Structure and Optical Properties of α–And γ-Cerium Sesquisulfide
René Windiks, Erich Wimmer, L Pourovskii, S Biermann, Antoine Georges
Journal of Alloys and Compounds 459(1-2), 438-446 (2008)
Structural and electronic properties of the α- and γ-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′ = 4 eV yields excellent results for crystal structures, bandgaps, and thermodynamic stability for both Ce₂S₃ allotropes. This approach gives insights in the difference in color of brownish-black α- Ce₂S₃ and dark red γ- Ce₂S₃. The calculations predict that both Ce2 S3 modifications are insulators with optical gaps of 0.8 eV (α-phase) and 1.8 eV (γ-phase). The optical gaps are determined by direct electronic excitations at k = Γ 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 μB per cerium cation, which is in agreement with measurements. The electronic energy of the α-phase is computed to be 6 kJ mol−1 lower than that of the γ-phase, which is consistent with the thermodynamic stability of the two allotropes.