Electronic supplementary information (ESI) available: CIF file; check cif; atomic coordinates and equivalent isotropic displacement parametersand selected bond distances and angles tables for Pb₃Ba₃Zn₆(BO₃)₈ and α-BaZn₂(BO₃)₂; table of the basic information of the existing anhydrous-zincoborates. CCDC 1949368 (Pb₃Ba₃Zn₆(BO₃)₈) and 1949369 (α-BaZn₂(BO₃)₂). For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c9qi01122f

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