F. J. Di Salvo

Charge-density waves and superlattices in the metallic layered transition metal dichalcogenides

Abstract The d1 layer metals TaS2, TaSe2, NbSe2, in all their various polytypic modifications, acquire, below some appropriate temperature, phase conditions that their electromagnetic properties have previously revealed as ‘anomalous’. Our present electron-microscope studies indicate that this anomalous behaviour usually includes the adoption, at some stage, of a superlattice. The size of superlattice adopted often is forecast in the pattern of satellite spotting and strong diffuse scattering found above the transition. Our conclusions are that charge-density waves and their concomitant periodic structural distortions occur in all these 4d1/5d1 dichalcogenides. We have related the observed periodicities of these CDW states to the theoretical form of the parent Fermi surfaces. Particularly for the 1T octahedrally coordinated polytypes the Fermi surface is very simple and markedly two-dimensional in character, with large near-parallel walls. Such a situation is known theoretically to favour the formation of...

Topochemical reactions of rutile related structures with lithium

Abstract The topochemical lithiation of rutile related MO 2 with n-BuLi, and in nonaqueous lithium electrochemical cells is reported. This series illustrates the importance of electronic conductivity and cell volume to substantial lithium incorporation. The Li x MO 2 compounds are metastable and decompose at temperatures less than 250°C.

Metal intercalation compounds of TaS2: Preparation and properties

We report the preparation of intercalation compounds of 2H–TaS2 with post transition period elements. In general we find that elements to the left of the metalloid line in the periodic table will intercalate TaS2. Crystallographic lattice parameters, the maximum amount intercalated per Ta, magnetic susceptibility, and superconducting transition temperatures are also given.

The low temperature electrical properties of 1T-TaS2

Abstract Measurements of the electrical resistivity of 1 T -TaS 2 to 0.03 K show that the increase in resistivity below ∼ 50 K is extrinsic.Below 2 K the resistivity is described by ϱ = ϱ 0 exp T 0 /T) 1 3 . Because of this fractional power law behavior, we conclude that the increase is due to Anderson localization by random impurity and/or defect potentials. Other difficulties in understanding the properties of 1 T -TaS 2 are also pointed out.

Cathodes for nonaqueous lithium batteries based on VS2

Abstract We present preliminary data on cathodes for room temperature, high energy density, reversible lithium batteries based on the intercalation of lithium into V 1−x M x S 2 (M = Fe or Cr). LiV 1−x M x S 2 is first prepared from the metal oxides or carbonates in flowing H 2 S. The lithium may then be removed either by reaction with iodine in acetonitrile solution, or directly by charging an electrochemical cell. Theoretical energy densities between 510 and 550 WHr/Kg are calculated for these cells.

Properties of HxTaS2: Correlation between the superconducting Tc and an electronic instability in layer compounds

The preparation of HxTaS2 (0 < x < 0.87) is described. The compounds are only marginally stable at room temperature, slowly evolving H2S and H2 (and possibly H2O in air). Magnetic susceptibility data show that a low temperature transformation in 2H⋅⋅⋅TaS2 (at 80°K) is suppressed with the addition of hydrogen, and at the same time the superconducting transition temperature Tc rises from 0.8 to ∼4.2°K at x = 0.11. Heat capacity measurements near this concentration show the superconductivity to be a bulk effect. Finally, by correlation of this data with susceptibility and Tc measurements in other intercalation compounds, we suggest that the rise of Tc (at low electron transfer) is due to suppression of the low temperature transformation and not due to an excitonic mechanism of superconductivity.

Preparation and properties of a new polytype of tantalum disulfide (4Hb-TaS2)

Abstract A new polymorph of the layered compound tantalum disulfide, 4Hb-TaS 2 , has been prepared in single crystal form. Measurements of magnetic susceptibility, electrical resistivity, and differential heat capacity show two first order transitions at 20 and 315°K. The 315°K transition involves only a small volume and enthalpy change, as in the previously observed transitions of another polymorph, 1T-TaS 2 . The resistivity measurements show metallic like conductivity, as in 2H-TaS 2 , when the current is parallel to the layers, but semiconducting like conductivity, as in 1T-TaS 2 , when perpendicular to the layers. The properties of 4Hb-TaS 2 are moderately well described as a ‘mix’ of those of 1T-TaS 2 and 2H-TaS 2 .

Mapping energy bands in layer compounds from the angular dependence of ultraviolet photoemission

Abstract Photoelectron energy spectra have been measured on the layer compounds 1 T − TaSe 2 and 1 T − TaS 2 at various polar angles of emission. The azimuthal angle was set to select only those photoelectrons propagating in a plane containing the ГMLA points of the Brillouin zone. The results are used to map energy-vs-wave-vector curves which are found to compare favorably with the first principles calculations of Mattheiss.

Preparation and properties of 1TTaSe2

Abstract The preparation and physical properties of 1TTaSe2 are described, including: magnetic susceptibility, electrical resistivity, heat capacity, pressure dependence and volume change of a first order intrapolytypic transformation at 473°K. Electron diffraction and X-ray measurements show that a superlattice exists in this compound containing 26 formula units per unit cell. The physical properties are consistent with the observation of a superlattice.

Heat capacity of 2HNbSe2 at the charge density wave transition

Abstract Measurements of the heat capacity and resistivity of 2HNbSe 2 are presented, showing distinct anomalies at the onset of the charge density wave ( T 0 = 33.5 K).