SOLID-STATE NMR STRUCTURAL STUDIES OF THE TERNARY MOLYBDENUM CLUSTER CHALCOGENIDES CDXMO6SE8 (X = 1, 2)

The structures of the ternary Chevrel phases CdMo6Se8 and Cd2Mo6Se8 ha ve been studied by several complementary Cd-111 NMR spectroscopic tech niques. Specifically, cadmium mobility and bonding properties are prob ed by temperature and frequency dependent measurements of static line shapes, magic angle spinning (MAS) NMR spectra, and spin-lattice relax ation rates. Furthermore, models for the spatial cadmium distribution are tested on the basis of Cd-111-Cd-111 dipole-dipole interactions, m easured by spin-echo decay spectroscopy on isotopically labeled materi als (97% Cd-111). CdMo6Se8 undergoes a phase transition near 130 K; th e cadmium ions are static on the NMR time scale over the whole tempera ture range in both phases. The spatial cation distribution is close to homogeneous, and it specifically excludes the presence of Cd-Cd dimer s. For the high-temperature phase, the Cd-111 spectra indicate a large degree of static disorder. In addition, the large Cd-111 chemical shi ft temperature coefficient and fast spin-lattice relaxation reveal str ong interactions between the intercalated cadmium atoms and the conduc tion band wave functions of the Mo6Se8 matrix. This behavior is typica l for charge-transfer intercalation compounds in which the conduction band is only partially filled. Cd2Mo6Se8, which crystallizes in the rh ombohedral <R(3)over bar> structure, shows the typical NMR signature o f a rigid, semiconducting compound. The intercalated metal species sho w no apparent interaction with conduction electron wave functions. Cd- 111 MAS and spin-echo decay data suggest a disordered atomic distribut ion of Cd2+ ions with a minimum Cd-Cd internuclear distance of 258 pm.