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 \ensuremath{\gamma}-${\mathrm{Ce}}_{2}$${\mathrm{S}}_{3}$ have been investigated using first-principles local-density-functional theory. The energy-band structures from augmented-spherical-wave calculations indicate that the S 3p\ensuremath{\rightarrow}Ce 5d interband transitions give rise to absorptions in the ultraviolet, whereas the observed red color of \ensuremath{\gamma}-${\mathrm{Ce}}_{2}$${\mathrm{S}}_{3}$ is associated with localized Ce 4f\ensuremath{\rightarrow}5d excitations. The cationic vacancies in \ensuremath{\gamma}-${\mathrm{Ce}}_{2}$${\mathrm{S}}_{3}$ (which is derived from the cubic ${\mathrm{Th}}_{3}$${\mathrm{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\ensuremath{\rightarrow}d transitions is shifted slightly to larger energies and becomes sharper, which is consistent with an observed change in the color of \ensuremath{\gamma}-${\mathrm{Ce}}_{2}$${\mathrm{S}}_{3}$ from maroon to red-orange upon doping with Na. \textcopyright{} 1996 The American Physical Society.