Article pubs.acs.org/cm
Electrical Properties of Epitaxial Thin Films of Oxyhydrides ATiO3−xHx (A = Ba and Sr) Guillaume Bouilly,† Takeshi Yajima,†,‡ Takahito Terashima,§ Wataru Yoshimune,† Kousuke Nakano,† Cédric Tassel,†,⊥ Yoshiro Kususe,∥ Koji Fujita,∥ Katsuhisa Tanaka,∥ Takafumi Yamamoto,† Yoji Kobayashi,† and Hiroshi Kageyama*,†,¶ †
Department of Energy and Hydrocarbon Chemistry, Graduate School of Engineering, Kyoto University, Kyoto 615-8510, Japan Institute for Solid State Physics, The University of Tokyo, Chiba 277-8581, Japan § Research Center for Low Temperature and Materials Sciences, Kyoto University, Sakyo-ku, Kyoto 606-8501, Japan ∥ Department of Material Chemistry, Graduate School of Engineering, Kyoto University, Kyoto 615-8510, Japan ⊥ The Hakubi Center for Advanced Research, Kyoto University, Yoshida-Ushinomiya-cho, Sakyo-ku, Kyoto 606-8302, Japan ¶ CREST, Japan Science and Technology Agency (JST), Kawaguchi, Saitama 332-0012, Japan ‡
ABSTRACT: We have studied electronic properties of perovskite oxyhydrides ATiO3−xHx (A = Ba, Sr). Epitaxial thin films of ATiO3−xHx with various hydride compositions, up to x = 0.58 for Ba and x = 0.45 for Sr, are prepared by the low-temperature CaH2 reduction of the corresponding oxide films deposited on (LaAlO3)0.3(SrAl0.5Ta0.5O3)0.7 (LSAT) substrates by pulsed laser deposition. Resistivity measurements for A = Sr show a metallic phase over a wide range of H− composition, implying a substantial stabilization of H 1s orbitals that should be distributed over O 2p orbitals. On the other hand, for A = Ba, a semiconducting behavior is seen up to ∼5− 8% of H− substitution. Interestingly, a similar contrasting behavior is observed in a Nb-substituted BaTiO3 and SrTiO3, which suggests that a local cation−off centering in lightly doped Ba films creates in-gap states in the band structure (as opposed to the Sr films), hindering the electron transport.
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INTRODUCTION Alkaline earth metal titanates ATiO3 (A = Ba, Sr, Ca) with the perovskite structure are materials of fundamental and technological importance because of a broad range of their novel chemical and physical properties. These properties are strongly dependent on the A-site cation (as well as the B-site cation). For example, BaTiO3 adopts a tetragonal structure at room temperature with a coherent displacement of the d0 Ti4+ cations along the c axis, relative to a cubic structure above 395 K, which accounts for ferroelectricity and piezoelectricity of this material.1,2 On the other hand, SrTiO3 stays cubic owing to nearly ideal size matching, but a marginal displacement of Ti4+ centers occurs below 105 K, and a giant photoconductivity related to a quantum paraelectricity of this phase has been reported.3,4 Although the stoichiometric A2+Ti4+O3 is a band insulator with an energy gap of ∼3.20 eV, tuning chemical composition allows injection of carriers (electrons) to the conduction band composed mainly of Ti 3d t2g orbitals. Aliovalent cation substitutions have been extensively examined, for example, using trivalent cations (e.g., La3+) at the A site5 or pentavalent cations (e.g., Nb5+ and Sb5+) at the Ti site.6,7 Oxygen deficient perovskite ATiO3−δ has been also studied.8−10 Although a © XXXX American Chemical Society
metallic phase is commonly achieved both in Ba and Sr systems by varying chemical compositions, qualitative and quantitative differences are observed between the two systems. For example, the concentration of dopant required to obtain metallic conductivity is significantly lower for SrTiO3 (n ∼ 1019 cm−3)11,12 than for BaTiO3 (n ∼ 1021 cm−3).7,10 In addition, superconductivity is observed in the rare-earth-substituted SrTiO3 and SrTiO3−δ with n close to 1020 cm−3,13,14 whereas it is absent for carrier doped BaTiO3.15 An interesting observation is coexistence of ferroelectricity and metallic conductivity in BaTiO3−δ,16,17 which is not fully understood yet, however. Therefore, revealing factors to differentiate the transport properties between Ba and Sr compounds is essential. An alternative and highly efficient approach to inject carriers in titanium perovskite oxides has been recently suggested, that is, an aliovalent anion substitution by hydride. A perovskite oxyhydride ATiO3−xHx (A = Ba, Sr, Ca) was topochemically synthesized in a powder form by a low temperature reaction with calcium hydride using the corresponding oxide precurReceived: June 21, 2015 Revised: September 1, 2015
A
DOI: 10.1021/acs.chemmater.5b02374 Chem. Mater. XXXX, XXX, XXX−XXX
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Chemistry of Materials sors.18,19 By varying synthetic conditions, the hydride (H−) concentration was widely controlled, up to 20% for A = Ba. We have subsequently succeeded in fabricating epitaxial oxyhydride thin films by the CaH2 reduction of ATiO3 films epitaxially deposited on (LaAlO3)0.3(SrAl0.5Ta0.5O3)0.7 (LSAT) substrate, and observed metallic behavior, irrespective of A.20 In this previous study, however, the hydride reduction of ATiO3−xHx films (A = Ba, Sr, Ca) was tested in a single reaction condition (530 °C, 24 h) and the anion composition was determined only for the Sr compound (SrTiO2.75H0.25) by secondary ion mass spectroscopy (SIMS) experiments because of the lack of the standard for Ba and Ca films. As mentioned above, the influence of the A-site cation on electrical conductivity in carrier doped ATiO3 materials remains unexplored. A systematic investigation of electronic properties in ATiO3−xHx as a function of the anion composition will provide a possibility to reveal the effect of the H− substitution and compare with the previously reported cation substitution. In this study, we report the evolution of the electrical conductivity against the carrier concentration in ATiO3−xHx (A = Sr, Ba) thin films. We found a remarkable difference between the Sr and Ba oxyhydride films: in SrTiO3−xHx a metallic state is stabilized in the x region examined, whereas in BaTiO3−xHx doping a semiconducting state is seen in a lightly doped region (up to x ≈ 0.20). A similar tendency has been seen in cationsubstituted ATiO3 materials (e.g., Nb5+).15,21 The observed difference in the physical properties between the Ba and Sr oxyhydride films is discussed in terms of differences in the local structure.
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Table 1. Reduction Conditions, Chemical Compositions, Hydride Concentration, and Carrier Concentration at Room Temperature for ATiO3−xHx Thin Films precursor
treatment
SrTiO3
450 °C, 2h 530 °C, 2h 530 °C, 1 day 530 °C, 5 days 450 °C, 2hours 530 °C, 2h 530 °C, 1 day 530 °C, 5 days
BaTiO3
chemical composition
H− concentration (cm−3)
carrier concentration (cm−3)
SrTiO2.95H0.05
8.20 × 1020
2.03 × 1019
SrTiO2.80H0.20
3.28 × 1021
1.85 × 1021
SrTiO2.75H0.25
4.10 × 1021
4.10 × 1021
SrTiO2.55H0.45
7.38 × 1021
2.28 × 1021
BaTiO2.86H0.14
2.29 × 1021
3.67 × 1017
BaTiO2.76H0.24
3.94 × 1021
2.10 × 1021
BaTiO2.65H0.35
5.76× 1021
4.90 × 1021
BaTiO2.42H0.58
9.54 × 1021
4.18 × 1021
The electric resistivity along the in-plane direction was measured by a four-probe method using a Quantum Design PPMS setup. Au (30 nm)/Ti (5 nm) electrodes were deposited by electron beam deposition.
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RESULTS AND DISCUSSIONS Out-of-plane XRD patterns indicated that the c axis oriented films of BaTiO3 and SrTiO3 were epitaxially grown on LSAT, as previously reported.20 Three diffractions peaks, 001, 002, and 003, from ATiO3 as well as those from the LSAT substrate are observed, as seen in Figure 1a and b. These peaks gave the outof-plane lattice constants to be 4.049(4) Å for BaTiO3 and 3.927(3) Å for SrTiO3. Rocking curves taken around the 002 reflection revealed that full widths at half-maximum (fwhm) are less than 0.01° for both BaTiO3 and SrTiO3, indicating the high quality of the films. The X-ray reciprocal space mapping around the 103 reflection of LSAT, conducted for the as-deposited films, is shown in Figure 1c. It is seen that the in-plane lattice parameter of SrTiO3 (d100 = 3.86(1) Å) matches well with the substrate one, indicating that the SrTiO3 film is fully strained. On the other hand, the in-plane lattice parameter of BaTiO3 (d100 = 3.98(4) Å) deviates significantly and is close to the a axis for the bulk system.25 This observation shows that the BaTiO3 film is nearly relaxed. Both as-deposited films exhibited sheet resistances higher than 2 × 103 MΩ at RT, assuring negligibly small amount of oxygen vacancies in the precursor oxide films. The CaH2 reduction of the transparent ATiO3 films is accompanied by a color change of the surface, reflecting a partial change in the oxidation state of Ti from +4 to +3. The absence of Bragg reflections from impurity phases, as shown in Figures 1a and b indicate no decomposition upon hydride reduction within the detection limit of the present instrumentation. The fwhm values obtained by the 002 rocking curve experiments for the films with different synthetic conditions are nearly invariant, from which one can exclude (substantial) deterioration of the film quality during the treatment. We observed that the hydride density in the oxyhydride films is controllable by varying synthetic conditions, namely, reaction temperature and time. Table 1 shows the amount of desorbed hydrogen species estimated from the TDS experiments. The
EXPERIMENTAL PROCEDURE
Polycrystalline targets were prepared using ATiO3 (A = Ba, Sr) powders (Rare Metallics, 99.99%), pelletized and sintered at 1200 °C for 48 h. Epitaxial thin films with a thickness of 100 nm were obtained by pulsed laser deposition (PLD) using a KrF excimer laser pulse (λ = 248 nm) with an energy density of 0.7 J/cm2. The substrate temperature and oxygen partial pressure in the chamber were kept at 750 °C and 5.0 × 10−2 Pa. As a substrate, we used a (001) oriented single-crystal (LaAlO3)0.3(SrAl0.5Ta0.5O3)0.7 (LSAT). LSAT was employed because of inertness against CaH2 treatment; although SrTiO3 (STO) and KTaO3 (KTO) have a smaller lattice mismatch than LSAT (∼3.6% with respect to BaTiO3), they are reduced by the CaH2 treatment and become metallic, which hampers acquiring intrinsic transport properties of the oxyhydride films.19,20 The sheet resistance of LSAT after the hydride reduction in the conditions used in this study was above the limit of our experimental setup (i.e., 2 × 103 MΩ at room temperature (RT)), indicating a negligible contribution of the substrate to the electrical properties. The as-deposited films were embedded with 0.25 g of CaH2 powder in an evacuated Pyrex tube, sealed under vacuum (530 °C) resulted in the formation of TiH2, indicating a (partial) decomposition of the film, whereas those at lower temperatures ( 2 K). The resistivity for the x = 0.05 sample exhibits a crossover to C
DOI: 10.1021/acs.chemmater.5b02374 Chem. Mater. XXXX, XXX, XXX−XXX
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Figure 3. Electrical properties as a function of the hydride concentration in ATiO3−xHx (A = Ba, Sr) thin films on LSAT substrate. The dashed line represents the transition between insulating state and metallic state as a function of the A-site composition. Data of Sr1−yBayTi1−xNbxO3 from the study by Page et al.21 are shown for comparison.
order Jahn−Teller (SOJT) distortion in the d0 perovskite oxides against electron doping (i.e., 12.5% of Ti3+ ions) is responsible for the localization of doped electrons in the system. Namely, local distortions in BaTi0.875Nb0.125O3 create polarons and induce in-gap states between the valence band and the conduction band, resulting in a semiconducting behavior.21,28 On the other hand, such local distortion is absent for SrTi0.875Nb0.125O3, and doped electrons are filled in the conduction band derived from the Ti 3d t2g orbitals to achieve metallic conductivity. Given the resemblance of the electronic properties between anion- and cation-substituted ATiO3 systems, we speculate that there is a local Ti disorder by SOJT effect in the lightly doped region of BaTiO3−xHx, whereas such distortion is absent for SrTiO3−xHx. If the above speculation is correct, then the H 1s orbitals in ATiO3−xHx should be stabilized substantially. The stability and the nature of the hydrogen atom in the lattice of ATiO3 have been investigated by the first-principles theoretical calculations. It is demonstrated that H− is the most stable when hydrogen is positioned at the anionic site, and the H 1s orbitals are distributed over O2 2p bands.29,30 The stabilization of H− in ATiO3 is ascribed to long-range interactions (e.g., Coulomb interactions in an extended lattice)31,32 and covalent bonding with titanium ions. Because of the orthogonality between the H 1s orbitals and the partially filled Ti 3d t2g orbitals, the H 1s orbitals are capable to form covalent bonding with unoccupied 3d eg orbitals, as also the case with Sr2VO4−xHx, SrCrO2H, SrVO2H, and La2Ti2As2H3.33−36 The maximum hydride density of the epitaxial films obtained in this study is 9.54 × 1021 cm−3 for Ba (x = 0.58) and 7.38 × 1021 cm−3 for Sr (x = 0.45). The large hydride concentration accessible for SrTiO3−xHx, compared to previous results obtained with bulk (x ≤ 0.12),19 indicates the importance of the particle size regarding the anion exchangeability in the ATiO3−xHx systems. We may enhance the solubility limit by adjusting parameters such as thickness to further improve the low temperature reactivity or substrate to induce a strain to the film. Oxyhydrides with higher hydride density might offer new perspectives to observe exotic physical properties that include a Mott insulating phase as reported in Sr1−xLaxTiO3 solid solution (x ≥ 0.95).5
temperature dependence of the resistivity for BaTiO2.86H0.14 (and other oxyhydride films) provides evidence for the absence of a structural phase transition involving coherent displacements of Ti centers. Therefore, BaTiO2.86H0.14 should be cubic from RT to 2 K, contrary to the low-doped region of oxidedeficient BaTiO3−x with several abrupt jumps in the resistivity, which has been attributed to the persistence of ferroelectricity.8,16 Most remarkably, the electronic phase diagram of our oxyhydride thin films qualitatively resembles with that of the Ti-site substituted perovskite oxides, ATi1−xNbxO3 perovskites (A = Ba, Sr).15,27 Although SrTi1−xNbxO3 displays metallic behavior for x ≥ 0.01, BaTi1−xNbxO3 remains semiconducting for x ≤ 0.20 and becomes metallic for higher x. Recent in-depth structural studies on the Nb-substituted samples of ATi1−xNbxO3 by Seshadri and co-workers have revealed a clue for the distinct behaviors between Ba and Sr.21 Pair distribution function (PDF) refinements on a semiconducting BaTi0.875Nb0.125O3 unveiled local and noncoherent cation offcentering in the Ti4+/Nb5+ site from the ideal B-site position of the cubic structure. The substantial tolerance of the second-
CONCLUSION Using epitaxial thin films deposited by PLD, we have examined electrical properties of perovskite oxyhydrides ATiO3−xHx (A = Sr, Ba) with various hydride contents. In SrTiO3−xHx, a metallic state is achieved with less than 2% of H− substitution, whereas BaTiO3−xHx stays semiconducting even for ∼5−8% of H− substitution. This difference between the Sr and Ba systems results likely from the distinct local structures, which has been reported to occur in ATi1−xNbxO3 where the presence of in-gap states (polarons) for the Ba compound localizes electrons.21 The resemblance in transport properties between A = Ba and Sr systems in ATiO3−xHx and ATi1−xNbxO3 suggests that a profound knowledge in structures and properties in extensively studied cation-substituted perovskite oxides might be applied to anion (hydride) substituted one in various aspects. Recent developments of PDF techniques to probe a local distortion in crystalline films37,38 will permit the confirmation of the proposed local disorder in our BaTiO3−xHx film, which will be done in future. There are a few but increasing number of studies on epitaxial thin films for perovskites with mixed-anion composition
semiconducting behavior below around 90 K, which indicates electron localization at low temperatures. Such a localization of doped electrons at low temperatures is also observed in cationsubstituted or oxygen deficient perovskite oxides7,11,14,26 and is attributed to the chemical disorder, in our case, introduced by the anion substitution. On the other hand, BaTiO2.86H0.14, a film with the lowest H content for A = Ba, is semiconducting in the whole temperature range examined, despite higher carrier concentration (n ∼ 3.1 × 1020 cm−3) compared with SrTiO2.95H0.05. With increasing x in BaTiO3−xHx, we observed a transformation from semiconducting to metallic states. A semiconductor-to-metal crossover appears around 200 K for BaTiO2.76H0.24, while the samples with x = 0.35 and 0.58 (A = Ba) show metallic behavior in an entire temperature range. It is noted that BaTiO2.42H0.58 and SrTiO2.55H0.45 display somewhat lower conductivities compared to SrTiO2.75H0.25 and BaTiO2.65H0.35, along with a slight upturn in resistivity near 50 K. We are not able to give a straightforward reason for this crossover. It may be due to a tiny partial decomposition of the film (to give impurity phase, e.g., TiH2), although such an impurity was not experimentally observed. The observed transport properties of ATiO3−xHx at RT are summarized in Figure 3. The lack of abrupt changes in the
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D
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Chemistry of Materials (oxynitride, oxyfluoride, or oxyhydride).39−44 In particular, the strain engineering of thin films has shown to be effective in controlling anion local structure41 and anion order/disorder.44 It would be interesting to utilize different single crystal substrates to investigate the strain effect in oxyhydrides epitaxial thin films.
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(16) Kolodiazhnyi, T.; Tachibana, M.; Kawaji, H.; Hwang, J.; Takayama-Muromachi, E. Persistence of Ferroelectricity in BaTiO3 through the Insulator-Metal Transition. Phys. Rev. Lett. 2010, 104, 147602. (17) Jeong, I. K.; Lee, S.; Jeong, S. Y.; Won, C. J.; Hur, N.; Llobet, A. Structural Evolution across the Insulator-Metal Transition in OxygenDeficient BaTiO3−δ studied using Neutron Total Scattering and Rietveld Analysis. Phys. Rev. B: Condens. Matter Mater. Phys. 2011, 84, 064125. (18) 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. E.; 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. (19) Sakaguchi, T.; Kobayashi, Y.; Yajima, T.; Ohkura, M.; Tassel, C.; Takeiri, F.; Mitsuoka, S.; Ohkubo, H.; Yamamoto, T.; Kim, J. E.; 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. (20) 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. (21) Page, K.; Kolodiazhnyi, T.; Proffen, T.; Cheetham, A. K.; Seshadri, R. Local Structural Origins of the Distinct Electronic Properties of Nb-Substituted SrTiO3 and BaTiO3. Phys. Rev. Lett. 2008, 101, 205502. (22) Castro, F. J.; Meyer, G. Thermal Desorption Spectroscopy (TDS) Method for Hydrogen Desorption Characterization (I): Theoretical Aspects. J. Alloys Compd. 2002, 330−332, 59−63. (23) Hirscher, M.; Becher, M.; Haluska, M.; Von Zeppelin, F.; Chen, X.; Dettlaff-Weglikowska, U.; Roth, S. Are Carbon Nanostructures an Efficient Hydrogen Storage Medium? J. Alloys Compd. 2003, 356−357, 433−437. (24) Nomura, K.; Hamiya, T.; Ohta, H.; Hirano, M.; Hosono, H. Defect Passivation and Homogenization of Amorphous Oxide ThinFilm Transistor by Wet O2 Annealing. Appl. Phys. Lett. 2008, 93, 192107. (25) Evans, H. T. An X-ray Diffraction Study of Tetragonal Barium Titanate. Acta Crystallogr. 1961, 14, 1019−1026. (26) Tomio, T.; Miki, H.; Tabata, H.; Kawai, T.; Kawai, S. Control of Electrical Conductivity in Laser Deposited SrTiO3 Thin Films with Nb-Doping. J. Appl. Phys. 1994, 76, 5886. (27) Marucco, J. F.; Ocio, M.; Forget, A.; Colson, D. Synthesis and Electrical Properties of Solid Solutions BaNb1−xTixO3 with 0 < x < 0.8. J. Alloys Compd. 2002, 262−263, 454−458. (28) Bubnova, O.; Khan, Z. U.; Wang, H.; Braun, S.; Evans, D. R.; Fabretto, M.; Hojati-Talemi, P.; Dagnelund, D.; Arlin, J. B.; Geerts, Y. H.; Desbief, S.; Breiby, D. W.; Andreasen, J. W.; Lazzaroni, R.; Chen, W. M.; Zozoulenko, I.; Fahlman, M.; Murphy, P. J.; Berggren, M.; Crispin, X. Semi-Metallic Polymers. Nat. Mater. 2013, 13, 190−194. (29) Iwazaki, Y.; Suzuki, T.; Tsuneyuki, S. Negatively Charged Hydrogen at Oxygen-Vacancy Sites in BaTiO3: Density-Functional Calculation. J. Appl. Phys. 2010, 108, 083705. (30) Iwazaki, Y.; Gohda, Y.; Tsuneyuki, S. Diversity of Hydrogen Configuration and its Roles in SrTiO3−δ. APL Mater. 2014, 2, 012103. (31) Takenana, H.; Singh, D. J. Bonding of H in O Vacancies of ZnO: Density Functional Calculations. Phys. Rev. B: Condens. Matter Mater. Phys. 2007, 75, 241102. (32) Morris, D. F. C.; Reed, G. L. Pauling Crystal Radius of the Hydride Ion. J. Inorg. Nucl. Chem. 1965, 27, 1715−1717. (33) Bang, J.; Matsuishi, S.; Hiraka, H.; Fujisaki, F.; Otomo, T.; Maki, S.; Yamaura, J.; Kumai, R.; Murakami, H.; Hosono, H. Hydrogen Ordering and New Polymorph of Layered Perovskite Oxyhydrides: Sr2VO4−xHx. J. Am. Chem. Soc. 2014, 136, 7221−7224. (34) Mizoguchi, H.; Park, S.; Hiraka, H.; Ikeda, K.; Otomo, T.; Hosono, H. An Anti CuO2-type Metal Hydride Square Net Structure
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by Grant-in-Aid for Scientific Research (A) (No. 25248016 and 25249090) and CREST.
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DOI: 10.1021/acs.chemmater.5b02374 Chem. Mater. XXXX, XXX, XXX−XXX
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DOI: 10.1021/acs.chemmater.5b02374 Chem. Mater. XXXX, XXX, XXX−XXX
