Peierls distortion, high temperature

The phase transition consists of a cooperative mechanism with charge-ordering, anion order-disorder, Peierls-like lattice distortion, which induces a doubled lattice periodicity giving rise to 2 p nesting, and molecular deformation (Fig. 11c). The high temperature metallic phase is composed of flat EDO molecules with +0.5 charge, while the low temperature insulating phase is composed of both flat monocations... 

The trisulphides (and triselenides) of Ti, Zr, Hf, Nb and Ta crystallize in onedimensional structures formed by MSg trigonal prisms that share opposite faces. Metal atoms in these sulphides are formally in the quadrivalent state, and part of the sulphur exists as molecular anions, M S2 S . TaSj shows a metal-insulator transition of the Peierls type at low temperatures (Section 4.9). NbSj adopts a Peierls distorted insulating structure suggesting the possibility of a transformation to a metallic phase at high temperatures, but does not transform completely to the undistorted structure. Electronic properties and structural transitions of these sulphides have been reviewed (Rouxel et al, 1982 Meerschaut, 1982 Rouxel, 1992). 

Electrical conductivity measurements of the [TTF]2[Ni(edt)2] and [TTT ]12[Ni(edt)2] systems show them to be semiconductors with conductivities of 10-3 and 30 Q-1cm-1, respectively. In both compounds, the direction of highest conductivity is along the stacks of organic molecules. Since the [TTT]i.2[Ni(edt)2] system contains chains of partially oxidized organic molecules that appear to be uniformly spaced along the stacking axis, a high conductivity would be expected. The low room-temperature conductivity (compared with other partially oxidized TTT salts) and its temperature dependence, which is typical of a semiconductor, are presumed to be a consequence of the postulated Peierls distortion. 

The states have an uncertainty in energy 6e v fi/x, where x is the mean free time for scattering by impurities, phonons or particle-particle interactions. A FrShlich-Peierls type transition, opening up a gap 2A with a distortion corresponding to 2k, is possible only if the states are sufficiently well defined so that 6e < m A. The same applies to the gap from hybridization of inverted bands on donor and acceptor chains. A 3D type band structure is appropriate if 6e < m tj. To the extent that 6e depends on thermal fluctuations a ID band structure may be appropriate at high temperatures and a 3D at low. 

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