Phase transition metal-insulator

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... 

Raman spectra were measured on fresh, chemically etched surfaces in quasi-backscattering configuration using a triple DILOR XY spectrometer, a liquid nitrogen cooled CCD detector, and a 514.5-nm Ar-ion laser. The laser beam of power level 20 mW was focused on an area of 0.1 mm2 on the mirror-like plane (it was the (ab) plane of the single crystals). The measurements were performed in a cryostat with a helium gas atmosphere in the temperature range 5-295 K below temperature of metal-insulator phase transition. 

Due to the second effect the system may reach a percolation threshold and consequently a metal-insulator phase transition may be induced by the magnetic field. It has to be stressed that the contributions to the effective magnetostriction of cobaltites mentioned above have a different dependence on temperature. An increase of temperature induces low-spin to intermediate-spin transitions. At the same time the volume of ferromagnetic clusters decreases with increasing temperature. The competition of these mechanisms leads to the unusual dependence of the effective magnetostriction of cobaltites on temperature. 

N. F. Mott, Metal-Insulator Phase Transitions, Taylor and Francis, London, 1974. 

An interesting feature that is common to all the quasi-one-dimensional organic conductors exhibiting metallic conductivity down to low temperatures is the existence of two partially filled bands of donor and acceptor stacks. The metal-to-insulator transition is usually associated with the opening of Peierls gap in at least one of these bands. Therefore it is of utmost interest to study alloys created by selective doping of different stacks in order to evaluate the effects of the two stacks on various physical properties and on the metal-insulator phase transition. Conclusions with regard to stabilization of the metallic phase to low temperatures will be presented. 

In elemental semiconductors and the polar faces of compound semiconductors, an odd number of electrons is formed per surface atom by the creation of a surface. The solid therefore undergoes a metal—insulator phase transition [82] to produce an even number of electrons per surface unit cell, thus reducing its symmetry in the plane of the surface. For non-polar faces of compound semiconductors, the simple truncated bulk geometry is already insulating in character because anionic and cationic species are electronically inequivalent. No distortions which reduce the symmetry are therefore necessary to provide stability, but the unbalanced ionic forces and unsaturated covalencies can produce quite large ( 0.5 A) atomic movements ( surface relaxation ). 

Peierls proposed the mechanism of the metal-insulator phase transition. 

Fig. 14. Metal-insulator phase transition temperature T as a function of anion radius (TMTSF)2X, T ...

The underlying physical mechanism has been found by Peierls in 1955. A 1-D metal is unstable, at zero Kelvin, with respect to a periodic modulation of the wave vector 2kp, opening a gap at kp in the reciprocal space This is a metal-insulator phase transition. 

The radical cation salts (BEDT-TTF)2l3 have drawn much attention, since one of them was found to be an organic superconductor. This series of salts can appear in many modifications, known as the a-, p-, 0-, K-phases. Under ambient pressure, the a-(BEDT-TTF)2l3 phase undergoes a metal-insulator phase transition at 135 K [1], while the p-, 0-, and K- (BEDT-TTF)2l3 become superconductors below -- 1.3 K, 3.6 K and 3.6 K, respectively [2-5]. After some particular pressure and temperature processing, the P-phase shows superconductivity at ambient pressure, up to a temperature as high as 8.1 K [6]. Unlike these phases, which are usually synthesized by an electrochemical method, two new phases, called p j - and -(BEDT-TTF)2l3, were synthesized recently by D. Zhu and co-workers by a diffusion method [7,8]. 

SmS is particularly interesting because of its first-order metal-insulator phase transition at about 6 kbar (Maple and Wohlleben 1971), and the fact that it was the first f material to be identified as having a valence transition. This phase transition may be understood as a sudden change in the size of the Sm ion, i.e., with a change in the Sm valence. By alloying with Y the lattice pressure in (Sm,Y)S can be arranged so that this change in valence... 

By adsorption of metal atoms, one can induce various reconstructed surfaces that show metal-insulator phase transitions. Some transitions apparently are of the Mott type [73], whereas the driving mechanism of others is still subject to discussion, particularly considering the role of defects. Glasslike, disordered states have been found, which are very similar to theoretical predictions for phase separation in correlated electron systems [105]. 

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