X-ray absorption studies of mixed valent rare earth-transition metal systems

Mixed valence is one of the most striking properties of some rare earth compounds. It intimately depends on the chemical environment as well as the rare earth itself. In the past, studies of the dependency of global valence trends on host chemical environment have been held back by the limitations of crystal structure stability. We have made systematic studies possible by implanting the rare earth ions (Ce, Eu or Yb) into essentially all the transition metals. Measurements of x-ray absorption near the L;The results are interpreted with a model based on the Anderson model. According to the Anderson model, the valence of rare earths depends on the 4f energy level and the width of 4f level. We suggest a scaling theory which shows that the 4f level energy of rare earths in a host simply depends on the host work function. By considering also the host density of states near the Fermi level, we can interpret these global trends of the rare earth valences.