The valence of cerium in metals (invited)

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.