Achieving fast oxygen diffusion in perovskites by cation ordering

Abstract

The oxygen-exchange behavior has been studied in half-doped manganese and cobalt perovskite oxides. We have found that the oxygen diffusivity in Gd0.5Ba0.5MnO 3−δ can be enhanced by orders of magnitude by inducing crystallographic ordering among lanthanide and alkali-earth ions in the A-site sublattice. Transformation of a simple cubic perovskite, with randomly occupied A-sites, into a layered crystal GdBaMn2O5+x (or isostructural GdBaCo2O5+x for cobalt oxide) with alternating lanthanide and alkali-earth planes reduces the oxygen bonding strength and provides disorder-free channels for ion motion, pointing to an efficient way to design new ionic conductors.PACS numbers: 66.30. Hs, 82.47.Ed Oxygen ion conductors -solids exhibiting very fast oxygen diffusion -constitute the basis for such emerging technologies as the membrane oxygen separation or the solid-oxide fuel cell (SOFC) power generation.1,2 . These technologies offer enormous economical and ecological benefits provided high performance materials can be developed: The scientific challenge is to design materials demonstrating high oxygen diffusivity at low enough temperature.In general, a crystal must meet two fundamental requirements to be a good oxygen-ion conductor: (i) it must contain a lot of vacancies in the oxygen sublattice, and (ii) the energy barrier for oxygen migration from one site to another must be fairly small, typically less than ∼ 1 eV. Only a few types of oxides, and perovskites ABO 3 (A is a rare-earth or an alkali-earth element and B is typically a transition metal) among them, have been found to possess these features. 1,2,3,4,5,6,7,8 Doped perovskites, which possess a high electronic conductivity in addition to the ionic one, are considered for using as electrodes in SOFC and as oxygen-selective membranes; for example, strontium-doped lanthanum manganese oxide, La 1−y Sr y MnO 3−δ , is a standard cathode material for SOFC applications operating at temperatures around 1000• C. 6 Recently, serious efforts are made to reduce the operation temperature of SOFC, and for the operation at 700 -800• C, strontium-doped lanthanum cobalt oxide, La 1−y Sr y CoO 3−δ , is considered to be the most promising cathode material.7,8 The performance of perovskite oxides has been already optimized as much as possible mostly by means of various ion substitutions in both A and B sublattices, 6,7,8,9 but they still fail to operate at low enough temperatures ∼ 500• C required for successful commercialization of the fuel cell technology.In this Letter, we show that the oxygen-ion diffusion in a doped perovskite can be enhanced by orders of magnitude if a simple cubic crystal [schematically shown in Fig.1(a)] transforms into a layered compound with ordered lanthanide and alkali-earth ions [ Fig. 1(b)], which reduces the oxygen bonding strength and provides disorder-free channels for ion motion.Recently, the A-site-ordered manganese and cobalt perovskite oxides have been synthesized by several , which demonstrate the doubling of the unit cell along the c axis ...

Defect equilibrium in PrBaCo2O5 at elevated temperatures

“…Co 2þ and Co 4þ cations, in LnBaCo 2 O 5+δ were reported earlier [7,8]. Accordingly, the coexistence of charged cobalt species in the cobaltite can be reflected in the formula PrBaCo 2þ n Co 3þ z Co 4þ p O 5þδ where symbols n, z and p denote concentration per formula unit of respective cobalt cations.…”

“…It is widely recognized that electric properties in cobaltites are strongly influenced by charge disproportionation of Co 3+ and appearance of Co 2+ and Co 4+ cations [7,8]. We have shown earlier [21] with the using of PrBaCo 2 O 5+δ as an example that all three charged cobalt species can simultaneously coexist in the double perovskite cobaltites at large variations of oxygen content.…”

“…Notice that the partial filling of the sites available for oxygen exchange is a prerequisite for large mobility of oxygen ions in the crystalline lattice. At the same time, the presence of cobalt cations that can easily change their oxidation state is favorable for electron conductivity [7][8][9]. Therefore, the double perovskite cobaltites have attracted considerable interest in recent years from the researchers in the field of catalysis, adsorption and cathode materials [10][11][12][13][14][15][16][17][18][19].…”

Crystal structure and oxygen content of the double perovskites GdBaCo2−Fe O6−δ

“…In particular, GdBaCo 2 O 6 À d was shown to possess sufficiently high oxygen ionic conductivity and oxygen chemical diffusion coefficient at temperatures as low as 400 1C [3]. Double perovskite GdBaCo 2 O 6 À d was recently found to exhibit excellent performance as a cathode for IT SOFCs [5][6][7] and oxygen permeable dense ceramic membrane [6].…”

“…Mixed ionic and electronic conducting oxides AA 0 Co 2 O 5 , where A is a rare earth element (RE), A 0 -Ba, with double perovskite structure have received great attention in past decade [1][2][3][4] due to their unique magnetic and transport properties at relatively low temperatures. In particular, GdBaCo 2 O 6 À d was shown to possess sufficiently high oxygen ionic conductivity and oxygen chemical diffusion coefficient at temperatures as low as 400 1C [3].…”