The reactivity of manganites in O-18 isotope exchange reactions

The effect of the structural properties and the oxidation state of Mn on th e O-18 isotope exchange behaviour of ternary manganites (La1-xSrxMnO3, La0. 5Sr1.5MnO4 and SrMnO3) has been studied. All types of (1)8O isotope exchang e homomolecular, partially and completely heteromolecular) take place on th e very active manganites with perovskite (LaMnO3 and La0.7Sr0.3MnO3) and pe rovskite-like (SrMnO3) structure, but not on the less active K2NiF4-structu re (La0.5Sr1.5MnO4). The highest O-18 exchange activity is observed for La0 .7Sr0.3MnO3, for which the completely heteromolecular O-18 exchange starts to occur at 520 K, already, a T(o)n which is typical for excellent redox ca talysts. The influence of the structural properties on the O-18 exchange an d oxygen diffusion behaviour of the manganites is much more pronounced than that of the Mn3+/Mn4+ ratio. The different reduction behaviour of the mang anites with perovskite and K2NiF4-structure can be explained by means of th e bond-valence model.