Switchable rare earth materials

Several binary lanthanide (Ln) materials have demonstrated spectacular reversible switching between a black semiconducting (S) and a shiny metallic (M) state under the influence of heat, alloying or pressure. At the origin of this first order SM-phase transition is a ground state configuration change from a [Xe]4fn  (4f in short) ground state to a [Xe]4fn-5d1 (5d in short) ground state.  In the rare occasion when the 4f and 5d sates are close in energy, physical properties become critically sensitive to internal or external perturbations that can trigger a ground state configuration change from 4f to a 5d. During this 4f-5d cross-over, electrons that were formerly localized in the 4f core shell are suddenly fully delocalized in the 5d–derived CB (or the other way around). The result is not only a Ln-ion valence change (from 2+ to 3+) and an associated large (~10%) lattice volume collapse but also a giant increase in charge carrier density and conductivity. We make thin films of SmS using a magnetron sputtering system shown below, starting from Sm2S3 and Sm-metal targets in a co-sputtering set-up to fundamentally understand the phase transition and explore applicability.

See E.G Rogers et al. in E Rogers et al 2010 J. Phys.: Condens. Matter 22 015005.
doi:10.1088/0953-8984/22/1/015005 (Please mail me for a copy).