On the Phase Transformation Ceγ \(\leftrightarrows\) Ceα
Cerium metal and SmS exhibit very interesting phase transformation. There is a large variation of the lattice parameter without any charge in the crystal structure1,2. To interpret these properties a change of valence was proposed eventually with fractional valence giving the notions of valence instabilities and intermediate valence. In the case of cerium the phase transformation Ceγ \(\leftrightarrows\) Ceα was first attributed to a change of valence from 3 in Cey to 4 in Ceα1. From extensive analyses of magnetic susceptibility and metallic radii different intermediate values of the valence were proposed3,4,5. But these notions have not led to a clear understanding of the problems. In our opinion one of the difficulties is that we must distinguish two aspects : on the one hand the existence of bonds each one involving one electron and on the other hand the strength of each bond. Shrinkage with large variation of the molecular or atomic volume must be attributed at least to one new bond involving the participation of one electron. Small variation of volume must be explained by variation of the intensity of the bonds. This last point arises from a comparison between the monochalcogenides of the rare earths and 3d metals6,7. In this paper we want to focus the attention on the cerium case.
KeywordsPositron Annihilation Atomic Volume High Pressure Phase Additional Bond Intermediate Valence
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