Abstract
The chemical accessibility of the CeIV oxidation state enables redox chemistry to be performed on the naturally coinage-metal-deficient phases CeM1- xSO (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) Å3) and a composition approaching CeSO is obtained by deintercalation of Ag from CeAg0.8SO. 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 HfCuSi2-type) structure (space group P4/ nmm: a = 3.98171(1) Å, c = 8.70913(5) Å, V = 138.075(1) Å3) and a composition close to LiCeSO, but with small amounts of residual Ag, is obtained by direct reductive lithiation of CeAg0.8SO 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 Ce4+ 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 Ce3+-containing LiCeSO, and the residual paramagnetism corresponds to the Ce3+ ions remaining due to the presence of residual Ag, which presumably reflects the difficulty of stabilizing Ce4+ in the presence of sulfide (S2-). Comparison of the behavior of CeCu0.8SO with that of CeAg0.8SO reveals much slower reaction kinetics associated with the Cu1- xS layers, and this enables intermediate CeCu1- xLi xSO phases to be isolated.
Highlights
CeCuSO1 and CeAg0.8SO2 crystallize in the tetragonal ZrCuSiAs structure (Figure 1), consisting of alternately stacked PbO-type CeO layers and anti-PbO-type CuS layers
We have previously shown that some samples reported as “CeCuSO” did not conform to the expectations of the lanthanide contraction because facile oxidation of CeCuSO occurs in moist air to form CeCu0.87SO and CuO, with Ce oxidized above the +3 oxidation state,[1] and similar behavior can be exploited in the property tuning of other oxide chalcogenides, notably Sr2CoO2Cu2S2 which oxidizes readily in moist air,[8] and Sr2MnO2Cu1.5S2 which may be oxidized to Sr2MnO2Cu1.3S2 using iodine solution,[9] resulting in significant changes in the details of the crystal structures and the magnetic ordering
Comparison of the unit cell volumes of LnAgSO and LnCuSO with our reported results for stoichiometric CeCuSO1 and the reported cell volume for the proposed stoichiometric “CeAgSO”2 is consistent with the results of our syntheses that the maximum Ag content obtainable using the high-temperature synthesis is close to CeAg0.9SO
Summary
CeCuSO1 and CeAg0.8SO2 crystallize in the tetragonal ZrCuSiAs structure (Figure 1), consisting of alternately stacked PbO-type CeO layers and anti-PbO-type CuS layers. Despite the removal of almost all the Ag+ from the tetrahedral sites in the sulfide layers, there is no evidence for the formation of S−S bonds following this oxidative deintercalation This contrasts with the case for compounds with trivalent lanthanides in which compounds of formula LnSO contain [S2]2− ions.[56] The preliminary analyses of diffraction patterns of CeSO samples measured using high-resolution synchrotron X-ray diffraction using peak shapes with only Gaussian and Lorentzian terms to account for particle size and strain broadening (as used in the refinement of the LiCeSO model) were inadequate (Figure S4). This likely enhances the mobility of Ag+ ions compared with the Cu+ ions in CeCu0.8SO, in line with the observations that the activation barrier for Ag+ motion in LaAgSO which is reported as 0.195 eV65 or 0.22 eV5 is significantly smaller than that reported for Cu+ (0.32 eV) in the analogue LaCuSO.[66]
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