Layer on the Band Gap - ACS Publications - American Chemical Society

Apr 27, 2016 - Information Materials and Devices, Xinjiang Technical Institute of Physics & Chemistry of CAS, 40-1 South Beijing Road, Urumqi. 830011 ...
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Effect of the [Ba2BO3F]∞ Layer on the Band Gap: Synthesis, Characterization, and Theoretical Studies of BaZn2B2O6·nBa2BO3F (n = 0, 1, 2) Hongping Wu,† Xin Su,†,‡ Shujuan Han,† Zhihua Yang,† and Shilie Pan*,† †

Key Laboratory of Functional Materials and Devices for Special Environments of CAS, Xinjiang Key Laboratory of Electronic Information Materials and Devices, Xinjiang Technical Institute of Physics & Chemistry of CAS, 40-1 South Beijing Road, Urumqi 830011, China ‡ University of Chinese Academy of Sciences, Beijing 100049, China S Supporting Information *

ABSTRACT: Two new zincoborate fluorides with the common formula BaZn2B2O6· nBa2BO3F (n = 1, 2) have been successfully synthesized for the relationship study between the band gaps and crystal structures in zinc-containing borate fluorides. Ba3Zn2B3O9F with n = 1 in the common formula belongs to the orthorhombic space group Pnma (No. 20), and Ba5Zn2B4O12F2 with n = 2 in the common formula crystallizes in the monoclinic space group C2/c (No. 62). They can both be seen as compounds with the n[Ba2BO3F]∞ (n = 1 or 2) layer inserted in the structure of BaZn2B2O6. UV−vis−near-IR diffuse-reflectance spectra show that the band gaps of BaZn2B2O6·nBa2BO3F (n = 0, 1, 2) gradually increase with more [Ba2BO3F]∞ layers inserted. The first-principles calculation indicates that the inserted n[Ba2BO3F]∞ layers play a positive effect in increasing the band gaps of zincoborate fluorides. Furthermore, the IR spectra, thermal behaviors, and refractive indices of these compounds are also studied.

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materials. In our previous study, we realized that the fluorine element can not only regulate the crystal structure but also cause the absorption edge to blue-shift. These are mainly because F− anions possess large electronegativity and they are more likely to be terminal instead of bridging compared with oxide anions. Therefore, in our current study, we hope to carry out a systematic study on the effect of the crystal structure on the band gaps of zincoborates. We successfully synthesized two new zincoborate fluorides, Ba3Zn2B3O9F and Ba5Zn2B4O12F2, using a high-temperature solution method. They can be seen as compounds with n[Ba2BO3F]∞ (n = 1, 2) layers inserted into the structure of BaZn2B2O6 synthesized by Smith and Keszler,14 and these three compounds satisfy a common formula, BaZn2B2O6·nBa2BO3F (n = 0, 1, 2). Interestingly, with an increase of the n values in the common formula, the band gaps of BaZn2B2O6·nBa2BO3F (n = 0, 1, 2) gradually increase. The first-principles calculation indicates that the increase of the band gaps in these materials is mainly attributed to the lower contribution of O atoms coordinated with Zn atoms on the band gaps with more n[Ba2BO3F]∞ layers inserted. Herein, we will present the synthesis, crystal structures, and structure− property relationship of this series of compounds.

INTRODUCTION Borates as an important family of functional materials have been received much attention owing to the various structural chemistry and wide application in photonic technologies and as ion exchangers, adsorbents, catalysts, etc.1−3 To enrich the structure chemistry of borate, TO4 (T = Be, Al, Si, P, Ge, Zn, etc.) tetrahedra have been widely introduced into borate as an important functional building block (FBB) over the past several years. Thus, many borate compounds with novel structures have been synthesized.4−12 In these borates containing TO4 tetrahedra, the zincoborates are impressive owing to the peculiar effect of ZnO4 tetrahedra in enriching the structure chemistry and enhancing functional properties. For example, in 2010 the Chen and Wu groups synthesized the first example of a zincoborate, KZnBO3, with edge-sharing BO4 tetrahedra under atmospheric pressure.6a,b The Halasyamani group combined the zincoborate with phosphate, obtaining a promising UV nonlinear-optical material, Ba3ZnB5O10PO4,13a and we used ZnO4 tetrahedra to substitute for BeO3F in KBe2BO3F2 (KBBF) and synthesized Cs3Zn6B9O21,13b which exhibits a KBBF-like crystal structure and possesses the largest second-harmonic-generation effect in the KBBF family but without a layer habit. These have suggested that introducing ZnO4 tetrahedra into borate is a good strategy for developing new functional materials. However, for the synthesis of new functional materials with ZnO4 tetrahedra, a problem will often be faced, that is, how to weaken the effect of ZnO4 tetrahdera on the band gap of the material, which is vital for the optical application of zincoborate © XXXX American Chemical Society

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EXPERIMENTAL SECTION

Reagents. PbO, BaCO3, BaF2, ZnO, and H3BO3 are analytical reagent grade and were used as received. Received: February 3, 2016

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DOI: 10.1021/acs.inorgchem.6b00300 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Synthesis. Polycrystalline samples of BaZn2B2O6, Ba3Zn2B3O9F, and Ba5Zn2B4O12F2 were synthesized by a conventional solid-state reaction method with stoichiometric starting reagents. They were elevated to sintering temperatures of up to 770 °C for BaZn2B2O6 (810 °C for Ba3Zn2B3O9F; 840 °C for Ba5Zn2B4O12F2). The purity of the samples was confirmed by powder X-ray diffraction (XRD), which was performed on a Bruker D2 PHASER diffractometer equipped with Cu Kα radiation at room temperature. The scanning step width of 0.02° and the scanning rate of 1 s/step were used to record diffraction patterns in the 2θ range from 10° to 70°. For these powders of BaZn2B2O6, Ba3Zn2B3O9F, and Ba5Zn2B4O12F2, the experimental XRD patterns well match the calculated ones from single-crystal XRD analysis (Figure S1 in the Supporting Information). Single crystals of Ba3Zn2B3O9F and Ba5Zn2B4O12F2 were obtained using a high-temperature solution method. The Ba3Zn2B3O9F crystal was grown from a mixture of PbO, BaF2, ZnO, and H3BO3 in a molar ratio of 2:5:1:3. The mixtures were heated to 900 °C, and this temperature was kept for 10 h to ensure that the solution was homogenized. Then the solution was allowed to cool to 700 °C with a rate of 3 °C/h and further cooled to room temperature with the power off. The Ba5Zn2B4O12F2 crystal was grown from a mixture of stoichiometric starting reagents with PbO in a molar ratio of 1:2. The mixtures were rapidly heated to 850 °C and held for 10 h. Then the solution was cooled to 600 °C at a rate of 3 °C/h, followed by a rate of 20 °C/h to room temperature. Thus, some submillimeter Ba3Zn2B3O9F and Ba5Zn2B4O12F2 were synthesized in a yield of about ca. 30% and 80% based on Ba, respectively. X-ray Crystallographic Studies. Crystals of Ba3Zn2B3O9F (0.08 × 0.08 × 0.10 mm3) and Ba5Zn2B4O12F2 (0.06 × 0.08 × 0.11 mm3) were selected for data collection of the crystal structure. Diffraction data were collected using monochromatic Mo Kα radiation at 296(2) K on a Bruker SMART APEX II 4K CCD diffractometer, and the data including the diffraction intensity, cell refinement, and other related parameters were integrated with the program SAINT.15 Both crystal structures were solved by direct methods using the program SHELXS97 and refined with the final least-squares program SHELXL.16 During the refinement, we found that, in the structure of Ba3Zn2B3O9F, the O(5) atom has a large thermal factor, and the SHELXS-97 program reminds us that the position of the O(5) atoms should be split. When refined with the split model, the results show that an O(5A) site with 51% occupancy and an O(5B) site with 49% occupancy become reasonable. This occupancy converges better R values and reasonable temperature factors. The program PLATON was used to check the structure, and no other higher symmetry was found.17 Table 1 gives the details of the crystal parameters, data collection, and structure refinement. Tables S1 and S2 in the Supporting Information include the final refined atomic coordinates and isotropic thermal parameters and the related bond distances (Å) and angles (deg) for both compounds. IR Spectra. The measurement of the IR spectra for these three compounds was carried out at room temperature on a Shimadzu IRAffinity-1 Fourier transform infrared spectrometer in the wavenumber range from 400 to 4000 cm−1. The polycrystalline samples were mixed thoroughly with dried KBr with a mass ratio of 1:100. UV−Vis−Near-IR (NIR) Diffuse-Reflectance Spectroscopy. Reflectance spectra were measured using a Shimadzu SolidSpec3700DUV spectrophotometer in the wavelength range from 190 to 2600 nm. Absorption (K/S) data were calculated based on the Kubelka−Munk function: F(R) = (1 − R)2/(2R) = K/S, where R is the reflectance, K is the absorption, and S is the scattering.18 Thermal Analysis. The thermal properties of three compounds were studied from room temperature to 1000 °C using a NETZSCH STA 449C simultaneous analyzer in nitrogen gas. Calculation Details. First-principle calculations on the electronic structures and optical properties of these three compounds were carried out using the total energy plane-wave pseudopotential method implemented in the CASTEP module in the Masterial Studio 5.5.19 The local density approximation20 with the Ceperley and Alder− Perdew−Zunger functional was chosen to describe the exchange− correlation effects. The following electrons were treated as valence

Table 1. Crystal Data and Structure Refinement for Ba3Zn2B3O9F and Ba5Zn2B4O12F2 empirical formula temperature (K) wavelength (Å) fw cryst syst space group, Z unit cell dimens a (Å) b (Å) c (Å) β (deg) volume (Å3) density (calcd) (Mg/m3) limiting indices

Ba3Zn2B3O9F 296(2) 0.71073 738.19 orthorhombic Pnma, 4

Ba5Zn2B4O12F2 296(2) 0.71073 1090.68 monoclinic C2/c, 4

8.425(5) 17.155(10) 7.040(4)

25.77(2) 7.257(6) 8.007(6) 105.518(8) 1442.6(19) 5.022

1017.5(10) 4.819

−9 ≤ h ≤ 11, −22 ≤ k ≤ −33 ≤ h ≤ 20, −9 ≤ k ≤ 20, −9 ≤ l ≤ 7 9, −9 ≤ l ≤ 10 5911/1252 [R(int)= 4347/1684 [R(int) = 0.0406] 0.0278] 99.6 98.8

reflns collected/ unique completeness to θ (%) GOF on F2 1.028 final R indices [Fo2 R1 = 0.0286, wR2 = 0.0581 > 2σ(Fo2)]a R indices (all data)a R1 = 0.0367, wR2 = 0.0621 extinction coeff 0.00200(13) largest diff peak and 1.622 and −1.357 hole (e/Å3)

1.069 R1 = 0.0236, wR2 = 0.0501 R1 = 0.0286, wR2 = 0.0521 0.00112(5) 1.072 and −0.914

R1 = ∑||Fo| − |Fc||/∑|Fo| and wR2 = [∑w(Fo2 − Fc2)2/∑wFo4]1/2 for Fo2 > 2σ(Fo2). a

electrons: Ba 5p65d06s2, Zn 3d104s2, B 2s22p1, O 2s22p4, and F 2s22p5. The plane-wave basis set energy cutoff was set at 800.0 eV for BaZn2B2O6, Ba3Zn2B3O9F, and Ba5Zn2B4O12F2 compounds. The kpoint sampling in the Brilliouin zone was set as 4 × 2 × 2. Our test showed that these parameters make good convergence in the present studies. The linear-optical properties for these three compounds were obtained through the dielectric function ε(ω) = ε1(ω) + iε2(ω). The imaginary part of the dielectric function ε2 is illustrated in the following equation:21 ε2(q → Oû , hω) =

2e 2π Ωε0

∑ |⟨φkc|u·r|φkv⟩|2 δ[Ekc − Ekv − E] kcv

In the equation, e represents the elementary charge, h represents Planck’s constant, u represents the vector of the incident polarization, r represents the position operator, Ω represents the unit cell volume, ε0 represents the dielectric constant, and φk represents the momentum matrix element transition. The energies of occupied and empty electronic states use Evk and Eck, respectively. On the basis of the imaginary part, the real part can be obtained using the Kramers− Kronig transformation. All of the other optical constants containing the absorption spectrum, refractive index, are calculated from ε1(ω) and ε2(ω).

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RESULTS AND DISCUSSION Crystal Structure. Ba3 Zn2 B3O 9F crystallizes in the orthorhombic crystal system with space group Pnma, and Ba5Zn2B4O12F2 crystallizes in the monoclinic crystal system with space group C2/c. In their structures, all B atoms possess one coordination model, BO3 triangles, and the Zn atoms are coordinated in ZnO4 tetrahedra. In the asymmetric unit of Ba3Zn2B3O9F, Ba, Zn, B, O, and F atoms each occupy two, one, two, five, and one crystalloB

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Figure 1. Frameworks of Ba3Zn2B3O9F (a) and Ba5Zn2B4O12F2 (b). All of the Ba−O bonds are omitted.

valence sums of each atom agree with the expected oxidation states. Comparison of the Structures among BaZn2B2O6, Ba3Zn2B3O9F, and Ba5Zn2B4O12F2. As described above, Ba 3Zn2B3O9F and Ba5 Zn2B4O12F2 both contain similar [BaZn2B2O6]∞ layers, and the layer has the same stoichiometric ratio as that of the BaZn 2B2O 6 compound. BaZn2B2O6 crystallizes in an orthorhombic system with noncentrosymmetric space group P212121. In its structure, two ZnO4 tetrahedra connect with two BO3 triangles to form the basic building unit [Zn2B2O6] rings, which further connect with each other to form a [Zn2B2O6]∞ 3D framework (Figure S3 in the Supporting Information). The Ba atoms coordinated with seven O atoms fill in the space of the [Zn2B2O6] framework. In the structure of Ba3Zn2B3O9F, the [Zn2B2O6]∞ framework in BaZn2B2O6 is separated into [Zn2B2O6]∞ layers by the [Ba2BO3F]∞ layers, and they connect with each other to form the 3D framework of Ba3Zn2B3O9F, while in the structure of Ba5Zn2B4O12F2, the [Ba2BO3F]∞ layers in Ba3Zn2B3O9F change to [Ba4(BO3)2F2]∞ double layers composed of two [Ba2BO3F]∞ layers sharing F atoms. It seems that a derivate rule exists in these compounds. The structural formulas of BaZn2B2O6, Ba3Zn2B3O9F, and Ba5Zn2B4O12F2 can be written as BaZn2B2O6·nBa2BO3F (n = 0, 1, 2). These suggest that a series of compounds with the common formula BaZn2B2O6· nBa2BO3F (n = 3, 4, ...) might exist, and they may be more effectively synthesized than we expect. It is interesting that when the n value increases in the common formula BaZn2B2O6·nBa2BO3F (n = 0, 1, 2), the space group of BaZn2B2O6·nBa2BO3F (n = 0, 1, 2) changes from P212121, Pnma, to C2/c. These may be attributed to the introduction of [Ba2BO3F]∞ layers in the structures, which leads to the transformation the of Zn2B2O6 framework from 3D in BaZn2B2O6 to two-dimensional (2D) in BaZn2B2O6· nBa2BO3F (n = 1, 2). Obviously, the 2D layer structure possesses a more flexible arrangement and orientation than the 3D framework. The flexible arrangement and orientation are favorable for the two compounds (Ba 3 Zn 2 B 3 O 9 F and Ba5Zn2B4O12F2) to crystallize in a centrosymmetric space group (Pnma and C2/c). Thermal Analysis. The samples used to measure the thermal behavior were characterized by powder XRD (Figure S1 in the Supporting Information), which shows that those samples are in a pure phase. The thermogravimetry (TG)/ differential scanning calorimetry (DSC) curves for these

graphically unique positions, respectively. The structure of Ba3Zn2B3O9F is shown in Figure 1a. It can be seen as a threedimensional (3D) framework consisted of [BaZn2B2O6]∞ layers represented by A and [Ba2BO3F]∞ layers represented by B. In the [BaZn2B2O6]∞ layers, the isolated B(1)O3 triangle shares its three vertices with three neighboring ZnO 4 tetrahedra; likewise each ZnO4 tetrahedron bonds with three B(1)O3 triangles to form [BaZn2B2O6]∞ layers, with the Ba(1) atoms filling the spaces of the layers. In the [Ba2BO3F] layers, all B(2), F, and Ba(2) atoms extend into the ac plane. Further, the A and B layers are alternately stacked along the b axis and connected with each other by the Zn−O and Ba−O bonds to form the 3D structure of Ba3Zn2B3O9F. In the structure, the Ba(1) atoms are bonded to five O atoms and two F atoms, while the Ba(2) atoms connect with eight O atoms and one F atom (Figure S2a in the Supporting Information). The Ba(1)O5F2 and Ba(2)O8F1 polyhedra connect with each other to form the 3D network. The Ba−O bond lengths range from 2.745(5) to 3.08(3) Å, while the interatomic distances of d(Ba−F) lie in a very narrow interval, 2.597(7)−2.739(4) Å. For BO3 triangles and ZnO4 tetrahedra, the bond distances range from 1.301(13) to 1.335(12) Å for B−O and from 1.886(5) to 1.952(5) Å for Zn−O. These values are in agreement with those of other borate compounds reported previously.22 The structure of Ba5Zn2B4O12F2 is shown in Figure 1b. It can also be seen as a complex framework composed of two main units, [BaZn2B2O6]∞ layer (A layer) and [Ba4(BO3)2F]∞ layer. Obviously, the [BaZn 2 B2 O 6 ] ∞ layer is similar to the [BaZn 2 B 2 O 6 ] ∞ layer in Ba 3 Zn 2 B 3 O 9 F, while the [Ba 4(BO3 )2 F2 ]∞ layer seems to be composed of two [Ba2BO3F]∞ layers of Ba3Zn2B3O9F sharing F atoms. The [BaZn2B2O6]∞ and [Ba4(BO3)2F2]∞ layers alternatively connect with each other by shared O atoms to generate the 3D network structure of Ba5Zn2B4O12F2. The Ba atoms have three coordination environments: Fsharing Ba(1)O7F2, Ba(2)O5F2, and Ba(3)O8F2 polyhedra (Figure S2b in the Supporting Information). Also, the Bacentered polyhedra connect with each other to form the 3D framework by sharing O and F atoms. The bond lengths of Ba− O(F) are from 2.670(4) to 3.178(4) Å [2.578(4)−2.620(3) Å]. Also, the B−O bond lengths range from 1.361(7) to 1.395(7) Å, and the Zn−O bond lengths range from 1.911(4) to 1.972(4) Å. These values are reasonable, and calculation of the bond valence sum for both compounds suggests that the C

DOI: 10.1021/acs.inorgchem.6b00300 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 2. TG/DSC curves for BaZn2B2O6 (a), Ba3Zn2B3O9F (b), and Ba5Zn2B4O12F2 (c).

Table 2. Assignments of the IR Absorption Peaks for BaZn2B2O6·nBa2BO3F (n = 0, 1, 2) mode description (cm−1)

BaZn2B2O6

Ba3Zn2B3O9F

Ba5Zn2B4O12F2

asymmetric stretching of B3−O out-of-plane bending of B3−O stretching and bending of Zn4−O

1200 728, 672, 611 1145, 439

1385 748, 615 1136, 436

1389 750, 600 1146, 451

Figure 3. Absorption spectra for BaZn2B2O6 (a), Ba3Zn2B3O9F (b), and Ba5Zn2B4O12F2 (c).

Figure 4. Calculated band structure of BaZn2B2O6 (a), Ba3Zn2B3O9F (b), and Ba5Zn2B4O12F2 (c).

compounds are shown in Figure 2. It is clear that there is only one endothermic peak on each DSC curve of BaZn2B2O6· nBa2BO3F (n = 0, 1, 2), and there is no weight loss on their TG curves. To furthermore confirm the thermal behavior, the pure samples of BaZn2B2O6·nBa2BO3F (n = 0, 1, 2) were melted at 1000 °C for 10 h and then slowly cooled to room temperature at a rate of 10 °C/h. Analysis of the powder XRD pattern of the solidified melt reveals that BaZn2B2O6 and Ba3Zn2B3O9F exhibit diffraction patterns different from those of the initial powder (Figure S4a,b in the Supporting Information),

demonstrating that BaZn2B2O6 and Ba3Zn2B3O9 are incongruently melting compounds. That is consistent with the phenomenon that a higher temperature will lead to Ba3Zn2B3O9F transforming to another unknown phase during the process of synthesizing a pure phase by a solid-state reaction. However, for Ba5Zn2B4O12F2, the powder XRD after melting exhibits the same diffraction pattern as that of the initial powder (Figure S4c in the Supporting Information), which suggests that Ba5Zn2B4O12F2 melts congruently. IR and UV−Vis−NIR Spectra. IR spectra of BaZn2B2O6· nBa2BO3F (n = 0, 1, 2) compounds are given in Figure S5 in D

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Figure 5. Density of states of the O orbitals of BaZn2B2O6 (a), Ba3Zn2B3O9F (b), and Ba5Zn2B4O12F2 (c).

are mixed with the p orbitals of Zn, B, and O. Thus, we can obviously observe that the bands around the Fermi level are predominantly Ba and O orbitals. These indicate that the Ba, Zn, and B−O groups are the dominant active unit factors on the optical properties. The Zn atoms also have an effect on the band gap, which can be further quantitatively proven by the results via ab initio calculations of the density of states and the highest occupied and lowest unoccupied orbitals. The difference in the absorption edge among these compounds can be elucidated by the electronic structure about the highest occupied and lowest unoccupied orbitals of BaZn2B2O6·nBa2BO3F (n = 0, 1, 2; Figure 5). From BaZn2B2O6 (n = 0) to Ba5Zn2B4O12F2 (n = 2), it can be observed that the effect of ZnO4 tetrahedra on the band gaps has gradually decreased with an increase in n from 0 to 2 (Figure S8 in the Supporting Information). In the compound BaZn2B2O6, the top of the valence band (VB) is mainly occupied by the O(1), O(2), O(4), and O(5) atoms shared by ZnO4 tetrahedra, while in the compound Ba3Zn2B3O9F, the top of the VB is mainly occupied by the O(5) atoms shared by ZnO4 tetrahedra and BO 3 triangles, and, furthermore, in the compound Ba5Zn2B4O12F2, the top of the VB is mainly occupied by the O atoms shared by BO3 triangles. These indicate less contribution of the O atoms coordinated with the Zn atoms on the band gaps with more n[Ba2BO3F]∞ layers inserted and also suggest that the Ba, B, O, and F atoms have a positive effect on the band gap. These factors, consequently, lead to a 47 nm blue shift of the absorption edge from BaZn2B2O6 (n = 0) to Ba5Zn2B4O12F2 (n = 2). The linear-optical properties of BaZn2B2O6·nBa2BO3F (n = 0, 1, 2) compounds were calculated from the complex dielectric function. The BaZn2B2O6·nBa2BO3F (n = 0, 1, 2) compounds crystallize in the 222, mmm, and 2/m point groups, respectively, and they all belong to negative biaxial crystals because ny − nx < nx − nz. During the calculation, confirmation of the principal dielectric axis is necessary. For BaZn2B2O6 and Ba3Zn2B3O9F, the corresponding relationship between the dielectric and crystallographic axes is X, Y, Z → a, b, c, and the birefringence can be calculated based on linear response functions, from which the anisotropy of the index of refraction is procured. For Ba5Zn2B4O12F2, the Y dielectric axis corresponds to the b crystallographic axis, but the principal dielectric X and Z axes are not superposed in any specific crystallographic directions. We can calculate the included angle between the crystallographic axis a and the principal dielectric axis X in the ac plane

the Supporting Information. Table 2 lists the absorption peaks.23,24 From the table, we can see that these spectra exhibit similar B−O and Zn−O vibrations. The results confirm the existence of BO3 triangles and ZnO4 tetrahedra in their structures and are consistent with the results obtained from the crystallographic structures. The optical diffuse-reflectance spectra of BaZn2B2O6· nBa2BO3F (n = 0, 1, 2) in the region 190−2600 nm are shown in Figure S6 in the Supporting Information. On the basis of diffuse-reflectance spectra, they were converted to absorbance based on the Kubelka−Munk function and deposited in Figure 3. It is clear that the experimental band gaps of BaZn2B2O6·nBa2BO3F (n = 0, 1, 2) change from 4.38 to 5.26 eV from n = 0 to 2 (4.38 eV for BaZn2B2O6; 4.93 eV for Ba3Zn2B3O9F; 5.26 eV for Ba5Zn2B4O12F2). Structure−Property Relationship. As described above, the band gaps of BaZn2B2O6·nBa2BO3F (n = 0, 1, 2) become larger with an increase of the n value. To better understand the band structure features of this series of compounds, firstprinciples calculation of the electronic structures was carried out (Figure 4). The three compounds are all direct-band-gap compounds with band gaps of 3.98 eV for BaZn2B2O6, 4.89 eV for Ba3Zn2B3O9F, and 5.03 eV for Ba5Zn2B4O12F2, all of which are smaller than the experimental values, just as expected from the density functional theory method, in which the band gaps are generally underestimated. This is mainly because the exchange correlation energy was inaccurately calculated.25 It is well-known that the valence electrons will play an important role in most optical properties of a compound, which are dominated by the band structures in the vicinity of the Fermi level. Thus, a more detailed investigation of the electronic structure in this range was carried out for analysis of its effect on the band gaps and optical properties. Figure S7 in the Supporting Information gives the partial density of states (PDOS) of the Ba, Zn, B, O, and F orbitals in the BaZn2B2O6· nBa2BO3F (n = 0, 1, 2) compounds. Obviously, the three compounds exhibit similar PDOS. Bands lower than −10 eV mostly consisted of 5p orbitals of Ba atoms mixing with B 2s2p, O 2s, and F 2s orbitals. In the range of −7.5 to −2.5 eV, there is obvious hybridization between the Zn, B, and O states, while the 2p orbitals of the F atoms are strongly localized around −5.0 to −3.0 eV in BaZn2B2O6·nBa2BO3F (n = 1, 2). From −3 to 0 eV in the BaZn2B2O6·nBa2BO3F (n = 0, 1, 2) compounds, mainly O 2p orbitals exist. The conduction bands of the three compounds are mainly composed of valence orbitals of Ba but E

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U1303392 and 51425206), the Western Light of CAS (Grant XBBS201214), and the Outstanding Young Scientists Project of CAS, Youth Innovation Promotion Association CAS (Grant 2015353).

(Figure S9 in the Supporting Information). On the basis of the principal dielectric axis coordinate system, the refractive indices of Ba5Zn2B4O12F2 are calculated. The obtained birefringence values against the wavelengths are plotted in Figure S10 in the Supporting Information. It is clear that BaZn2B2O6·nBa2BO3F (n = 0, 1, 2) possess birefringence with Δn = 0.044, 0.037, and 0.035 at 532 nm, respectively.

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CONCLUSION Ba3Zn2B3O9F and Ba5Zn2B4O12F2 have been synthesized by a high-temperature solution method. Both structures contain the [BaZn2B2O6]∞ layers, separated by the n[Ba2BO3F]∞ (n = 1, 2) layers. Furthermore, a more detailed structural comparison among BaZn2B2O6, Ba3Zn2B3O9F, and Ba5Zn2B4O12F2 shows that their structural formulas can be written as BaZn2B2O6· nBa2BO3F (n = 0, 1, 2), and they also suggest that a series of compounds with the common formula BaZn2B2O6·nBa2BO3F (n = 3, 4, ...) may be more effectively synthesized than we expect. Thermal analysis shows that BaZn 2 B 2 O 6 and Ba3Zn2B3O9F are incongruently melting compounds, while Ba5Zn2B4O12F2 melts congruently. More interestingly, the band gaps of BaZn2B2O6·nBa2BO3F (n = 0, 1, 2) gradually increase with more [Ba2BO3F]∞ layers inserted. Theoretical calculation shows that the different band excitations are mostly from the O 2p to Ba 5d orbitals in BaZn2B2O6·nBa2BO3F (n = 0, 1, 2), and the contribution of the shared O atoms of ZnO4 tetrahedra on the top of the VB gradually decreases with an increase of the n value in the common formula. More research about zincoborate will be carried out in the near future.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b00300. CIF file (CIF) CIF file (CIF) Tables of atomic coordinates and displacement parameters and selected bond distances and angles for Ba3Zn2B3O9F and Ba5Zn2B4O12F2, coordination environments of the Ba atoms for Ba 3 Zn 2 B 3 O 9 F and Ba5Zn2B4O12F2, structure of BaZn2B2O6, experimental and calculated XRD patterns of BaZn2B2O6·nBa2BO3F (n = 0, 1, 2), TG/DSC curves for BaZn2B2O6, IR spectra for BaZn2B2O6·nBa2BO3F (n = 0, 1, 2), UV−vis−NIR spectra, band structures, total density of states and the corresponding partial density of states, highest occupied and lowest unoccupied orbitals, and calculated birefringence indices (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: (+86)991-3674558. Fax: (+86)991-3838957. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the Xinjiang Program of Cultivation of Young Innovative Technical Talents (Grant 2014711001), the 973 Program of China (Grant 2014CB648400), the National Natural Science Foundation of China (Grants F

DOI: 10.1021/acs.inorgchem.6b00300 Inorg. Chem. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.inorgchem.6b00300 Inorg. Chem. XXXX, XXX, XXX−XXX