A comparative study of the surface and bulk properties of lanthanum-strontium-manganese oxides La1-xSrxMnO3 +/-delta as a function of Sr-content, oxygen potential and temperature

The properties of the perovskite-type La1-xSrxMnO3+/-delta oxides (0 less t han or equal to x less than or equal to 1) are related to the reversible re dox reaction of manganese ions Mn3+ <-> Mn4-. This reaction plays an essent ial role both in the interfacial transfers and transport properties and dep ends on relative cation concentrations and experimental conditions. In this paper we present experimental investigations aiming to determine, to compa re and to control some of the surface and bulk properties of these oxides. The bulk and surface Mn4+ contents were investigated by thermogravimetry an alysis (TGA) in hydrogen and X-ray photoelectron spectroscopy (XPS) respect ively. With a finely divided La0.8Sr0.2MnO3 powder, the chemical compositio n in the surface layers was found to be practically independent of the cond itions of a high temperature treatment in oxygen. The same is true for the mean oxidation state of manganese always close to Mn4+. With pellets of var ious compositions (x = 0, 0.2, 0.5 and 1), enrichment in strontium with res pect to the bulk composition was detected in oxygen at high temperature and found to depend on the strontium concentration in the bulk. Moreover, for any chemical composition, deficit in manganese were systematically evidence d in the surface layers. Surface strontium concentration and mean oxidation state of manganese was correlated as expected. By comparing the results ob served with very finely divided powder and pellets respectively, it was ded uced that the grain boundaries have an important effect on the surface comp osition. Data on the structural evolution of the initial compounds during t heir hydrogen reduction are also presented. (C) 1999 Elsevier Science B.V. All rights reserved.