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

Hydrothermal Synthesis of Pyrochlore-Type Pentavalent Bismuthates Ca2Bi2O7 and Sr2Bi2O7 Md Saiduzzaman,† Takahiro Takei,† Sayaka Yanagida,† Nobuhiro Kumada,*,† Hena Das,‡,§ Hirokazu Kyokane,‡ Shogo Wakazaki,‡ Masaki Azuma,‡ Chikako Moriyoshi,⊥ and Yoshihiro Kuroiwa⊥ †

Center for Crystal Science and Technology, University of Yamanashi, 7-32 Miyamae-cho, Kofu 400-8511, Japan Laboratory for Materials and Structures, Tokyo Institute of Technology, 4259 Nagatsuta, Midori, Yokohama 226-8503, Japan § World Research Hub Initiative, Institute of Innovative Research, Tokyo Institute of Technology, 4259 Nagatsuta, Midori, Yokohama 226-8503, Japan ⊥ Department of Physical Science, Hiroshima University, 1-3-1 Kagamiyama, Higashi-Hiroshima, Hiroshima 739-8526, Japan

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‡

S Supporting Information *

pyrochlores. For the first time, we succeeded in synthesizing two new Bi5+-based pyrochlores by hydrothermal reactions, where only Bi5+ occupies the B site. In the previous research37 of hydrothermal reactions between hydrate sodium bismuthate with alkaline-earth-metal nitrates (Ca, Sr), mixed bismuth valence pyrochlore-type oxides with a CO3 group contained in their crystal structures were produced. However, alkaline-earthmetal hydroxides (Ca, Sr) yield pyrochlore-type oxides with only Bi5+, which is confirmed by the temperature-dependent gas evolution. In this work, we report the synthetic procedure of two new metallic pyrochlore-type pentavalent bismuthates. Their crystal structure refinement was performed by the Reitveld refinement method using synchrotron powder X-ray diffraction (SPXRD) data. Their thermal stabilities were checked by thermogravimetric and differential thermal analysis (TG−DTA). Their temperature-dependent gas evolution was measured by thermogravimetric mass spectrometry (TG−MS). Scanning electron microscopy (SEM) reveals the surface morphologies of particles. UV−vis spectroscopy gives an idea of the absorption spectra. The density of states (DOS) curves generated by density functional theory (DFT) calculations show the contribution of each orbital of all of the elements. The temperature-dependent electrical resistivity and magnetic susceptibility were measured by a standard four-probe method (PPMS, Quantum Design) and SQUID magnetometer, respectively. The PXRD patterns of the hydrothermally synthesized Ca2Bi2O7 and Sr2Bi2O7 samples can be indexed as cubic pyrochlore-type oxides (space group Fd3̅m), as shown in Figure S1. The XRD pattern of Sr2Bi2O7 shifted to a lower angle compared to Ca2Bi2O7 because of the larger ionic radius of Sr2+. The very weak peaks for Sr2Bi2O7 at 25.24° and 25.81° are from traces of Sr(OH)2·8H2O, as shown in Figure S2. Timedependent XRD patterns indicated that the pyrochlore phase started to grow from 4 h, and a single phase was obtained at 24 h for both compounds Ca2Bi2O7 and Sr2Bi2O7, as shown in Figures S3 and S2, respectively. SEM observation indicated that

ABSTRACT: The pyrochlore-type Ca 2 Bi 2 O 7 and Sr2Bi2O7 have been synthesized from a low-temperature hydrothermal route using NaBiO3·nH2O as a starting material. The crystal structures of these compounds were refined using synchrotron powder X-ray diffraction data. The cell parameters were found to be a = 10.75021 (5) Å and 10.94132 (6) Å for Ca2Bi2O7 and Sr2Bi2O7, respectively. Density functional theory calculations showed the metallic band structure, but the negligible mixing of O2 2p bands with the A-site alkaline-earth-metal states and weak overlap with the conduction bands result in the semiconducting behavior.

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he low-temperature hydrothermal method is an attractive synthetic route for the preparation of new inorganic compounds with various crystal structures.1−10 Basically, pyrochlore-type oxides (A2B2O6O′) get much attention because of the broad range of elements adapted in their crystal structures. The A-site cations are 8-fold distorted-cubic-coordinated with two different oxygen sites (O and O′), and the B-site cations are 6-fold octahedral-coordinated with oxygen (O).11 Because of the wide range of element adaptations, pyrochlore-type oxide structures exhibit versatile physical and chemical properties, allowing them a wide range of applications such as nuclear waste disposal,12 thermal barrier coatings,13 dielectric materials,14 laser materials,15 lithium-ion batteries,16 optical materials,17 photocatalysts,18 solid electrolytes,19 oxygen evolution,20 and superconductors.21 Ruthenium- and iridium-based pyrochlore oxides get much attention by researchers because of their metallic behavior.22−25 Bouchard and Gillson26 synthesized Bi3+-based Bi2Ir2O7 and Bi2Ru2O7 metallic pyrochlores for the first time. After that, many researchers investigated these metallic compounds27−34 and tried to synthesize new metallic pyrochlore-type compounds M3+2Ir2O7 and M3+2Ru2O7 (M3+ = Pr, Nd, Sm, Eu, Lu)35,36 by replacing Bi3+ with other M3+ elements. Bi3+ is generally adopted in the A site but unable to be located in the B site because of its large ionic radius for the octahedral metal position. In this case, Bi5+ could be a good choice for the B site. Until recently, however, only Bi5+ has not been reported in the B site for © XXXX American Chemical Society

Received: December 25, 2018

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DOI: 10.1021/acs.inorgchem.8b03596 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

Inorganic Chemistry

represent O1 (48f), and red balls represent O2 (8b). The Bi−O bond lengths in the BiO6 octahedra are 2.082 (13) and 2.09 (14) Å for Ca2Bi2O7 and Sr2Bi2O7, respectively. These values are in agreement with other pentavalent bismuthates (2.08−2.116 Å).38−41 The distances of Ca−O (2.56 Å) and Sr−O (2.65 Å) are somewhat slightly larger than the antimony-based cubic pyrochlores Ca2Sb2O7 (Ca−O = 2.49 Å)42 and Sr1.36Sb2O7 (Sr− O = 2.61 Å).43 Figure S6 displays the local environments of the Ca, Sr, and Bi atoms. The Ca and Sr atoms occupy the 16d site with 8-fold distorted-cubic coordination containing six O1 (48f) and two O2 (8b) atoms. Meanwhile, the Bi atom occupies the 16c site with 6-fold octahedral coordination surrounded by only six O1 (48f). The TG curves (Figure S7) of both pyrochlore-type compounds indicated that these compounds are not thermally stable above 300 °C, which is a behavior similar to that of other pentavalent bismuthates38−41 because of the instability of Bi5+ at high temperature. The DTA curves indicated that the mass loss is mainly related to the endothermic reactions. This mass loss is caused by oxygen evolution, as confirmed by TG−MS measurements (Figure S8), which corresponded to the reduction of Bi5+ to Bi3+. The mass losses for both pyrochloretype compounds are 5.53% (Ca2Bi2O7) and 4.25% (Sr2Bi2O7). Those values are close to the calculated values of 5.25% (Ca2Bi2O7) and 4.54% (Sr2Bi2O7). After the TG−DTA experiment (600 °C), Ca2Bi2O7 (deep brown) becomes Ca2Bi2O544 (light yellow) and Sr2Bi2O7 (dark green) becomes Sr2Bi2O545 (deep yellow), as shown by the XRD patterns in Figure S9. The optical absorption spectra for both pyrochlores indicated that they can absorb visible region wavelengths, as shown in Figure S10. The band-gap energies for both pyrochlore-type compounds are small, and the values are 1.09 and 1.07 eV for Ca2Bi2O7 and Sr2Bi2O7, respectively, as shown in Figure S11. In order to obtain an accurate description of the electronic structures for Ca2Bi2O7 and Sr2Bi2O7, we performed DFT calculations using the Heyd−Scuseria−Ernzerhof (HSE06) hybrid functional.46 The calculated DOSs are shown in Figure 3. The qualitative nature of the electronic structure is not significantly different compared to that of the generalized gradient approximation (GGA) DOS (Figure S12). According to the ionic model, a Bi atom is expected to donate five electrons to O, forming 6s and 6p conduction bands. However, in these systems, the Bi 6s states hybridize strongly with the O1 2p states, forming valence bands in the energy window from ∼−12 to −8 eV and conduction bands on top. The contribution of the Bi 6s states is higher for the valence bands compared to the conduction bands, which indicates stabilization of the hole on O. Similar phenomena were observed in various post-transitionmetal base oxides, such as the Bi3+/Bi5+-ordered BiNiO3 under pressure47 and the β-PbO2 structure.48 The O1 2p states also hybridize with the Bi 6p states and form valence bands in the energy range from ∼−7 to −2 eV. The 2p bands crossing the Fermi level are purely contributed by the O2 atoms that are locally coordinated by four nearest-neighbor A-site cations. The systems show metallic band structures. However, the O2 2p bands show negligible mixing with the A-site alkaline-earthmetal states (bandwidth ∼1.5 eV) and weak overlap with the conduction bands (see the insets of Figures 3a,b and S13), which is a probable cause of the relatively high resistivity compared to a conventional metal and the semiconducting temperature dependence of the resistivity of Ca2Bi2O7 and Sr2Bi2O7, as shown in Figure 4.

the Sr2Bi2O7 particles are larger in size than the Ca2Bi2O7 particles, as shown in Figure S4. Parts a and b of Figure 1 display the Rietveld refinement patterns of Ca2Bi2O7 and Sr2Bi2O7, respectively, obtained from

Figure 1. Rietveld refinement patterns from the SPXRD data for Ca2Bi2O7 (a) and Sr2Bi2O7 (b). The markers and solid lines are for the experimental and calculated profiles, respectively. In the middle portion, the short vertical lines denote the positions of possible Bragg reflections.

the SPXRD data. The lattice parameters of Ca2Bi2O7 and Sr2Bi2O7 derived from crystal structure refinement of the SPXRD data were a = 10.75021 (5) Å and 10.94132 (6) Å, respectively. Both pyrochlore-type compounds fall on the linear relationship for the unit-cell volume against the entire range of the sum of the cationic ionic radii with A2+2B5+2O7 and A3+2B4+2O7 pyrochlore-type compounds, as shown in Figure 2.

Figure 2. Unit-cell volume versus the sum of the ionic radii of the A and B cations for A2+2B5+2O7 and A3+2B4+2O7 pyrochlore-type structures.

The final R factors in the Rietveld analysis of these compounds led to reasonable values of Rwp = 5.53% and Rp = 4.28% (Ca2Bi2O7) and Rwp = 6.40% and Rp = 4.84% (Sr2Bi2O7). The crystal data and structural parameters are summarized in Tables S1/S2 and S3/S4 for Ca2Bi2O7 and Sr2Bi2O7, respectively. Figure S5 shows the unit cells of Ca2Bi2O7 and Sr2Bi2O7 with ash and light-blue BiO6 octahedra, respectively. Small black balls B

DOI: 10.1021/acs.inorgchem.8b03596 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

space group Fd3̅m. The crystal structures of both compounds were refined using the SPXRD data. Theoretical data indicated that the metallic behavior for both pyrochlore compounds and their electrical and magnetic behavior exhibited paramagnetic semiconduting properties. These compounds showed that a decrease of the concentration of phenol under dark conditions, unlike the other pentavalent bismuthates (NaBiO3 and BaBi2O6), which have high photocatalytic activity under visible-light irradiation40,41 and a decrease of the phenol concentration, is probably caused not by photocatalytic activity but adsorption. Also, they are not reactive for soft chemical reactions such as lithium insertion.

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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.8b03596. XRD and time-dependent XRD patterns, crystal and structural parameter data, bond valence sum, SEM, TG− DTA and TG−MS curves, XRD after TG−DTA,UV−vis absorption spectra, Tauc plot, GGA DOS, calculated electron localization function, and temperature-dependent magnetic susceptibility curves (PDF) Accession Codes

CCDC 1840517 and 1840535 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 3. Calculated DOSs for Ca2Bi2O7 (a) and Sr2Bi2O7 (b) using the HSE06 functional.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +81-55-220-8615. Fax: +81-55-220-8270. ORCID

Md Saiduzzaman: 0000-0001-8003-9914 Takahiro Takei: 0000-0002-5624-2899 Sayaka Yanagida: 0000-0002-4719-5023 Nobuhiro Kumada: 0000-0002-0402-5809 Masaki Azuma: 0000-0002-8378-321X

Figure 4. Temperature dependence of the resistivity of Ca2Bi2O7 (a) and Sr2Bi2O7 (b) between 1.8 and 300 K.

Author Contributions

The electrical resistivity shows 0.1 and 0.03 Ω·m for highly pressed samples of Ca2Bi2O7 and Sr2Bi2O7, respectively, at room temperature, as shown in Figure 4. The lower resistivity of strontium bismuthate compared to that of calcium bismuthate is consistent with the tendency toward delocalization of the O2 2p bands for the bigger cations (see the insets of Figure 3a,b). Temperature-dependent magnetic susceptibility curves (Figure S14) of both compounds at zero-field cooling had very small negative values, which could be attributed to the core diamagnetism, in agreement with the absence of magnetic ions. The small upturn at low temperature is due to the unavoidable magnetic impurity. No transitions are observed for both compounds over the entire temperature range from 2 to 400 K. In summary, two new pyrochlore-type oxides with Bi5+ (Ca2Bi2O7 and Sr2Bi2O7) have been prepared by using a lowtemperature hydrothermal method. XRD confirms that these compounds are the cubic pyrochlore-type structure with the

N.K. conducted this research work in collaboration with M.A., C.M., and Y.K. M.S. performed the XRD experiments. C.M. and Y.K. performed the SPXRD experiments. M.S. performed the structural refinements with help from N.K. TG−DTA, TG−MS, SEM, UV−vis, impedance analysis, electrical resistivity, magnetic susceptibility, and oxidizing activity experiments were performed by M.S., S.W., S.Y., T.T., and N.K. H.D., H.K. and M.A. performed the DOS calculations. All authors discussed the results; M.S. wrote the manuscript with comments from the coauthors. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The experiments at SPring-8 were performed with the approval of the Japan Synchrotron Radiation Research Institute (Proposal 2017B1343). C

DOI: 10.1021/acs.inorgchem.8b03596 Inorg. Chem. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.inorgchem.8b03596 Inorg. Chem. XXXX, XXX, XXX−XXX