The valence of cerium in metals (invited)

Abstract
The valence of Ce in solids can be measured by LIII x‐ray absorption. One finds γ‐ and α‐like materials with Ce valences clustering near v≳3 and near v≊3.25, respectively. There are no Ce metals with valence four. The valence of Ce is determined by a competition between the elastic strain energy of the mixture of three and four valent Ce atoms and the free energy due to configurational mixing. The latter has a minimum near valence 3.25, which decreases strongly with decreasing volume and eventually traps the valence of all α‐like materials near its bottom.