Temperature-Independent, Large Dielectric Constant Induced by

for Materials and Structures, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan. Chem. Mater. , 2016, 28 (15), pp...
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Temperature-Independent, Large Dielectric Constant Induced by Vacancy and Partial Anion Order in the Oxyfluoride Pyrochlore Pb2Ti2O6−δF2δ Kengo Oka,*,† Hajime Hojo,‡ Masaki Azuma,‡ and Katsuyoshi Oh-ishi† †

Department of Applied Chemistry, Faculty of Science and Engineering, Chuo University, Bunkyo-ku, Tokyo 112-8551, Japan Laboratory for Materials and Structures, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan

‡

S Supporting Information *

ABSTRACT: In mixed-anion systems, partial anion order can be the key to realizing enhanced dielectric properties. A novel A2B2X6X′0.5-type oxyfluoride pyrochlore, Pb2Ti2O5.4F1.2, was prepared through the conventional solid-state reaction. Characterization of the oxyfluoride pyrochlore by synchrotron X-ray diffraction and electron diffraction revealed the existence of partial O2−/F− order at the X site, in which F− is involved in fac-type Ti(O/F)6 coordination, associated with the O2−/vacancy order at the X′ site. Although Pb2Ti2O5.4F1.2 adopts a centrosymmetric structure, the refined occupancy factors suggested the existence of local polarization. Partial O2−/F− anion order stabilized the large displacement of Ti4+, leading to a high dielectric constant (as high as 800) with a low-temperature coefficient. These results suggest that mixing heteroanions in pyrochlores provides a basis for exploring novel dielectric materials.

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INTRODUCTION Mixed-anion systems exhibit various attractive properties, such as superconductivity in Sr2CuO2F2+δ,1 LaFeAsO1−xFx,2 and Ca 2−x Na x CuO 2 Cl 2 ; 3 water-splitting photocatalysis by (Ga1−xZnx)(N1−xOx)4 and BaTaO2N;5 high electric constants with low-temperature coefficients in BaTaO 2 N and SrTaO2N;6−8 helical spin order in MnTaO2N;9 and colossal magnetoresistivity in EuNbO2N10 and EuWO2N.11 These properties originate from differences in the valences, electronegativities, and ionic radii of the heteroanions. Because the cation/anion ratio dictates the crystal structures of ionic compounds, mixing heteroanions can realize a structure that cannot be stabilized by oxides (O2−), nitrides (N3−), and fluorides (F−) alone. For example, the pyrochlore Sr2Ta2O7 transforms to the perovskite SrTaO2N by ammonolysis.12 Mixing oxygen and nitrogen anions changes the stable cation/ anion stoichiometry from A2+2B5+2O2−7 to A2+2B5+2O2−4N3−2(A2+B5+O2−2N3−). This fact has also been confirmed in a Pb- and Ti-containing oxyfluoride system. According to our previous study, Pb2Ti4O9F2 is isostructural with a paraelectric phase of Bi2Ti4O11.13 The differences in the valences, electronegativities, and ionic radii of the heteroanions often result in anion order in the structure. For example, the oxypnictide LaFeAsO and the oxychloride Ca2−xNaxCuO2Cl2 adopt a two-dimensional layered structure resulting from anion order, which is closely correlated with their superconducting nature. Precise structural analysis aids interpretation of the high dielectric constants observed in SrTaO2N and BaTaO2N.6 BaTaO2N adopts a © 2016 American Chemical Society

centrosymmetric cubic structure; however, a neutron-pair distribution function study has suggested a local ciscoordination of N atoms around Ta.14 The lack of inversion symmetry induced by the O2−/N3− order can result in the displacement of Ta5+, leading to local polarization, and the first principle calculation performed by Hinuma et al. indicated that the anion order can be the origin of relaxor-type behavior in the oxynitride perovskites.15 Hence, the high dielectric constants of the aforementioned oxynitride perovskites are attributed to partial anion order. The same scenario should be available for oxyfluorides. Octahedral coordination with mixed O2− and F− anions leads to noncentrosymmetric off-center displacement of metal cations, and ordering behavior of the octahedra can induce exotic electronic or optical properties.16−22 The oxyfluoride perovskite PbFeO2F has a high dielectric constant, which can reach 300 at room temperature, although this oxyfluoride is centrosymmetric, with the Pm3̅m space group.23 Its dielectric property is attributed to the random displacement of Pb2+ induced by the stereochemical effect of 6s2 lone-pair electrons. No evidence of long-range anion order has been found by electron diffraction (ED); however, 57Fe Mössbauer spectroscopy suggested the preference of the FeO4F2 octahedra for trans-type coordination.24 Stabilization of the local distortion by partial O2−/F− order is expected; however, the negligible contrast between O2− and F− in both X-ray and Received: June 21, 2016 Revised: July 17, 2016 Published: July 18, 2016 5554

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Chemistry of Materials neutron diffraction obscures their distinct anion sites for Rietveld refinement. Hence, calculation of bond valence sums (BVSs)25 and determination of electron density distribution through the maximum entropy method (MEM)26−28 hold promise for investigation of the O2−/F− anion order.13 Mixed-anion systems have potential use as dielectric materials with low-temperature coefficients. However, difficulties associated with their synthesis will hinder their applications. BaTaO2N and SrTaO2N have been prepared by ammonolysis, in which reactants are always heated in a powdered form.6,12,29 Therefore, the sintered pellets were produced by subsequent pressing and heat treatment under NH3. PbFeO2F has been synthesized under high pressure (6 GPa);24 such a highpressure technique requires the use of a special apparatus and limits the volume of reactants. In this work, we report a novel oxyfluoride pyrochlore with a high dielectric constant, Pb2Ti2O6−δF2δ. The sample was synthesized by the conventional solid-state reaction. The random displacement of cations induced by lone-pair electrons observed in Bi3+-containing pyrochlore oxides such as (Bi1.5Zn0.5−δ) (Zn0.5Nb1.5)O7−δ is considered to be the origin of the high dielectric constants of these pyrochlores.30−33 Thus, the combined stereochemical effect of the 6s2 lone-pair electrons and local polarization induced by mixing heteroanions are expected to enhance the dielectric property. Our structural analysis revealed the coexistence of O2−/F− anion order and O2−/vacancy order in the oxyfluoride Pb2Ti2O6−δF2δ. The temperature dependence of the dielectric permittivity showed a high dielectric constant and a low-temperature coefficient, which are comparable to those of BaNbO2N and SrNbO2N.

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Figure 1. Structural characterization of Pb2Ti2O5.4F1.2 by Rietveld refinement using the SXRD pattern (λ = 0.41951(1) Å) collected at room temperature. The space groups used for the refinement were (a) Fd3̅m and (b) F4̅3m. The red crosses, black solid line, and blue solid line respectively represent the observed, calculated, and difference intensities. The green ticks indicate the positions of the Bragg peaks.

EXPERIMENTAL SECTION

Pb2Ti2O6−δF2δ was prepared through a solid-state reaction using a stoichiometric mixture of PbO (99.9%), PbF2 (99.9%), and TiO2 (rutile, 99.9%) powders. The pelletized mixture was sealed in an evacuated Pyrex tube and treated at 873 K for 12 h. Synchrotron X-ray diffraction (SXRD) patterns were collected using an imaging plate with a large Debye−Scherrer camera installed at beamline BL02B2 of SPring-834 and analyzed by the Rietveld method using the RIETAN-FP program.35 Electron density distributions were determined by MEM using the Dysnomia program and visualized using the VESTA program.36,37 ED patterns were recorded by transmission electron microscopy (JEOL, JEM-2100F). The temperature dependence of the dielectric permittivity and the impedance spectra were obtained using a precision LCR meter (Agilent, 4284A) and an impedance analyzer (Solartron, 1260), respectively. The electrodes were gold, deposited on opposite faces of the samples. Impedance spectra obtained at 10−2 to 107 Hz were interpreted by equivalent-circuit analysis using the Zview software.

Pb2Ti2O5.4F1.2 at room temperature and the results of Rietveld refinement. The pyrochlore A2B2X6X′ structure, in which A and B are cations and X and X′ are anions, consists of cornerconnected BX6 octahedra and X′A4 tetrahedral networks; A, B, X, and X′ ions occupy the 16d, 16c, 48f, and 8b sites, respectively.40 Because XRD cannot distinguish between O2− and F−, these were considered to be a single species in the refinement. The refinement was converged to RWP = 3.924%. Occupancy factors for anion sites were refined to 1.018(5) for the X site and to 0.48(1) for the X′ site. These factors indicate that an anion vacancy is present at the X′ site (see Table 1). In A2B2X6X′0.5 compounds, in which A is a polarizable cation such as Pb2+ or Bi3+, the oxygen vacancy ordering at the X′ site accompanied the Pb2+ or Bi3+ displacement, thereby lowering the symmetry from the space group Fd3m ̅ to F43̅ m (no. 216).40−42 ED patterns viewed along the [001] and [101] zone axes (Figure 2) show h00 reflections with h = 2n, which are forbidden in Fd3̅m and allowed in F4̅3m. This model was introduced next. Figure 1b shows the results of Rietveld refinement based on the F4̅3m space group. Reliability factors improved from 3.924 to 3.715% for RWP and from 4.955 to 2.816% for RI. Table 1 summarizes the refined crystallographic parameters for both space groups. Lowering the symmetry from Fd3̅m to F4̅3m divided the X and X′ sites into two distinct sites (these are denoted as X1, X2, X′1, and X′2 in the refinement) and permitted the displacement of cations along the [111] direction. The occupancy factors were refined to 0.90(10) for X′1 and 0.16(10) for X′2 sites, indicating vacancy order, in which O2− and vacancy occupy the X′1 and X′2 sites, respectively. Such vacancy order has been observed in other

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RESULTS AND DISCUSSION Pb2Ti2O6−δF2δ samples with various δ values were prepared. The single phase of the pyrochlore structure was obtained at δ = 0.6 (Pb2Ti2O5.4F1.2) as a pale-yellow sintered pellet; deviation from this composition resulted in the formation of other phases, such as Pb2Ti4O9F2. Pb2Ti2O5.4F1.2 was stable in air and transformed to the perovskite PbTiO3 by releasing F2 above 600 °C. No structural transition was observed by SXRD at temperatures of 100−500 K. Because the pyrochlore Bi2Ti2O7−δ is also present as a metastable phase,38,39 the Pb− Ti−O−F system behaves analogously to the Bi−Ti−O system. We first assumed the ideal pyrochlore structure with the Fd3̅m space group (no. 227) and refined the structural parameters. Figure 1a shows the SXRD pattern of 5555

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Table 1. Refined Occupancy Factors, Atomic Coordinates, And Atomic Displacement Parameters for Pb2Ti2O5.4F1.2 with the a Space Groups Fd3m ̅ and F43̅ m atom

site

g

Pb Ti X X′

16d 16c 48f 8b

1 1 1.018(5) 0.48(1)

Pb Ti X1 X2 X′1 X′2

16e 16e 24f 24g 4b 4d

1 1 1 1 0.90(10) 0.16(10)

x Fd3̅m, RWP = 3.924% , RI = 4.955% 0.625 0.125 0.4404(3) 0.5 F43̅ m, RWP = 3.715%,RI = 2.816% 0.6251(1) 0.1352(4) 0.1807(13) 0.4480(13) 0.5 0.25

y

z

B (Å2)

0.625 0.125 0.25 0.5

0.625 0.125 0.25 0.5

1.62(1) 1.33(3) 1 1

0.6251 0.1352 0 0.25 0.5 0.25

0.6251 0.1352 0 0.25 0.5 0.75

1.64(1) 0.24(11) 0.39(27) 0.71(29) 1 1

a = 10.3768(1) Å. Atomic displacement parameters for X′, X′1, and X′2 sites are fixed at 1. Refinement was performed using the scattering factor of O2− for the anion sites. The refined composition corresponds to Pb2Ti2(O/F)6.59(2) and Pb2Ti2(O/F)6.53(20) for Fd3̅m and F4̅3m, respectively. a

Figure 2. ED patterns of Pb2Ti2O5.4F1.2 viewed along the [001] and [101] zone axes.

Pb-containing pyrochlores, such as Pb2Ru2O6.5,43 Pb2Ir2O6.5,41 Pb2(Ga0.5Sb1.5)O6.5, and Pb2(Ni0.333Sb1.667)O6.5.42 Figure 3a shows the refined crystal structure of Pb2Ti2O5.4F1.2. Notably, the displacement of Pb2+ (0.02(2) Å) was almost negligible, whereas that of Ti4+ (0.18(1) Å) was pronounced. Influence of the lone-pair cations on the intraoctahedral distortion in d0 transition metal has been suggested;44 however, the corner-connected BX6 octahedra and X′A4 tetrahedral networks in pyrochlore structure are separated and weakly interact. Indeed, the displacement of the B cation in other Pb-containing F4̅3m pyrochlore oxides is almost negligible.41−43 Thus, the large displacement of Ti4+ is not owing to the lone-pair electrons on Pb2+. The displacements of the cations resulted in enhanced dielectric properties, as discussed later. Figure 3b shows the distorted Ti(O/F)6 octahedron, which consists of three long and three short Ti− O/F bonds. Table 2 summarizes the M−O/F bond lengths and the BVSs for the anion sites. According to Pauling’s second crystal rule, the BVS for each site indicates the charge on an anion or a cation.45 The BVSs for the X1 and X2 sites are substantially different. The X1 site exhibited a value plausible for F− rather than for O2−, suggesting fac-type O2−/F− anion order in the Ti(O/F)6 octahedron. As shown in Figure 3c, further investigation based on the determination of the electronic distribution by MEM demonstrated the difference between the Ti−X1 and Ti−X2 bonds. The stronger Ti−X2 bonds suggested the presence of O2− at the X2 site, confirming the fac-type O2−/F− anion order. Anion ordering in the Ti(O/ F)6 octahedron leads to the large displacement of Ti4+ along the [111] direction. Nevertheless, the composition does not

Figure 3. (a) Crystal structure of Pb2Ti2O5.4F1.2, as well as (b) coordination state and (c) electron density distribution of the Ti(O/ F)6 octahedron. The isosurface level is 3.6 e−/Å3.

Table 2. Selected M−(O/F) Bond Lengths and BVSs for the Anion Sitesa site X1 X2 X′1 X′2

bond Ti−X1 Pb−X1 Ti−X2 Pb−X2 Pb−X′1 Pb−X′2

×2 ×2 ×2 ×2 ×4 ×4

bond length (Å)

BVS (O2−)

BVS (F−)

2.039(10) 2.726(7) 1.893(7) 2.596(7) 2.248(1) 2.245(1)

1.47

1.09

2.16

1.61

2.77 2.79

2.22 2.24

BVSs for O2− and F− denote the values calculated from the respective parameters of the anions.

a

allow the formation of fully ordered TiO3F3 octahedra. Thus, the above results imply that F− is selectively located, albeit randomly distributed, at the X1 site. This result is in contrast to that observed with other oxyfluoride pyrochlores, such as 5556

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Chemistry of Materials Li2xCa1.5‑xM2O6F (M = Nb, Ta),46 LiLa0.67Ta2O6F, and Li1.25La0.58Nb2O6F,47 in which 19F solid-state NMR spectroscopy revealed that F− ion selectively occupies the X′ site in these oxyfluoride pyrochlores. The vacancy order in oxygen-deficient Pb-containing pyrochlores (F4̅3m) can be interpreted as the result of a structural distortion. The anions at X′1 and X′2 sites are coordinated with four nearest-neighbor Pb2+ ions and six nextnearest-neighbor anions at X1 or X2 sites and, thus, are located at the center of the cage structures. Figure 4 shows the cage

ratio changed from A2B2X6.5 to A2B2X6.6. To maintain the valences of Pb2+ and Ti4+, the chemical composition was fixed to Pb2Ti2O5.4F1.2. The O2−/F− anion order and vacancy order were considered to be associated with each other in the oxyfluoride pyrochlore Pb2Ti2O5.4F1.2. Our structural analysis revealed the large displacement of Ti4+ and the partial anion order in the Ti(O/F)6 octahedra. These structural characteristics can enhance the dielectric permittivity. Figure 5 shows the temperature dependence of the

Figure 4. Cage structures around the X′1 and X′2 sites in Pb2Ti2O5.4F1.2.

structures around the X′1 and X′2 sites. The difference in the stability of O2− ions located at these two cages is essential for the vacancy order. In Pb2B2O6.5 oxides, Pb2+ is displaced toward the [111] direction.40−42 This displacement enlarges the Pb4 tetrahedron in the Pb4X16 cage, while that in the Pb4X26 cage contracts. The enlargement in the X′1Pb4 tetrahedron relaxes the instability of O2− at the X′1 site. Thus, O2− ions selectively occupy the X′1 site, resulting in the O2−/vacancy order. However, the contraction of the vacancy-centered Pb4 tetrahedron in the Pb4X26 cage increases the Coulomb repulsion energy between Pb2+ ions. Another mechanism to induce vacancy order without increasing the Coulomb repulsion energy can be realized by the anion order in Pb2Ti2O5.4F1.2. The O2−/F− anion order, in which F− ions selectively occupy the X1 sites, was revealed by calculation of the BVSs and determination of the electron density distribution by the MEM. Substitution of O2− by F− can decrease the Coulomb repulsion force between corresponding anions. Table 3 summarizes the interatomic distances between the X1, X2,

Figure 5. Temperature dependence of the dielectric constant and dielectric loss of Pb2Ti2O5.4F1.2 at various frequencies.

dielectric constant and dielectric loss. A dielectric constant as high as 800 with a low dielectric loss below 0.10 was observed at 300 K. The former magnitude is much larger than those for other Bi-containing pyrochlores.30,31,33 The temperature coefficient was estimated to be 578(4) ppm/K at a frequency of 1 kHz, which is comparable to those of BaTaO2N (1140 ppm/K) and SrTaO2N (−730 ppm/K). The dielectric properties of the bulk region were evaluated by ac impedance analysis. Figure 6 shows the impedance spectra recorded at 300 K. The spectra feature two semicircles, which represent the bulk and grain-boundary contributions, respectively. For the nonlinear curve fitting of the spectra, a corresponding equivalentcircuit model was used. Here, each of two serial components was taken as a resistor and a constant-phase element (CPE) in parallel. Fitting yielded the resistivity and dielectric constants (Rbulk = 1.6 MΩ·cm, εbulk = 4180, Rgb = 8.2 MΩ·cm, and εgb = 2.83 × 105, where the subscripts “bulk” and “gb” denote the values for bulk and grain boundary, respectively). This result confirms that the observed high dielectric constant is an intrinsic property of Pb2Ti2O5.4F1.2.

Table 3. Selected Interatomic Distances between the Anions and Vacant Sites sites X1(F/O)−X′1 (occupied) X1(F/O)−X′2 (vacant) X2(O)−X′1 (occupied) X2(O)−X′2 (vacant)

bond length (Å) ×6 ×12 ×12 ×6

3.314(1) 3.739(10) 3.708(2) 3.134(14)

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occupied X′1, and vacant X′2 sites. To minimize the Coulomb repulsion force, O2− tended to avoid anions and stayed close to the vacancy; the opposite behavior was observed with F−. The interatomic distances between X1(F−/O2−) and X′1 sites were longer than those of X2(O2−) and X′2 by 6%, indicating reduced Coulomb repulsion force at the X′1 site. Thus, O2− ions selectively occupy the X′1 sites, and O2−/vacancy ordering can take place without the displacement of Pb2+. The absence of the shrinkage of Pb4X′2 tetrahedra can allow the partial occupation of O2− at the X′2 site, and thus the cation/anion

CONCLUSION The oxyfluoride pyrochlore Pb2Ti2O5.4F1.2 was prepared by the conventional solid-state reaction. Our structural study revealed the lowering of the symmetry from Fd3̅m to F4̅3m due to the vacancy order in the X′ site. The lowered symmetry allowed the displacement of the cations along the [111] direction. Ti4+ showed a large displacement, whereas that of Pb2+ was almost negligible. The BVSs obtained from the refined crystallographic 5557

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ACKNOWLEDGMENTS

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REFERENCES

We thank Prof. Mitsuru Itoh of the Laboratory for Materials and Structures, Tokyo Institute of Technology for help with the dielectric property measurement. This work was supported by JSPS KAKENHI Grant Nos. 26800180 and 16K05731 and the Collaborative Research of Laboratory for Structures and Materials, Tokyo Institute of Technology. The synchrotronradiation experiments were performed at SPring-8 with the approval of the Japan Synchrotron Radiation Research Institute (Grant Nos. 2015A1788 and 2015B1796).

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Figure 6. Impedance spectra of Pb2Ti2O5.4F1.2 obtained at 300 K in the frequency range of 10−2 to 107 Hz. The inset shows the equivalentcircuit model used for curve fitting.

parameters and the electron density distribution as determined by MEM suggest an O2−/F− anion order with fac-type coordination in the Ti(O/F)6 octahedron. F− was selectively located, albeit it was randomly distributed, at the X1 site. The O2−/F− anion order can stabilize O2− located at the X′1 site, leading to O2−/vacancy ordering. Such partial order can induce the enhancement of the dielectric properties. Dielectric measurements of Pb2Ti2O5.4F1.2 showed that it has a high dielectric constant and a low-temperature coefficient. The magnitude of the dielectric constant is much larger than those of other Bi-containing pyrochlores and comparable to those of BaTaO2N and SrTaO2N. These results provide evidence of partial anion order concurrent with vacancy order in pyrochlores, which enhance the dielectric properties. They indicate that mixing heteroanions in pyrochlores provides a basis for exploring novel dielectric materials.

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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.chemmater.6b02509. CIF file for Pb2Ti2O5.4F1.2(CIF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Author Contributions

K. Oka and K. Oh-ishi designed the research. K. Oka carried out the synthesis, structural analysis, and dielectric measurement of the sample and wrote the manuscript. H.H. and M.A. carried out the electron diffraction experiment. Notes

The authors declare no competing financial interest. 5558

DOI: 10.1021/acs.chemmater.6b02509 Chem. Mater. 2016, 28, 5554−5559

Article

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DOI: 10.1021/acs.chemmater.6b02509 Chem. Mater. 2016, 28, 5554−5559