The α-γ Phase Transitions in Ce-Th Alloys under High Pressure
In the past few years a great deal of research has been done on rare earth metals and compounds of them which exhibit non-integral valences.1–3 The rare earth ion in these materials undergoes a change of valence in response to a temperature, pressure, or alloying variation. In the Sm mixed valence systems the atomic-like 4f state is usually situated energetically below the 5d-6s conduction band. As the temperature, pressure, or composition is varied, the 5d-6s bands broaden and move down in energy relative to the 4f level and finally, at some critical value, the 4f level is pinned at the Fermi level. In this latter state, the system is defined as a mixed valence material.2 Historically, Ce was the first material found to exhibit a valence change. X-ray studies revealed that when Ce was compressed to modest pressures (> 8 kbar at 300 K)4 or cooled to low temperature (< 100 K at 1 atm)5 a large (> 12%) volume contraction (the α-γ transformation) took place while the crystal structure (fcc) remained unchanged.
KeywordsVolume Contraction Kondo Effect Valence Change Thorium Concentration Alamos Scientific Laboratory
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