Transport properties and the large anisotropic magnetoresistance of CuxNbS2 single crystals
The transport properties of CuxNbS2 (x = 0.09, 0.44 and 0.55) single crystals were systematically studied. The in-plane and out-of-plane resistivities decrease with increasing Cu content, and a transition with hysteresis shows up for the crystals with x = 0.44 and 0.55. The thermopower and Hall coefficient of CuxNbS2 show opposite signs, indicating that there are two kinds of carriers in this system. The angular dependences of the in-plane magnetoresistance (MRab = (ρab(H)−ρab(0))/ρab(0) × 100%) at different temperatures were also studied. The single crystals with x = 0.44 and 0.55 show a strong anisotropic MRab. For the x = 0.55 sample, MRab reaches 80% with a magnetic field of 14 T applied along the c-axis, while MRab is less than 5% for the magnetic field applied within the ab-plane. These results can be well understood in the light of the anisotropic Fermi surface in the multiband system.
Stoichiometric two-dimensional non-van der Waals AgCrS2 with superionic behaviour at room temperature
Layered materials have attracted tremendous interest for accessing two-dimensional structures. Materials such as graphite or transition metal dichalcogenides, in which the layers are held together by van der Waals interactions, can be exfoliated through a variety of processes in a manner that retains the structure and composition of the monolayers, but this has proven difficult for solids with stronger interlayer interactions. Here, we demonstrate the exfoliation of AgCrS2, a member of the AMX2 family (where A is a monovalent metal, M is a trivalent metal and X is a chalcogen), through intercalation with tetraalkylammonium cations, chosen for their suitable redox potential. The as-exfoliated nanosheets consist of Ag layers sandwiched between two CrS2 layers, similar to their structure in the bulk. They show superionic behaviour at room temperature, with an ionic conductivity of 33.2 mS cm−1 at 298 K that originates from Ag+ ions rapidly hopping between neighbouring tetrahedral interstices; in the bulk, this behaviour is only observed above 673 K. Layered materials held together by weak interactions can be exfoliated into monolayers that retain the structure and composition of their bulk counterpart, but this has remained challenging to achieve for non-van der Waals materials. Now, AgCrS2 has been exfoliated into such [CrS2]Ag[CrS2] nanosheets through intercalation with tetraalkylammonium cations chosen for their suitable redox potential. The nanosheets show superionic behaviour at room temperature.
Structure and superconductivity in lithium-intercalated niobium dichalcogenides
The series LixNbS2 (0 ≤ x ≤ 0.50) and LixNbSe2 (0 ≤ x ≤ 0.67) have been prepared by high-temperature reaction of Li2S, Nb, and S or Li2Se, Nb, and Se. Structures have been determined by analysis of X-ray powder patterns with polytypism observed in the sulfide series. Evidence of staging and Li ordering is reported, and crystal parameters are proposed for a second-staged sample, Li0.2NbSe2. In the series LixNbSe2, Tc exhibits a monotonic decrease with increasing lithium content, falling to less than 2 K by x = 0.25. In contrast, in the LixNbS2 series, Tc exhibits a more complicated pattern with Tc plummeting to below 2 K by x = 0.05, only to rebound to 3.7 K at x = 0.33 and then fall to below 2 K by x = 0.4. © 1985 American Chemical Society.