Rational Synthesis and Characterization of a New Family of Low Thermal Conductivity Misfit Layer Compounds ((PbSe)_(0.99))_m(WSe2)_n

摘要

We describe here a general synthesis approach for the preparation of new families of misfit layer compounds and demonstrate its effectiveness through the preparation of the first 64 members of the (PbSe)_(0.99))_m(WSe2)_n family of compounds, where m and n are integers that were systematically varied from 1 to 8. The new compounds (PbSe)_(1+y)_m(WSe2)_n were synthesized by annealing reactant precursors containing m layers of alternating elemental Pb and Se followed by n layers of alternating elemental W and Se, in which the thickness of each pair of elemental layers was calibrated to yield a structural bilayer of rock salt structured PbSe and a trilayer of hexagonal WSe2. The compounds are kinetically trapped by the similarity of the composition profiles and modulation lengths in the precursor and the targeted compounds. The structural evolution from initial reactant of layer elements to crystalline misfit layer compounds was tracked using X-ray diffraction. The crystal structures of new compounds were probed using both analytical electron microscopy and X-ray diffraction. The c-axis of the misfit layer compound is perpendicular to the substrate, with a c-axis lattice parameter that changes linearly with a slope of 0.612-0.615 nm as m is changed and n is held constant and with a slope of 0.654-0.656 nm as n is varied and m is held constant. The in-plane lattice parameters did not change as the individual layer thicknesses were increased and a misfit parameter of y = -0.01 was calculated, the first negative misfit parameter among known misfit layer compounds. Analytical electron microscopy images and X-ray diffraction data collected on mixed hkl reflections revealed rotational (turbostratic) disorder of the a-b planes.