Local electronic structures around hydrogen and acceptor ions in perovskite-type oxide, 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,where YZr′ is a Y3+ ion at the Zr4+ site and VO⋅⋅ 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 Hi⋅, are introduced into the crystal. This is expressed as,where OO× is an oxygen ion at the oxygen site. As a result, protonic conductivity through the interstitial Hi⋅ 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+YZr, SrZrO3+H(1) and SrZrO3+YZr+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 YZr ion or ScZr ion for ZrZr 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.
References (28)
- et al.
Solid State Ionics
(1981) - et al.
Solid State Ionics
(1995) - et al.
Solid State Ionics
(1995) - et al.
Solid State Ionics
(1997) - et al.
Solid State Ionics
(1997) - et al.
Solid State Ionics
(1996) - et al.
Solid State Ionics
(1995) - et al.
Solid State Ionics
(1997) - et al.
Solid State Ionics
(1999) - et al.
Progr. in Surface Sci.
(1983)
Solid State Ionics
Solid State Ionics
Solid State Ionics
Solid State Ionics
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