Three Mixed-Alkaline Borates: Na2M2B20O32 (M = Rb, Cs) with Two

Oct 9, 2017 - Na 2s22p63s1, Rb 4s24p65s1, Cs 5s25p66s1, B 2s22p1, and O 2s22p4 were chosen as the valence electrons. .... angles of neighboring BO3 tr...
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Three Mixed-Alkaline Borates: Na2M2B20O32 (M = Rb, Cs) with Two Interpenetrating Three-Dimensional B‑O Networks and Li4Cs4B40O64 with Fundamental Building Block B40O77 Maierhaba Abudoureheman,†,‡ Shujuan Han,*,† Ying Wang,† Qiong Liu,†,‡ Zhihua Yang,† and Shilie Pan*,† †

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

ABSTRACT: Three new mixed-alkaline borates Na2M2B20O32 (M = Rb, Cs) and Li4Cs4B40O64 with unique structures were synthesized by the spontaneous nucleation method, and their structures were determined by single-crystal X-ray diffraction. The B-O networks of Na2M2B20O32 (M = Rb, Cs) are constructed by two independent interpenetrating three-dimensional (3D) frameworks, which is the first case in anhydrous mixed-alkaline borate systems. In addition, Li4Cs4B 40 O 64 shows high B-O polymerization with a new fundamental building block B40O77. Meanwhile, detailed structure comparisons containing the cation effect on the framework have been discussed. UV−vis−NIR diffuse reflectance spectra and the infrared spectra were measured. The band structures and the density of states were performed using density functional theory calculation.

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INTRODUCTION Owing to the abundant structural chemistry and excellent optical properties of metal borates, they have been widely used as the nonlinear optical materials (NLO), birefringent crystals, catalysis, ionic conductivity, photoluminescence materials, etc.1−11 The impressive structural diversity of borates12−15 can be attributed to three/four-coordinated of boron, which can be further connected to form isolated units, chains, layers, and three-dimensional (3D) frameworks. As one of the popular research systems, alkali and mixedalkaline borates have attracted considerable interest. The reasons are analyzed as follows: on the one hand, there is no d−d or f−f electron transitions for alkali and mixed-alkaline, which is helpful to obtain the borate crystal with short cutoff edge.16 On the other hand, the introduction of cations with large atomic radii (Rb or Cs) may lead to special crystal structures.17 On the basis of this, several borates with outstanding properties, such as MB3O5 (M = Li, Cs),18,19 CsLiB6O10,20 Li6Rb5B11O22,21 Li4CsB5O10,22 Li5M2B7O14 (M = Rb, Cs),22,23 and Li3Cs2B5O10,24 have been reported. Hence, it is meaningful to continue the investigation on the alkali and mixed-alkaline borates, especially the compounds containing the Rb or Cs atoms. However, how to obtain this kind of compounds? To date, several effective strategies are used: (1) increasing the number © XXXX American Chemical Society

of the B-O clusters or decreasing the ratio of A/B (A = alkaline metals, B = boron);25 (2) mutually substituting the metal cations using the ones with similar atomic radii, coordination, and bond valence in the host crystal structure. Inspired by the above ideas, three new crystals Na2M2B20O32 (M = Rb, Cs) and Li4Cs4B40O64 with unique structures have been discovered. Na2M2B20O32 (M = Rb, Cs) are the first cases of anhydrous mixed-alkaline borates whose structures are constructed by two independent, interpenetrating 3D frameworks. The Li4Cs4B40O64 compound contains the largest B-O building block after the Cs3B7O12 compound26 that presents a new type of the B-O fundamental building blocks (FBBs). In this Article, all of the anhydrous mixed-alkaline borates are summarized; the detailed structure comparisons among compounds with interpenetrating 3D frameworks and cation effect have also been discussed. Moreover, optical properties and the theoretical calculations were presented.

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EXPERIMENTAL SECTION

Synthesis. The single crystals of the title compounds were synthesized by the spontaneous nucleation method. The starting materials NaF, RbF\CsF, and B2O3 with the molar rate of 2:2:10 were Received: August 23, 2017

A

DOI: 10.1021/acs.inorgchem.7b02168 Inorg. Chem. XXXX, XXX, XXX−XXX

a

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

1558.70(7) 2.014 2.48−27.47° −15 ≤ h ≤ 15, −15 ≤ k ≤ 13, −14 ≤ l ≤ 14 6619/1774 [R(int) = 0.0406] 99.1 1.071 R1 = 0.0346, wR2 = 0.0699 R1 = 0.0477, wR2 = 0.0752 0.452 and −0.528 e/Å3

volume (Å3) density (calcd) (mg/m3) θ range for data collection limiting indices reflections collected/unique completeness θ (%) goodness of fit on F2 final R indices [F02 > 2σ(F02)]a R indices (all data)a largest diff. peak and hole

Na2Rb2B20O32 945.12 monoclinic C2/c, 2 a = 12.0194(3) Å b = 12.4188(3) Å c = 11.4969(3) Å β = 114.731(2)°

formula weight crystal system space group, Z unit cell dimensions

empirical formula

1583.19(7) 2.182 2.46−27.47° −15 ≤ h ≤ 15, −15 ≤ k ≤ 13, −14 ≤ l ≤ 14 6725/1790 [R(int) = 0.0471] 98.9 1.044 R1 = 0.0327, wR2 = 0.0588 R1 = 0.0449, wR2 = 0.0627 0.696 and −0.929 e/Å3

1040.00 monoclinic C2/c, 2 a = 12.2701(3) Å b = 12.2905(3) Å c = 11.5174(3) Å β = 114.285(2)°

Na2Cs2B20O32

Table 1. Crystal Data and Structural Refinement for Na2Rb2B20O32,, Na2Cs2B20O32, and Li4Cs4B40O64 Li4Cs4B40O64 2015.80 triclinic P1̅, 2 a = 11.0442(2) Å b = 11.0981(2) Å c = 23.7594(4) Å α = 87.4400(10)° β = 77.3770(10)° γ = 76.5900(10)° 2764.26(8) 2.422 0.88−27.52° −14 ≤ h ≤ 13, −14 ≤ k ≤ 14, −30 ≤ l ≤ 30 44393/12581 [R(int) = 0.0529] 98.9 1.100 R1 = 0.0501, wR2 = 0.1011 R1 = 0.0730, wR2 = 0.1083 2.546 and −1.095 e/Å3

Inorganic Chemistry Article

B

DOI: 10.1021/acs.inorgchem.7b02168 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 1. (a) FBB in the structure of Na2Rb2B20O32. (b) Connection of FBBs. (c) Topological structure of Na2Rb2B20O32.

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weighed for Na2Rb2B20O32 and Na2Cs2B20O32. After the substitution of alkaline metals, we have got Li4Cs4B40O64 using the mixtures of LiF, CsF, and B2O3 with the molar rate of 4:4:20. All of the mixtures were placed into a Pt crucible and heated to 650 °C slowly. Two hours later, the melts were cooled to 550 °C (2 °C/h), cooled to 450 °C (3 °C/ h), and finally cooled to room temperature (10 °C/h). A solid-state reaction method was used to get the polycrystalline samples of Na2Rb2B20O32, Na2Cs2B20O32, and Li4Cs4B40O64. The stoichiometric mixtures of Li2CO3\Na2CO3, Cs2CO3\Rb2CO3, and B2O3 were presintered at 300 °C for 24 h. With several times of intermediate grindings, we raised the temperature to 560 °C for the Na2Rb2B20O32 compound and held at this temperature for 15 days. The temperature was raised to 620 °C for Na2Cs2B20O32, 690 °C for Li4Cs4B40O64, and held at the chosen temperatures for 3 days. A Bruker D2 PHASER diffractometer (2θ range from 5° to 70°) was used to collect the powder X-ray diffraction (XRD) data to confirm the purity of the compounds. As shown in Figure S1a,b in the Supporting Information (SI), the experimental XRD data of Na2Rb2B20O32 and Na2Cs2B20O32 were in good agreement with the calculated ones. Two small peaks at 2θ = 12°, 21° were found in the XRD pattern of Li4Cs4B40O64, which belong to the LiCsB6O10 compound (Figure S1c in the SI). We failed to avoid the small amount of LiCsB6O10, although we have tried to synthesize pure phase of Li4Cs4B40O64 at various temperatures and with different sintering times. Characterization. In order to determine the structure, an APEX II CCD diffractometer (monochromatic Mo Kα radiation) was used for data collection at 123 K. The SAINT program,27 SHELXTL software,28 and PLATON program29 were used to integrate data, solve structures, and check the symmetry of structures, respectively. Table 1 and Tables S1 and S2 in the SI give the results of structure refinements, and crystal data information such as isotropic thermal parameters, atomic coordinates, and selected bond lengths and angles of the three compounds. The corresponding CCDC nos. are 1561509−1561511 for Li4Cs4B40O64, Na2Cs2B20O32, and Na2Rb2B20O32, respectively. The optical diffuse reflectance data were performed by a Shimadzu SolidSpec-3700 UV−vis−NIR spectrophotometer at room temperature. Infrared (IR) spectra were collected on a Shimadzu IRAffinity-1 spectrometer with the range of 400−4000 cm−1. Calculation Details. The experimental crystal data of Na2Rb2B20O32 and Na2Cs2B20O32 were used to perform the theoretical calculation. The electronic structures of Na2Rb2B20O32 and Na2Cs2B20O32 were obtained by using density functional theory calculation with the CASTEP code.30 The generalized gradient approximation (GGA) with the Perdew−Burke−Ernzerhof (PBE) functional was chosen as the exchange-correlation functional and the normconserving pseudopotential (NCP) was chosen as the pseudopotential. Na 2s22p63s1, Rb 4s24p65s1, Cs 5s25p66s1, B 2s22p1, and O 2s22p4 were chosen as the valence electrons. The k-point separation of each material was set as 0.04 Å in the Brillouin zone. Other parameters were set as the default values in the CASTEP.

RESULTS AND DISCUSSION Description of Crystal Structure. Na2Rb2B20O32 and Na2Cs2B20O32 are isostructural and crystallize in the monoclinic symmetry with the C2/c (No.15) space group. Hence, the structure of Na2Rb2B20O32 is shown as a representative. The main outstanding feature of Na2Rb2B20O32 is that its structure is made up of two interpenetrating 3D B-O nets with pseudochannels, while the Na+ and Rb+ cations reside in the cavities (Figure S2 in the SI). To the best of our knowledge, Na2M2B20O32 (M = Rb, Cs) are the first samples where the two interpenetrating 3D B-O nets coexist in one anhydrous mixed-alkali metal borate structure. The asymmetric unit includes one Na atom, one Rb atom, five B atoms, and eight O atoms. The B(1,2,4,5) atoms are coordinated to three O atoms and held the triangular coplanar sites with the B−O distances ranging from 1.327(3) to 1.390(3) Å, while the B(3) atom resides in the tetrahedral site with the bond lengths in the range of 1.451(4)−1.499(4) Å. Four BO3 triangles and one BO4 tetrahedron are corner-connected to give a B5O10 unit (Figure 1a). On the basis of the classification proposed by Burns et al. and summarized by Touboul et al.,31,32 the B5O10 unit can be written as {5:∞3[(5:4Δ+T)}. Each B5O10 unit is surrounded by four B5O10 units through terminal O atoms. Topologically, the B5O10 units can be considered as 4connected nodes (Figure 1b), and the connection of the B-O groups can be described as two interpenetrating diamond-like nets (Figure 1c) with the Schläfli symbol33 of 66, which is analyzed by the TOPOS 4.0 program.34 Interestingly, this connection of the B-O groups results in two independent interpenetrating [B5O8]∞ 3D frameworks, which are named as A and A′ (Figure S2 in the SI). It can be seen that the network A generates network A′ through rotating 180° along the c axis. Two 1D [B4O9]∞ chains with different arrangements are linked by the isolated B(3)O4 tetrahedra via the O atoms to construct the network A (Figure S3 in the SI). Further, the 3D networks A and A′ interpenetrate with each other to present pseudochannels with a size of 8.42 × 5.02 Å (Figure S4 in the SI), which are along the c axis and filled by the Rb atoms (Figure 2). The other pseudo-channels are filled by the Na atoms (Figure 2). The Rb atoms are connected with 10 O atoms with the Rb− O bond lengths ranging from 2.955(1) to 3.408(2) Å, while the Na atoms are six-fold coordinated (2.330(1)−2.696(2) Å). Li4Cs4B40O64 crystallizes in the space group P1̅ (No. 2), and the asymmetric unit contains 4 Li atoms, 4 Cs atoms, 40 B atoms, and 64 O atoms. The shorthand notation of the 3D borate anion can be classified as{40:∞3[5(5:4Δ+T)+8Δ+T+ 2(3:2Δ+T)}; to the best of our knowledge, it is the biggest and C

DOI: 10.1021/acs.inorgchem.7b02168 Inorg. Chem. XXXX, XXX, XXX−XXX

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parallel direction, which are almost perpendicular to the 1D [B16O33]∞ chains. As shown in Figure 3, the Cs(1) atoms (nine-coordinated) are edge-sharing with the Cs(4) atoms (nine-coordinated) to form [Cs2O16] dimers, while seven-coordinated Cs(2) atoms and eight-coordinated Cs(3) atoms are corner-sharing O atoms to become [Cs2O14] dimers. The Cs−O distances range from 3.081(3) to 3.545(4) Å. Each of the Li atom is fourcoordinated, and the Li−O distances are from 1.942(1) to 2.180(1) Å. Influence of Cations on the Structures. Interestingly, the ratio of the BO3 triangles and the BO4 tetrahedra is 4:1 in the structures of Na2M2B20O32 (M = Rb, Cs) and Li4Cs4B40O64. However, their FBBs are different; the shorthand notation is {40:∞3[5(5:4Δ+T)+8Δ+T+2(3:2Δ+T)} for Li4Cs4B40O64 and {5:∞3[(5:4Δ+T)} for Na2M2B20O32 (M = Rb, Cs), which results in the first compound belonging to the space group P1̅ (No. 2) and the latter compounds crystalline in the space group C2/c (No. 15). The reasons for this difference can be attributed to the different cationic elements, different coordinations, as well as different M−O (M = Li, Na) bond distances. For comparison, if the Li cations are substituted by the larger Na cations in the structure of Li4Cs4B40O64, there is a strain generated. To reduce this structural strain, two kinds of the [B8O17]∞ chains and the [B16O33]∞ chains in the framework of Li4Cs4B40O64 become two kinds of [B4O9]∞ chains in that of Na2Rb2B20O32 or Na2Cs2B20O32. More importantly, in the structure of Na2M2B20O32 (M = Rb, Cs), there are two independent 3D B-O networks, while there is only one network in the structure of Li4Cs4B40O64. In addition, effects of cations also can be found on the two structures of Na2Rb2B20O32 and Na2Cs2B20O32. In the structure of Na2Cs2B20O32, the B−B−B angles of neighboring BO3 triangles in the [B4O9]∞ chains 1 are 127.2, 165.1, 126.2°, and the B−B−B angles are 127.3, 166.9, 126.2, 165.1° in the [B4O9]∞ chains 2 (Figure S5 in the SI). After replacing Cs+ by the Rb+ cations, these values of angles become a little smaller, such as, in the structure of

Figure 2. 3D crystal structure of Na2Rb2B20O32.

new fundamental building blocks (FBBs) among the structure of an anhydrous mixed-alkali metal borate system (Figure S4 in the SI). The new FBBs are corner-sharing to reveal a 3D B-O network with channels, which are filled by the Cs atoms and the Li atoms. (Figure 3). Touboul et al. reported the Cs3B7O12 compound, which contains the FBB formed by 63 boron atoms. However, the structure shows that 18 Cs atoms are split and have great displacement. A large amount of the B and O atoms show highly disordered; in this work, there is neither split nor disordered atoms. In the structure, a notable feature is that two kinds of [B8O17]∞ chains and [B16O33]∞ chains are linked by isolated BO4 tetrahedra to form a 3D network (Figure 4). The B(26, 36, 29, 23, 8, 17, 20, 31)O3 triangles are connected via O corners to form the 1D [B8O17]∞ chain 1, while another 1D [B8O17]∞ chain 2 is constructed by the B(5, 3, 22, 34, 19, 14, 18, 30)O3 triangles. The remaining BO3 triangles are also linked by corner-sharing to form the 1D [B16O33]∞ chains. Two kinds of 1D [B8O17]∞ chains extend themselves along the

Figure 3. 3D structure of Li4Cs4B40O64. D

DOI: 10.1021/acs.inorgchem.7b02168 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 4. 3D B-O network of Li4Cs4B40O64.

Table 2. Anhydrous Alkaline and Mixed-Alkaline Borates Containing Interpenetrating 3D Framework no.

compounds

space group

A/B ratio

FBBs

1 2 3 4 5 6 7 8 9 10 11 12 13 14

α-K2B10O1647 β-KB5O848 K2O(B2O3)549 β-RbB5O850 β-CsB5O851 δ-CsB5O852 α-Na2B6O1053 α-Na2B8O1354 β-Na2B8O1355 α-CsB9O1456 Cs2O(B2O3)957 Cs2O(B2O3)958,59 Li2B4O760 Li2B6O9F261,62

Pbca (61) Pbca (61) Pbca (61) Pbca (61) Pbca (61) Pccn (56) P21/c (14) P21/a (14) P21/c (14) P2221 (17) P2221 (17) P4122 (91) I41cd (110) Cc (9)

0.2 0.2 0.2 0.2 0.2 0.2 0.33 0.25 0.25 0.11 0.11 0.11 0.5 0.33

{5:∞3[(5:4Δ+T)} {5:∞3[(5:4Δ+T)} {5:∞3[(5:4Δ+T)} {5:∞3[(5:4Δ+T)} {5:∞3[(5:4Δ+T)} {10:∞3[(5:4Δ+T)+2(2.5:2Δ+0.5T)} {9:∞3[(5:4Δ+T)+(4:2Δ+2T} {8:∞3[(5:4Δ+T)+(3:2Δ+T} {8:∞3[(5:4Δ+T)+(3:2Δ+T} {9:∞3[2(3:3Δ)+(3:2Δ+T} {9:∞3[2(3:3Δ)+(3:2Δ+T} {9:∞3[2(3:3Δ)+(3:2Δ+T} {4:∞3[(4:2Δ+2T)} {5:∞3[(5:4Δ+T)}+B2F2

(Q = Cl, Br),44 and 1D B-O chains, isolated B-O groups can be found in Li2B3O4F3,45 Na2B4O7 (P1̅),46 respectively, although the A/B value is less than 1. During the investigation, the interpenetrating 3D framework attracts our attention. After carefully checking the structures of anhydrous alkaline, mixed-alkaline borates (including alkaline and mixed-alkaline halogen borates), only 14% of them (13 alkaline borates and 1 fluorooxoborate) are built up by the interpenetrating 3D frameworks. However, no interpenetrating 3D framework can be documented in the anhydrous mixedalkaline borates. The first case containing an interpenetrating framework is KB5O8 reported by Krogh-Moe.47 Since then, the other 13 compounds that have interpenetrating frameworks or interlocking double networks have been reported (Table 2). As shown in Table 2, the A/B ratio of those compounds are smaller than 0.5. β-MB5O8 (M = K, Rb, Cs), α-K2B10O16, and K2O(B2O3)5 belong to the same space group, and the FBBs of them can be written as{5:∞3[(5:4Δ+T)}. If the B5O10 units are considered as nodes, the structure can be represented as two interpenetrating infinite 3D frameworks with a Schläfli symbol of 66, which is the same as that of Na2Rb2B20O32 and Na2Cs2B20O32. Although, the δ-CsB5O8 compound with the BO building block of {10:∞3[(5:4Δ+T)+2(2.5:2Δ+0.5T)} has the same formula as the above-mentioned compounds, but a different interpenetrating B-O configuration can be seen in the structure. The interpenetrating 3D framework is built up of equal amounts of the [B5O8] and [B4O9] groups for α-

Na2Rb2B20O32, the B−B−B angles of neighboring BO3 triangles in the [B4O9]∞ chains 1 are 125.9, 166.4, 125.9°, and the B− B−B angles are 167.7, 125.9, 166.4, 125.9° in the [B4O9]∞ chains 2 (Figure S6 in the SI). Interpenetrating Phenomenon in Anhydrous Alkaline and Mixed-Alkaline Borates. According to the investigation of the Inorganic Crystal Structure Database (ICSD-3.5.0) and recent publications, anhydrous alkaline and mixed-alkaline borates are summarized in Table S3 in the SI, which shows the B-O framework configuration and the ratio of A/B. With the A/B ratio increasing, the dimension of the B-O framework is almost decreased except for some special compounds. For example, γ-LiBO2 (I4̅2d space group) is a high-pressure phase.35 The layer framework structure of α-LiBO2 (P21/n space group) is found to be compressed easily along the direction of the c axis, resulting in the formation of tetracoordinated BO4 units (γ-LiBO2). The B-O framework in the structure shows an isolated or 2D layered configuration, while the A/B ratio ranges from 1 to 3. Except that, the LiBO2 compound with space group I4̅ 2 d shows a 3D B-O configuration with an equal ratio of the Li and B atoms. When the A/B value is less than 1, most of them show a 3D framework and contain high-polymerized B-O groups. However, the B-O groups display 2D layered configurations in the structure of M2Cs2B10O17 (M = Na, K),36 CsB5O8 (P21/c, P21/ n),37,38 M3B7O12 (M = Rb, Cs) (P1̅, C2/c),39,26 Cs3B13O21,40 RbB3O5 (C2/c),41 Rb2B4O7 (P1)̅ ,42 LiB6O9F,43 Na11B21O36Q2 E

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Figure 5. Topological structures of (a) α-Na2B6O10; (b) α,β-Na2B8O13; (c) CsB9O14 and two phases of Cs2O(B2O3)9; (d) Li2B6O9F2.

Na2B6O10, whose FBBs can be described as {9:∞3[(5:4Δ+T)+ (4:2Δ+2T}. Topologically, this FBB can be regarded as 6-c nodes and the structure shows an interpenetrating double network with a Schläfli symbol of 412.63 (Figure 5a); The interleaved B-O networks of α,β-Na2B8O13 consist of [B5O8] and [B3O7] with shorthand notation: {8:∞3[(5:4Δ+T)+(3: 2Δ+T}. As shown in Figure 5b, the topological structure of the two compounds display two 6-c interpenetrating nets with a point symbol of 412.63. When the A/B ratio is 1:9, CsB9O14 and two phases of Cs2O(B2O3)9 have been reported, whose structures consist of two infinite, independent 3D frameworks with {9:∞3[2(3:3Δ)+(3:2Δ+T}. The topological structures of those compounds are the same as the description above, containing a 6-c net with a point symbol of 412.63 (Figure 5c). Two interlocking B-O networks can be found in the structure of Li2B4O7 with the basic unit of {4:∞3[(4:2Δ+2T)}. The structure of fluorooxoborate compound Li2B6O9F2 can be viewed as two neighboring B5O8 groups interlinked by a BO2F2 tetrahedron to form interpenetrating B-O networks. Different from the other 13 compounds in Table 2 and Na2M2B20O32 (M = Rb, Cs), Li2B6O9F2 contains three interpenetrating B-O networks with 4-c nodes, in which the Schläfli symbol is 66 (Figure 5d). In one word, the title compounds Na2M2B20O32 (M = Rb, Cs) are the first mixed-alkaline borates, whose structures are composed of two infinite, separate, interpenetrating B-O 3D frameworks. Optical Properties. The UV−vis−NIR diffuse reflectance spectra of both compounds are shown in Figure S7 in the SI. It can be seen that there are 58% of reflectance at 250 nm for Na2Rb2B20O32 and 45% at 220 nm for Na2Cs2B20O32, respectively. In other words, the UV cutoff edges are below 250 and 220 nm for Na2Rb2B20O32 and Na2Cs2B20O32, respectively. The IR spectra of Na2M2B20O32 (M = Rb, Cs) and the assignment of the absorption bands are shown in Figure S8 and Table S4 in the SI. The peaks observed around 1380, 1225 cm−1 for Na2Rb2B20O32 and 1378, 1225 cm−1 for Na2Cs2B20O32 can be assigned to asymmetric stretching of B-O in BO3. The peaks at 1093, 1036 cm−1 for Na2Rb2B20O32 and 1097, 1041 cm−1 for Na2Cs2B20O32 belong to asymmetric stretching of B-O in BO4.22 Several peaks obtained at 931, 867, 815, 785, 761 cm−1 for Na2Rb2B20O32 and 936, 864, 822, 786, 761 cm−1 for Na2Cs2B20O32 are attributed to symmetric stretching of B−O in BO3 and BO4;23 the peaks around 693, 665, 634, 561, 524, 496 cm−1 for Na2Rb2B20O32 and 691, 634, 561, 522, 498 cm−1 for Na2Cs2B20O32 arise from out-of-plane bending modes of BO3 and BO4.21 Band Structures and Density of States. The band structures of Na2Rb2B20O32 and Na2Cs2B20O32 are shown in Figure S9 in the SI. Na2Rb2B20O32 and Na2Cs2B20O32 exhibit direct band gaps of 5.5 and 5.4 eV at the G point, respectively. Compared with the experimental results, there is a little

underestimation, which can be attributed to the exchangecorrelation energy used in the calculation.63,64 The partial density of states are shown in Figure S10 in the SI; it is clear that, in the two compounds, the orbits of the Na, B, and O atoms are almost located at the same range. From −10 to −5 eV, the bands are mainly contributed by O 2p, B 2s, 2p and Rb 4p (Cs 5p) states. States below the Fermi level are determined by the O 2p states and B 2p states. The bottom of the conduction bands are mainly made up by the B 2p states. The above results indicate that the gaps in the two compounds are determined by the B-O groups.

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CONCLUSIONS By introducing different alkali metals in the B-O framework, three new mixed-alkaline borates Na2M2B20O32 (M = Rb, Cs) and Li4Cs4B40O64 have been synthesized. Na2Rb2B20O32 and Na2Cs2B20O32 are isostructural (space group C2/c), and the structures of them are constructed by two independent interpenetrating 3D B-O networks with pseudo-channels filled by the cations. Structural comparisons indicate that Na2M2B20O32 (M = Rb, Cs) are the first samples where the two interpenetrating 3D B-O nets coexist in one anhydrous mixedalkali metal borate structure. For Li4Cs4B40O64, it crystallizes in another different space group (P1̅) and its B-O network is built up by new FBBs with B40O77 atoms with channels, in which the Li and Cs atoms are located. The UV−vis−NIR diffuse reflectance spectra indicate that the UV cutoff edges are below 220, 250 nm for Na2M2B20O32 (M = Rb, Cs) compounds, which are close to the corresponding calculated gaps of 5.5 and 5.4 eV. The absorption bands in the infrared spectra confirm the existence of the BO3 and BO4 groups.

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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.7b02168. Crystal data information, XRD patterns, the figures of crystal structures, the summarization table of anhydrous alkaline and mixed-alkaline borates, UV−vis−NIR and IR spectra, the table of assignment of the absorption bands and electronic band structure (PDF) Accession Codes

CCDC 1561509−1561511 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033. F

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (S.H.). *E-mail: [email protected] (S.P.). ORCID

Ying Wang: 0000-0001-6642-543X Shilie Pan: 0000-0003-4521-4507 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the Urumqi Science and Technology Plan (Grant Nos. P151010004, P161010003), the Xinjiang Program of Cultivation of Young Innovative Technical Talents (Grant No. QN2016BS0344), the National Natural Science Foundation of China (Grant Nos. 51425206, 91622107, 21501194), the National Key Research Project (Grant Nos. 2016YFB1102302, 2016YFB0402104), the Xinjiang Key Research and Development Program (Grant No. 2016B02021), the National Basic Research Program of China (Grant No. 2014CB648400), and the Major Program of Xinjiang Uygur Autonomous Region of China during the 13th Five-Year Plan Period (Grant No. 2016A02003).

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