Color of pure and alkali-doped cerium sulfide: A local-density-functional study

The electronic structure and mechanisms for the optical excitations of pure and Na-doped {γ}-Ce₂{S}₃$ have been investigated using first-principles local-density-functional theory. The energy-band structures from augmented-spherical-wave calculations indicate that the S 3p{→}Ce 5d interband transitions give rise to absorptions in the ultraviolet, whereas the observed red color of {γ}-Ce₂{S}₃is associated with localized Ce 4f→5d excitations. The cationic vacancies in γ-{Ce}₂S₃ (which is derived from the cubic Th₃{P}₄$ structure) lead to the formation of vacancy bands, which are split off from the top of the S 3p valence band. Na doping removes these vacancy bands and homogenizes the band edges of the valence and conduction bands. As a consequence, the onset of the f{→}d transitions is shifted slightly to larger energies and becomes sharper, which is consistent with an observed change in the color of {γ}-Ce₂{S}₃$ from maroon to red-orange upon doping with Na. {} 1996 The American Physical Society.