Hydride Reduction of BaTiO3 − Oxyhydride Versus O Vacancy

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Article Cite This: ACS Omega 2018, 3, 11426−11438

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Hydride Reduction of BaTiO3 − Oxyhydride Versus O Vacancy Formation Reji Nedumkandathil,† Aleksander Jaworski,† Jekabs Grins,† Diana Bernin,‡ Maths Karlsson,‡ Carin Eklöf-Ö sterberg,‡ Alexandra Neagu,† Cheuk-Wai Tai,† Andrew J. Pell,† and Ulrich Häussermann*,† †

Department of Materials and Environmental Chemistry, Stockholm University, SE-10691 Stockholm, Sweden Department of Chemistry and Chemical Engineering, Chalmers University of Technology, SE-41296 Gothenburg, Sweden

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‡

S Supporting Information *

ABSTRACT: We investigated the hydride reduction of tetragonal BaTiO3 using the metal hydrides CaH2, NaH, MgH2, NaBH4, and NaAlH4. The reactions employed molar BaTiO3/H ratios of up to 1.8 and temperatures near 600 °C. The air-stable reduced products were characterized by powder X-ray diffraction (PXRD), transmission electron microscopy, thermogravimetric analysis (TGA), and 1H magic angle spinning (MAS) NMR spectroscopy. PXRD showed the formation of cubic productsindicative of the formation of BaTiO3−xHxexcept for NaH. Lattice parameters were in a range between 4.005 Å (for NaBH4-reduced samples) and 4.033 Å (for MgH2-reduced samples). With increasing H/ BaTiO3 ratio, CaH2-, NaAlH4-, and MgH2-reduced samples were afforded as two-phase mixtures. TGA in air flow showed significant weight increases of up to 3.5% for reduced BaTiO3, suggesting that metal hydride reduction yielded oxyhydrides BaTiO3−xHx with x values larger than 0.5. 1H MAS NMR spectroscopy, however, revealed rather low concentrations of H and thus a simultaneous presence of O vacancies in reduced BaTiO3. It has to be concluded that hydride reduction of BaTiO3 yields complex disordered materials BaTiO3−xHy□(x−y) with x up to 0.6 and y in a range 0.04−0.25, rather than homogeneous solid solutions BaTiO3−xHx. Resonances of (hydridic) H substituting O in the cubic perovskite structure appear in the −2 to −60 ppm spectral region. The large range of negative chemical shifts and breadth of the signals signifies metallic conductivity and structural disorder in BaTiO3−xHy□(x−y). Sintering of BaTiO3−xHy□(x−y) in a gaseous H2 atmosphere resulted in more ordered materials, as indicated by considerably sharper 1H resonances.

1. INTRODUCTION

It then came as a surprise when in 2012 Kobayashi et al. reported that the reaction of CaH2 with the archetypical perovskite BaTiO3 affords BaTiO3−xHx with large amounts (x < 0.6) of hydrogen incorporated.10 The cubic perovskite oxyhydride BaTiO3−xHx is remarkable in several respects. It represents a defect-/vacancy-free solid solution of O2− and H− ions, which commonly form the octahedral environment around Ti that is now in a mixed IV/III oxidation state (Figure 1a). BaTiO3−xHx is stable in air and water. Further, it is stable at elevated temperatures of up to approximately 400 °C above which hydrogen is released. When present, oxygen is scavenged and BaTiO3 is retained. In inert gas atmospheres containing D2, a hydride exchange H/D occurs at hydrogen release temperatures. Also, oxynitrides BaTiO3−δNy may be prepared by heating BaTiO3−xHx under N2 flow at 400−600

Hydride reduction has developed into a versatile method for modifying transition metal oxides, yielding highly reduced products with unusual coordination environments and exotic electronic and magnetic properties.1−4 Especially fruitful precursors are oxides that relate to perovskite and Ruddlesden−Popper phases. Specific prominent examples are the LaNiO3 to LaNiO2, SrFeO3 to SrFeO2, and YBaCo2O5 to YBaCo2O4.5 reductions.5−7 In a typical hydride reduction, a ternary transition metal oxide is reacted with CaH2, NaH, or LiH at relatively mild, low temperature (“chimie douce”) conditions (150−600 °C). The reduction is accompanied with O removal (i.e., O vacancy formation), and frequently, there is a strict topotactic relationship between the original oxide and its reduced form. In rare cases, transition metal reduction and hydride ion insertion occur simultaneously, leading to oxyhydrides.8 This scenario is thought to be highly unusual because of the incompatibility of O2− and strongly reducing H− in forming a common anion substructure.9 © 2018 American Chemical Society

Received: June 18, 2018 Accepted: August 29, 2018 Published: September 19, 2018 11426

DOI: 10.1021/acsomega.8b01368 ACS Omega 2018, 3, 11426−11438

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Figure 1. (a) Structure of BaTiO2.75H0.25 represented as the 2 × 2 × 2 supercell of cubic BaTiO3 in which two O atoms were randomly replaced by two H atoms. The right-hand side shows the local coordination of H in BaTiO3−xHx. The Ti is in a mixed IV/III oxidation state. (b) Anion-ordered tetragonal structure of SrVO2H.15 The alkaline earth metal, O, and H atoms are depicted as gray, red, and green spheres, respectively.

°C.11 These observations led to the conclusion that the hydride species in BaTiO3−xHx is labile and that the material represents a versatile precursor toward new mixed-anion compounds.12 Sakaguchi et al.13 and Yamamoto et al.14 showed that perovskite oxyhydrides are also obtained with CaTiO3, SrTiO3, and EuTiO3 although with smaller hydride contents x ( 0.02.36,37 The hexagonal form can maintain a maximum x of about 0.15.38 Characteristic for all forms O-deficient BaTiO3−x is their blue/black color. Cubic BaTiO3−x is synthesized at temperatures 900−1250 °C in a flowing mixture of 5% H2/ 95% N2. Reduction at higher temperatures results in the hexagonal form.37 Mixtures of tetragonal and cubic BaTiO3−x are also obtained when sintering BaTiO3 in the reducing environment of an SPS apparatus at 1200 °C,39,40 and the hexagonal form can be synthesized when heating tetragonal BaTiO3 in a graphite-vacuum furnace to 1300−1500 °C.38 Against this background, the existence of cubic phases BaTiO3−xHy□(x−y) with x values exceeding 0.5 is surprising. Clearly, the incorporation of H plays an important role in stabilizing these highly O-deficient variants of BaTiO3, which in contrast to previously reported forms are accessible at comparatively low temperatures (∼600 °C). The two-phase behavior observed for most reductions may correlate with a segregation of BaTiO3−xHy□(x−y) into a vacancy and hydriderich composition. Interestingly, our defect samples show a similar reactivity toward the formation of mixed-anion derivatives as that reported for BaTiO2.5H0.5.12 For example, BaTiO3−xHy□(x−y) can be easily converted into green-colored nitridized BaTiO3 (BaTiO3−δNy) under a stream of N2 gas at temperatures above 450 °C.

Table 5. Compositions of 1.2-H Samples from Hydride Reductions during 2 Days at 600 °C sample

xH from TGa

x□ from TGb

y from NMR

formula BaTiO3−xHyx−y

CaH2 MgH2 NaAlH4 NaBH4

0.24 0.41 0.39 0.36

0.23 0.39 0.37 0.34

0.16 0.14 0.15 0.04

BaTiO3.76H0.16□0.08 BaTiO3.60H0.14□0.26 BaTiO3.62H0.15□0.23 BaTiO3.65H0.04□0.31

xH refers to a reaction BaTiO3−xHx + 0.75xO2 → BaTiO3 + 0.5xH2O. bx□ refers to a reaction BaTiO3−x + 0.5xO2 → BaTiO3.

a

during their synthesis of BaTiO3−xHxfor unknown reasonsa pressurized H2 atmosphere. Following this conjecture, we subjected the CaH2-, MgH2-, and NaAlH4-reduced samples to pressurized H2 atmospheres (30−50 bar) at 600 °C, which is the temperature applied in the preceding hydride reduction. The results are summarized in Table S1 (Supporting Information) and Figure 13. Interestingly, during annealing

4. CONCLUSIONS Following previous reports on the reaction of tetragonal BaTiO 3 with CaH 2 yielding cubic oxyhydrides BaTiO3−xHx,10,13 we investigated hydride reduction of BaTiO3 with various metal hydrides as reducing agents. At the applied temperature of 600 °C, we find that NaH acts only as a weak reducing agent. A conversion into a cubic product is not obtained. This is attributed to the low decomposition temperature of NaH, instead of making H− gaseous H2 the reducing species. Apart from the originally employed CaH2, also MgH2 and the ternary hydrides NaBH4 and NaAlH4 reduce BaTiO3 to a blue-/black-colored cubic product. However, at variance with the original reports, we find that only a small concentration of H replaces O in the anionic substructure of BaTiO3. Instead, highly O-deficient disordered cubic phases BaTiO3−xHy□(x−y) with x up to 0.6 and y in a range 0.04−0.25 are obtained. The existence of such phases is surprising, and the incorporation of H supposedly plays an important role in stabilizing these highly O-deficient variants of BaTiO3. These defective samples show high reactivity toward the formation of mixed-anion derivatives, such as oxynitrides. Finally, we emphasize the important role of NMR spectroscopy for characterizing both H species and H contents in reduced BaTiO3 samples. The negative shift of hydridic H is attributed to the metallic nature of the phases BaTiO3−xHy□(x−y).

Figure 13. (a) 1H MAS NMR spectra of posthydrogenated samples. (b) Comparison of relative strengths and shift contributions to the 1H MAS NMR signal integral for regular and posthydrogenated 1.2-H samples.

under pressurized H2, the two-phase samples essentially became single phase, with a lattice parameter closely corresponding to the previous majority phase (Table S1, cf. Table 2). The 1H NMR spectra (Figure 13) revealed that the annealing procedure did not lead to an increased hydride content. However, the signal narrowed significantly, which indicates that more ordered phases BaTiO3−xHy□(x−y) were obtained (cf. Figure 11a and Supporting Information, part V, where spectra of post- and prehydrogenated samples are directly compared). Apparent again is a correlation of the size of the lattice parameter and the chemical shift of the 1H resonance: the larger the parameter, the more negative the shift is. The TGA weight increases are by about 20% lower after the annealing procedure. This coincides with a lower H content of the posthydrogenated samples. The diminished hydridic H content may be due to a slight oxidation of the sample from traces of oxygen in the autoclave setup. 11436

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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/acsomega.8b01368.

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Evolution of the lattice parameters during heating and cooling of 1.2 H MgH2-reduced BaTiO3 in air; PXRD patterns of products obtained from the hydride reduction of BaTiO3 at 600 °C during 2 day experiments using NaH, MgH2, NaAlH4, and NaBH4; 1H MAS NMR spectra of products obtained from reduction with 1.2 H CaH2 at 600 °C during 1, 2, 4, and 7 day experiments; characteristics of products from hydrogenating various reduced samples at 30−50 bar for 24 h at 600 °C; refinement details and Rietveld plots for the products from NaH, CaH2, CaD2, MgH2, NaAlH4, and NaBH4 reductions; details on the study of the effect of different washing agents (0.1 M NH4Cl/MeOH, 0.1 M HAc, 0.1 M HCl) on a 1.2-H-CaH2 (600 °C, 2 day synthesis) reference sample; details of the oxidation study of samples 1.2-H-CaH2 and 1.2-H-MgH2 in air by in situ PXRD; and details on the analysis of products from annealing in a pressurized (30−40 bar) H2 atmosphere for 24 h at 600 °C (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (U.H.). ORCID

Diana Bernin: 0000-0002-9611-2263 Maths Karlsson: 0000-0002-2914-6332 Cheuk-Wai Tai: 0000-0001-7286-1211 Andrew J. Pell: 0000-0002-2542-8113 Ulrich Häussermann: 0000-0003-2001-4410 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Swedish Research Council (VR) through grants #2014-03980 and #2016-03441. REFERENCES

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