Livermore researchers, working with colleagues from the University of California at Davis, have elucidated the behavior of electrons in gadolinium (Gd) metal as a function of compression. Physicists have known for some time that a number of lanthanide and actinide metals exhibit phase transitions at high pressures characterized by unusually large changes in volume. They have theorized that such transitions are associated with the delocalization of f electrons due to the dramatic changes in properties, such as crystal structure, that occur in going from low to high pressures. Now, new experiments on compressed Gd metal have revealed prolonged and continuous delocalization of the 4f electrons with volume over the entire pressure range (from ambient to 113 gigapascals). This result suggests that the volume-collapse transition, which occurs at 59 gigapascals in Gd, is only part of the phenomenon affecting the behavior of electrons with increasing compression.

Using a 7-kiloelectronvolt x-ray beam at the Advanced Photon Source at Argonne National Laboratory, the scientists measured resonant inelastic x-ray scattering and x-ray emission spectra from Gd samples compressed to pressures up to 113 gigapascals. These techniques probe the electronic and magnetic properties of the metal as a function of pressure. The high pressures were achieved by loading Gd samples, together with mineral oil for the pressure medium and small ruby chips for pressure determination, into a Livermore-designed piston-cylinder diamond anvil cell.

The measured inelastic x-ray scattering spectrum provides information about the number of electrons occupying the 4f shell in Gd. At ambient pressure, in the strongly localized limit, this occupation number is seven. The changes in the spectrum with increasing pressure show that, under compression, the 4f occupation effectively increases due to electron transfer between the 4f and valence shells. Furthermore, this increase is continuous and occurs over the entire pressure range. At the highest compression reached in the experiment, approximately one sixth of the atoms, on average, have eight electrons in the 4f shell. Another interesting result comes from the x-ray emission spectrum, which shows essentially no variation of the spin magnetic moment of the 4f electrons with compression, even at the volumecollapse transition. This disagrees with the predictions of electronic structure calculations. These results appear in an article published in the June 2, 2006, issue of Physical Review Letters.

Contact: Andy McMahan (925) 422-7198 (

Andy McMahan