Designing Three Fluorooxoborates with a Wide Transmittance

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

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Designing Three Fluorooxoborates with a Wide Transmittance Window by Anionic Group Substitution Dequan Jiang,†,‡,§ Guopeng Han,†,‡,§ Ying Wang,† Hao Li,†,‡ Zhihua Yang,† and Shilie Pan*,† †

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 ‡ Center of Materials Science and Optoelectronics Engineering, University of Chinese Academy of Sciences, Beijing 100049, China

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valence can be widely used.46−50 For the [B3O6]3− anion, the terminal O atoms resist ultraviolet (UV) transmittance.51 According to the previous studies, the [BO2F2]3− and [PO4]3− units exhibit larger HOMO−LUMO gaps than the [BO3]3− unit.25,52 The anionic group substitution of the [BO3]3− by [BO2F2]3− or [PO4]3− units in the anion [B3O6]3− could improve the UV-transparent region. The same chemical valence of the [BO3]3−, [BO2F2]3− and [PO4]3− units is one factor for the successful anionic group replacement. Besides, groups with different valences can also substitute each other using various charge-compensation mechanisms. In this Communication, three fluorooxoborates, K0.42Rb2.58B3O3F6 and M3B2PO5F4 (M = K, Cs), with sixmembered oxofluoride anions [B3O3F6]3− or [B2PO5F4]3− were successfully obtained for the first time according to the above ideas. They were all synthesized using the high-temperature solution method in the open system. In comparison, most fluorooxoborates were obtained in the closed environment. In order to introduce the [BO2F2]3− unit, the raw materials KBF4 and CsBF4 were selected, which can effectively offer B−F bonds. In addition, KBF4 and CsBF4 can act in the role of the flux that can prompt the raw materials to melt at a relatively low temperature, which is beneficial for synthesizing fluorooxoborates in the open air. Because KPO3 and NH4H2PO4 can generally be used as reagents for synthesizing phosphates, they were added to the reaction to introduce the [PO4]3− unit. The powder samples were obtained by grinding single crystals from spontaneous crystallization. The experimental powder X-ray diffraction (PXRD) pattern of K0.42Rb2.58B3O3F6 contains several small peaks of RbBF4 (PDF 18-1131), marked by green triangles, and the other peaks correspond well to the calculated pattern (Figure S1a). The experimental PXRD patterns of M3B2PO5F4 (M = K, Cs) can match well with the calculated ones, and each experimental pattern has an unknown phase, which was marked by green triangles (Figure S1b,c). Thermogravimetry (TG) and differential scanning calorimetry (DSC) of M3B2PO5F4 were performed. There is no obvious mass loss during the whole process of heating and cooling. Each compound possesses an endothermic peak around 460 °C, which corresponds to the melting point. In addition, Cs3B2PO5F4 owns an exothermic peak around 354 °C. Because the solution of K3B2PO5F4 exhibits great viscosity, there is no

ABSTRACT: Three fluorooxoborates, K0.42Rb2.58B3O3F6 and M3B2PO5F4 (M = K, Cs), were designed and synthesized under the open system. One of the common features is that the title compounds consist of sixmembered oxofluoride anions. The oxofluoride anions [B3O3F6]3− and [B2PO5F4]3− display structures similar to the boroxine [B3O6]3−. Anionic group substitution using the [BO2F2]3− and [PO4]3− units can improve the optical property of the [B3O6]3− anion and help to generate a wide transmittance window.

F

luorine, as the maximum electronegativity element in the Periodic Table, has captured the attention of chemical researchers for a long time.1−7 Involving fluorine in inorganic compounds can not only create novel chemical structures8,9 but also lead to unusual physical properties, such as ionic conductivity,10,11 nonlinear optics,12−14 and birefringence.15,16 Therefore, it is an effective strategy to explore new functional materials by introducing the F atoms.17−22 Introducing three novel units, [BO3F]4−, [BO2F2]3−, and [BOF3]2− ([BOF] units), into the structures can enrich the structural diversity of the borate chemistry.23,24 These [BOF] units were found to generate the large highest occupied molecular orbital (HOMO)−lowest unoccupied molecular orbital (LUMO) gap, polarizability anisotropy, and hyperpolarizability, which are beneficial for the design of functional materials with a large band gap, birefringence, and second-order susceptibility.25 Meanwhile, the existence of terminal F atoms could cut the three-dimensional anionic framework and create more flexible anionic arrangements. Recently, a series of functional fluorooxoborate materials were synthesized, such as Li2B3O4F3,10 MIB4O6F (MI = Na, NH4, Rb, Cs),11,19,26,27 MIIB5O7F3 (MII = Ca, Sr),28,29 MII2B10O14F6 (MII = Ca, Sr),30 BiB2O4F,31,32 MIIB2O3F2 (MII = Pb, Sn),33,34 and BaB4O6F2.35,36 Their excellent optical properties have captured the attention of chemical researchers and become a hotspot for discovering new functional materials. Borophosphates also exhibit abundant structural chemistry and promising properties;37,38 however, few compounds are reported in the system of fluorinated borophosphates, i.e., (C2H10N2)[BPO4F2],39 Na3B2PO5F4,40 MIBPO4F (MI = NH4, K, Rb, Cs)41−44 and LiB(PO2F2)4.45 In the world of inorganic chemistry, anionic group substitution with the same chemical © XXXX American Chemical Society

Received: January 21, 2019

A

DOI: 10.1021/acs.inorgchem.9b00197 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

Inorganic Chemistry

and phosphates. The anionic group [B2PO5F4]3− can only be found in Na3B2PO5F4.40 The M cations connect with the O and F atoms of the isolated anions [B2PO5F4]3− to form the threedimensional crystal structures (Figure 1b,c). In their crystal structures, the K−O and K−F distances range from 2.671(4) to 3.428(6) Å with an average bond length of 2.893 Å in K3B2PO5F4. By contrast, the Cs−O and Cs−F distances range from 2.965(4) to 3.657(5) Å with an average bond length of 3.257 Å in Cs3B2PO5F4. The Cs cations need a larger space than the K cations to accommodate in the crystal structure. Therefore, the arrangement mode of the anionic group [B2PO5F4]3− in the unit cell is changed because of the cation size effect (Figure S5), which further causes the difference between the two compounds. The BVS calculation shows that all atoms of M3B2PO5F4 are in the rational range of the value states (Tables S3 and S4).54 Upon investigation of the Inorganic Crystal Structure Database, seven compounds with six-membered oxofluoride anions [B3O3F6]3− or [B2PO5F4]3− have been reported up to now, as shown in Table S8. In order to investigate their structural features, the maximum torsion angles and structural formations of the six-membered rings B3O3 and B2PO3 in the [B3O3F6]3− and [B2PO5F4]3− anionic groups were summarized (Table S8 and Figure S6). It is found that Na3B2PO5F4 (P21/ n)40 establishes the largest torsion angle O(2)−P(1)−O(1)− B(1) = 47.691°. It is revealed that anions [B2PO5F4]3− generally possess larger maximum torsion angles than anions [B3O3F6]3− in these compounds [except disordered Na3B2PO5F4 (Cmcm),40 which is occupied by B/P and O/F in its anions [B2PO5F4]3−]. It is noted that the B3O3 and B2PO3 six-membered rings exhibit different structural formations, such as the plane, chair, and boat conformations. It can be found that [B3O3F6]3− and [B2PO5F4]3− exhibit structures similar to that of [B3O6]3−. The diagram of structural evolution among [B3O6]3−, [B3O3F6]3−, and [B2PO5F4]3− is presented in Figure 2. From [B3O6]3− to [B3O3F6]3−, the [BO3]3− units are changed by the [BO2F2]3− units. From [B3O3F6]3− to [B2PO5F4]3−, the anionic group substitution of one [BO2F2]3− with one [PO4]3− unit happens. It can be found that the [BO3]3−, [BO2F2]3−, and [PO4]3− units exhibit the same chemical valence of 3−, and the gradual replacement among the three units broadens the UV transmittance range of the borates with the original anion [B3O6]3−, owing to the larger HOMO− LUMO gaps of the [BO2F2]3− and [PO4]3− units. This idea of structural evolution using [BO3]3−, [BO2F2]3−, and [PO4]3− units can be further applied in the design of new function materials. Finally, this strategy can further be expanded in other six-membered anionic groups, for example, [B3O9]9−, [P3O9]3−, [Si3O9]6−, and [Ge3O9]6−. In order to probe the coordination geometry of anionic groups, the IR spectra of K0.42Rb2.58B3O3F6 and M3B2PO5F4 (M = K, Cs) are recorded from 400 to 4000 cm−1 (Figures S7 and S8). The assignments of the absorption peaks observed in the IR spectra of K0.42Rb2.58B3O3F6 and M3B2PO5F4 are presented in Tables S9 and S10, according to the reported compounds K3B3O3F623 and Na3B2PO5F4.40 UV−vis−near-IR diffusereflectance spectra of K0.42Rb2.58B3O3F6 and M3B2PO5F4 are displayed in Figure S9. It is obvious that K0.42Rb2.58B3O3F6 and M3B2PO5F4 possess short absorption edges under 220 nm (corresponding to 5.64 eV), which imply that the title compounds have a wide UV-transmittance range. The electronic structures of K 0.42 Rb 2.58 B 3 O 3 F 6 and M3B2PO5F4 (M = K, Cs) were calculated using density

exothermic peak on the TG−DSC curve (Figure S2a,b). Energydispersive X-ray (EDX) spectroscopy was operated on single crystals of the title compounds to verify the presence of the F and O atoms (Figure S3). K0.42Rb2.58B3O3F6 crystallizes in the orthorhombic crystal system with the space group Pbcn. In its asymmetric unit, there is one Rb atom, one Rb/K atom, two B atoms, two O atoms, and three F atoms. In the structure, all of the B atoms choose a fourcoordination environment with two O and two F atoms to generate the [BO2F2]3− unit. The ranges of the B−O and B−F distances in the [BO2F2]3− units are 1.421(5)−1.432(5) and 1.435(6)−1.455(5) Å, respectively, which can match with those of the reported fluorooxoborates. Furthermore, three [BO2F2]3− units polymerize to form the six-membered oxofluoride anion [B3O3F6]3− by sharing the corner O atoms (Figure 2b). These types of anionic groups can also be found in Na3B3O3F653 and K3B3O3F6.23 The Rb and Rb/K cations connect with O and F atoms, forming the [RbO3F8]13− and [Rb/KO2F8]11− polyhedra, respectively (Figure S4). Finally, these polyhedra connect with the zero-dimensional anions [B3O3F6]3−, generating the whole crystal structure (Figure 1a). Bond-valence-sum (BVS) calculation was carried out for all atoms to verify the rationality of this structure, and all atoms are in the reasonable range of valences (Table S2).54

Figure 1. Crystal structures of (a) K0.42Rb2.58B3O3F6, (b) K3B2PO5F4, and (c) Cs3B2PO5F4.

Although M3B2PO5F4 (M = K, Cs) belong to the monoclinic crystal system with the same space group of P21/n, they are not isomorphic compounds. They exhibit similar structures that are composed of the six-membered oxofluoride anions [B2PO5F4]3−. In their asymmetric units, there are three M atoms, one P atom, two B atoms, five O atoms, and four F atoms. The anion [B2PO5F4]3− is made up of two [BO2F2]3− units and one [PO4]3− unit (Figure 2c). The lengths of the B−O, B−F, and P−O bonds are in the range of 1.412(7)−1.464(6), 1.404(6)−1.433(6) Å, and 1.453(4)−1.569(4) Å, respectively, which are consistent with those in the reported fluorooxoborates

Figure 2. Structural evolution among the anions (a) [B3O6]3−, (b) [B3O3F6]3−, and (c) [B2PO5F4]3−. B

DOI: 10.1021/acs.inorgchem.9b00197 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

Inorganic Chemistry

Figure 3. Calculated band structures of (a) K0.42Rb2.58B3O3F6, (b) K3B2PO5F4, and (c) Cs3B2PO5F4.

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functional theory. It is clear that K0.42Rb2.58B3O3F6 exhibits a direct band gap of 5.5 eV, whereas M3B2PO5F4 owns indirect band gaps of 5.4 and 5.0 eV, respectively (Figure 3). The density of states of the title compounds are described in Figure 3. It is obvious that the largest orbital contributions in the valence band maximum (VBM) are due to the B−O units in K0.42Rb2.58B3O3F6 or the B−O and P−O units in M3B2PO5F4. Although the F states (at ∼−5 to −2 eV) are not involved in the VBM, the introduction of F atoms could eliminate terminal O atoms, which is helpful for increasing the band gap. The electronic structures of α-BaB2O455 are also shown in Figure S10. It is found that α-BaB2O4 possesses a smaller band gap of 4.58 eV than the title compounds, which can be due to the existence of π-conjugated orbitals and terminal O atoms in [B3O6]3−. Therefore, the advantage of anionic group substitution using anions [BO2F2]3− and [PO4]3− is beneficial for the wide band gap. In summary, three fluorooxoborates, K0.42Rb2.58B3O3F6 and M3B2PO5F4 (M = K, Cs), were successfully obtained using a high-temperature solution method in open air. They all contain the six-membered oxofluoride anions [B 3 O 3 F 6 ] 3− or [B2PO5F4]3−, which display structures similar to that of [B3O6]3−. The [BO2F2]3− and [PO4]3− units exhibit larger HOMO−LUMO gaps than the [BO3]3− units, and the πconjugated orbitals in the anions [B3O6]3− are eliminated through the introduction of [BO2F2]3− and [PO4]3− units, which finally leads to the wider UV-transmittance ranges of the title compounds compared with that of α-BaB2O4. The results demonstrate that the introduction of F atoms in anionic groups could deter the formation of terminal O atoms and further cause the wide band gaps of the title compounds.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Ying Wang: 0000-0001-6642-543X Shilie Pan: 0000-0003-4521-4507 Author Contributions §

These authors contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grants 91622107, 51602341, and 51425206), the West Light Foundation of the CAS (Grant 2015-XBQN-B-11), the Natural Science Foundation of Xinjiang (Grant 2016D01B061), the National Key Research Project (Grant 2016YFB0402104), and the Xinjiang Scientific and Technological Innovation Talents Project (Grant QN2016YX0339).

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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.9b00197. Experimental section, crystal data, PXRD patterns, TG− DSC curves, EDX spectra, crystal structural discussion, optical spectra of K0.42Rb2.58B3O3F6 and M3B2PO5F4 (M = K, Cs), and calculated band structures of α-BaB2O4 (PDF) Accession Codes

CCDC 1886108−1886110 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. C

DOI: 10.1021/acs.inorgchem.9b00197 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

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DOI: 10.1021/acs.inorgchem.9b00197 Inorg. Chem. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.inorgchem.9b00197 Inorg. Chem. XXXX, XXX, XXX−XXX