TEM STUDY OF THE MISFIT INTERCALATION COMPOUNDS ALPHA-HG1.19TAS2 AND BETA-HG1.3TAS2

Transmission electron microscopy has been employed to study the in-pla ne structure of intercalated Hg guest layers in the stage 1 misfit int ercalation compounds alpha-Hg1.19TaS2 and beta-Hg1.3TaS2. Both phases are remarkable for their complex composite crystal structures and for the very different Hg guest layer structures that they exhibit in spit e of their similar Hg uptakes, Micro diffraction patterns of alpha-Hg1 .19TaS2 suggest a chainlike Hg arrangement. Each Hg atom is surrounded by two nearest neighbor atoms at 2.78 Angstrom and four next nearest neighbors at 3.2 Angstrom. The Hg chains form an ordered sublattice th at shares two commensurate axes with the TaS2 host matrix (a and c axe s) but is incommensurate along the Hg chain direction (b axis), In the case of beta-Hg1.3TaS2, hexagonal close-packed Hg layers are formed. Here each Hg atom encounters six nearest neighbors at a 2.9 Angstrom d istance. The Hg sublattice is rotated by about 30 degrees with respect to the TaS2 host lattice. Thus, the total structure falls close to co mmensurability: the TaS2 host layers can be viewed as a 2 x 2 superstr ucture built upon the hexagonal closed packed Hg layers. The Hg arrang ements observed in alpha-Hg1.19TaS2 and beta-Hg1.3TaS2 bear a strong r esemblance to the (101) lattice planes in solid state beta-Hg and the hexagonal Hg layers in Hg(3)MF(6) (M = Ta, Nb), respectively. This ana logy allows us to draw some instructive conclusions about the presence of the Hg metal bonding and its influence on the resulting crystal st ructures.