Abstract
The structures of the ternary Chevrel phases CdMo{sub 6}Se{sub 8} and Cd{sub 2}Mo{sub 6}Se{sub 8} have been studied by several complementary {sup 111}Cd NMR spectroscopic techniques. Specifically, cadmium mobility and bonding properties are probed by temperature and frequency dependent measurements of static line shapes, magic angle spinning (MAS) NMR spectra, and spin-lattice relaxation rates. Furthermore, models for the spatial cadmium distribution are tested on the basis of {sup 111}Cd-{sup 111}Cd dipole-dipole interactions, measured by spin-echo decay spectroscopy on isotopically labeled materials (97% {sup 111}Cd). CdMo{sub 6}Se{sub 8} undergoes a phase transition near 130 K; the cadmium ions are static on the NMR time scale over the whole temperature range in both phases. The spatial cation distribution is close to homogeneous, and it specifically excludes the presence of Cd-Cd dimers. For the high-temperature phase, the {sup 111}Cd spectra indicate a large degree of static disorder. In addition, the large {sup 111}Cd chemical shift temperature coefficient and fast spin-lattice relaxation reveal strong interactions between the intercalated cadmium atoms and the conduction band wave functions of the Mo{sub 6}Se{sub 8} matrix. This behavior is typical for charge-transfer intercalation compounds in which the conduction band is only partially filled. Cd{sub 2}Mo{sub 6}Se{sub 8}, whichmore » crystallizes in the rhombohedral R{bar 3} structure, shows the typical NMR signature of a rigid, semiconducting compound. The intercalated metal species show no apparent interaction with conduction electron wave functions. {sup 111}Cd MAS and spin-echo decay data suggest a disordered atomic distribution of Cd{sup 2+} ions with a minimum Cd-Cd internuclear distance of 258 pm.« less
Published Version
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