Structure and optical properties of α- and γ-cerium sesquisulfide

Structural and electronic properties of the alpha- and gamma-phases of cerium sesquisulfide, Ce(2)S(3), 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' = 4 eV yields excellent results for crystal structures, band gaps, and thermodynamic stability for both Ce(2)S(3) allotropes. This approach gives insights in the difference in color of brownish-black alpha-Ce(2)S(3) and dark red gamma-Ce(2)S(3). The calculations predict that both Ce(2)S(3) modifications are insulators with optical gaps of 0.8 eV (a-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 Ce(2)S(3) modifications is explained by different coordinations of the cerium cations by sulfur anions. For both Ce(2)S(3) 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(-1) lower than that of the gamma-phase, which is consistent with the thermodynamic stability of the two allotropes. (C) 2007 Elsevier B.V. All rights reserved.