Phase structural: electronic Peierls

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). 

For the experimentalist, the fundamental point is that the metallic state may not be stable down to very low temperatures. A phase transition might occur, which may or may not be a structural phase transition. In the first case, we will get an electronic Peierls distortion at 2 kp for a free electron gas, or a magnetic analog at 4 kp in the large-U approximation [28] (see Fig. 3). In the second case (AF and SC), there is a cooperative effect, which is manifested through different physical properties. 

The octet principle, primitive as it may appear, has not only been applied very successfully to the half-metallic Zintl phases, but it is also theoretically well founded (requiring a lot of computational expenditure). Evading the purely metallic state with delocalized electrons in favor of electrons more localized in the anionic partial structure can be understood as the Peierls distortion (cf. Section 10.5). 

The structure of MnP is a distorted variant of the NiAs type the metal atoms also have close contacts with each other in zigzag lines parallel to the a-b plane, which amounts to a total of four close metal atoms (Fig. 17.5). Simultaneously, the P atoms have moved up to a zigzag line this can be interpreted as a (P-) chain in the same manner as in Zintl phases. In NiP the distortion is different, allowing for the presence of P2 pairs (P ). These distortions are to be taken as Peierls distortions. Calculations of the electronic band structures can be summarized in short 9-10 valence electrons per metal atom favor the NiAs structure, 11-14 the MnP structure, and more than 14 the NiP structure (phosphorus contributes 5 valence electrons per metal atom) this is valid for phosphides. Arsenides and especially antimonides prefer the NiAs structure also for the larger electron counts. 

Peierls showed 74 [41,42] that an instability in a one-dimensional chain, with one electron per site, driven by electron-phonon interactions, can lead to a subtle structural distortion and to a first-order Peierls phase transition, at and below a finite temperature TP (the Peierls temperature) [42], For instance, at and below Tp either a dimerization into two sets of unequal interparticle distances d and d" (such that d + d" = 2d) or some other structural distortion must occur. The electronic energy of the metallic chain may also be lowered by the formation of a charge-density wave (CDW) of amplitude p(x) ... 

Phase transitions. Low-dimensional conductors undergo several types of specific structural phase transitions, such as the Peierls distortion (electron-phonon coupling), the spin-Peierls distortion (spin-phonon coupling), anion-ordering transitions, and so on. These first have to be detected and then measured and understood. However, the foregoing distortions may be very small and difficult to observe, and up to now, only a few lattice distortions have been fully measured and described. 

BEDT-TTF)4M(CN)4, M=Pt, Ni (6,7) In these compounds [6], the "ET" molecules form tetramerized slipped stacks (Fig. lb), already observed in other P-type "ET" salts, which present a tendency to a 2-D electronic structure. These salts exhibit also the two characteristic electronic (CT) absorption bands (Fig. 2) associated with strong vibronic IR modes, which are temperature-dependent [15]. Indeed, we have shown that there is a variation of the electron-molecular vibration (e-mv) coupling effect, associated with a "Peierls-like" phase transition around 200 K for both salts. These results are confirmed by the investigation of the electrical and the magnetic properties of the Ni(CN)4 salt, which indicate a metal-insulator transition in the same temperature range [9]. Nevertheless, low-temperature structural investigations are needed to fully characterize this phase transition. 

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