The gamma-to-alpha transition in cerium

The phase diagrams of solids frequently exhibit structural transitions in which the symmetry of the structure is lowered as the temperature is lowered. The gamma-to-alpha (γ → α) transition in cerium, fi rst discovered by Andrew Werner Lawson and Ting-Yuan Tang in 1947, provides a notable exception to this trend because the atoms retain their ordering in a face-centered cubic structure while the volume changes by almost 17 percent. Moreover, the hightemperature γ phase has a large and temperature-dependent Curie– Weiss susceptibility, whereas the low-temperature α phase has a small and almost temperature-independent susceptibility.

Because cerium is the first element in the lanthanide series, it could be expected that the anomalous change in the volume may be related to the anomalous lattice vibrations of uranium. However, a major difference between cerium and uranium occurs because the electrons in the 4f shell of atomic cerium are shielded by the outer 5d and 6s electrons. Soon after Lawson and Tang’s discovery, Linus Pauling and William H. Zachariasen independently interpreted the transition in terms of the promotional model, in which all the electrons are being squeezed out of the innermost 4f shell into the 5d–6s conduction band to participate in the bonding.


Karl Gschneidener, Jr. and co-workers at Ames Laboratory later modifi ed the promotional model by proposing that not all the electrons were squeezed out of the 4f shell. In the late 1960s, however, Dan Gustafson and co-workers at Wayne State University performed positron annihilation experiments on cerium and found that the change in the number of conduction electrons was inconsistent with the change invoked in the promotional model.

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