Sr₃Ir₂O₇ is an unusual manifestation of a bilayer square lattice of spins (as illustrated at top of the Figure) due to the extended exchange inherent to its 5d electron valence states and their large orbital extent, not only within but also between bilayer planes. As a result, this system potentially exists in a unique parameter space where the strength of magnetic exchange coupling between these planes can approach or exceed the strength of the coupling within them.

The formation of superlattices is a fascinating mesoscale phenomenon governed by the interplay of a range of thermodynamic and kinetic factors. Long-time collaborators Detlef Smilgies, CHESS, and Tobias Hanrath, Chemical and Biomolecular Engineering, have recently summarized the role of time-resolved X-ray scattering techniques in combination with in-situ sample environments to gain unique insights into the relevant processes.

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.