Robert B. Somoano

Physics and chemistry of MoS2 intercalation compounds

Abstract Molybdenum disulfide is a layered diamagnetic semiconductor. Trigonal prismatic coordination exists between the Mo and the S atoms, which are primarily covalently bonded. The layers can stack to form either a hexagonal (2H), or a rhombohedral (3R) polymorph. The reaction of MoS 2 with alkali and alkaline-earth metals dissolved in liquid ammonia results in a series of metallic superconducting intercalation compounds. The alkali and alkalline-earth intercalates, A x MoS 2 (A = Na, K, Rb, Cs, Ca, and Sr) can be separated into two groups according to their stoichiometry, structure and superconducting properties. The first group (I) consists of the Na, Ca, and Sr intercalates, which have nonhexagonal structures, less well-defined stoichiometries, small intercalate ion diameters, and superconduct at temperatures of 3.6, 4.0, and 5.6 K, respectively. The uncertainties in the stoichiometries in this group are Na x MoS 2 (0.3 ⩽ x ⩽ 0.6), Ca x MoS 2 (0.05 ⩽ x ⩽ 0.07), and Sr x MoS 2 (0.06 ⩽ x ⩽ 0.1). The second group (II) consists of the K, Rb, and Cs intercalates which form compounds of definite stoichiometry ( x ≌ 0.3) , have a well-ordered hexagonal crystal structure, larger intercalate ion diameters, and superconduct at 6.9 K. The compound Li x MoS 2 differs from the above groups in that NH 3 or NH 2 intercalates along with Li, and the structure exibits considerable disorder. It superconducts at 3.7 K. The electronic energy band structure of MoS 2 results from strong intralayer hybridization between metal d z 2 and d xy , d x 2 -y 2 bands resulting in at least a 1 eV gap within the d band manifold. The lowest d sub-band is filled in MoS 2 , and upon intercalation the Fermi level is raised into the upper d sub-band. The high density of states at the Fermi energy in the intercalates is believed to be the major reason for superconductivity up to 7 K. The superconducting critical magnetic fields are highly anisotropic, and are very large (greater than 20 Tesla for group II) for fields parallel to the layer planes. The critical field angular dependencies can be fitted to either a model for thin film superconductivity (group I), or to an anisotropic effective mass model (group II). An unusual positive curvature in the critical field vs. temperature plots is found in all compounds.

Stability of polyacetylene films

Abstract The oxidative and radiation stability of polyacetylene films have been determined. The radiation stability of the material is found to be quite good but the intrinsic thermal oxidative stability is poor and presents a serious obstacle to widespread terrestrial or space use.

Disorder and the ground state of bis(tetrathiatetracene) triodide TTT2I3

Abstract The low temperature ground state of TTT2I3 with varying amounts of iodide chain disorder has been studied using specific heat and infrared and far infrared reflectance and absorbance. Our results support the model of TTT2I3 as an organic conductor at room temperature which undergoes a metal to insulator transition near 50 K. At low temperatures we find a semiconducting gap of 80 cm−1 independent of disorder.

Examination of design options for 35 A h ambient temperature Li-TiS2 cells

Abstract Some of the design options for a rechargeable 35 A h Li-TiS2 cell have been examined. A specific energy of 80 – 100 W h kg−1 at the 2 h rat

Capacity—cycle life behavior in secondary lithium cells

Abstract The practical utilization of high energy density rechargeable lithium cells is dependent upon maintaining high capacity for the duration of the required cycle life. However, a critical, yet generic problem with room temperature lithium systems is that the capacity often declines considerably during the early stages of cycling. We report the results of our studies on electrolyte degradation which we observe after cells have undergone 300 and 700 deep cycles with 3-methylsulfolane- and 2-methyltetrahydrofuran—LiAsF6 electrolytes, respectively.

Nordheim-Gorter like behavior and one carrier conduction in a quasi-one-dimensional system

Abstract We report the extension and observation of Nordheim-Gorter type scaling of the thermoelectric power with the d.c. conductivity for the system of quasi-one-dimensional (QOD) conductors, (TTF) 12 (SeCN) 7 , (TTF) 12 (SCN) 7 , (TTF) 3 (Cl) 2 and (TTF) 7 (I) 5 . The consideration of additional scattering processes for one carrier transport leads to a thermopower which scales linearly with conductivity. Furthermore, such a TEP vs conductivity (or resistivity) analysis will identify single carrier (or bi-carrier) transport in other QOD systems as well.

Effects of disorder on the thermoelectric power and electrical conductivity of TTT2I3

(Tetrathiatetracene)2(iodide )3 is a quasi-one-dimensional organic metal consisting of segregated stacks of (TTT) cation radicals and polyiodide chains. we report the results of a study of the thermoelectric power and electrical conductivity measurements on single crystals of TTT2I3 in which the degree of disorder is varied. Effects of disorder on the phase transition and low temperature conductivity are discussed.