Oxide Exchange in SrTiO3 by Introduction of Anion

Oct 25, 2017 - Promoted Hydride/Oxide Exchange in SrTiO3 by Introduction of Anion Vacancy via Aliovalent Cation Substitution. Fumitaka Takeiri† ... ...
3 downloads 14 Views 970KB Size
Download PDF
Article pubs.acs.org/IC

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

Promoted Hydride/Oxide Exchange in SrTiO3 by Introduction of Anion Vacancy via Aliovalent Cation Substitution Fumitaka Takeiri,† Kohei Aidzu,† Takeshi Yajima,‡ Toshiaki Matsui,† Takafumi Yamamoto,† Yoji Kobayashi,† James Hester,§ and Hiroshi Kageyama*,†,∥ †

Department of Energy and Hydrocarbon Chemistry, Graduate School of Engineering, Kyoto University, Nishikyo-ku, Kyoto 615-8510, Japan ‡ Institute for Solid State Physics, The University of Tokyo, Kashiwa, Chiba 277-8581, Japan § Australian Centre for Neutron Scattering, Australian Nuclear Science and Technology Organisation, Locked Bag 2001, Kirrawee DC, NSW 2232, Australia ∥ CREST, Japan Science and Technology Agency, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan S Supporting Information *

ABSTRACT: We investigated topochemical anion exchange reactions for a ScIIIsubstituted SrTiIVO3 perovskite, Sr(Ti1−yScy)O3−y/2□y/2 (y ≤ 0.1), using CaH2. It was found that the initial introduction of a small amount of anion vacancies (y/2) is crucial to enhance the anion (H−/O2−) exchangeability. For example, hydride reduction of Sr(Ti0.95Sc0.05)O2.975 yielded the oxyhydride SrTi0.95Sc0.05O2.56H0.41 in which the hydride concentration is increased by 33% with respect to pristine SrTiO3 (leading to SrTiO2.76H0.24). This observation highlights the importance of anion vacancies to improve anion (H−/O2−) diffusion, which is a well-known strategy for improving oxide anion conductivity, and suggests that such a vacancy-assisted reaction could be applied to other anion exchange reactions (e.g., F−/O2− and N3−/O2−) to extend the solubility range.

1. INTRODUCTION Titanium perovskite oxyhydrides ATiO3−xHx (A = Ba, Sr, Ca, Eu),1−3 which are prepared by topochemical reductions of the corresponding oxides with calcium hydride, have shown novel chemical and physical properties that are distinct from those of other transition metal oxyhydrides.4−8 For instance, metallic conductivity is achieved only in the titanium system, with a carrier concentration that can be widely tuned by the hydride content x.9,10 Metallic EuTiO3−xHx undergoes a ferromagnetic transition at 12 K, which is much higher than those of cationsubstituted oxides.3 Furthermore, hydride exchange reactions using titanium oxyhydrides as precursors have been developed; the labile nature of hydride anion allows access to various mixed-anion oxides. Oxynitrides ATiO3−xN2x/3 result when oxyhydrides ATiO3−xHx are heated under NH311 or even N212 gas at around 400 °C. To further improve these properties or explore new phenomena in titanium oxyhydride perovskites, it is important to precisely control the hydride concentration x and possibly extend its solubility range. However, the solubility range appears to vary depending on various factors. Among polycrystalline samples of ATiO3−xHx, the maximum hydrogen incorporation has been achieved for A = Ba, where the oxygen site can be substituted up to x = 0.6, while the substitution limits for other A look much smaller (x ≤ 0.3 for Sr and Eu and 0.1 for Ca).2,3 The reason could be related to the more electropositive nature of Ba, a factor which may stabilize H 1s © XXXX American Chemical Society

orbitals and increase the solubility range. Alternatively, the larger lattice for Ba may facilitate anion diffusion to assist the hydride/oxide exchange. The particle size of precursor oxides also significantly affects the hydride concentration: for particle sizes of 170 nm and 30 μm, x in BaTiO3−xHx reaches 0.6 and 0.19, respectively.1,2 The latter two factors underline kinetic aspects of the anion (H−/O2−) exchange, as pointed out in our study on (La,Sr)Ti(O,N)3.13 In the field of solid-state ionics, aliovalent cation substitution in oxides is generally used to create oxygen vacancies, which enhances oxide conductivity. Representative examples are Zr1−xYxO2−x/2 (YSZ)14−16 and La1−xSrxGa1−yMgyO3−δ.17−20 In this study, we used this strategy to improve the anion (H−/O2−) exchange property. A ScIIIsubstituted SrTiIVO3, Sr(Ti1−yScy)O3−y/2 (y ≤ 0.1), was employed as a precursor for CaH2 reduction. We observed that the introduction of only a small amount of anion vacancies significantly improves the anion exchangeability and successfully expands the solubility range.

2. EXPERIMENTAL DETAILS Sample Preparation. The precursor oxides, Sr(Ti1−yScy)O3−y/2 (y = 0, 0.02, 0.05, 0.1), were obtained by the polymerized complex method (citric acid method). Ti(OiPr)4 (97%, Sigma-Aldrich) and Received: July 20, 2017

A

DOI: 10.1021/acs.inorgchem.7b01845 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

Sc3+ (vs 0.605 Å for Ti4+).22 It should be noted that neither diffraction patterns nor lattice parameters of Sr(Ti1−yScy)O3−y/2 have been presented previously.23,24 The Sc/Ti ratios estimated by EDX analysis (Figure S2) support the successful preparation of the solid solutions. TEM images of these oxides (Figure S2) show primary particles (10−50 nm in diameter) that aggregated into granular larger particles that were ca. 1 μm in diameter for all of the compositions. This enabled us to discuss the y dependence of the reactivity of these samples with CaH2 without an effect of the particle size. All of the PXRD patterns of samples after the reaction with CaH2 at 580 °C for 4 days can also be indexed by the cubic perovskite structure, but with a slight shift of peaks toward lower angles (Figure S1), indicating the reduction of titanium ions, as is also the case with ATiO3−xHx (A = Ba, Sr, Ca, Eu).1−3 A small amount of TiH2 (2−5 wt % as determined later) was observed in each pattern. As shown in Figure 1a, the lattice parameters after the reduction are clearly expanded. Like their precursors, the reduced samples show a linear evolution of the lattice constant with increasing y, but the slope Δa/Δy is obviously steeper. In Figure 1b, we show the volume expansion relative to the precursor, Vafter/Vbefore, which can be regarded as a measure of the degree of reduction, as a function of y. Compared to y = 0, Vafter/Vbefore increases by 7%, 33%, and 56% for y = 0.02, 0.05, and 0.1, respectively, indicating that the reducibility is surprisingly enhanced with y. As previously reported,2 the CaH2 reduction of SrTiO3 results in SrTiO3−xHx with negligible anion vacancies. As expected, we observed a significant amount of hydrogen from the QMS measurement of y = 0 (Figure 2). A Rietveld

anhydrous citric acid were dissolved in methanol, followed by addition of ethylene glycol under continuous stirring. Then Sr(NO3)2 (99.9%, Rare Metallic) and Sc2O3 (99.9%, Kojundo Chemical) were added according to the target stoichiometry. After nitric acid was added to dissolve Sc2O3, the mixture was stirred at 150 °C until it became transparent, and upon continuous heating, the colorless solution became a deep brown gel. The gel was carbonized at 350 °C in a mantle heater to afford a black solid mass, and finally it was calcined at 600 °C in air for 24 h. For comparison, we prepared samples with a larger particle size using standard high-temperature solid-state reactions. Stoichiometric mixtures of SrCO3 (99.99%, Rare Metallic), TiO2 (anatase, 99.99%, Rare Metallic), and Sc2O3 were pelletized and preheated in air for 12 h at 900 °C. Subsequently, the pellets were ground, repelletized, and heated for 24 h at 1350 °C twice. Then the obtained precursor oxides were mixed with a 3 molar excess of CaH2 (99%, Sigma-Aldrich) and pelletized in a nitrogen-filled glovebox (H2O, O2 < 0.1 ppm). The pellets were sealed in an evacuated Pyrex tube with a residual pressure of less than 4 × 10−2 Pa and reacted at 480−580 °C for 1−5 days. The residual CaH2 and the CaO byproduct were removed by washing with an NH4Cl/methanol solution. Characterization. Powder X-ray diffraction (PXRD) patterns of precursors and reduced compounds were collected at room temperature on a Bruker D8 Advance diffractometer equipped with a Cu Kα source. The synchrotron PXRD experiments were performed at room temperature using a Debye−Scherrer camera installed at beamline BL02B2 (JASRI, SPring-8) with an imaging plate as a detector. The wavelengths of X-rays employed were λ = 0.41964, 0.42004, and 0.77470 Å. The samples, inserted in a glass capillary tube with an inner diameter of 0.1 mm, were rotated during measurements to avoid the effect of preferential orientation. The powder neutron diffraction (PND) experiment on the reduced sample of y = 0.05 was carried out at room temperature on the Wombat diffractometer (λ = 1.594 Å) at the Australian Nuclear Science and Technology Organisation (ANSTO). The synchrotron PXRD and PND data were refined by the Rietveld method using the RIETAN-FP program.21 Energydispersive X-ray (EDX) spectra were collected with an Oxford Inca Xact detector mounted on a Hitachi S-3400N scanning electron microscope. Transmission electron microscopy (TEM) images were recorded on a JEOL JEM-1400 transmission electron microscope. Thermogravimetric analysis (TGA) was conducted under flowing O2 gas (300 mL/min) with a Bruker TG-DTA2000 analyzer. Hydrogen release was monitored by a Bruker MS9610 quadrupole mass spectrometer (QMS) under Ar gas flowing at a rate of 300 mL/min.

3. RESULTS AND DISCUSSION The laboratory PXRD patterns of the precursor oxides, Sr(Ti1−yScy)O3−y/2 (y = 0, 0.02, 0.05, 0.1), show the singlephase formation of the cubic perovskite structure (Figure S1). All of the specimens are white, suggesting that the oxidation state of Ti is +4. As shown in Figure 1a (blue circles), the lattice constant increases linearly as a function of Sc content y. This trend is reasonable given the larger ionic radius of 0.745 Å for

Figure 2. H2 gas evolution from Sr(Ti1−yScy)O3−y/2□y/2 (y = 0, 0.02, 0.05, 0.1) during heating under Ar flowing at 300 mL/min.

Figure 1. (a) Lattice parameters of Sr(Ti1−yScy)O3−y/2□y/2 (y = 0, 0.02, 0.05, 0.1) before (blue) and after (red) CaH2 reduction at 580 °C for 4 days. (b) Volume expansions Vafter/Vbefore (solid symbols, left axis) and hydrogen contents (open symbols, right axis) of the reduced oxyhydrides.

refinement of PXRD data assuming an oxygen-deficient cubic structure SrTiO3−x gave an oxygen content of 2.76(1) (Figure 3a and Table S1), which is fairly consistent with the TGA result (Figure S3). Thus, we concluded the reduced specimen to be SrTiO2.76H0.24. The greater hydride content compared with the previous study2 is likely a result of the difference in particle size (1 μm in this study vs 5−30 μm in the previous one; Figure S4), while it is smaller than the value of 0.45 measured for an epitaxial thin film with a thickness of 100 nm.10 The QMS measurements for Sc-substituted specimens after reduction also revealed a significant amount of hydrogen release at around 400 °C, as shown in Figure 2. We performed a Rietveld refinement on synchrotron X-ray diffraction data for y = 0.05 using the ideal perovskite structure (Pm3̅m), where Ti B

DOI: 10.1021/acs.inorgchem.7b01845 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

Figure 3. Observed and refined synchrotron PXRD patterns for the reduced samples of Sr(Ti1−yScy)O3−y/2□y/2 (y = 0, 0.02, 0.05, 0.1). Red crosses, green solid lines, and blue solid lines represent observed, calculated, and difference intensities, respectively. The upper and lower green ticks represent the positions of the calculated Bragg reflections of the target phases and TiH2, respectively. A minor unidentified peak at around 2θ = 13° in (a) and 7° in (b−d) was removed from the data and fitting.

and Sc atoms were placed randomly at Wyckoff position 1a according to the nominal composition and Sr and O atoms were placed at 1b and 3c, respectively. TiH2 was included as a secondary phase. The refinement converged reasonably (Figure 3c), with agreement indices of Rp = 1.19%, Rwp = 1.83%, and RB = 4.73%, and yielded an oxygen content of 2.56(2). The amount of TiH2 was estimated as 2 wt %. The TGA data for the same specimen (Figure 4) were analyzed with the contribution

Figure 5. Observed and refined powder neutron diffraction patterns for the reduced sample SrTi0.95Sc0.05O2.56H0.41(2) (reduced sample of y = 0.05). Red crosses, the green solid line, and the blue solid line represent observed, calculated, and difference intensities, respectively. The upper and lower green ticks represent the positions of the calculated Bragg reflections of the product and TiH2 (secondary phase), respectively.

Figure 4. TGA results for the reduced sample with y = 0.05 under O2 gas flowing at 300 mL/min.

despite the low level of Sc substitution (5%) or initial anion vacancy (0.83%). Since the neutron refinement assuming the stoichiometric (i.e., fixed) anion composition of Sr(Ti0.95Sc0.05)(O2.56H0.44) gave similar reliability factors of Rp = 1.16%, Rwp = 1.52%, and RB = 1.00%, we are not able to conclude whether the anion vacancy of 0.83% initially present in the y = 0.05 precursor remained after hydride reduction. Thus, for the sake of simplicity, let us hereafter assume that the anion vacancy is NOT present in these reduced oxyhydride phases. For y = 0.02

of TiH2 (to be oxidized to TiO2) included. This yielded an oxygen content of 2.60, which agrees well with the X-ray refinement. A neutron refinement was conducted to mainly determine the hydride concentration (Figure 5), where the oxygen content was fixed at 2.56 (i.e., SrTi0.95Sc0.05O2.56Hx). This resulted in SrTi0.95Sc0.05O2.56H0.41(2) with Rp = 1.15%, Rwp = 1.52%, and RB = 0.99%. The refined parameters are summarized in Table 1. It is remarkable that this hydride content is much larger than that of the Sc-free (y = 0) sample, C

DOI: 10.1021/acs.inorgchem.7b01845 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry Table 1. Refined Neutron (and Synchrotron PXRD) Structural Parameters of Sr(Ti0.95Sc0.05)(O2.56(2)H0.41(2)□0.03)a

a b

atom

site

x

y

z

gb

Biso (Å2)

Sr Ti/Sc O H

1a 1b 3c 3c

0 0.5 0 0

0 0.5 0.5 0.5

0 0.5 0.5 0.5

1 0.95/0.05 0.854d (0.854(6)) 0.137(5)

0.92(4) (1.08(2)) 0.27(4) (0.47(3))c 1d (1.5(1)) 1d

Space group Pm3m ̅ (No. 221), a = 3.9267(1) Å (3.92688(9) Å). Numbers without errors in parentheses were fixed during the final refinement. Site occupancies. cLinear constraint: Biso(Ti) = Biso(Sc). dFixed in ND refinement.

and 0.1, we performed synchrotron X-ray refinements using the anion-vacancy-free model and obtained Sr(Ti0.98Sc0.02 )O2.68(1)H0.32(1) and Sr(Ti0.9Sc0.1)O2.38(2)H0.62(2) (Figure 3b,d and Table S1). As shown in Figure 1b (right axis), the estimated hydrogen content increases in proportion with y, demonstrating the improved hydride/oxide exchangeability resulting from the introduction of anion vacancies in the precursor oxide. It is noted that the doped scandium cation itself would not play a key role to promote the exchange reaction since the hydrogen content is much larger than the scandium content y. It would be possible that the initial anion vacancy around the Sc3+ center persistently stays, as its valence is unvaried; then the vacancy amount (y/2) difference will have a large influence on the anion (O2−/H−) diffusion and thus the final hydride content. The hydride/oxide exchangeability for the 10% Scsubstituted SrTiO3 with the initial anion vacancy of 1.67% is comparable to that of BaTiO3 (which leads to BaTiO2.4H0.6).1 This observation could mean from the thermodynamic point of view that ATiO3 can in principle be hydridized up to ATiO2.4H0.6 irrespective of the alkaline-earth metal (A = Ba, Sr, Ca), with a titanium valence of +3.4. We have previously discussed that during the hydride reduction of ATiO3 at ∼580 °C, a tiny yet finite anion vacancy is created, allowing O2− and H− anion to hop from one site to other neighboring sites.1 The lower anion exchangeability in pristine SrTiO3 and CaTiO3 means that the smaller lattices of these compounds compared with BaTiO3 substantially restrict the anion (O2−, H−) diffusion. In such a case, the initial presence of anion vacancies due to ScIII-for-TiIV substitution could be crucial in facilitating anion diffusion, giving access to the composition expected from the thermodynamic consideration. These results are broadly coherent with the thin film result,10 where the kinetic aspect is less important than the bulk: the epitaxial SrTiO3 film can be hydridized up to SrTiO2.55H0.45, which is much larger than the corresponding bulk (SrTiO2.76H0.24),10 while for BaTiO3 there is nearly no difference between the thin film (BaTiO2.42H0.58) and bulk (BaTiO2.4H0.6).1,10 To gain further insight into the effect of the anion vacancy on the hydride/oxide exchangeability, we investigated the time dependence of the CaH2 reaction at several different temperatures. For example, SrTiO3 (y = 0) and Sr(Ti0.9Sc0.1)O2.95 (y = 0.1) were reacted with hydride at 580 °C for 1−5 days. Here we anticipated that the cell expansion ratio Δ[V(t)/V(0)]/Δt (corresponding to the anion exchange ratio) for y = 0.1 would become larger than that for y = 0. However, it was unexpectedly observed that the cell expansion ratio at the initial stage of the reaction (t ≤ 1 day) was almost independent of y, as shown in Figure 6, indicating that the anion exchangeability is more or less the same for samples with a low hydride density. An obvious difference appears in the middle stage (t ≥ 2 days), where the cell constant for y = 0.1 increases more prominently before showing a tendency to saturate. The apparent activation

Figure 6. Time dependence of the cell expansion of Sr(Ti1−yScy)O3−y/2□y/2 (y = 0, 0.1) during CaH2 reaction at 580 °C.

energy was estimated using a radial diffusion model based on Fick’s law25,26 and the Avrami−Erofe’ev model,27,28 and we obtained 1.8(2) and 1.11(6) eV for y = 0 and 2.1(1) and 1.32(1) eV for y = 0.1 (Figure S5). These values are within the range of activation energies for oxygen vacancy migration in SrTiO3 (0.3−2.1 eV).29 We note that the apparent activation energy in our case should be dependent on the hydride content, as recently shown in BaTiO3−xHx, where the activation energy decreases as a function of x.30 Hence, the observed values shown here are only rough estimates. The observation that the reactivity difference appears only at the middle stage is interpreted by the difference of the (electro)chemical potential E(Ti4+/Ti3+) in the samples with respect to CaH2, though the (electro-)chemical potential differences of hydride and vacancies may also be at play. At the initial stage of reaction (i.e., a small hydride concentration), the relative difference in the potential would be large enough to enable fast H−/O2− exchange irrespective of Sc substitution. As the exchange reaction proceeds (the samples become reduced), however, the difference in potential becomes smaller, which makes the kinetic aspects, such as the presence of anion vacancies, more significant, resulting in a better exchange property for the Sc-substituted case. Although our experiments cannot provide clear evidence for anion deficiency during the reduction, these considerations suggest that the initial amount of vacancies created by Sc−Ti substitution (y/2) remains, at least partially, and this plays a crucial role in promoting the anion exchange. Supposing that all the anion vacancies remain, we can rewrite the final composition as Sr(Ti1−yScy)(O3−y/2−xHx). The bond valence sum (BVS) calculation31,32 for the reduced sample of y = 0.1 yielded a titanium valence of +3.46, which agrees well with the value of +3.42 calculated from the composition. Together with the study on BaTiO3,1 the present study shows that as far as thermodynamic factors are considered, ATiO3 is reducible with the titanium valence down to +3.4. In view of the very low oxidation state of +1.7 in LaSrCoO3H0.7 (Co1.7+),4 the “normal” titanium valence appears peculiar. Further investigations, including theoretical calculaD

DOI: 10.1021/acs.inorgchem.7b01845 Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

■

tions, are necessary in order to clarify why the reduction stopped at Ti3.4+. We performed CaH2 reduction of Sr(Ti1−yScy)O3−y/2□y/2 (y = 0, 0.02, 0.05) with a larger particle size of >10 μm prepared by high-temperature solid-state reactions. We also observed that the cell expansion ratio Δ[V(t)/V(0)]/Δt increases in proportion to y, as shown in Figure S6. However, the value of Δ[V(t)/V(0)]/Δt is much smaller than that shown in Figure 1b, and XRD peaks of the reduced specimens showed an asymmetric broadening, indicating a gradual distribution of hydride concentration in a grain from a more reduced region near the surface to a less reduced region at the grain core, as observed in BaTiO3−xHx with a particle size of 20−30 μm.2 The exchangeability will be suppressed in the grain interior with a greater distance from the surface (or CaH2) even if anion vacancies are present. It should be noted that the particles used in this work are too large to expect a nanosize effect, implying that anion diffusion in the bulk mainly affects the exchange reaction. In the course of the study on a topochemical synthesis of oxynitrides, we observed a remarkable difference in the reactivities of EuTiO3 and EuTiO2.82H0.18, yielding EuTiO2.25N0.75 and EuTiO2N, respectively.33 We proposed that in the latter case, an in situ-created vacancy (in EuTiO2.82N0.12□0.06) during a low-temperature ammonolysis reaction promotes the anion exchange of N3−/O2−. Together with our previous study, the present study suggests that the creation of anion vacancies is important in general to enhance anion exchangeability in a variety of mixed-anion systems.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Hiroshi Kageyama: 0000-0002-3911-9864 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was supported by CREST (JPMJCR1421), JSPS KAKENHI (JP16H02267), and Grants-in-Aid for Scientific Research on Innovative Areas (JP16H06439, JP16H06440, and 16K21724). The synchrotron XRD experiments were performed at beamline BL02B2 of SPring-8 with the approval of JASRI (2013B1117, 2014B1360, and 2016A1050). F.T. was supported by a JSPS Grant for Young Scientists.

■

REFERENCES

(1) Kobayashi, Y.; Hernandez, O. J.; Sakaguchi, T.; Yajima, T.; Roisnel, T.; Tsujimoto, Y.; Morita, M.; Noda, Y.; Mogami, Y.; Kitada, A.; Ohkura, M.; Hosokawa, S.; Li, Z.; Hayashi, K.; Kusano, Y.; Kim, J.; Tsuji, N.; Fujiwara, A.; Matsushita, Y.; Yoshimura, K.; Takegoshi, K.; Inoue, M.; Takano, M.; Kageyama, H. An Oxyhydride of BaTiO3 Exhibiting Hydride Exchange and Electronic Conductivity. Nat. Mater. 2012, 11, 507−511. (2) Sakaguchi, T.; Kobayashi, Y.; Yajima, T.; Ohkura, M.; Tassel, C.; Takeiri, F.; Mitsuoka, S.; Ohkubo, H.; Yamamoto, T.; Kim, J.; Tsuji, N.; Fujihara, A.; Matsushita, Y.; Hester, J.; Avdeev, M.; Ohoyama, K.; Kageyama, H. Oxyhydrides of (Ca,Sr,Ba)TiO3 Perovskite Solid Solutions. Inorg. Chem. 2012, 51, 11371−11376. (3) Yamamoto, T.; Yoshii, R.; Bouilly, G.; Kobayashi, Y.; Fujita, K.; Kususe, Y.; Matsushita, Y.; Tanaka, K.; Kageyama, H. An Antiferro-toFerromagnetic Transition in EuTiO3−xHx Induced by Hydride Substitution. Inorg. Chem. 2015, 54, 1501−1507. (4) Hayward, M. A.; Cussen, E. J.; Claridge, J. B.; Bieringer, M.; Rosseinsky, M. J.; Kiely, C. J.; Blundell, S. J.; Marshall, I. M.; Pratt, F. L. The Hydride Anion in an Extended Transition Metal Oxide Array: LaSrCoO3H0.7. Science 2002, 295, 1882−1884. (5) Helps, R. M.; Rees, N. H.; Hayward, M. A. Sr3Co2O4.33H0.84: An Extended Transition Metal Oxide-Hydride. Inorg. Chem. 2010, 49, 11062−11068. (6) Denis Romero, F.; Leach, A.; Möller, J. S.; Foronda, F.; Blundell, S. J.; Hayward, M. A. Strontium vanadium oxide−hydrides: “squareplanar” two-electron phases. Angew. Chem., Int. Ed. 2014, 53, 7556− 7559. (7) Bang, J.; Matsuishi, S.; Hiraka, H.; Fujisaki, F.; Otomo, T.; Maki, S.; Yamaura, J.; Kumai, R.; Murakami, Y.; Hosono, H. Hydrogen Ordering and New Polymorph of Layered Perovskite Oxyhydrides: Sr2VO4−xHx. J. Am. Chem. Soc. 2014, 136, 7221−7224. (8) Tassel, C.; Goto, Y.; Kuno, Y.; Hester, J.; Green, M.; Kobayashi, Y.; Kageyama, H. Direct Synthesis of Chromium Perovskite Oxyhydride with a High Magnetic-Transition Temperature. Angew. Chem., Int. Ed. 2014, 53, 10377−10380. (9) Yajima, T.; Kitada, A.; Kobayashi, Y.; Sakaguchi, T.; Bouilly, G.; Kasahara, S.; Terashima, T.; Takano, M.; Kageyama, H. Epitaxial Thin Films of ATiO3−xHx (A = Ba, Sr, Ca) with Metallic Conductivity. J. Am. Chem. Soc. 2012, 134, 8782−8785. (10) Bouilly, G.; Yajima, T.; Terashima, T.; Yoshimune, W.; Nakano, K.; Tassel, C.; Kususe, Y.; Fujita, K.; Tanaka, K.; Yamamoto, T.; Kobayashi, Y.; Kageyama, H. Electrical Properties of Epitaxial Thin Films of Oxyhydrides ATiO3−xHx (A = Ba and Sr). Chem. Mater. 2015, 27, 6354−6359. (11) Yajima, T.; Takeiri, F.; Aidzu, K.; Akamatsu, H.; Fujita, K.; Yoshimune, W.; Ohkura, M.; Lei, S.; Gopalan, V.; Tanaka, K.; Brown, C. M.; Green, M. A.; Yamamoto, T.; Kobayashi, Y.; Kageyama, H. A

4. CONCLUSION We have studied the CaH2 reactivity of titanium perovskite oxides Sr(TiIV1−yScIIIy)O3−y/2□y/2 (y ≤ 0.1) with anion vacancies introduced by the aliovalent cation substitution. It is observed that a small amount of anion vacancies significantly improves the anion (H−/O2−) exchangeability, with an 33% increase in the hydride content for y = 0.05 in comparison with y = 0 (SrTiO3). The creation of anion vacancies by aliovalent cation substitution, a well-established strategy in solid-state ionics to improve oxide anion conductivity, could also be useful to improve the anion exchangeability and widen the solubility range in oxyhydride materials and more generally in mixedanion compounds such as oxynitrides and oxyfluorides. For example, a F−/O2− exchange rate in layered perovskite compounds (e.g., RbLaNb2O7) using poly(tetrafluoroethylene)34 might be improved by an aliovalent cation substitution (e.g., RbLaNbV2−yTiIVyO7−y/2).

■

Article

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b01845. Supporting figures and tables (PDF) Accession Codes

CCDC 1563512 contains 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 e-mailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, U.K.; fax: +44 1223 336033. E

DOI: 10.1021/acs.inorgchem.7b01845 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry Labile Hydride Strategy for the Synthesis of Heavily Nitridized BaTiO3. Nat. Chem. 2015, 7, 1017−1023. (12) Masuda, N.; Kobayashi, Y.; Hernandez, O.; Bataille, T.; Paofai, S.; Suzuki, H.; Ritter, C.; Ichijo, N.; Noda, Y.; Takegoshi, K.; Tassel, C.; Yamamoto, T.; Kageyama, H. Hydride in BaTiO2.5H0.5: A Labile Ligand in Solid State Chemistry. J. Am. Chem. Soc. 2015, 137, 15315− 15321. (13) Takeiri, F.; Yajima, T.; Yamamoto, T.; Kobayashi, Y.; Matsui, T.; Hester, J.; Kageyama, H. Suppression of H−/O2− Exchange by Incorporated Nitride Anions in the Perovskite Lattice. J. Solid State Chem. 2017, 256, 33−37. (14) Badwal, S. Zirconia-based Solid Electrolytes: Microstructure, Stability and Ionic Conductivity. Solid State Ionics 1992, 52, 23−32. (15) Goff, J.; Hayes, W.; Hull, S.; Hutchings, M.; Clausen, K. N. Defect Structure of Yttria-Stabilized Zirconia and its Influence on the Ionic Conductivity at Elevated Temperatures. Phys. Rev. B: Condens. Matter Mater. Phys. 1999, 59, 14202. (16) Goodenough, J. B. Oxide-Ion Electrolytes. Annu. Rev. Mater. Res. 2003, 33, 91−128. (17) Ishihara, T.; Matsuda, H.; Takita, Y. Doped LaGaO3 Perovskite Type Oxide as a New Oxide Ionic Conductor. J. Am. Chem. Soc. 1994, 116, 3801−3803. (18) Huang, P.-n.; Petric, A. Superior Oxygen Ion Conductivity of Lanthanum Gallate Doped with Strontium and Magnesium. J. Electrochem. Soc. 1996, 143, 1644−1648. (19) Ishihara, T.; Kilner, J. A.; Honda, M.; Takita, Y. Oxygen Surface Exchange and Diffusion in the New Perovskite Oxide Ion Conductor LaGaO3. J. Am. Chem. Soc. 1997, 119, 2747−2748. (20) Khan, M.; Islam, M.; Bates, D. Dopant Substitution and Ion Migration in the LaGaO3-based oxygen ion conductor. J. Phys. Chem. B 1998, 102, 3099−3104. (21) Izumi, F.; Momma, K. Three-Dimensional Visualization in Powder Diffraction. Solid State Phenom. 2007, 130, 15−20. (22) Shannon, R. D. Revised Effective Ionic Radii and Systematic Studies of Interatomic Distances in Halides and Chalcogenides. Acta Crystallogr., Sect. A: Cryst. Phys., Diffr., Theor. Gen. Crystallogr. 1976, 32, 751−767. (23) Higuchi, T.; Tsukamoto, T.; Sata, N.; Ishigame, M.; Kobayashi, K.; Yamaguchi, S.; Ishiwata, Y.; Yokoya, T.; Fujisawa, M.; Shin, S. Hole-State and Defect Structure of Proton Conductor SrTiO 3 Observed by High-Resolution X-ray Absorption Spectroscopy. Solid State Ionics 2000, 136-137, 261−264. (24) Higuchi, T.; Tsukamoto, T.; Sata, N.; Ishigame, M.; Kobayashi, K.; Yamaguchi, S.; Shin, S. Electronic Structures of Protonic Conductors SrTiO3 and SrCeO3 by O 1s X-ray absorption spectroscopy. Solid State Ionics 2002, 154-155, 735−739. (25) Crank, J. The Mathematics of Diffusion; Oxford University Press: Oxford, U.K., 1979. (26) Jordá, J. L.; Rey, F.; Sastre, G.; Valencia, S.; Palomino, M.; Corma, A.; Segura, A.; Errandonea, D.; Lacomba, R.; Manjón, F. J.; Gomis, Ó .; Kleppe, A. K.; Jephcoat, A. P.; Amboage, M.; RodríguezVelamazán, J. A. Synthesis of a Novel Zeolite through a PressureInduced Reconstructive Phase Transition Process. Angew. Chem. 2013, 125, 10652−10656. (27) Avrami, M. Kinetics of phase change. I. General theory. J. Chem. Phys. 1939, 7, 1103−1112. (28) Erofe’ev, B. Generalized Equation of Chemical Kinetics and its Application in Reactions Involving Solids. C. R. Acad. Sci. USSR 1946, 52, 511−514. (29) De Souza, R. A.; Metlenko, V.; Park, D.; Weirich, T. E. Behavior of oxygen vacancies in single-crystal SrTiO3: equilibrium distribution and diffusion kinetics. Phys. Rev. B: Condens. Matter Mater. Phys. 2012, 85, 174109. (30) Tang, Y.; Kobayashi, Y.; Shitara, K.; Konishi, A.; Kuwabara, A.; Nakashima, T.; Tassel, C.; Yamamoto, T.; Kageyama, H. On Hydride Diffusion in Transition Metal Perovskite Oxyhydrides Investigated via Deuterium Exchange. Chem. Mater. 2017, 29, 8187−8194.

(31) Brown, I. D.; Altermatt, D. Bond-Valence Parameters Obtained from a Systematic Analysis of the Inorganic Crystal Structure Database. Acta Crystallogr., Sect. B: Struct. Sci. 1985, 41, 244−247. (32) Brese, N. E.; O’Keeffe, M. Bond-Valence Parameters for Solids. Acta Crystallogr., Sect. B: Struct. Sci. 1991, 47, 192−197. (33) Mikita, R.; Aharen, T.; Yamamoto, T.; Takeiri, F.; Ya, T.; Yoshimune, W.; Fujita, K.; Yoshida, S.; Tanaka, K.; Batuk, D.; Abakumov, A. M.; Brown, C. M.; Kobayashi, Y.; Kageyama, H. Topochemical Nitridation with Anion Vacancy-Assisted N3−/O2− Exchange. J. Am. Chem. Soc. 2016, 138, 3211−3217. (34) Kobayashi, Y.; Tian, M.; Eguchi, M.; Mallouk, T. E. IonExchangeable, Electronically Conducting Layered Perovskite Oxyfluorides. J. Am. Chem. Soc. 2009, 131, 9849−9855.

F

DOI: 10.1021/acs.inorgchem.7b01845 Inorg. Chem. XXXX, XXX, XXX−XXX