Charge transfer in silicon clathrates studied by Compton scattering

High-resolution Compton profiles of the clathrates Na(8)Si(46), K(7.6)Si(46), and Si(136) were measured to study the charge transfer of the alkali-metal guest atoms to the clathrates silicon host lattice. The experiment was accomplished utilizing the Compton spectrometer at beamline ID15B of the European Synchrotron Radiation Facility with an incident energy of 56.7 keV and a momentum space resolution of 0.16 a.u. The valence electron contribution to the total Compton profile of the sodium and potassium guest atoms was extracted and is compared with calculations employing density-functional theory using Becke exchange and Perdew-Wang correlation. Good overall agreement between experiment and theory is found and proves the quality of the underlying wave functions used for the theoretical predictions. On this basis, the charge transfer in these systems is estimated by a Mulliken population analysis and discussed for the different guest atoms.