  Structural and electronic properties of the alpha- and gamma-phases of cerium
sesquisulfide, Ce2S3, are examined by first-principles calculations using the
GGA+U extension of density functional theory. The strongly correlated
f-electrons of Ce are described by a Hubbard-type on-site Coulomb repulsion
parameter. A single parameter of $U^/prime$=4 eV yields excellent results for
crystal structures, band gaps, and thermodynamic stability for both Ce2S3
allotropes. This approach gives insights in the difference in color of
brownish-black alpha-Ce2S3 and dark red gamma-Ce2S3. The calculations predict
that both Ce2S3 modifications are insulators with optical gaps of 0.8 eV
(alpha-phase) and 1.8 eV (gamma-phase). The optical gaps are determined by
direct electronic excitations at k=Gamma from localized and occupied Ce
4f-orbitals into empty Ce 5d-states. The f-states are situated between the
valence and conduction bands. The difference of 1 eV between the optical gaps
of the two Ce2S3 modifications is explained by different coordinations of the
cerium cations by sulfur anions. For both Ce2S3 modifications the calculations
yield an effective local magnetic moment of 2.6 $mu_B$ per cerium cation, which
is in agreement with measurements. The electronic energy of the alpha-phase is
computed to be 6 kJ/mol lower than that of the gamma-phase, which is consistent
with the thermodynamic stability of the two allotropes.
