Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
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The Rubidium Barium Borate Resulting from B7O15 Fundamental Building Block Exhibits DUV Cutoff Edge Yun Yang,† Xiaoyu Dong,† Shilie Pan,* and Hongping Wu CAS Key Laboratory of Functional Materials and Devices for Special Environments, Xinjiang Technical Institute of Physics & Chemistry, CAS Xinjiang Key Laboratory of Electronic Information Materials and Devices, 40-1 South Beijing Road, Urumqi 830011, China
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S Supporting Information *
ABSTRACT: The first rubidium barium borate, RbBaB7O12, has been prepared. Through analysis of single crystal structures, RbBaB7O12 is built from a three-dimensional [B7O12]∞ framework with two types of channels which are occupied by Rb+ and Ba2+ ions. The connection style of the fundamental building block (FBB) B7O15 is different from that of B−O FBBs existing in the other borates, which can be determined as the unprecedented unit. The structural performance relationship can be better understood by combining first-principles calculations and experimental results. For the RbBaB7O12, the indirect energy gap is 5.96 eV, which matches the experimental data (wavelength absorption < 190 nm). IR spectroscopy and thermal analysis have also been characterized. What’s more, for all the available anhydrous hepta-borates, the structure comparisons about FBBs were carried out.
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INTRODUCTION
structural configurations and excellent performances might also exist incorporating alkali/alkaline-earth metal borate. Inspired by the above idea, in Pan’s group, alkali/alkalineearth metal borate systems are chosen and persevered in deep research. In this work, a new rubidium barium borate RbBaB7O12 was prepared for the first time. Interestingly, a FBB B7O15 was found in RbBaB7O12 structure, then the title compound features a novel anionic framework [B7O12]∞. In addition, characterization, including thermal and optical analysis, and theoretical calculations of RbBaB7O12 are also presented.
In past decades, the design, configuration, and elucidation based on borate crystals have attracted quite a lot interest from structural chemists and application engineers.1−6 The drives arise not only from the pragmatic viewpoint to acquire functional crystals, but also from the scientists’ interest to reveal the diversity of framework topologies and the architectures that can be gathered and to reveal the regulations of structural-performance relationships.7−20 The characterization of alkali metal/alkaline-earth metal borate materials is an area of particular interest due to their excellent performance in creating intriguing structural diversity and potential functionality in optical parameter oscillators, laser micromachining, photolithography, and signal processing.1 Among these, LiB3O5,1b β-BaB2O4,1c LiCsB6O10,19 and KBe2BO3F21d are most widely used as nonlinear optical (NLO) materials. These materials show excellent performance based on their special structures. Various oxoborate clusters are constructed by BO3 or BO4 units, and widely different B−O fundamental building blocks (FBBs) will be generated. And these FBBs formed by B−O groups have been summarized by several excellent researchers,2−6 which has played a very positive role in control, design, and synthesis of borate compounds. In recent years, more compounds have been prepared, and some new FBBs have been discovered, among which several new materials retain the structural advantages and related outstanding properties of well-known classical compounds. In borates, it is entirely reasonable for researchers to believe that new and interesting materials with novel © XXXX American Chemical Society
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EXPERIMENTAL SECTION
Synthesis. The high temperature solid phase reaction was used to prepare the RbBaB7O12 polycrystalline sample. A stoichiometric ratio of Rb2CO3, BaCO3, and B2O3 about 1:2:7 (2.31 g Rb2CO3, 3.95 g BaCO3, and 4.87 g B2O3) was mixed thoroughly. The following general chemical equation can describe this process of synthesis: Rb2CO3 + 2BaCO3 + 7B2O3 → 2RbBaB7O12 + 3CO2 ↑ To decompose the carbonate, the mixed sample was held at about 400 °C over 12 h first. Then, it was elevated to about 600 °C for 24 h. Afterward, it was increased to 750 °C for 1 week with intermittent mixing and grinding. In the end, powder X-ray diffraction (PXRD) analyses were applied, which shows the PXRD pattern of RbBaB7O12 is a good match to the one calculated form crystal data (Figure S1 in the Supporting Information), proving that the pure phase of RbBaB7O12 has been obtained. Received: July 13, 2018
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DOI: 10.1021/acs.inorgchem.8b01960 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry RbBaB7O12 crystals were synthesized from a reaction molar ratio of RbCO3/Ba2CO3/PbO/Na2CO3/B2O3 = 3:1:0.5:1:6. The mixed powder was ground and mixed thoroughly in a small platinum crucible. It was melted to solution when increased to 800 °C and held for 8 h until clear and transparent. As the solution cooled at a rate of 2° per hour, several crystals floated on the surface of the solution. Finally, the sample was cooled to room temperature (RT). Using polarizers, some small and transparent crystals were selected for a single-crystal test. Powder X-ray Diffraction. PXRD patterns were confirmed with a Bruker D2 PHASER diffractometer equipped with Cu Kα radiation (λ = 1.5418 Å, T = 296 K). The 2θ range was from 10 to 70° (scanning step = 0.02°). X-ray Crystallographic Studies. On a Bruker Smart APEX II single-crystal diffractometer, a crystal of RbBaB7O12 with a size of 0.12 × 0.10 × 0.09 mm3 was selected for measurement using a monochromatic Mo Kα radiation (λ = 0.71073 Å) at 296(2) K.21 All the calculations were processed using the SHELXTL crystallographic software package.22 The full-matrix least-squares method in the SHELXL-97 systems was used to refine structure.23 The missing symmetry elements of the structure were checked with PLATON.24 In Table 1, structure refinement information and the crystal data of
A Shimadzu IR Affinity−11 Fourier transform infrared spectrometer was used to record the IR spectrum in the 400−4000 cm−1 range. The test polycrystalline sample was fully mixed with dried potassium bromide, and the transparent sheet for testing was prepared using a laminator. Theoretical Calculations. The electronic structures and optical properties of RbBaB7O12 were performed using the DFT method implemented in the CASTEP module.26 The exchange-correction functional was Perdew−Burke−Emzerhof (PBE) within the generalized gradient approximation (GGA). The valence electron configurations for diverse electron orbital pseudopotentials were Rb 4s24p65s1, Ba 5s25p66s2, B 2s22p1, and O 2s22p4. The plane-wave energy cutoff was set at 800 eV. The Monkhorst−Pack k-point was sampled with a separation of less than 0.03 Å−1, which was 4 × 4 × 3 for the Brillouin Zone (BZ).
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RESULTS AND DISCUSSION Structural Description. For the structure of RbBaB7O12, it crystallizes into the P21/c space group (Figure 1). RbBaB7O12
Table 1. Crystal Data and Structure Refinement Information empirical formula temperature formula weight crystal system space group unit cell dimensions
volume Z density (calculated) absorption coefficient F(000) θ range for data collection index ranges reflections collected/unique completeness to theta = 27.51 refinement method data/restraints/parameters goodness-of-fit on F2 final R indices [Fo2 > 2σ(Fo2)]a R indices (all data)a largest diff. peak and hole
RbBaB7O12 296(2) 490.48 monoclinic P21/c a = 10.6461(5) Å α = 90° b = 8.5090(5) Å β = 112.569(3) ° c = 12.4701(6) Å γ = 90° 1043.13(9) Å 3 4 3.123 g/cm3 8.508 mm−1 896 2.98 to 27.51° −13 ≤ h ≤ 13, −7 ≤ k ≤ 10, −15 ≤ l ≤ 16 7814/2364 [R(int) = 0.0370] 98.6% semiempirical from equivalents 2364/6/191 1.037 R1 = 0.0302, wR2 = 0.0542 R1 = 0.0481, wR2 = 0.0593 0.745 and −0.790 e·A−3
Figure 1. Extended asymmetric unit of RbBaB7O12.
is built from three-dimensional [B7O12]∞ framework with two types of channels, which are occupied by Rb+ and Ba2+. As shown in Figure 2a, by corner-sharing, a fundamental building block (FBB) B7O15 was configured by the interconnection of four BO3 and three BO4, which consists of three rings connected alternately and can be written as 4Δ3T:- according to the definition given by Filatov and Bubonova et al.3,5 As far as we know, B7O15 has never been reported previously, so it is a new FBB. The infinite [B7O14]∞ chains are formed by the B7O15 FBB along the a axis by B(5)O3 by sharing the O(6) atom with B(6)O4 (Figure 2b). It is obvious to see that the adjacent [B7O14]∞ chains opposite are exactly parallel to each other along the a axis, and these chains form a [B7O13]∞ layer lying on the a−c plane (Figure 2c,d). Via sharing O atoms, the 2D [B7O13]∞ layers are arranged along the b axis to form a 3D [B7O12]∞ framework exhibiting a feature of 3D tunnels (Figure 2e), which is constructed of B7O15 units and can be classified into two types (Figures 2e). The Rb atom is coordinated to 11 O atoms and a distorted Rb(1)O11 polyhedron was formed. Every two Rb(1)O11 polyhedra are sharing edges into a dimer, which are interconnected via bridged O(3) atoms to format 1D straight chains along the c axis (Figure S2, Supporting Information).
R(F) = ∑||Fo| − |Fc||/∑|Fo|·wR(Fo2) = [∑w(Fo2 − Fc2)2/ ∑w(Fo2)2]1/2.
a
RbBaB7O12 are listed. Atomic coordinates, equivalent isotropic displacement parameters (Å2), selected bond distances (Å) and angles (deg), and the bond valence sum (BVS) for RbBaB7O12 are listed in Tables S1, S2, and S3 in the Supporting Information, respectively. Thermal Analysis. A NETZSCH STA 449C thermal analyzer was used to analyze the thermal behavior (heating rate = 10 °C min−1, flowing N2) from 25 to 950 °C. Spectroscopic Analysis. A SolidSpec-3700DUV spectrophotometer was used to collect the UV−vis−NIR diffuse-reflectance data from 190 to 2600 nm. According to the Kubelka−Munk equation, F(R) = (1 − R)2/2R (R = reflectance),25 the absorbance data were converted from reflectance data. B
DOI: 10.1021/acs.inorgchem.8b01960 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry
Figure 2. (a) B7O15 unit. (b) The chains of [B7O14]∞ on a−c plane. (c) Perspective views of the [B7O13]∞ layer down the b axis. (d) [B7O13]∞ layer down the a axis. (e) The whole 3D framework. Figure 3. Configurations of hepta-borate FBBs (a) B7O14, (b) B7O16, (c) B7O17, (d) B14O30, (e) B63O125.
The Ba(1) atoms are 10-fold coordinated to form Ba(1)O10 polyhedra. By shared edges and corners, every six Ba(1)O10 polyhedra form a rectangle, which is further interconnected to form a 2D layer (Figure S3, Supporting Information) parallel to the bc plane. Then, 1D RbO10 chains act as bridges to connect the 2D BaO9 layer to generate a Rb−O−Ba 3D network (Figure S4, Supporting Information). A wide region of the Ba−O bond lengths vary from 2.655(3) to 3.251(3) Å. Rb(1)−O bond distances range from 2.939(3) to 3.597(3) Å. And B−O bond lengths are from 1.359(5) to 1.517(6) Å (Table S2 in the Supporting Information). In Table S3, Supporting Information, the calculated27 value of the bond valence sums for each atom are listed. The results of BVS values (Rb, 0.9318; Ba, 2.0591; B, 3.0190−3.0629) agree with the reported oxidation states. FBBs Comparison among the Hepta-Borates. For the crystal structure RbBaB7O12, based on the molecular formula, it is a hepta-borate. Several anhydrous borates having a formula which contains seven B atoms are known: Li3B7O12,28 Na 3 B 7 O 12 , 2 9 Rb 3 B 7 O 12 , 30 Cs 3 B 7 O 12 , 3 1 AgSrB 7 O 12 , 32 Li4Cs3B7O14,33 Li4Rb3B7O14,34 KBa7Mg2B14O28F5,12b and NaMg2Ba7B14O28F5.35 Although all of their molecular formulas are composed of seven B atoms, the distinction of configurations is obvious. And the differences between these FBBs are analyzed below. For Li4Cs3B7O14, Li4Rb3B7O14, KBa7Mg2B14O28F5, and NaMg2Ba7B14O28F5, in the case where all cations are deleted and only boron atoms are left, the four compounds contain the same FBB, isolated B 7 O 14 (5Δ2T:-)3 (Figure 3a and Figure S5, Supporting Information), which is formed by two B3O7 and one B3O8 through sharing two BO4 units. For another six hepta-borates with similar formulas of Li3B7O12, Na3B7O12, Rb3B7O12, Cs3B7O12, AgSrB7O12, and the title compound RbBaB7O12, the FBBs configurations are flexible and variable. For RbBaB7O12, the FBB is B7O15 (4Δ3T:--). A B3O8 ring, a B3O7 ring, and a bridged BO3 form the B7O15 unit through sharing vertex O atoms. The connection style of this FBB is different from that of FBBs existing in the other hepta-borates. The FBB existing in Na3B7O12 (and Li3B7O12) is B7O16
(4Δ3T:-Δ;3 Figure 3b, Figure S6, Supporting Information). The FBB consists of three parts including the B3O7 ring, B3O8 ring, and BO3 (Figure S6, Supporting Information). Every two B7O16 units are connected end to end by two oxygen atoms to constitute a 20-MR (Figure S6b, Supporting Information), while the rings are connected by another two oxygen atoms to build a 1D chain along the c axis, and another 16-MR ring is formed simultaneously. Next, these 1D chains form the 2D layer, and the whole 3D framework shares oxygen atoms (Figure S6c and d, Supporting Information). In AgSrB7O12, a FBB B7O17 (4Δ3T:1Δ1T< 2Δ1T >1Δ1T)3 was found (Figure 3c and Figure S7, Supporting Information). The FBB consists of three groups, one B3O7 ring and two B3O6 chains, which are first connected to a 1D chain (Figures S7b, Supporting Information) and then further linked to the two-dimensional (2D) network along the b axis (Figure S7c and d, Supporting Information). For Rb3B7O12, the FBB is 8Δ6T:- 1T-3 (Figure 3d and Figure S8, Supporting Information) containing eight BO3 and six BO4. In this FBB, there are two double-ring B5O11 bridged by two BO4, one of which is included in a B3O7 ring. By sharing oxygen atoms, FBBs are arranged in parallel to form a 2D layer (Figure S8, Supporting Information). Finally, in Cs3B7O12, it owns the biggest FBB with 63 boron atoms (Figure 3e and Figure S9, Supporting Information). The double-ring B5O11, triple-ring B7O16, and double-ring B5O12 are found in Cs3B7O12 and all of these lead to the FBBs 63∞2:2−8 + 10 8 + 8 + T)]3a (Figure 3e). And the structures of Rb3B7O12, Cs3B7O12, and AgSrB7O12 heptaborates present all 2D FBBs. Thermal Analysis. Through thermodynamic analysis, we can determine the thermal stability of the compound and whether it requires the participation of a solvent when growing large sized crystals. Only one endothermic peak is around 784 °C (Figure S10, Supporting Information) on the DSC heating curve. PXRD was used to substantiate the thermal behavior of the RbBaB7O12 further. Powder of RbBaB7O12 was placed into C
DOI: 10.1021/acs.inorgchem.8b01960 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry a small platinum crucible heated to 850 °C until melted. After that, it was slowly cooled to RT. The PXRD pattern of the melted RbBaB7O12 residual presents a different pattern from that of the initial RbBaB7O12 sample (Figure S1, Supporting Information), which proves that it is an incongruent melting compound. If the large crystals are to be grown, the participation of flux is required. Optical Analysis. The IR spectrum36,37 of RbBaB7O12 is shown in Figure S11 in the Supporting Information. The main infrared absorption region at 1137 to 988 cm−1 is attributed to asymmetry stretching of BO4 groups. The bands from 921 to 779 cm−1 are owed to symmetry stretching of the BO3 and BO4. At 736 and 617 cm−1, the bands belong to the out-ofplane bending of BO3. The peaks which originate from the bending modes of the BO3 and BO4 are at 570 and 494 cm−1. For the optical applications of materials, the ultraviolet cutoff edge is an important parameter. The cutoff edge of RbBaB7O12 is
