Layered CeSO and LiCeSO Oxide Chalcogenides Obtained via Topotactic Oxidative and Reductive Transformations

The chemical accessibility of the Ce IV oxidation state enables redox chemistry to be performed on the naturally coinage-metal-deficient phases CeM 1- x SO (M = Cu, Ag). A metastable black compound with the PbFCl structure type (space group P4/ nmm: a = 3.8396(1) Å, c = 6.607(4) Å, V = 97.40(6) Å 3 ) and a composition approaching CeSO is obtained by deintercalation of Ag from CeAg 0.8 SO. High-resolution transmission electron microscopy reveals the presence of large defect-free regions in CeSO, but stacking faults are also evident which can be incorporated into a quantitative model to account for the severe peak anisotropy evident in all the high-resolution X-ray and neutron diffractograms of bulk CeSO samples; these suggest that a few percent of residual Ag remains. A straw-colored compound with the filled PbFCl (i.e., ZrSiCuAs- or HfCuSi 2 -type) structure (space group P4/ nmm: a = 3.98171(1) Å, c = 8.70913(5) Å, V = 138.075(1) Å 3 ) and a composition close to LiCeSO, but with small amounts of residual Ag, is obtained by direct reductive lithiation of CeAg 0.8 SO or by insertion of Li into CeSO using chemical or electrochemical means. Computation of the band structure of pure, stoichiometric CeSO predicts it to be a Ce 4+ compound with the 4f-states lying approximately 1 eV above the sulfide-dominated valence band maximum. Accordingly, the effective magnetic moment per Ce ion measured in the CeSO samples is much reduced from the value found for the Ce 3+ -containing LiCeSO, and the residual paramagnetism corresponds to the Ce 3+ ions remaining due to the presence of residual Ag, which presumably reflects the difficulty of stabilizing Ce 4+ in the presence of sulfide (S 2- ). Comparison of the behavior of CeCu 0.8 SO with that of CeAg 0.8 SO reveals much slower reaction kinetics associated with the Cu 1- x S layers, and this enables intermediate CeCu 1- x Li x SO phases to be isolated.