Solid-State NMR Structural Studies of the Ternary Molybdenum Cluster Chalcogenides Cd_xMo₆Se₈ (x = 1, 2)

Abstract

The structures of the ternary Chevrel phases CdMo 6 Se 8 and Cd 2 Mo 6 Se 8 have been studied by several complementary 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 spinlattice relaxation rates. Furthermore, models for the spatial cadmium distribution are tested on the basis of 111 Cd-111 Cd dipole-dipole interactions, measured by spin-echo decay spectroscopy on isotopically labeled materials (97% 111 Cd). CdMo 6 Se 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 111 Cd spectra indicate a large degree of static disorder. In addition, the large 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 6 Se 8 matrix. This behavior is typical for charge-transfer intercalation compounds in which the conduction band is only partially filled. Cd 2 Mo 6 Se 8 , which crystallizes in the rhombohedral R3 h structure, shows the typical NMR signature of a rigid, semiconducting compound. The intercalated metal species show no apparent interaction with conduction electron wave functions. 111 Cd MAS and spin-echo decay data suggest a disordered atomic distribution of Cd 2+ ions with a minimum Cd-Cd internuclear distance of 258 pm.

New Insight on the Unusually High Ionic Mobility in Chevrel Phases

“… a The interpretation of the experimental data related to Cd 2+ insertion into Mo 6 S 8 differs from that of ref , , and (see text). …”

“…This suggestion agrees well with most of the synthetic results, which showed that CPs of group I could be obtained only at high temperatures, in contrast to CPs of group II, which can be produced by chemical or electrochemical reversible reactions as well at ambient temperatures. However, our analysis of available experimental data ,− ,,− ,− (Tables and ) demonstrates that the ion-transport behavior in CPs is much more complex; it includes (i) apparent immobility of the large M cations like Pb 2+ , Sn 2+ , Ag + in the ternary phases, MMo 6 T 8 ; (ii) coupled M + M′ diffusion in the quaternary phases, M x M′ y Mo 6 T 8 , where both large and small cations can assist; (iii) cation trapping in the Mg−Mo 6 S 8 , Cd−Mo 6 S 8 , and Na−Mo 6 T 8 systems; (iv) a combination of low and high rate diffusion kinetics at the first and last intercalation stages, respectively, for the Cu−Mo 6 S 8 , Mn−Mo 6 S 8 , and Cd−Mo 6 Se 8 systems; and (v) a fast ionic transport for small cations like Ni 2+ , Zn 2+ , and Li + . Thus, contrary to general expectation, there is no unambiguous correlation between the cation mobility and its size.…”

“…One of the major problems of such systems is slow transport of the respective ions in inorganic hosts that can serve as cathodes for such batteries. Until now, only one family of materials, namely Chevrel phases (CPs), M x Mo 6 T 8 (M = metal, T = S, Se), is known to allow for a fast and reversible insertion of various cations, monovalent (Li + , Na + , Cu + ) as well as divalent: Zn 2+ , Cd 2+ , Ni 2+ , Mn 2+ , Co 2+ , Fe 2+ , and Mg 2+ at ambient temperatures, − which is associated with high electronic conductivity of the intercalation compounds.…”

Crystallography of Chevrel Phases, MMo6T8 (M = Cd, Na, Mn, and Zn, T = S, Se) and Their Cation Mobility

“…The remarkable ionic conductivity of Chevrel phases (CPs), M x Mo 6 T 8 (M = metal, T = S, Se,Te) remains in the shadow of their striking superconductivity. However, CPs are, at present, the only materials that allow fast and reversible insertion of various cations, monovalent (Li + , Na + , Cu + ) as well as divalent Zn 2+ , Cd 2+ , Ni 2+ , Mn 2+ , Co 2+ , Fe 2+ , and Mg 2+ at ambient temperatures. − Moreover, the insertion of an M cation into metal-containing CPs, M′ x Mo 6 T 8 ( x ≠ 0), may result in the displacement reaction associated with an unusual coupled M + M′ cation motion in the same crystal structure. , The key question here is what is so special in CPs that ensures the exceptionally high mobility of multivalent cations?…”

Cd111andCd113spin-lattice relaxation inBeckmann1, Bai2, Dybowski3 2005Phys. Rev. B

“…There have been very few reports of solid-state 111 Cd and/or 113 Cd nuclearmagnetic-resonance ͑NMR͒ relaxation studies. Spin-lattice relaxation times T 1 have been reported in the pure metal, 4,5 in Cd x Mo 6 Se 8 ͑x =1,2͒, 6 and in a variety of doped semiconductor crystals. 7,8 In the experiments reported here, 111 Cd and 113 Cd spectra and magnetization recoveries for CdMoO 4 were observed using static samples with a Bruker MSL-300 NMR spectrometer at a magnetic field of 7.049 T, where the proton resonance frequency is 300.130 MHz.…”