Synthesis, Characterization and Optical Properties of Misfit Layer Chalcogenides [(PbSe)_1+y]_m(NbSe₂)_n and [(SnSe)_1+y]_m(VSe₂)_n

Misfit layer dichalcogenide compounds [(MX)_1+y]_m(TX₂)_n consist of a stacking of rock salt (MX) and transition metal dichalcogenide (TX₂) layers (M = Sn, Pb, Bi, rare earth; X = S, Se; T = Ti, V, Nb, Ta, Cr) where at least one lattice parameter differs between the two sublattices, and m and n give the stacking arrangement of the MX and TX₂ layers.[1] We have synthesized single crystals of [(PbSe)_1+y]_m(NbSe₂)_n with m=1 and n=1, 2 and 3 via the iodine vapour transport technique. The structure and composition of the single crystals were investigated by X-ray diffraction and Energy Dispersive X-ray spectroscopy respectively and the crystals were further characterized by magnetization and resistivity measurements. The parent compound 2H-NbSe₂ is a type II superconductor with a Tc of 7.2 K and a known charge density wave transition at 35 K.[2] As expected the superconducting transition temperature of [(PbSe)_1+y]_m(NbSe₂)_n was found to increase with increasing n, the number of dichalcogenide layers inserted between rock salt layers ( n=∞ for NbSe₂). We measured the room temperature absolute reflectance over a wide wavenumber range with the goal of elucidating trends in the optical properties via Drude-Lorentz fitting and Kramers Kronig analysis. We found an approximate isosbestic point in the optical conductivity which is common in strongly correlated materials. We have also attempted to synthesize [(SnSe)_1+y]_m(VSe₂)_n using the same technique. The parent compound VSe₂ exhibits a charge density wave transition near 110 K. We are interested to investigate the effect of the coupling between the two sublattices in the misfit layer compound on the charge density wave state. Results of our studies on these two misfit layer dichalcogenide systems will be presented.

[1] D.R. Merrill, et al. Materials, 8:2000 (2015).

[2] M. Naito and S. Tanaka, J. Phys. Soc. Jpn. 51:219-227 (1982).