Role of f Electrons in the Optical and Photoelectrochemical Behavior of Ca(La 1– x Ce x ) 2 S 4 (0 ≤ x ≤ 1)

This study focuses on a solid solution series, Ca(La1–xCex)2S4 (0 ≤ x ≤ 1), where the f electron density is absent in CaLa2S4 and is progressively increased until it is maximized in CaCe2S4. Correspondingly, these samples, synthesized by a sealed ampule method, showed progressive variations in color ranging from gray for CaLa2S4 to orange-red for CaCe2S4. The crystal structural nuances of both the end members and three solid solutions with x = 0.25, 0.50, and 0.75 were established with the complementary use of synchrotron X-ray diffraction and neutron scattering. Interestingly, these data were consistent with a two-phase composition centered around each nominal solid solution stoichiometry. Optical characterization via diffuse reflectance spectroscopy and Tauc analyses showed a shrinking of the energy band gap (from the UV to vis range) when Ce was progressively introduced into the host CaLa2S4 structure. These data were in concert with electronic band structure calculations, using density functional theory, which showed the progressive formation of an intermediate f band when Ce was introduced intro the structure. Photoelectrochemical measurements in an aqueous redox electrolyte, as well as surface photovoltage and Kelvin probe measurements, revealed all samples to be n-type semiconductors. The valence and conduction band edge positions of the end members and the three solid solutions could be mapped, on both the redox and vacuum reference energy scales, by combining these measurements with the optical data58745534560COORDENAÇÃO DE APERFEIÇOAMENTO DE PESSOAL DE NÍVEL SUPERIOR - CAPES88881.131530/2016-01We acknowledge O. Gourdon for many useful discussions. R.T.M. and P.S. acknowledge NSF CAREER Award 1541230 and ACS PRF New Directions Award. C.L. and M.T.G. gratefully acknowledge the CAPES−PDSE program for financial support (88881.131530/2016-01). M.N.H. was supported by National Science Foundation Award No. DMR-1609811. We gratefully acknowledge the computations time awarded by Texas Advanced Computing Center (TACC). Use of the Advanced Photon Source at Argonne National Laboratory was supported by the U. S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357. A portion of this research used resources at the Spallation Neutron Source, a DOE Office of Science User Facility operated by the Oak Ridge National Laborator