Pressure Induced Valence Transition in Cerium Studied by Positron Annihilation Spectroscopy
The low pressure fcc γ-phase of Ce at ambient temperature is known to possess the properties of a trivalent rare earth metal. On the application of pressure at ~ 8 kbar, it undergoes a first order isostructural transition to the a-phase, accompanied by a decrease in lattice parameter from 5.16 to 4.85 Å. At ~ 55 kbar, Ce transforms to another close packed structure (α’ phase) with a lattice parameter of 4.66 Å. Our earlier understanding of the mechanism of the pressure induced γ → α transition was on the basis of the 4f-promotional model . This assumes that the 4f-level in the γ-phase is well localised and occupied by one electron per atom and that this electron is completely or partially (0.7 electron) transferred to the conduction band in the α-phase. Recent spectroscopic measurements such as photo-emission , x-ray absorption  and Compton scattering  provided evidence against the validity of the 4f-promotional model and raised a controversy on the question of valence in the collapsed phase. The detailed mechanism of the valence transitions and the nature of 4f-delocalisation in the high pressure phases are still not fully understood.
KeywordsCarbide Tungsten Bismuth Cerium
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