Local electronic structures around hydrogen and acceptor ions in perovskite-type oxide, SrZrO3

doi:10.1016/S0167-2738(99)00274-X

Solid State Ionics

Volume 127, Issues 1–2, 1 January 2000, Pages 109-123

https://doi.org/10.1016/S0167-2738(99)00274-X Get rights and content

Abstract

Local electronic structures around hydrogen and acceptor ions in SrZrO₃ are simulated by the DV-Xα molecular orbital method. In pure SrZrO₃, there is a band gap of about 6 eV between the O–2p valence band and the Zr–4d conduction band, in agreement with experiments. When Y or Sc is doped into SrZrO₃, an acceptor level appears just above the valence band. When hydrogen is introduced into SrZrO₃, a donor level appears below the conduction band. Also, an oxygen ion vacancy makes the defect level below the conduction band. It is shown that charge compensation takes place among these acceptor, donor and defect levels. In addition, local electronic structure around hydrogen is found to change largely with the acceptor dopants, Sc and Y. For example, the metal–oxygen bond strength changes in the order, Y–O<Zr–O<Sc–O, which probably modifies the thermal vibrational amplitude of oxygen ions near hydrogen, and affects protonic conductivity in the doped SrZrO₃.

Introduction

It is known that many perovskite-type oxides (e.g., SrTiO₃, SrCeO₃) show protonic conduction at high temperatures when acceptor ions are doped into them [1]. These oxides have a large potential for use in fuel cells, steam electrolysis and hydrogen gas sensors [2], [3], [4].

When trivalent cations such as Y³⁺ are substituted for tetravalent ions such as Zr⁴⁺ in SrZrO₃, oxygen ion vacancies are introduced into the crystal to keep the charge balance. Following the Kröger-Vink notation, it is expressed as, $Y+Zr_{Zr}^{×} + 12 O_{O}^{×} →Y_{Zr} ′+ 12 V_{O}^{⋅⋅} +Zr+ 14 O_{2},$ where Y_Zr′ is a Y³⁺ ion at the Zr⁴⁺ site and V_O^⋅⋅ is an oxygen ion vacancy. In a wet atmosphere, some of this oxygen ion vacancy is readily filled with an oxygen ion, and interstitial protons H_i^⋅, are introduced into the crystal. This is expressed as, $H_{2} O+V_{O}^{⋅⋅} =O_{O}^{×} +2H_{i}^{⋅},$ where O_O^× is an oxygen ion at the oxygen site. As a result, protonic conductivity through the interstitial H_i^⋅ is induced in such oxides.

Lots of experiments have been performed in order to elucidate the transport mechanism of protons in the perovskite-type oxides [5], [6], [7], [8], [9]. For example, it has been shown experimentally that protons in a Sc-doped SrZrO₃ exist preferentially on the site with a relatively large trapping energy [10]. Also, the position of proton in a Sc-doped SrTiO₃ has been investigated using a neutron scattering technique [7], and it has been found that the proton exists in the neighborhood of the oxygen ion, in agreement with the result of a infrared absorption experiment [8]. In addition, in order to understand the mechanism of protonic conduction, quantum molecular dynamics simulations have been carried out in several oxide systems. For example, it has been pointed out that local relaxation of the oxygen lattice is needed for proton transfer in LaAlO₃ [11], [12]. In fact, a large oxygen vibrational amplitude may facilitate proton transfer [13]. Recently, we have shown from the calculation of the electronic structure of SrTiO₃ that charge compensation takes place among the impurity levels which appear in the band gap due to the presence of the proton, oxygen ion vacancy and acceptor dopant in the oxide [14]. In this study, the electronic states in the another perovskite-type oxide, SrZrO₃, are calculated by the DV-Xα molecular orbital method in order to obtain further evidence of the charge compensation among the impurities, and also to get a clue into the mechanism for protonic conduction in the perovskite-type oxides.

Section snippets

DV-Xα cluster method

The DV-Xα cluster method [15], [16] is a molecular orbital calculating method, assuming the Hartree-Fock-Slater (HFS) approximation. In this calculation, the exchange-correlation between electrons is given by the Slater’s Xα potential, and the self-consistent charge approximation is used. The matrix elements of the Hamiltonian and the overlap integrals are calculated by a random sampling method. The molecular orbitals are constructed by a liner combination of numerically generated atomic

Partial electron density of states

The calculated partial densities of states for pure SrZrO₃, SrZrO₃+Y_Zr, SrZrO₃+H₍₁₎ and SrZrO₃+Y_Zr+H₍₁₎, are shown in Fig. 3(a)–(d), respectively. For convenience sake, in each figure, the position of the HOMO (Highest Occupied Molecular Orbital) level is indicated by an arrow (←). For pure SrZrO₃, the HOMO level lies on the top of the O–2p valence band as shown in Fig. 3(a). This HOMO level for pure SrZrO₃ is set to be zero and used as a reference of the energy. As shown in Fig. 3(a), the

Correlation between local electronic structure and activation energy for protonic conduction in SrZrO₃

The present simulation is not dynamic but static, so it may be difficult for the dynamical behavior of protonic conduction to be treated following the present result alone. However, it still provides us information of the local electronic structure around hydrogen and dopant ions. As explained earlier, the doping of acceptor elements modifies the ionicities of oxygen ions and the bond orders between metal and oxygen ions in a small octahedron where an acceptor dopant is located in the center.

Conclusion

The electronic structures of hydrogen in SrZrO₃ are simulated by the DV-Xα molecular orbital method. It is found that an acceptor level is formed above the valence band by the substitution of Y_Zr ion or Sc_Zr ion for Zr_Zr ion, whereas a donor level is created below the conduction band by the introduction of hydrogen into it. Also, the defect level due to the oxygen ion vacancy appears below the conduction band. It is confirmed that charge compensation indeed takes place among these acceptor,

Acknowledgements

The authors acknowledge the Computer Center, Institute for Molecular Science, Okazaki National Institutes for the use of the SX-3/34R computer. This study was supported in part by the Grant-in-Aid for Scientific Research from the Ministry of Education, Science, Sports and Culture of Japan.

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Local electronic structures around hydrogen and acceptor ions in perovskite-type oxide, SrZrO3

Abstract

Introduction

Section snippets

DV-Xα cluster method

Partial electron density of states

Correlation between local electronic structure and activation energy for protonic conduction in SrZrO3

Conclusion

Acknowledgements

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Local electronic structures around hydrogen and acceptor ions in perovskite-type oxide, SrZrO₃

Correlation between local electronic structure and activation energy for protonic conduction in SrZrO₃