Electronic structure of proton conducting SrCeO₃–SrZrO₃ thin films

Abstract

The electronic structure of the protonic conductor Sr[Ce_1−xZr_x]_0.95Yb_0.05O_3−δ thin film has been studied by soft-X-ray spectroscopy. The valence band is composed mainly of the O 2p state. The conduction band is composed of Ce 4f and Zr 4d states. The holes and acceptor level are observed at the top of the valence band and just above the Fermi level (E_F), respectively. In H₂-annealed Sr[Ce_1−xZr_x]_0.95Yb_0.05O_3−δ thin films, their intensities decrease and the hydrogen-induced level is created at just below E_F. The energy separation between the bottom of the hydrogen-induced level and the top of the valence band agrees with the activation energy estimated from the electrical conductivity.

Introduction

Perovskite-type oxides SrCeO₃ and SrZrO₃ exhibit high protonic conductivity at sufficiently high temperatures when they are doped with acceptor ions [1], [2], [3], [4], [5], [6], [7], [8]. These protonic conductors are important materials for a wide range of electrochemical applications such as fuel cells and hydrogen sensors. For this purpose, many theoretical and experimental investigations have been reported in the case of ceramics and single crystals [9], [10], [11], [12], [13], [14], [15], [16], [17], [18]. The local lattice distortion due to acceptor doping plays an important role in the proton migration in these protonic conductors. Sata et al. attempted to fabricate a locally distorted thin film of protonic conductors by the pulsed laser deposition (PLD) method. They found that the lattice distortion and the protonic conductivity depend on the film thickness ratio of SrZr_0.95Yb_0.05O_3−δ and SrCe_0.95Yb_0.05O_3−δ [19], [20], [21], [22]. In spite of this interesting property, the numbers of both theoretical and experimental studies on protonic conductor thin films are very few.

In this study, the electronic structure of protonic conductor Sr[Ce_1−xZr_x]_0.95Yb_0.05O_3−δ thin films has been investigated by X-ray absorption spectroscopy (XAS). The XAS is related directly to the electronic structure of unoccupied states [23], [24], [25]. This is a rather local process, because of the localized core state. It is governed by the dipole selection rules so that XAS gives the spectrum relating to the site- and symmetry-selected density-of-state (DOS). Thus, the holes at the top of valence band and acceptor level just above the Fermi level (E_F), which provide the electrical conductivity, might be observed by careful measurements using high-resolution and high-brightness XAS. A soft-X-ray emission spectroscopy (SXES) spectrum, which reflects the electronic structure of occupied states [23], has been also measured. In this paper, we present high-resolution XAS spectra of Sr[Ce_1−xZr_x]_0.95Yb_0.05O_3−δ thin films and show how the XAS peaks found in the band gap region are related to the protonic conductivity.