Elsevier

Solid State Ionics

Volume 177, Issues 19–25, 15 October 2006, Pages 1779-1783
Solid State Ionics

Mixed conducting perovskite-like ceramics on the base of lanthanum gallate

https://doi.org/10.1016/j.ssi.2006.05.046Get rights and content

Abstract

Ceramic perovskite solid solutions (La0.9Sr0.1)[(Ga1−xMx)0.8Mg0.2]O3−y, 0 ≤ x ≤ 0.5, M = Fe, Ni, Cr (systems I–III) and brownmillerite solid solutions (La0.2Sr1.8)[Ga(Fe1−xMgx)]O5−z, 0 ≤ x ≤ 0.5, (system IV) have been prepared. The samples have been studied by X-ray diffraction and electron microscopy methods, dielectric spectroscopy and permeability measurements. The correlation between the composition, unit cell parameter changes, electrical transport and oxygen permeation properties has been revealed. Introduction of transition metals (Fe, Ni, or Cr), substituting for gallium, ensures the enhancement of the electronic constituent of the conductivity in the perovskite systems I–III. Stabilization of the transition metal high valence states 4+ or 5+ has been suggested for compositions I and III. This leads to a unit cell volume contraction and provides a decrease in the concentration of oxygen vacancies. The oxygen permeability reaches its maximum values in compositions I–III with x ∼ 0.3. On the contrary, increasing concentration of the doping element with lower valence state (magnesium), substituting for iron, determines the expansion of the brownmillerite unit cell volume and provides an increase of the oxygen vacancy concentration, which in turn, favors the enhancement of oxygen permeability of composition IV.

Introduction

The perovskite-like ionic and mixed conducting oxides have wide prospects for application as membranes for pure oxygen separation, catalysts for exhaust gases, etc. Earlier, the perovskites (La,A)(Co,Fe)Oy were intensively studied as the most promising for oxygen separation membranes [1]. However, in spite of the high ionic conductivity, low thermodynamic stability of these materials hinders applications [2]. These compositions stability was improved when cations with stable valence Ba2+ or Sr2+ and Ga3+ or Zr4+ substituted for La3+ and Co3+ in the perovskite lattice, respectively. However, materials with characteristics which meet the criteria for oxygen separating membranes were not developed till now [3], [4], [5], [6], [7].

High oxygen ion conductivity in combination with stability in a wide range of oxygen partial pressure values was revealed in anion deficient perovskites (La,Sr)(Ga,Mg)Oy [8], [9], [10]. Additional doping of these compositions with the transition metals Fe, Ni, Co, Cr resulted in materials with high mixed ionic–electronic conductivity [11], [12], [13], [14], [15].

The highest ionic conductivity was revealed in ceramics of the composition (La0.9Sr0.1)(Ga0.8Mg0.2)O2.85 [8], [9], [10], [16]. Formation of solid solutions on the base of this composition modified with iron, nickel or manganese cations was studied [17], [18], [19], [20], [21]. It was found that iron and manganese substituted for gallium in the whole concentration range, while nickel could substitute for gallium only up to 40 at.%.

In this work, the unit cell parameter changes, electrical transport and oxygen permeation properties of perovskite ceramics (La0.9Sr0.1)[(Ga1−xMx)0.8Mg0.2]O3−y, where M = Fe (system I), Ni (system II), Cr (system III) with x = 0–0.4, and brownmillerite ceramics (La0.2Sr1.8)[Ga(Fe1−xMgx)]O5−z (system IV) with x = 0–0.5 have been studied. The correlation between the composition, conductivity and oxygen permeation characteristics has been established.

Section snippets

Experimental

The gas tight ceramic samples were prepared by the solid-state reaction method at T1 = 700–1000 °C (6 h) and T2 = 1450–1500 °C (1–4 h). Starting materials of the chemical grade “pure” were used (La2O3, Ga2O3, Ni2O3 Fe2O3, Cr2O3 and SrCO3). MgCO3·Mg(OH)2·3H2O was used as starting reagent which converted upon heating to highly dispersed and consequently highly reactive MgO [16], [18]. The samples were studied using X-ray diffraction, electron microscopy, and dielectric spectroscopy methods in wide

Structural changes

Pure perovskite phase compositions were obtained in the systems I–III (Fig. 1). The X-ray diffraction pattern of the base composition with x = 0 in systems I–III was indexed in an I-centered monoclinic structure with a = 7.815 (± 0.002), b = 5.525, c = 5.536 (± 0.001), β = 90.016 (± 0.002) with space group I2/a. Compositions I and III with x < 0.4 and compositions II with x < 0.2 were also indexed in monoclinic structure, while compositions II with x > 0.2 were indexed in rhombohedral structure [22]. The

Conclusions

The results obtained confirmed the correlation of ionic and mixed conducting properties of the lanthanum gallate-based ceramics with perovskite (I–III) and brownmillerite (IV) structure with their oxygen permeation characteristics. Evidence for the enhancement of the electronic constituent of conductivity in systems I–III with the transition metals Fe, Ni, or Cr introduction is received. The stabilization of the transition metal high valence state 4+ or 5+ has been suggested for compositions I

Acknowledgments

The financial support of the project by the Russian Fund for Basic Research (N 04-03-32094) and the Ministry of Science and Technology, Republic of Korea and Russia Korea Center for Energy and Environment Technology is acknowledged.

S.V. Zaitsev acknowledges the Haldor Topsoe firm for the scholarship grant awarded.

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