Physical Properties and Behavior of Allotropically Pure α-, β- and γ-Ce

Abstract

Cerium was the first material found to undergo a valency change. X-ray studies in 1949–1950 revealed that when Ce was compressed to modest pressures (> 8 kbar at 300 K)¹ or cooled to low temperature (< 100 K at 1 atm)² a large volume contraction occurred (> 12%) while the crystal structure remained the same. When these authors discussed this unusual behavior with their colleagues, both Zachariasen³ and Pauling⁴ independently suggested that this volume contraction corresponded to a valence change of 3 for γ-Ce to 4 for α-Ce, i.e. the 4f electron of γ-Ce was promoted to the valence band, leaving the 4f level of α-Ce empty. Several other authors subsequently reanalyzed the physical properties of the phases involved in the valence change and they concluded that about 0.5 to 0.67 of an electron per atom is transferred from the 4f level to the valence band.⁵ Today the consensus of scientists favors the 0.67 value. Furthermore, Gschneidner and Smoluchowski point out that the valences of the two phases involved (γ- and α-Ce) change with temperature and pressure. As a result of this valence change, the γ-α pressure-temperature phase boundary ends at a critical point. Ce is the only solid known to have a critical point.