Phase metal-insulator

Both phases exhibit metalhc character at RT with <7rt — 90 and 20-70 2 cm for the H2O- and C4H802-derived materials, respectively. The water pseudoiso-morph remains metallic down to 4.2 K while the C4H8O2 pseudoisomorph shows a rather sharp metal-insulator transition below c. 100 K. In this case the metallic state is restored down to 4.2 K by application of hydrostatic pressures of 15 kbar. 

In the C4H8O2 case the metal-insulator phase transition seems to originate from structural modihcations as a function of temperature. Dimerization would explain such a transition because of the induced opening of the gap. 

Ota A, Yamochi H, Saito G (2002) A novel metal-insulator phase transition observed in (ED0-TTF)2PF6. j Mater Chem 12 2600-2602... 

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). 

In principle, then, the zero-temperature free energy of the system, plotted against volume or (in an alloy) composition, both denoted by x, must show a kink as illustrated in Fig. 4.2 at the metal-insulator transition. If x is the volume and this is decreased by pressure then there will be a discontinuous change of volume between B and A. If x denotes the composition then between B and A the alloy will be unstable, and will in equilibrium separate into two phases. The behaviour... 

In materials in which a metal-insulator transition takes place the antiferromagnetic insulating state is not the only non-metallic one possible. Thus in V02 and its alloys, which in the metallic state have the rutile structure, at low temperatures the vanadium atoms form pairs along the c-axis and the moments disappear. This gives the possibility of describing the low-temperature phase by normal band theory, but this would certainly be a bad approximation the Hubbard U is still the major term in determining the band gap. One ought to describe each pair by a London-Heitler type of wave function... 

Fig. 63 Generalized phase diagram of the metal-insulator transition in V203 as a function of doping with Cr or Ti and as a function of pressure, showing the critical point...

Clearly the metal-insulator transition is by no means so sharp as predicted for a first-order change of phase, but much disorder broadening is observed in the Raman spectra (Lemos et al. 1980). 

The bronzes with composition MxW03, where M is an alkali metal, show metallic behaviour, with conductivities of about 500 Q 1 cm 1 near the transition (Lightsey 1973), rising to 104-105 Q 1 cm 1 for higher values of x. The metal-insulator transition is associated with one or more changes of phase to different structures with insulating properties for details see Hagenmuller (1971) and Doumerc et al (1985). This can, however, be avoided by the addition of fluorine or tantalum (see below). 

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