Transformation of the B–O Units from Corner-Sharing to Edge-Sharing

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Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

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Transformation of the B−O Units from Corner-Sharing to EdgeSharing Linkages in BaMBO4 (M = Ga, Al) Fengjiao Guo,†,‡,§ Jian Han,†,§ Shichao Cheng,† Sujuan Yu,† Zhihua Yang,† and Shilie Pan*,† †

CAS Key Laboratory of Functional Materials and Devices for Special Environment, Xinjiang Technical Institute of Physics and Chemistry, CAS, Xinjiang Key Laboratory of Electronic Information Materials and Devices, 40-1 South Beijing Road, Urumqi 830011, China ‡ Center of Materials Science and Optoelectronics Engineering, University of Chinese Academy of Sciences, Beijing 100049, China

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S Supporting Information *

ABSTRACT: Two barium-containing borates BaMBO4 (M = Al, Ga) were synthesized via the solid-state method under atmospheric pressure. The 3D configurations of BaGaBO4 and BaAlBO4 are 2 2 comprised of ∞ [Ba 4 O 16 ] 24− / ∞ [Ga 4 O 10 ] 8− /[B 2 O 5 ] 4− and 24− 2 8− 8− 3 /∞[Al4O10] /[B4O10] , respectively, of which the ∞[Ba4O16] [B4O10]8− units possess unusual edge-sharing [BO4]5− tetrahedra. From BaGaBO4 to BaAlBO4, the B−O units are transformed from corner-sharing to edge-sharing linkages, which arises from the directional shrinkage caused by the Ba−O and M−O skeletons. The phonon spectra of these two compounds do not show imaginary frequency at any wave vectors, indicating that both of them are kinetically stable.

containing edge-sharing [BO4]5− tetrahedra that were prepared under ambient pressure. To enrich the structure of borates, introducing some cations (Me = Al3+, Ga3+, Be2+, Mg2+, Zn2+, Cd2+, etc.) with comparatively strong covalent Me−O bonds is a feasible strategy. These Me−O groups and B−O groups can connect to get Me−B−O clusters as the structural building units (SBUs). For example, excellent deep-ultraviolet (UV) nonlinear optical 2 KBe 2 BO 3 F 2 (KBBF) has ∞ [Be 4 BO 3 F 2 ] − layers. 3 1 2 Ba 3 Mg 3 (BO 3 ) 3 F 3 has ∞ [Mg 3 O 2 F 3 (BO 3 ) 2 ] 7− layers. 32 K2Al2B2O7 has 2∞[Al2B2O7]2− layers.33 Cs3Zn6B9O21 shows 3− 2 layers.34,35 NaBa4(GaB4O9)2X3 (X = Cl, Br) ∞[Zn2BO3O2] 3 features ∞[GaB4O9]3− networks, and Na4Ga3B4O12(OH) possesses [Ga6(BO3)4] cages.36,37 On the basis of these ideas, systematic explorations for aluminoborates (ABOs) and galloborates (GBOs) were carried out, and two compounds BaMBO4 (M = Al and Ga) were obtained. To the best of our knowledge, the 2∞[M4O10]8− layers of BaAlBO4 and BaGaBO4 were first found in ABOs and GBOs. Interestingly, BaAlBO4 possesses edge-sharing [BO4]5− tetrahedra, which are rare in borates, especially those synthesized under atmospheric pressure. The structure comparisons were performed to further understand the structural features of ABOs and GBOs. Moreover, infrared (IR) spectroscopy, UV−vis−NIR spectra, and thermal analyses were measured. In addition, the electronic structures of title compounds were calculated.

1. INTRODUCTION Since the properties of materials are closely related to their structures, it is important to explore crystal structures.1−7 Diversiform structures endow borates with multiple physical and chemical properties for wide applications as birefringent materials,8,9 electrode materials,10,11 phosphors,12,13 nonlinear optical materials,14−17 etc. Borates with the structural variety on account of 3-fold coordinated [BO3]3− triangles and 4-fold coordinated [BO4]5− tetrahedra can adopt corner- or edgesharing linkages to form diverse fundamental building blocks (FBB) and frameworks.18−24 By striving to explore borates with excellent properties and omnifarious structures, chemists and material scientists have gained more than 2000 synthetic compounds. Therein, corner-sharing types of borates are predominant, while edge-sharing type ones are rare. This discrepancy stems from the different energies produced by two kinds of linkages. According to Pauling’s third rule, more energies were required to overcome the repulsion caused by two [BO4]5− blocks linked by sharing a common edge.25,26 Ross and Edwards put forward that the B−O blocks can only interconnect by sharing corners.27,28 However, in 2002, Huppertz and von der Eltz synthesized the first borate Dy4B6O15 with edge-sharing [BO4]5− tetrahedra and broke that hypothesis.29 In the following years, a dozen borates containing edge-sharing [BO4]5− tetrahedra were discovered via a high-pressure technique, and it is widely believed that this linkage type of [BO4]5− units can only be generated under high pressure. Until 2010, the first borate KZnB3O6 with edgesharing [BO4]5− tetrahedra was synthesized under ambient pressure.30 To date, there are only three cases of borates © XXXX American Chemical Society

Received: April 19, 2019

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DOI: 10.1021/acs.inorgchem.9b01157 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

patterns for BaAlBO4 and BaGaBO4 are plotted in Figure 2, which exhibit that the experimental patterns fit well with the calculated ones. 2.4. Infrared Spectroscopy. Infrared (IR) spectra were measured using a Shimadzu IR Affinity-1 Fourier transform IR spectrometer in the range from 400 to 4000 cm−1 with a resolution of 2 cm−1 at room temperature. KBr was used as the reference pellet, and the samples were mixed thoroughly with it. 2.5. UV−vis−NIR Diffuse Reflectance Spectroscopy. The UV−vis−NIR diffuse reflectance spectra for BaAlBO4 and BaGaBO4 were measured on a Shimadzu SolidSpec-3700DUV spectrophotometer with a wavelength range from 180 to 2600 nm at room temperature. The reflectance spectra were converted to absorption based on the Kubelka−Munk function: F(R) = (1 − R)2/2R = K/S (R = reflectance; K = absorption; S = scattering).41 2.6. Thermal Analyses. Thermal gravimetric analyses (TG) and differential scanning calorimetry (DSC) were performed on a NETZSCH STA 449F3 simultaneous analyzer at a rate of 5 °C/ min under flowing nitrogen gas. 2.7. Computational Methods. The band structures, partial density of state (PDOS), and phonon spectra of BaAlBO4 and BaGaBO4 were calculated by utilizing the CASTEP program. Under the norm-conserving pseudopotential (NCP), the following orbital electrons were treated as valence electrons: Ba, 5s2 5p6 6s2; Al, 3s2 3p1; Ga, 3d10 4s2 4p1; B, 2s2 2p1; O, 2s2 2p4. The functional developed by Perdew−Burke−Ernzerhof (PBE) in generalized gradient approximation (GGA) form was used to describe the exchange-correlation energy. A kinetic energy cutoff of 880 eV is chosen with Monkhorst− Pack k-point meshes spanning less than 0.07/Å in the Brillouin zone.

2. EXPERIMENTAL SECTION 2.1. Synthesis. Single crystals of BaAlBO4 and BaGaBO4 were obtained from high-temperature solution by using Li2O as the flux. The mixtures of Ba(NO3)2, Al2O3/Ga2O3, B2O3, and Li2CO3 in molar ratios of 3:1:4:1.5 were thoroughly ground and then loaded into platinum crucibles. The platinum crucibles were heated to 840 °C for the reagents melt and held at this temperature for 10 h to ensure homogeneities. The solutions were cooled slowly to 700 °C at a rate of 1 °C/h and then cooled quickly to room temperature at a rate of 10 °C/h; colorless crystals were obtained. Polycrystalline samples of BaAlBO4 and BaGaBO4 were obtained by employing solid-state techniques with stoichiometric amounts of BaCO3, Al2O3/Ga2O3, and H3BO3. The mixtures were gradually raised to the appropriate temperatures (BaGaBO4, 730 °C; BaAlBO4, 810 °C) with several intermediate grindings. The samples were held at selected temperatures for 2 days to ensure complete reactions. The purities of two polycrystalline samples were verified by employing powder X-ray diffraction (XRD) measurements. 2.2. Structure Determination. The single crystal X-ray diffraction data were collected at 296 K on a Bruker APEX II CCD diffractometer equipped with a monochromatic Mo Kα radiation source (λ = 0.71073 Å). The SAINT program was employed to carry out the data integration, cell refinement, and absorption corrections.38 The crystal structures were solved via the SHELXTL crystallographic software package.39 The structures were checked with the PLATON program for eliminating symmetry elements.40 Crystallographic data and structure refinement information on BaAlBO4 and BaGaBO4 are given in Table 1. The atomic coordinates, isotropic thermal

3. RESULT AND DISCUSSION 3.1. Crystal Structure of BaGaBO4. BaGaBO4 crystallizes into centrosymmetric structure with a space group of P21/c and has a 3D 3∞[Ga4B4O16]8− framework, which is made up of 2 8− layers and [B2O5]4− groups, simultaneously ∞[Ga4O10] forming two types of tunnels where the Ba atoms are residing. Within the asymmetric unit of BaGaBO4, there are two, two, two, and eight crystallographically independent Ba, Ga, B, and O atoms, respectively. Tetradentate Ga(1) and Ga(2) atoms are coordinated to the O atoms to form the [Ga(1)O4]5− and [Ga(2)O4]5− tetrahedra with Ga−O bond lengths ranging from 1.789 (5) to 1.874 (5) Å. The [Ga(1)O4]5− and [Ga(2)O4]5− tetrahedra are alternately connected by sharing corners to build a 2D 2∞[Ga4O10]8− layer (Figure 1g), which is the first example in GBOs. With respect to the B atoms, the B(1) and B(2) atoms are three-coordinated; the [B(1)O3]3− and [B(2)O3]3− triangles are jointed using the O(5) atoms as nodes to form isolated [B2O5]4− dimers (Figure 1h). The Ga− O and B−O groups are interlinked to give a 3D 3 8− framework (Figure 1c). The Ba(1) and ∞ [Ga4B4O16] Ba(2) atoms are 10- and 8-fold coordinated, respectively. As presented in Figure 1a and b, the [Ba(1)O10]18− and [Ba(2)O8]14− polyhedra are interconnected to gain an involuted 2∞[Ba4O16]24− layer, and the adjacent layers are stacked in parallel along the c axis. 3.2. Crystal Structure of BaAlBO4. BaAlBO4 crystallizes into a centrosymmetric structure with a space group of P21/c and exhibits a 3D 3∞[Al4B4O16]8− skeleton which is composed of 2∞[Al4O16]8− layers and [B4O10]8− groups. Its asymmetric unit contains two, two, two, and eight crystallographically independent Ba, Al, B, and O atoms, respectively. The tetrahedra centered with 4-coordinated Al atoms interconnect to form 2D 2∞[Al4O10]8− layers, which are similar to the 8− 2 layers in BaGaBO4. The bond lengths of Al−O ∞[Ga4O10] fall in the range of 1.727−1.777 Å. Unlike BaGaBO4, the Ba and B atoms in BaAlBO4 adopt 9-fold and 3-/4-fold

Table 1. Crystal Data and Structure Refinement for BaAlBO4 and BaGaBO4 empirical formula fw (g/mol) cryst syst space group a (Å) b (Å) c (Å) β (deg) vol (Å3) Z abs coeff (mm−1) F(000) θ range for data collection (deg) limiting indices reflns collected/ unique completeness (%) GOF on F2 final R indices [Fo2 > 2σ(Fo2)]a R indices (all data)a largest diff peak and hole (e/Å3)

BaAlBO4

BaGaBO4

239.13 monoclinic P21/c 5.1488(7) 8.6055(11) 17.688(2) 106.155(9) 752.77(17) 8 10.644 848 2.398 to 27.569

281.87 monoclinic P21/c 5.1766(16) 8.681(3) 18.266(5) 106.383(8) 787.5(4) 8 16.646 992 2.324 to 27.488

−6 ≤ h ≤ 5, −10 ≤ k ≤ 11, −22 ≤ l ≤ 23 7233/1730 [R(int)= 0.0399] 99.9 1.015 R1 = 0.0321, wR2 = 0.0804 R1 = 0.0400, wR2 = 0.0855 1.870 and −1.913

−6 ≤ h ≤ 6, −11 ≤ k ≤ 11, −10 ≤ l ≤ 23 4864/1802 [R(int)= 0.0343] 100.0 1.050 R1 = 0.0270, wR2 = 0.0628 R1 = 0.0305, wR2 = 0.0649 1.342 and −1.092

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

a

parameters, and the values of the bond valence sum (BVS) of BaAlBO4 and BaGaBO4 are listed in Tables S1 and S2, respectively. Interatomic bond lengths are listed in Tables S3 and S4. 2.3. Powder X-ray Diffraction. The powder X-ray diffraction (XRD) data were recorded by using a Bruker D2 PHASER diffractometer equipped with a monochromatized Cu Kα radiation source (λ = 1.5418 Å) at room temperature. The powder XRD B

DOI: 10.1021/acs.inorgchem.9b01157 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 1. Ba−O units, Ba−O frameworks, holistic structures, and B−O groups of BaGaBO4 (a, b, c, h) and BaAlBO4 (d, e, f, i). The 2∞[M4O10]8− (M = Ga, Al) layers (g) in BaGaBO4 and BaAlBO4.

Figure 2. XRD patterns and TG-DSC curves: (a, c) BaAlBO4 and (b, d) BaGaBO4.

coordination modes, respectively. The [BaO9]16− polyhedra share O atoms to build a 3D 3∞[Ba4O16]24− network (Figure 1d and e). As shown in Figure 1i, two [B(1)O4]5− units are linked by sharing a common edge to form a [B2O6]6− ring, then two [B(2)O3]3− units connect with this [B2O6]6− ring through the O(5) atoms to get a [B4O10]8− group, which is the FBB of BaAlBO4 and first found in borates. Isolated [B4O10]8− groups act as bridges to connect the parallel 2∞[Al4O10]8− layers, forming a 3D 3∞[Al4B4O16]8− skeleton with tunnels where the Ba atoms are located (Figure 1f).

The B−O blocks in BaAlBO4 exhibit two connection types, those are corner-sharing [B(2)O3]3−−[B(1)O4]5− and edgesharing [B(1)O4]5−−[B(1)O4]5−. For distorted [B(1)O4]5−, its B−O bond lengths vary from 1.420 to 1.607 Å. Largedistortion [BO4]5− groups can be observed in known borates, such as Co7B24O42(OH)2·2H2O (1.407−1.587 Å), α-Gd2B4O9 (1.378−1.602 Å), Co6B22O39·H2O (1.432−1.606 Å), and Cs2B4SiO9 (1.396−1.671 Å).42−46 The O−B−O angle and transannular B···B distance inside the [B2O6]6− ring are 94.062° and 2.098 Å, respectively, and these values are C

DOI: 10.1021/acs.inorgchem.9b01157 Inorg. Chem. XXXX, XXX, XXX−XXX

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Ba3[B10O17(OH)2]54 were obtained via a high-pressure synthesis technique (type I), as well as KZnB 3 O 6 ,30 Ba4Na4Zn4(B3O6)2(B12O24),55 and Li4Na2CsB7O1426 were prepared under ambient pressure (type II). With respect to their anionic configurations, the B−O groups of type II exhibit 0D clusters, while those of type I are likely to polymerize into high-dimension structures from 2D layers to 3D frameworks owing to the high-pressure synthesis. There are some similarities and differences between BaAlBO4 and BaGaBO4. Compared with BaGaBO4, BaAlBO4 has a more compact structure, the reasons are as follows: (1) the stacking of the Ba−O framework in BaAlBO4 is more intensive than that in BaGaBO4 (Figure 1b and e), and (2) the bond lengths of Al−O (average 1.746 Å) are shorter than those of Ga−O (average 1.826 Å). This compact agreement of cationic frames squeezes the holistic structure, resulting in the distances of its tunnels decreasing. Simultaneously, directional shrinkage shortens the distances of the B atoms. As shown in Figure 1c and f, the distance of two B(2) atoms in tunnel 1 for BaGaBO4 is 2.9311 Å, and the distance of two B(1) atoms in tunnel 1′ for BaAlBO4 is 2.0987 Å. The compact structure of BaAlBO4 can be visually represented in its crystallographic data. The lattice constants of BaAlBO4 (a = 5.1488(7) Å, b = 8.6055(11) Å, c = 17.688(2) Å, V = 752.77(17) Å3) are obviously smaller than those of BaGaBO4 (a = 5.1766(16) Å, b = 8.681(3) Å, c = 18.266(5) Å, V = 787.5(4) Å3). In particular, the lattice of BaAlBO4 undergoes a contraction along the c axis. 3.3. Structure Comparison. To investigate the structure diversities of ABOs and GBOs, all the available anhydrous and disorder-free compounds were chosen based on a survey of the Inorganic Crystal Structure Database. The following details were obtained by comparing the structures and summarizing the rules. (1) Taking all the ABOs and GBOs (95 and 34 cases, respectively) into consideration, the 0D Al−O and Ga−O groups account for the majorities (51.6 and 70.6%, respectively), and the 2D ones are extremely rare. UO2[B3Al4O11(OH)] is the sole aluminoborate which features 2D 2∞[Al8O23]22− layers.56 BaAlBO4 and BaGaBO4 in this work possess distinctive 2D 2∞[M4O10]8− layers which are first found in ABOs and GBOs. Furthermore, 2∞[Ga4O10]8− is the first 2D case in GBOs. (2) In aluminophosphates (APOs), Al/P ratios play an important role in crystal frames and dimensionalities, but M/B (M = Al, Ga) ratios in ABOs and GBOs do not explicitly affect the holistic structures and dimensionalities. For both ABOs

Figure 3. IR spectra of BaAlBO4 and BaGaBO4.

Table 2. Assignments of the IR Peaks for BaAlBO4 and BaGaBO4 BaAlBO4 asymmetric stretching vibrations of [BO3]3− (cm−1) symmetric stretching vibrations of [BO3]3− (cm−1) bending vibrations of [BO3]3− (cm−1) asymmetric stretching vibrations of [BO4]5− (cm−1) symmetric stretching vibrations of [BO4]5− (cm−1) bending vibrations of [BO4]5− (cm−1)

BaGaBO4

1400, 1339, 1222, 1128 923, 831

1390− 1113 950−833

750, 727, 640 1182, 1074

777−615

991, 971, 852 701, 679

consistent with the ones found in other borates containing edge-sharing [BO4]5− tetrahedra, such as Dy4B6O15 (94.1°, 2.072 Å),29 HP-NiB2O4 (93.5°, 2.088 Å),47 HP-CoB2O4 (93.3°, 2.090 Å),48 and β-FeB2O4 (93.4°, 2.083 Å).49 Similar to the edge-sharing [BO4]5− tetrahedra in other borates, [B4O10]8− in BaAlBO4 possesses a C2 axis as well. Up to now, borates containing edge-sharing [BO4]5− have been rare, of which Hp-MB2O4 (M = Ni, Fe, Co),47−49 αRE4B6O15 (RE = Dy, Ho),29,50 RE2B4O9 (RE = Eu, Gd, Dy),43 HP-AB3O5 (A = K, NH4, Rb, Tl),51−53 M6B22O39·H2O (M = Fe, Co),44 Co7B24O42(OH)2·2H2O,42 and α-

Figure 4. UV−vis−NIR spectra: (a) BaAlBO4 and (b) BaGaBO4. D

DOI: 10.1021/acs.inorgchem.9b01157 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 5. Phonon spectra, electronic band structures, and partial densities of (a, b) BaGaBO4 and (c, d) BaAlBO4.

[B2O5]4− B−O groups. Similarly, the different coordination numbers of strontium atoms in SrAlBO4/β-SrGaBO4 and SrAl2B2O7/SrGa2B2O7 result in different Sr−O, M−O, and B− O frames and holistic structures. That is to say, the cationic configuration can affect the crystal structure. The above analyses indicate that rich architectures of ABOs and GBOs can be obtained by reasonably adjusting the M/B ratios and introducing diversiform cationic groups. 3.4. Thermal Behavior Analysis. To investigate the thermal behaviors of BaAlBO4 and BaGaBO4, TG and DSC experiments were performed, and the results are plotted in Figure 2. Obviously, there is an endothermic peak (899 °C for BaAlBO4 and 843 °C for BaGaBO4) on each heating DSC curve of two compounds. For the cooling DSC curves, BaAlBO4 shows a weak exothermic peak at about 802 °C, but BaGaBO4 has no obvious exothermic peak due to high viscosity. Both TG curves of two compounds show inconspicuous weight loss. To further know their thermal behaviors, their polycrystalline samples were heated to recrystallize. As shown in Figure 2, the samples of BaAlBO4 and BaGaBO4 before and after recrystallizing are identical, indicating that these two compounds melt congruently. However, high viscosities hinder the crystal growth of these two compounds; the flux should be introduced to overcome this obstacle. 3.5. IR Spectra. BaAlBO4 and BaGaBO4 exhibit different IR spectra (Figure 3). Assignments of the respective IR peaks are listed in Table 2. The results indicate that only the [BO3]3− units exist in GaBO4, while BaAlBO4 contains both [BO3]3− and [BO4]5− units.69−71 It is difficult to undoubtedly distinguish the absorption peaks below 600 cm−1 on account of the overlaps of the stretching modes of M−O (Al, Ga) groups and the bending modes of B−O groups in the low frequency vibrations.72−74 3.6. UV−vis−NIR Diffuse Reflectance Spectra. The UV−vis-NIR spectra of two title compounds are plotted in Figure 4. Obviously, BaAlBO4 and BaGaBO4 possess broad

and GBOs, the M−B−O groups are inclined to form 3D frameworks, although M/B ratios vary from 0.08 to 7. Nevertheless, the M/B ratio is a significant factor for affecting the structures and dimensionalities of M−O groups. For instance, all the 3D Al−O groups only appear in the crystals with large M/B ratios (>1.75). These rules are more intuitively reflected in ABOs and GBOs containing alkaline-earth metal ions (Table S5 in the Supporting Information). The M/B ratios in these compounds operate the structures of M−O blocks, but do not work for the M−B−O groups. The M−O blocks tend to construct low-dimensional architectures with M/B ratios below 1, and 1D M−O groups can be observed when the M/B ratios increase to 1. (3) Besides Al3+ and Ga3+, other cations in the materials also have influence on the crystal structures. In the absence of any cation other than Al3+ or Ga3+, the M−O groups in ABOs and or GBOs only exhibit 3D networks, such as GaBO3 and Al4B2O9.57,58 Although CaGaBO4, SrGaBO4, PbGaBO4, and BaGaBO4 share the common formula QGaBO4 (Q = Ca, Sr, Pb, and Ba), they exhibit different structures. CaGaBO4, SrGaBO 4 , and PbGaBO 4 show 1D Q−O groups (1∞[Ca2O8]12−, 1∞[Sr2O8]12−, and 1∞[PbO3]4− chains, respec1 [GaO3]3−, tively), corresponding to 1D Ga−O chains (∞ 1 6− 1 5− ∞ [Ga 2 O 6 ] , and ∞ [GaO 4 ] , respectively) and 1D 2− 1 chains.59−61 For BaGaBO4 in this work, it ∞[GaBO4] 2 features 2D ∞ [Ba4O16]24− layers, corresponding to 2D 2 8− [Ga O ] layers and a 3D 3∞[Ga4B4O16]8− network. Table ∞ 4 10 S6 in the Supporting Information lists 23 pairs of ABOs and GBOs; each pair of them share a common formula. Except βSrGaBO4/SrAlBO4,62 SrGa2B2O7/SrAl2B2O7,63,64 and BaGa2B2O7/BaAl2B2O7,65 the other 20 pairs of compounds show isomorphic structures for each pair because the cations in each pair have the same coordination numbers. As listed in Table S7 in the Supporting Information, BaAl2B2O7 and BaGa2B2O7 have 12- and 8-coordinated barium atoms, respectively, corresponding to 2∞[BaO6]10−/1∞[Ba2O10]16− Ba−O configurations, [Al2O7]8−/1∞[Ga2O6]6− M−O frames, and [BO3]3−/ E

DOI: 10.1021/acs.inorgchem.9b01157 Inorg. Chem. XXXX, XXX, XXX−XXX

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transmission windows from UV to IR regions with cutoff edges at 186 and 208 nm, respectively, corresponding to band gaps of about 6.44 and 5.52 eV, respectively. 3.7. Electronic Structures Calculations. The firstprinciple calculations were used to explore the microscopic structural features of BaAlBO4 and BaGaBO4. The phonon spectra of these two compounds do not show imaginary frequency at any wave vectors, indicating that both of them are kinetically stable (Figure 5a and c). The band structures and PDOS of two compounds are plotted in Figure 5b and d and exhibit some similarities and differences. Similarly, BaAlBO4 and BaGaBO4 are direct bandgap materials with band gaps of 4.56 and 3.61 eV, respectively, and these values are less than the experimental ones due to the discontinuity of the exchange-correlation energy functional.75−77 Moreover, the energy regions at about 4−8, −8−0, and −10 eV for both of them are mainly contributed from Ba 5d, O 2p, and Ba 5p orbitals, respectively. In addition, their tops of valence bands (VB) are occupied by O 2p orbitals. Differently, the bottoms of the conduction bands (CB) of BaAlBO4 and BaGaBO4 are occupied by Ba 6s and Ga 4s orbitals, respectively (embedded figures).

Shilie Pan: 0000-0003-4521-4507 Present Address §

F.G. and J.H. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the Natural Science Foundation of Xinjiang, China (Grant No. 2019D01A93), Key Research Project of Frontier Science of CAS (Grant No. QYZDB-SSW-JSC049), CAS Pioneer Hundred Talents Program, Shanghai Cooperation Organization Science and Technology Partnership Program (Grant No. 2017E0113), the National Natural Science Foundation of China (Grant Nos. 11774414, 61835014, 51425206), and the National Key Research Project (Grant No. 2016YFB0402104).

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4. CONCLUSION In conclusion, two Ba-containing borates BaAlBO4 and BaGaBO4 were synthesized by using a solid-state method at atmospheric pressure. Though these two compounds share a common formula of BaMBO4 (M = Al, Ga), they exhibit different structures. BaGaBO4 features a 3D framework consisting of 2∞[Ba4O10] 24− layers, 2∞[Ga4O10]8− layers, and [B 2 O 5 ] 4− groups. Compared with BaGaBO 4 , BaAlBO 4 possesses a more compact configuration caused by compression of cationic frameworks. The 3D 3∞[Ba4O16]24− network and 2D 2∞[Al4O10]8− layers with short Al−O bond lengths in BaAlBO4 directionally compress the structural skeleton, shortening the distances of B atoms in tunnels 1′ and forming 2 edge-sharing [BO4]5− tetrahedra. Moreover, ∞ [M4O10]8− layers were first found in ABOs and GBOs. In addition, structure comparisons show that introduction of cations with flexible coordination can diversify the structures of compounds.

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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.9b01157. Listings of crystal data, ABOs and GBOs containing alkaline-earth metal ions, ABOs and GBOs with general formulas (PDF) Accession Codes

CCDC 1908445−1908446 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.

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DOI: 10.1021/acs.inorgchem.9b01157 Inorg. Chem. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.inorgchem.9b01157 Inorg. Chem. XXXX, XXX, XXX−XXX