Decades-old mystery solved
The concept of “valence” – the ability of a particular atom to combine with other atoms by exchanging electrons – is one of the cornerstones of modern chemistry and solid-state physics. Valence controls crucial properties of molecules and materials, including their bonding, crystal structure, and electronic and magnetic properties.
Four decades ago, a class of materials called “mixed valence” compounds was discovered. Many of these compounds contain elements near the bottom of the periodic table, so-called “rare-earth” elements, whose valence was discovered to vary with changes in temperature in some cases. Materials comprising these elements can display unusual properties, such as exotic superconductivity and unusual magnetism.
But there’s been an unsolved mystery associated with mixed valence compounds: When the valence state of an element in these compounds changes with increased temperature, the number of electrons associated with that element decreases, as well. But just where do those electrons go?
Image: Illustration of ytterbium (Yb) atoms in YbAl3, where electrons transform from localized states (bubbles surrounding the yellow orbitals) to itinerant states (hopping amongst orbitals), as a function of temperature.