Abrupt Structural Transformation in Asymmetric ABPO4F (A = K, Rb, Cs)

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Abrupt Structural Transformation in Asymmetric ABPO4F (A = K, Rb, Cs) Qingran Ding,†,‡ Sangen Zhao,*,† Lina Li,† Yaoguo Shen,† Pai Shan,† Zhenyue Wu,† Xianfeng Li,† Yanqiang Li,†,§ Shuai Liu,† and Junhua Luo*,† †

State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, China ‡ University of Chinese Academy of Sciences, Beijing 100049, China § Shanghai Tech University, Shanghai, 14423, China Inorg. Chem. Downloaded from pubs.acs.org by TULANE UNIV on 01/17/19. For personal use only.

S Supporting Information *

CsB4O6F to hexagonal Cs0.5Rb0.5B4O6F. Nevertheless, all members of the AB4O6F family keep similar 2D-layered structures. Evidently, although the aforementioned asymmetric compounds show a large transformation from a monoclinic space group of low symmetry to even a hexagonal space group of considerably high symmetry, their basic structural configurations do not show obvious changes.26 To date, the cation-tuning strategy to design asymmetric compounds is merely effective in a very limited sense.7,19,27 So far, the sole available deep-UV (wavelengths below 200 nm) NLO material, KBe2BO3F2 (KBBF), contains highly toxic element beryllium; moreover, its weak interlayer bonding (dominated by K−F ionic bonds) caused another serious issue of layered growth habit.28−30 Consequently, many scientists made great efforts to discover the candidates of deep-UV NLO materials with superior performance. A number of borates and some newly reported phosphates were thus synthesized and primarily characterized.31,32 Recently, Mi and coauthors reported a novel fluorinated borophosphate KBPO4F, which has the similarly layered structure of KBBF.33 This relatively strong interlayer bonding of KBPO4F is expected to improve the layered growth tendency in comparison with KBBF. Herein, by cation substitution from KBPO4F, we obtained two asymmetric compounds, RbBPO4F (Ι) and CsBPO4F (II). Both compounds belong to cubic space group P213 (No. 198) with three-dimensional (3D) gismondine-like structures that are composed of BO3F and PO4 tetrahedra. Both structures are distinct from that of the monoclinic 2D layered KBPO4F. Such an abrupt structural transformation caused by alkaline cations is first reported in all-inorganic asymmetric compounds.31,34,35 Single crystals of Ι and II were obtained through a fluoriderich mild hydrofluorothermal reaction (detailed descriptions are given in the Supporting Information (SI)). The powder X-ray diffraction (XRD) patterns of the resulting products well match the simulated ones determined from single-crystal XRD analysis (Figure S1). The element analysis results revealed that the molar ratios of I and II are Rb:B:P = 1.08:0.91:0.87 and Cs:B:P = 1.02:0.93:0.9, respectively. The crystal structures of I and II belong to asymmetric cubic space group P213 (No. 198). I and II have the general formula

ABSTRACT: An asymmetric structure is the necessary requirement for second-order nonlinear-optical (NLO) materials, which have important applications in modern science and technology. Here we report two isostructural asymmetric compounds, RbBPO4F and CsBPO4F. Both compounds crystallize in cubic space group P213 (No. 198) with three-dimensional (3D) gismondine-like structures. Remarkably, in spite of the same basic structural units BO3F and PO4, both structures are distinct from the previously reported derivative KBPO4F, which crystallizes in a monoclinic space group Cc (No. 9) with a two-dimensional (2D)-layered structure. Careful structural analysis reveals that this structural transformation (from a monoclinic 2D structure to cubic 3D structures) should be aroused by the different alkaline ionic radii. To the best of our knowledge, such an abrupt structural transformation by alkaline elements is reported in all-inorganic asymmetric compounds for the first time. The structural transformation from 2D to 3D structures is favorable to eliminate the layered growth habit. This study will shed deep insight in the structural modulation of asymmetric compounds.

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n asymmetric structure is the prerequisite for second-order nonlinear-optical (NLO) materials,1−4 which have very important applications in modern science and technology.5−8 In the past decades, scientists have made great efforts to explore new asymmetric compounds with the desirable functional properties.9−11 One common strategy to obtain asymmetric compounds, cation tuning, was proposed after extensive investigations.12−15 For instance, by the selection of different alkaline cations, a family of asymmetric compounds ABe2BO3F2 (A = Na, K, Rb, Cs) can be obtained.16−20 These compounds undergo structural transformation from monoclinic space group C2 (No. 5) to trigonal space group R32 (No. 155), but they all feature similar two-dimensional (2D)-layered structures, with the cations (Na+, K+, Rb+, or Cs+) residing between the layers.21 Recently, Pan and coauthors reported a new AB4O6F (A = Na, Rb, Cs, Cs0.5K0.5, Cs0.5Rb0.5) family by selecting different alkaline cations.22−25 The AB4O6F family has a large structural transition from monoclinic NaB4O6F to orthorhombic RbB4O6F and © XXXX American Chemical Society

Received: September 29, 2018

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DOI: 10.1021/acs.inorgchem.8b02754 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

Inorganic Chemistry

coordination numbers, forming a [RbO9F3] polyhedron in I (Table S5). Second, I crystallizes in the space group of P213 (No. 198), which is of much higher symmetry than that of monoclinic Cc (No. 9) of KBPO4F. Third, KBPO4F features a 2D structure in which the layered units are further linked to each other by K−O and K−F bonds; in contrast, I features a 3D gismondine-like framework (Figure 1d).41 Such an abrupt structural transformation within a structural family is extremely rare. To the best of our knowledge, it is first reported in allinorganic asymmetric compounds.19 Moreover, because I is not completely layered, it would be much more favorable than the layered KBPO4F to eliminate the layered growth habit that KBBF greatly suffers. In fact, the as-grown crystals of I (Figrue S4) are blocklike and do not show any layered growth habit. The abrupt structural transformation in the ABPO4F family may be attributed to the dramatic change of the ionic radii of the alkaline cations.42,43 As shown in Figure S5, the cation radii versus atomic number curve has a significant change in slope between Rb+ and K+.44−46 On the one hand, because of the increase in the distances of the A−O (F) bonds (Table S5) with increasing cation radii, the bonding force of the adjacent layers will reduce after the replacement of K+ by Rb+.45,47 On the other hand, the coordination numbers of the alkaline-metal cations increase from 9 to 12 (Table S5) after the replacement of K+ by Rb+, which leads to different linkages of the anion units, such as six-membered rings in KBPO4F and helical chains in RbBPO4F. In RbBPO4F, all helical chains show single-stranded righthanded helices, which give rise to asymmetric 3D chiral porous frameworks. Therefore, the replacement of K+ by Rb+ leads to the fracture of the six-membered rings (Figure 1e) and the formation of larger open helical channels to reside on Rb+. The average void space increases with an increase of the cation size (Table S5). The bond valence sums (BVSs) of all cations (Table S6) were calculated.48,49 In the ABPO4F family, the calculated BVSs of the B and P atoms are consistent with the expected oxidation state, and there is almost no difference in the BVSs of the B and P atoms between different compounds. However, the BVSs of alkali metals are distinct. In the layered structure of KBPO4F, the BVS of the K atom (+1.15) evidently deviates from the expected value (+1), indicating that the layered structure may be unstable while replacing the K atom with the even larger Rb atom. In fact, the crystal structure becomes 3D for RbBPO4F (Figure 1f), and the calculated oxidation state of the Rb atom is in good agreement with the expected value. Compared to RbBPO4F, although the BVS of the Cs atom deviates from the expected value, the close connection between the [BO3F]4− and [PO4]3− groups maintains the stability of the 3D framework structure. Thermogravimetric analysis of I and II indicates that decomposition took place at around 923 and 823 K, respectively (Figure S6). The powder second-harmonic-generation (SHG) measurements of Ι and II were found to be approximately 0.3 times that of KH2PO4 (Figure 2a). These results confirm that I and II have asymmetric structures. Figure 2b shows that there is no obvious absorption from 200 to 800 nm. Especially, the reflectance is >83% at 200 nm, indicating that I and II should be transparent below 200 nm. In order to further understand the relationship between the structure and properties, we performed first-principles calculations.42,43 The calculations indicate that the indirect energy gap of I is approximately 6.35 eV, which is consistent with the UV−vis−near-IR diffuse-reflectance spectrum (Figure 3a). The total density of states (DOS) and partial DOS are shown in

ABPO4F and are isostructural. Therefore, I is chosen as a representative for discussion. The crystal structure of I is a gismondine-like 3D open framework (Figure 1a) with helical

Figure 1. Structural comparison of RbBPO4F and KBPO4F. (a) 3D framework structure of RbBPO4F. (b) One right-handed helix. (c) Topological structure in RbBPO4F. (d) 2D-layered structure of KBPO4F. (e) Six-membered rings. (f) Topological structure of the [BPO4F]− layer in KBPO4F.

channels running along the three cubic axes.36,37 The helical channels are formed by the alternative linkages of BO3F and PO4 tetrahedra. The connectivity of BO3F and PO4 tetrahedra by their common vertices shows right-handed helices arranged alternately along the [1 0 0] direction (Figure 1b). All helical chains in the 3D porous framework show single-stranded righthanded helices. As illustrated in Figures 1c and S2, each PO4 group is connected with neighboring three BO3F tetrahedra and vice versa. Thus, the identical helical channels run along the three cubic axes by the linkages in the helical chains of BO3F and PO4 tetrahedra and further connect each other to generate a 3D chiral framework with Rb+ cations occupied. Each B atom is coordinated to three O atoms and one F, forming tetrahedron BO3F with a B−O bond length of 1.465 Å and a B−F bond length of 1.397 Å. Notably, the distance of the B−F bond is slightly smaller than that of the B−O bonds, which is consistent with the reported results.23 Each P atom is coordinated with one terminal O atom with a P−O bond length of 1.471 Å and three O atoms with a P−O bond length of 1.557 Å. These bond distances of P−O, B−O, and B−F are in agreement with the previously reported results.25,28,33 The Fourier transform infrared spectrum of I38 (Figure S3) illustrates that the strong peak at 1239 cm−1 is attributed to the asymmetric stretching modes of the B−O bonds. Stretching vibrations of the B−F bonds are found at 774 and 626 cm−1. The weak doublets observed at 980 and 945 cm−1 can be assigned to symmetric stretching modes of the P−O bonds. The asymmetric bending mode of the P−O bonds of PO4 groups appear at 513 cm−1. The characteristic antisymmetric mode of the P−F bonds of PO3F groups is not observed, further verifying the existence of BO3F and PO4 groups.39,40 Interestingly, I and KBPO4F have similar formulas and are composed of the same BO3F and PO4 tetrahedra, but their crystal structures are quite different. First, each K atom is coordinated to six O atoms and three F atoms, forming a [KO6F3] polyhedron in KBPO4F, while each Rb atom has more B

DOI: 10.1021/acs.inorgchem.8b02754 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

Inorganic Chemistry

does not exhibit obvious layered growth habit that hinders the practical applications of notable KBBF. Such an abrupt structural transformation caused by alkali-metal cations is reported for the first time in all-inorganic asymmetric compounds. Notably, it is probably aroused from the abrupt slope change on the cation radii versus atomic number curve between K+ and Rb+ in the alkaline main group. We propose that using the abrupt change of the cation sizes in the same elemental group may be a new effective strategy to designing and synthesizing new asymmetric compounds with different structures and the resultant improved properties.

Figure 2. (a) SHG measurements of RbBPO4F and CsBPO4F. (b) Diffuse-reflectance spectra of RbBPO4F and CsBPO4F.

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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.8b02754. Experimental details, tables of atomic coordinates, and first-principles calculations for RbBPO4F and CsBPO4F (PDF) Accession Codes

Figure 3. (a) Electron-band structure of RbBPO4F. (b) DOS and partial DOS plots of RbBPO4F.

CCDC 1833578 and 1833852 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.

Figure 3b.50,51 The orbitals of the Rb atoms are mainly located at −7.7 eV, which are away from the Fermi level.52,53 The top of the valence band from −5 to 0 eV primarily consists of the O 2p and F 2p orbitals and small proportions of the P 3s and P 3p states. It is obvious that the bottom of the conduction band is dominantly composed of P 3s, P 3p, and B 2p orbitals and small contributions of the O 2p orbitals. Therefore, it can be concluded that the [BO3F]4− and [PO4]3− groups make considerable contributions to the band gap and NLO properities, whereas the contribution of the Rb atoms to the NLO response is negligibly small. The above-mentioned analysis is also in agreement with the anionic group theory.54 The calculated band gap, total DOS, and partial DOS of II are also provided, as shown in Figure S7. Figure 2b and the figure in a previous work33 show that the absorption edges of the ABPO4F family are less than 200 nm, which is consistent with the results based on electron-band structures and experiments. Clearly, these band gaps do not have great differences. Compared with a previous work,33 the powder SHG response of KBPO4F (about 1.1 × KDP) is obviously larger than that of I and II (about 0.3 × KDP). Compounds I and II crystallize in the same nonpolar cubic space group P213, so the consistent [BO3F]4− and [PO4]3− units themselves have opposite directions, leading to the nonpolarity of the macroscopic crystals. According to the anionic group theory,54 the microscopic NLO coefficients of the [BO3F]4− and [PO4]3− units are partially offset, resulting in relatively small macroscopic SHG responses. On the contrary, compound KBPO4F crystallizes in polar space group Cc. The second-order polarizability of the [BO3F]4− and [PO4]3− units is superimposed in the polar axis directions, and thus the resulting SHG response is relatively large. In conclusion, we successfully synthesized novel asymmetric compounds I and II by alkaline cation tuning. Interestingly, the substitution of alkaline cations for the ABPO4F family causes an abrupt structure transformation from monoclinic space group Cc (No. 9) with 2D-layered structure to cubic space group P213 (No. 198) with a 3D gismondine-like framework. As a result, I

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Sangen Zhao: 0000-0002-1190-684X Junhua Luo: 0000-0002-7673-7979 Author Contributions

All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Science Foundation of China (Grants 21833010, 21571178, 21525104, 51502288, and 91622118) and the Strategic Priority Research Program of the Chinese Academy of Sciences (Grants XDB20000000 and XDB20010200). S.Z. is grateful for support from the National Science Foundation for Distinguished Young Scholars of Fujian Province (Grant 2016J06012) and Young Elite Scientists Sponsorship Program by CAST (Grant 2017QNRC001).

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