Optical properties of γ- and α-Ce by spectroscopic ellipsometry

A rotating analyzer type ellipsometer with an ultra high vacuum sample chamber was built. The dielectric functions of γ- and α-Ce were measured in the energy range from 1.5 to 5.4 eV to investigate the role of the 4f electron in the isostructural phase transition. γ- and α-Ce were made by evaporation of Ce on sapphire substrates at room temperature and 25 K. All measurements were performed inside a UHV chamber at pressures lower than l x 10-10Torr. The measured dielectric functions showed a thickness dependence. The thicker sample has the smaller optical conductivities. Both overlayer thickness and void fraction increase as sample thickness increases. Repeating cooling-heating-cooling or heating-cooling-heating cycles causes the sample surface to become rougher but the relative volume fractions of both phases and the void fraction in the bulk remain unchanged. The optical conductivity increases upon entering the a-phase but the number of electrons per atom, Neff/NA, contributing to the optical conductivity does not change. The valence electrons lose oscillator strength in the above energy range due to volume collapse. This reduces Neff/NA but the increased 4f-sd valence band hybridization exactly compensates the reduced oscillator strength. Therefore the net effects of the γ→α isostructural phase transition are an increase of optical conductivity and constancy of Neff/NA