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 gamma-Cep(2)S(3) have been investigated using first -principles local-density-functional theory. The energy-band structure s from augmented spherical-wave calculations indicate that the S 3p--> Ce 5d interband transitions give rise to absorptions in the ultraviole t, whereas the observed led color of gamma-Ce2S3 is associated with lo calized Ce 4f-->5d excitations. The cationic vacancies in gamma-Ce2S3 (which is derived from th cubic Th3P4 structure) lead to the formation of vacancy bands, which are split off from the top of the S 3p valenc e band. Na doping removes these vacancy bands and homogenizes the band edges of the valence and conduction bands. As a consequence, the onse t of the f-->d transitions is shifted slightly to larger energies and becomes sharper, which is consistent with an observed change in the co lor of gamma-Ce2S3 from maroon to red-orange upon doping with Na.