The chemistry in air of the rare-earth-metal sesquioxides. Comparative study of hexagonal and cubic neodymia samples

The evolution in air of four different neodymia samples, two of them with hexagonal structure and the other two cubic, has been studied. Upon exposure to air the four samples become heavily hydrated. Likewise, carbonation, though much less intense, does affect the bulk oxides. The behaviour of the hexagonal samples on the one hand, and that of the cubic oxides on the other, was found to be quite different. Dissimilarities were observed in the rates of the aging processes, being much faster for the cubic oxides, in the nature of the carbonated phases, and in the thermal evolution of samples stabilized in air. In the case of the cubic neodymium oxides, a crystalline carbonate hydroxide-like phase, isostructural with that observed for cubic samaria and europia samples aged in air, was found. By contrast, the carbonated phase of the hexagonal neodymias seems to be similar to those detected for hexagonal lanthana, as well as for a monoclinic sample of samaria. The results, in addition to showing that the behaviour in air of the rare-earth-metal oxides is much more complex than presumed earlier, give further support to the tentative classification of these oxides into three groups. (author)