Article pubs.acs.org/IC
Observation of Anion Order in Pb2Ti4O9F2 Kengo Oka*,† and Katsuyoshi Oh-ishi† Department of Applied Chemistry, Faculty of Science and Engineering, Chuo University, Bunkyo-ku, Tokyo 112-8551, Japan S Supporting Information *
ABSTRACT: The observation of anion order is indispensable for the investigation of oxyfluorides. However, the negligible contrast between O2− and F− in both X-ray and neutron diffraction obscures the distinct anion sites for Rietveld refinement. Therefore, the difference in the chemical bonding of M−O2− and M−F− is the key to determining anion order. In this study, bond-valence-sum calculations and determination of the electron density distribution by the maximum entropy method illustrated anion order in the newly synthesized oxyfluoride Pb2Ti4O9F2. These results demonstrate a promising method to determine anion order in mixed anion systems.
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INTRODUCTION The search for mixed anion compounds has contributed to the development of the chemistry and physics of solids. A variety of functions have been discovered in oxynitrides, oxyfluorides, oxychlorides, and oxypnictides, such as superconductivity in Sr2CuO2F2+δ,1 WO2.6F0.4,2 LaFeAsO1−xFx,3 and Ca 2−x Na x CuO 2 Cl 2 , 4 water-splitting photocatalysis in (Ga1−xZnx)(N1−xOx)5 and BaTaO2N,6 high electric permittivity in BaTaO2N and SrTaO2N,7 helical spin order in MnTaO2N,8 and colossal magnetoresistivity in EuNbO2N9 and EuWO2N.10 These functions originate from the differences in the valence, electronegativity, and ionic radius of the heteroanions. A unique valence state or cation−anion ratio can be realized by mixing anion sites with different valences to give exotic properties. The study of mixed anion systems requires precise structural analysis because differences in anions can lead to anion order in the structure and anion order can lower the symmetry. For example, a two-dimensional-layered structure resulting from anion order is adopted in the oxypnictide LaFeAsO1−xFx and the oxychloride Ca2−xNaxCuO2Cl2 and closely correlated with their superconductive nature. Anion order can lower the symmetry to a noncentrosymmetric one, as observed in KNaNbOF5,11 and important physical properties such as pyroelectricity, ferroelectricity, piezoelectricity, and second harmonic generation are observed in the noncentrosymmetric structures. For oxyfluorides and oxynitrides, the difference in the ionic radius is less significant than that for oxychlorides and oxypnictides. Because the anions can occupy the same crystallographic site, N/O or F/O order can vary from longrange to random.12 For example, F/O order is observed in the K2NiF4-type layered perovskites Ba2InO3F13 and Sr2FeO3F,14 but it is absent in K2NbO3F15 and Ba2ScO3F.13 Although determining the anion order is indispensable, the negligible contrast between N3−, O2−, and F− in X-ray diffraction (XRD) obscures their distinct anion sites for Rietveld refinement. Moreover, oxygen and fluorine exhibit comparable scattering © 2015 American Chemical Society
lengths for neutron diffraction. Thus, investigation of bond valence sums (BVSs) based on Pauling’s second rule16 and the electron density distribution by the maximum entropy method (MEM)17 are promising methods for determining the anion order. However, because reliability of the MEM depends on the estimated structure factors, quality of the sample and accuracy of the diffraction data are required. In this study, we focused on the PbO−PbF 2 −TiO 2 oxyfluoride system as a candidate for an anion-ordered system. The coupling of the stereochemical effect of the 6s2 lone-pair electrons of Pb2+ or Bi3+ and the second-order Jahn−Teller effect induced by the d0 electronic configuration of Ti4+ is suggested to stabilize the distorted structure led by anisotropy in M−O bonds,18 and noncentrosymmetric structures are found in PbTiO3 and Bi4Ti3O12. The presence of anisotropy in cation−anion interactions is the key for realizing anion order, as discussed in terms of the electronic potentials and chemical hardness in inorganic−organic hybrids.19 The distorted structure should lower the symmetry and lead to an increase in the number of distinct anion sites, and anion order is founded in Pb2Nb3O7F5, Pb3Ta4O12F2, Pb3Ta5O9F13, and Pb 12Ta 9O20F2920 containing both Pb2+ and d0 cations. Bi2Ti4O4F2 and BiNbO5F adopt layered structures with an ordered array of O2− and F−.21 Accordingly, O/F order should be present in the PbO−PbF2−TiO2 system. We synthesized the novel oxyfluoride compound Pb2Ti4O9F2 and performed precise structural analysis using the synchrotron XRD (SXRD) technique. The obtained BVSs and electron density indicated that oxygen and fluorine ions occupy distinct sites, revealing the presence of long-range O/F order in Pb2Ti4O9F2. Received: July 3, 2015 Published: October 20, 2015 10239
DOI: 10.1021/acs.inorgchem.5b01496 Inorg. Chem. 2015, 54, 10239−10242
Article
Inorganic Chemistry
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EXPERIMENTAL SECTION
Pb2Ti4O9F2 was prepared by solid-state reaction of a mixture of PbO (99.9%), PbF2 (99.9%), and TiO2 (rutile, 99.9%) powders. The powders were weighed to have a slightly fluorine-rich composition to compensate for the loss of fluorine during the reaction. The pelletized mixture was sealed in an evacuated Pyrex tube and treated at 823 K for 12 h. XRD pattern matching was performed using the integrated powder XRD software package PDXL (Rigaku). The SXRD patterns were collected with a large Debye−Scherrer camera installed at beamline BL02B2 of SPring-822 and analyzed by the Rietveld method using the RIETAN-FP program.23 The chemical composition was analyzed using an X-ray fluorescence (XRF) spectrometer (Rigaku, ZSX Primus III+). The electron density distributions were determined by the MEM using the Dysnomia program.24 The dielectric permittivity was measured with a precision LCR meter (Agilent, 4284A) using a disk-shaped sintered pellet. The electrodes were gold deposited on opposite faces of the sample.
Figure 2. Temperature dependence of the relative permittivity of Pb2Ti4O9F2 upon cooling from 300 K.
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Table 2. BVSs of Pb2Ti4O9F2 and the Paraelectric Phase of Bi2Ti4O1125
RESULTS AND DISCUSSION A white sintered pellet was obtained, and the Pyrex tube was etched a little bit during the treatment but kept stiff. Figure 1
Pb2Ti4O9F2
Bi2Ti4O11
site
O
F
site
O
O1/F1 O2/F2 O3/F3 O4/F4 O5/F5 O6/F6
2.10 2.06 1.97 2.12 1.79 1.51
1.80 1.77 1.69 1.79 1.53 1.24
O1 O2 O3 O4 O5 O6
2.06 2.03 1.96 1.94 2.02 2.11
The BVSs were calculated with the summation of Ti1−O1/F1 × 2, Ti2−O1/F1, and Pb/Bi−O1/F1 × 2 for O1/F1 site, Ti1−O2/F2, Ti2−O2/F2 × 2, and Pb/Bi−O2/F2 × 2 for the O2/F2 site, Ti1− O3/F3 × 2 and Pb/Bi−O3/F3 × 4 for the O3/F3 site, Ti2−O4/F4 × 2 and Pb/Bi−O4/F4 × 2 for the O4/F4 site, Ti1−O5/F5, Ti2−O5/ F5, and Pb/Bi−O5/F5 × 2 for the O5/F5 site, and Ti1−O6/F6 and Pb/Bi−O6/F6 × 3 for the O6/F6 site. Figure 1. Structural characterization of Pb2Ti4O9F2 by Rietveld refinement of the SXRD pattern (λ = 0.42045 Å) collected at room temperature. Red crosses, the black solid line, and the blue solid line represent observed, calculated, and difference intensities, respectively. The green ticks indicate the positions of the Bragg peaks. The inset shows a magnified view from 2 to 10°.
Table 1. Refined Crystallographic Parameters of Pb2Ti4O9F2 at Room Temperaturea atom
site
x
y
z
B (Å2)
Pb1 Ti1 Ti2 O1/F1 O2/F2 O3/F3 O4/F4 O5/F5 O6/F6
4i 4i 4i 4i 4i 2b 4i 4i 4i
0.2415(1) 0.4470(3) 0.0769(2) 0.4529(7) 0.4232(6) 0.5 0.5995(6) 0.2710(7) 0.6460(6)
0 0 0 0.5 0 0 0.5 0 0
0.1555(1) 0.7833(3) 0.4568(3) 0.7536(9) 0.5713(9) 0 0.6593(9) 0.6484(9) 0.9329(9)
1.12(2) 0.97(5) 0.14(5) 0.25(7) 0.25 0.25 0.25 0.25 0.25
a
Space group C2/m (No. 12), Z = 2, a = 14.6247(10) Å, b = 3.8275(1) Å, c = 10.7514(8) Å, β = 135.577(3)°, V = 421.24(5) Å3, RWP = 3.65%, and RI = 2.05%.
Figure 3. (a) Crystal structure of Pb2Ti4O9F2. (b) Coordination states of Ti1 and Ti2. The integers represent x in Ox/Fx sites.
shows the SXRD pattern of Pb2Ti4O9F2 at room temperature. The pattern-matching program suggested that the hightemperature paraelectric phase of Bi2Ti4O11 (C2/m, a = 14.6412(12) Å, b = 3.8032(3) Å, c = 10.7824(6) Å, β = 136.135(5)°, and T = 573 K)25 has a similar pattern. Rietveld refinement was performed based on the structural parameters
and reached convergence. The results are shown in Figure 1, and the refined structural parameters are summarized in Table 1. The reliability of the refinement was enough for the MEM. Because the XRD method cannot distinguish between O2− and F−, these were considered to be a single species in the refinement. To check the oxygen/fluorine content, we varied 10240
DOI: 10.1021/acs.inorgchem.5b01496 Inorg. Chem. 2015, 54, 10239−10242
Article
Inorganic Chemistry
This type of structure is rare: only the paraelectric phase of Bi2Ti4O11 is listed in the Inorganic Crystal Structure Database. This suggests that the presence of 6s2 lone-pair electrons is essential for stabilizing this structure. Because Bi2Ti4O11 exhibits a structural transition from the high-temperature paraelectric phase to the low-temperature antiferroelectric phase [C2/c, a = 14.5999(6) Å, b = 3.8063(2) Å, c = 14.9418(8) Å, and β = 93.129(4)°] at 523 K,25 the temperature dependence of the SXRD pattern was investigated. However, no sign of a structural transition was observed upon cooling to 100 K. Further investigation was carried out by measuring the dielectric permittivity because Bi2Ti4O11 exhibits an anomaly accompanied by the structural transition.26 Figure 2 shows the temperature dependence of the dielectric permittivity of Pb2Ti4O9F2. Paraelectric behavior was maintained down to 5 K, and any anomaly indicating a structural transition was absent. Although O2− and F− cannot be distinguished by XRD, their different electronic properties allow the investigation of anion order. According to Pauling’s second crystal rule, the BVS for each site should indicate the charge on an anion or cation. The BVSs for the distinct anion sites in Pb2Ti4O9F2 are summarized in Table 2, and significant differences are apparent in their values. O6/F6 exhibits a BVS that is plausible for F− rather than O2− compared with the other sites that showed reasonable values for O2−. This result indicates the presence of anion order. The BVSs based on this order model gave 1.92+, 4.09+, and 4.11+ for Pb, Ti1, and Ti2, respectively. For comparison, the BVSs of Bi2Ti4O11 do not show significant deviation from 2−. Such a difference in the BVSs indicates that anion order is also found in other oxyfluorides.13,21 These results strongly indicate the presence of anion order with F− selectively occupying the O6/F6 site, which gives the chemical formula Pb2Ti4O9F2. Clear evidence of anion order should be provided by investigation of the chemical bonds. Because O2− and F− have different electronegativities (O = 3.44 and F = 3.98 in the Pauling scale) and empirical anion polarizabilities in a compound [α0(O2−) = 1.988 Å3 and α0(F−) = 1.295 Å3],27 the M−O and M−F (M = metal cation), bonds should have different bond lengths and covalencies. Figure 3 shows the crystal structure of Pb2Ti4O9F2 and the coordination states of the Ti1 and Ti2 sites. The Ti1−O6/F6 bond has the longest bond length among all of the bonds, indicating the lowest covalency. This result agrees with the anion-order model suggested by the BVSs. Further investigation was performed by calculating the electron density distribution. Figure 4 shows a two-dimensional electron density map sliced along the (010) plane. This clearly shows that the electron density at the O6/F6 site is isolated and illustrates its ionic nature, confirming our anion-order model. Considering BVS, substitution of O2− by F− requires elongation of metal−anion bond lengths. The O6/F6 site occupies the open site in the crystal structure and can adopt elongation of metal−anion bonds. Actually, elongation of the Pb/Bi−O6/F6 bonds was observed between Pb2Ti4O9F2 and Bi2Ti4O11, as summarized in Table 3. Hence, F− occupies O6/ F6 selectively, leading to anion order. The electron density distribution confirms its unique structure. In Figure 4, covalent bonding between the Pb and the O4/F4 sites seems to be present. This is clear in the twodimensional electron density map for a plane parallel to (001) in Figure 5. It should be noted that such a covalency between Pb and O is also observed in PbTiO3.17c The distortion because
Figure 4. Two-dimensional electron density map of the electron density sliced along the (010) plane. The integers represent x in Ox/ Fx sites.
Table 3. Selected Bond Lengths (Å) for the O6/F6 Site in Pb2Ti4O9F2 and the Paraelectric Phase of Bi2Ti4O1125 Ti1−O6/F6 Pb/Bi−O6/F6 Pb/Bi−O6/F6 Pb/Bi−O6/F6
Pb2Ti4O9F2
Bi2Ti4O11
2.089(5) 2.562(7) 2.562(7) 2.418(9)
2.195(7) 2.410(5) 2.410(5) 2.143(4)
Figure 5. (a) Illustration of the sliced plane parallel to (001) and (b) its two-dimensional electron density map. The integers represent x in Ox/Fx sites.
the occupancy factors of the anion sites. These converged to 1.04(1) for O1/F1, 1.01(1) for O2/F2, 0.95(1) for O3/F3, 0.99(1) for O4/F4, 1.06(1) for O5/F5, and 0.99(1) for O6/F6, indicating negligible anion deficiency. The chemical composition was further confirmed by the XRF technique. The wavelength-dispersive spectra gave the atomic composition of 11.85(4)% of Pb, 23.60(8)% of Ti, 52.80(19) of O, and 11.75(12)% of F, corresponding to the chemical formula of Pb2.01(1)Ti4.01(1)O8.98(3)F2.00(2). 10241
DOI: 10.1021/acs.inorgchem.5b01496 Inorg. Chem. 2015, 54, 10239−10242
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Inorganic Chemistry of the 6s2 lone-pair electrons of Pb2+ is suggested to support anion order by increasing the number of distinct anion sites.
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CONCLUSION We have synthesized the new oxyfluoride compound Pb2Ti4O9F2 and conducted precise structural analysis. The crystal structure is isostructural with the high-temperature paraelectric phase of Bi2Ti4O11, and structural transitions were absent down to 5 K. Investigation of the BVSs and electron density distribution provided evidence for anion order with F selectively occupying the O6/F6 site. The electron density distribution also showed the presence of a covalent bond between Pb and O, as observed in PbTiO3. Determining the anion order is important for investigating mixed anion systems, but the similar properties of oxygen and fluorine make them difficult to distinguish by XRD. We have succeeded in the direct observation of anion order by determining the electron densities. This demonstrates a promising procedure for investigating anion order in mixed anion systems.
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ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.5b01496. CIF file (CIF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Author Contributions †
K.O. and K.O.-i. designed the research. K.O. carried out all experiments and wrote the manuscript. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We thank Prof. Mitsuru Itoh of the Materials and Structures Laboratory, Tokyo Institute of Technology, for help in the dielectric property measurement and Rigaku Corp. for help in the XRF measurement. This work was partially supported by Grants-in-Aid for Young Scientists (B) (Grant 26800180). The synchrotron-radiation experiments were performed at SPring-8 with the approval of the Japan Synchrotron Radiation Research Institute (Grant 2014B1224).
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REFERENCES
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DOI: 10.1021/acs.inorgchem.5b01496 Inorg. Chem. 2015, 54, 10239−10242
