Communication Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
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BaPO3Cl: a Metal Phosphate Chloride with Infinite [PO3]∞ Chains Jianghe Feng, Chun Li Hu, Yuan Lin, and Jiang Gao Mao* State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002, P. R. China
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but no 1D [PO3]∞ chains on the basis survey in the inorganic crystal structure database (ICSD, 2017-1, version 1.9.9, by Fachinformatiionszentrum Karlsruhe, Germany). It is noted that the [POn]x− groups change from an isolated [PO4]3− unit in Ba5(PO4)3Cl to a [P3O10]5− trimer in Ba3P3O10Cl with a decrease in the ratio of Ba2+/Cl−.33,34 Therefore, we hope to further increase the Cl− content to induce more condensation of the (POn)x− group into 1D chains and achieve large Δn. Guided by this idea, we replaced one O2− in Ba3P3O10Cl by two Cl− and obtained the first metal phosphate chloride with infinite [PO3]∞ chains, namely, BaPO3Cl, which owns relatively large Δn (0.021 at 1064 nm) compared with those (∼0.00−0.01) of other reported phosphates.29 Herein, we report its synthesis, crystal structure, optical properties, and theoretical calculations. Needle-shaped crystal samples with the largest size of ∼5 mm were obtained by the high-temperature solid-state reactions of BaCl2, BaO, P2O5, and KI (as a flux) with a molar ratio of 1:1:1:1.5 at 700 °C in a fused quartz tube. The pure phase was confirmed by powder X-ray diffraction (PXRD) analysis (Figure 1). Also, the energy-dispersive spectroscopy (EDS) elemental
ABSTRACT: We adopted a chemical substitution strategy to design a particular anionic structure with large optical anisotropy in phosphate. Specifically, we replaced one O2− in Ba3P3O10Cl by two Cl−, leading to the formation of a new compound of BaPO3Cl (P21/c). Notably, this compound is the first metal phosphate chloride featuring infinite [PO3]∞ chains, and it displays good chemical and thermal stability, as well as a short UV cutoff edge ( 3) rings, © XXXX American Chemical Society
Figure 1. Simulated and experimental PXRD patterns for BaPO3Cl (the inset is the image of BaPO3Cl crystals).
analysis gave an average Ba/P/Cl molar ratio of 1.0:1.05:1.11 (Figure S1), which is consistent with that obtained from singlecrystal XRD study. BaPO3Cl crystallizes in the centrosymmetric (CS) monoclinic space group of P21/c (Table S1). Its structure features a 3D network composed of isolated 1D [PO3]∞ chains, which are further bridged by BaO3Cl6 and BaO6Cl3 polyhedra (Figure 2c). Each asymmetric unit contains two crystallographic Ba, P, and Cl atoms, as well as six O atoms (Table S2); all of them are located at general positions. P atoms are four-coordinated by Received: November 7, 2018
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DOI: 10.1021/acs.inorgchem.8b03111 Inorg. Chem. XXXX, XXX, XXX−XXX
Communication
Inorganic Chemistry
bonds, which are connected by BaO8F polyhedra rather than the PO4 units within 1D [PO3]∞ chains for BaPO3Cl. In addition, the simple [PO3]∞ chains in BaPO3Cl are composed of two independent PO4 units, being different from those in other polyphosphates, which contain more PO4 tetrahedra and various second anionic building units, such as [P4O12]4− for CsLa(PO3)4,35 [P5O15]5− for RbBa2(PO3)5, 1 [P9O27]9− for Cs6Mg6(PO3)18,36 etc. To evaluate the water resistance, an important property for phosphates, crystalline samples of BaPO3Cl were soaked in distilled water at 70 °C for 48 h. After being dried, little change in their shapes and transparencies was observed (Figure S2), denoting that BaPO3Cl is of good water resistance. According to thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) studies of BaPO3Cl, there is an obvious weight loss on the TGA curve higher than 800 °C and a corresponding strong endothermic peak at 870 °C on the DSC curve (Figure S3). On the basis of EDS analysis (without any information on Cl; Figure S4), the observed weight loss can be attributed to the release of Cl2, and the weight loss is estimated to be 12.5% at 1000 °C, which is close to the calculated value of 14.08%. Moreover, the final residues were confirmed to be Ba2P2O7 and BaO by PXRD analysis (Figure S5). The UV−vis−near-IR diffuse reflectance is shown in Figure S6. There is no obvious absorption peak in the range of 200− 2500 nm, and the reflectance at 200 nm is still higher than 93%. Therefore, the relevant band gap is larger than 6.2 eV and would be close to that (6.89 eV) of the parent compound Ba3P3O10Cl. Besides, the observed IR absorption peaks (Figure S7) at 1305 and 1278 cm−1 can be assigned to the asymmetrical stretching vibration of O−P−O; peaks at 1150 1088, and 1016 cm−1 belong to the typical symmetrical stretching vibration of O−P− O, and those beside 855, 768, and 687 cm−1 can be attributed to the asymmetrical stretching vibration of the O−P−O modes. In addition, the symmetrical O−P−O and P−O−P bending vibrations are revealed by the peaks of 555 and 494 cm−1. All of the assignments are consistent with the reported results of the [PO3]∞ chains.36,37 To better elucidate the structure−property relationship, firstprinciple calculations were performed. The calculated band structure of BaPO3Cl (Figure S8a) indicates its indirect-bandgap characteristic because of the valence-band (VB) maximum (0.00 eV) at B point and the conduction-band (CB) minimum (5.34 eV) at Γ point (Table S5). All of the points on the path through the zone area are also depicted in Figure S8b. The calculated band gap is smaller than the experimental value (>6.2 eV) because the exchange and correlation functional of Perdew−Burke−Ernzerhof generalized gradient approximation makes the CB levels intrinsically lower.38 Because Ba3P3O10Cl and BaPO3Cl have similar theoretical band gaps (5.43 and 5.34 eV, respectively), indicating that their experimental band gaps would be very close, a scissors vibration at 1.55 eV was applied during calculations of the optical property (Eg = 6.89 eV for Ba3P3O10Cl). The density of states of BaPO3Cl are depicted in Figure S9. VBs lower than −7 eV are mainly occupied by Ba 5p and Cl 3s mixed with minor P 3p3s and O 2p states. The energy range from −6 to 0 eV mostly consisted of the Cl 3p, O 2p, and P 3p states, disclosing strong P−O bond interaction. The bottom of the CB originates from the P 3p3s, Ba 5d, and O 2p states mixed with a small amount of Cl 3p and Ba 6s. Thus, the band gap is determined by the Cl and P atoms.
Figure 2. View of the PO4 units (a), 1D zigzag chains of [PO3]∞ (b), the 3D structure of BaPO3Cl down the b axis, (c) and the BaO3Cl6 and BaO6Cl3 polyhedra (d).
four O atoms in tetrahedral geometries (Figure 2a), the P−O bond lengths are in the range of 1.4801(21)−1.6256(18) Å (Table S3), and the O−P−O angles are in the range of 96.38(10)−121.24(14)° (Table S4). These bonds lengths and angles are comparable with those in Ba3 P 3 O 10 Cl and Ba5(PO4)3Cl. Distortions of the PO4 units [32.67° for P(1)O4 and 35.4° for P(2)O4] are slightly larger than those (20.41− 30.9°) of the parent Ba3P3O10Cl but much larger than that (8.81°) of Ba5(PO4)3Cl. The two Ba atoms are both ninecoordinated with O and Cl atoms to form Ba(1)O3Cl6 and Ba(2)O6Cl3 polyhedra (Figure 2d), respectively. The Ba−O and Ba−Cl bond distances are in the ranges of 2.7010(20)− 2.9881(17) and 3.1020(7)−3.3053(6) Å, respectively. The calculated bond valence sums of 2.136−2.331 (Ba) and 4.697− 4.714 (P) revealed that the oxidation states of the Ba and P atoms are 2+ and 5+, respectively. The P(1)O4 and P(2)O4 tetrahedra are alternatively interconnected to each other via the corner-sharing of O(4) and O(5) atoms to form 1D zigzag chains of [PO3]∞ along the b direction (Figure 2b). These chains are further bridged by Ba(1)O3Cl6 and Ba(2)O6Cl3 polyhedra (Figure 2d) into a 3D network. Interestingly, BaPO3Cl can be viewed as derived from Ba3P3O10Cl through the substitution of one O2− by two Cl−, which induced the 0D [P3O10]5− trimers in Ba3P3O10Cl to condense into the [PO3]∞ chains without any mirror plane symmetry in BaPO3Cl (Figure 3). Unfortunately, when [PO3]∞ chains are linked together by Ba−Cl/O-based polyhedra, it results in a CS structure.
Figure 3. View of 3D structures of Ba3P3O10Cl and BaPO3Cl and the transition of their structures.
A comparison of the structures of Ba5(PO4)3Cl and Ba3P3O10Cl with BaPO3Cl indicates that the [POx]n− anionic units change from 0D [PO4]3− tetrahedra in Ba5(PO4)3Cl to an isolated [P3O10]5− trimer in Ba3P3O10Cl and finally to a 1D [PO3]− chain in BaPO3Cl with a decreasing ratio of Ba2−/Cl−. In addition, the structure of BaPO3Cl is also different from that of BaPO3F, although their molecular formulas are very similar.32 Because of the largest electronegativity of the F element, the structure of BaPO3F features 0D PO3F groups with strong P−F B
DOI: 10.1021/acs.inorgchem.8b03111 Inorg. Chem. XXXX, XXX, XXX−XXX
Inorganic Chemistry
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The population analyses are shown in Table S3. The populations of the P−O bonds are in the range of 0.4−0.75, indicating their strong covalent characteristic, and those of the Ba−O and Ba−Cl bonds are much smaller and fall in the ranges of 0.02−0.14 and 0.09−0.16, respectively, because of their ionic bonding nature. Because, for the monoclinic space group P21/c, the dielectric principal axes are not consistent with the crystallographic axes in the ac plane, aprincipal axis transformation (rotation angle of −24.789628°) was performed.39,40 The imaginary and real parts of the dispersion curves of the dielectric function were calculated (Figure S10), which reveals their strong anisotropies. Figure S11 presents the calculated averaged dielectric function over different directions, from which we can see that the calculated static dielectric constant is 2.90 eV. The strongest absorption peak around 9.88 eV in the εave 2 (ω) curve is mainly dominated by the transitions from the occupied O 2p and Cl 3p states to the unoccupied P 3p3s and Ba 5d states. The refractive indices calculated from n2(ω) = ε(ω) are shown in Figure 4, displaying a
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The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b03111. Experimental details and characterization, including crystal data, chemical, thermal, and optical property measurements, and density functional theory calculation results (PDF) Accession Codes
CCDC 1874820 contains the supplementary crystallographic data of BaPO3Cl for this paper. The 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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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Fax: (+86)591−63173121. ORCID
Jianghe Feng: 0000-0003-1640-4221 Jiang Gao Mao: 0000-0002-5101-8898 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 21701173, 21875248, and 91622112), the “Strategic Priority Research Program” of the Chinese Academy of Sciences (Grant XDB20000000), and the “100 Talents Project” of Fujian Province. The authors also thank Prof. Zhi-Hua Yang of Xinjiang Technical Institute of Physics & Chemistry for help in birefringence calculations.
Figure 4. Calculated refractive indices and birefringence.
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trend of nz > nx > ny during the wavelength range of 200−2000 nm. In addition, the nx, ny, and nz values at 1064 nm are calculated to be 70538, 1.6996, and 1.72083 , respectively, and the birefringence is 0.021, being comparable to those of Ba3P3O10Cl (0.028) and Ba2NaClP2O7 (0.017)41 and those of fluorophosphates with PO3F units (∼0.02−0.03).29 However, it is clear that birefringence of BaPO3Cl is much larger than those of phosphates (∼0.00−0.01), such as KLa(PO3)4 (0.008),42 CsLa(PO3)4 (0.006),35 BPO4 (0.007).6 Cs2LiPO4 (
