CePS4 electronic structure and optical properties

The structure of CePS4 has been determined from single-crystal X-ray diffraction data (I4(1)/acd space group, a = b = 10.9228(3) Angstrom, c = 19.3998(6) Angstrom, V = 2314.5(2) Angstrom(3), Z = 16). The refinement, conducted from 547 independent reflections (I > 3 sigma(I) and 30 variables) led to R-F (%) = 1.83 and R-wF (%) = 2.36. Diffuse reflectance spectra of CePS4 and of the La1-xCexPS4 series indicated that the electronic absorption mechanism in CePS4, a 4f(1) --> 5d(0) transition on the Ce-III ions, is the same as in gamma-Ce2S3, with only the band gap width being modified. To this transition corresponds an absorption threshold responsible for the yellow hue of the thiophosphate at about 2.5 eV. Band structure calculation (TB-LMTO-ASA) confirms the experimental evidence of the Ce-III 4f(1) --> 5d(0) transition. Comparisons between CePS4 and gamma-Ce2S3 through their electronic localized functions revealed the inductive effect of the P-S covalent bonding on the Ce-S bond, with the resulting increase of the Ce-III 4f(1) --> 5d(0) transition in the thiophosphate as compared to the sulfide. This increase is ascribed, in part, to a lowering of the 4f block because of the decreased screening effect of the more ionic Ce-S bonding in CePS4.