Transitions disorder increase

Fig. 8 Temperature dependence of the four NMR peaks of squaric acid [20]. Note how the four peaks coalesce to one above the phase transition, but that the average of the peak positions does not stay constant, as required for a pure order/disorder transition. It increases around the transition temperature, emphasizing an additional displacive component, coexisting with the order/disorder one...

Gast and co-workers (Gast 1996 McConnell et al. 1993,1995) have used SAXS to probe ordered micellar structures in PS-PI block copolymers dissolved in decane, which is a preferential solvent for PI. They determined the form factor of a range of diblocks by performing SAXS in dilute solution (McConnell et al. 1993). The same diblocks in more concentrated solutions were found to form cubic micellar structures, and the gelation (micellar disorder-order) transition on increasing concentration was determined. Remarkably, both BCC and FCC struc-... 

At low temperatures, rj will be unity because all of the Cu atoms will be localized on A sites. 1 But the degree of disorder increases as the temperature increases until the Cu and Zn atoms are mixed randomly on the two sublattices and 77 = 0. This process, called a positional (order + disorder) transition, is often described as a cooperative phenomenon because it becomes easier to produce additional disorder once some disorder is generated. In the vicinity of a critical temperature, the order parameter rj behaves like the density difference (pi — pg) near the gas-liquid critical point. Thus,... 

The resistivity, p(T), and the resistivity ratio, = p(l. 4 K)/p(300 K), have been successfully used to quantify the relative disorder in different samples and for sorting out the various regimes [60,248-251]. In general, as the disorder increases, the materials become more insulating, and the conductivity decreases more rapidly upon lowering the temperature i.e. Pr increases. In fact, the resistivity ratio (p ) has proven to be useful as a elfective order parameter for the metal-insulator (M-I) transition in condncting polymers [60,252]. 

T0 5 bar ). The main transition is accompanied by a well pronounced 3 % change in volume, which is mainly due to changes of the chain cross-sectional area, because chain disorder increases drastically at the transition. The volume change at the main transition decreases slightly with increasing temperature and pressure along the main transition line. 

Another criterion for metallicity arises from electron localization induced by disorder proposed by Anderson. Electrons diffuse when the disorder is small, but at a critical disorder they do not diffuse (giving rise to zero conductivity). A transition from the metallic to the insulating state occurs as the disorder increases. Mott proposed that there exists a minimum metallic conductivity, a j , or maximum metallic resistivity, pmax. for which the material may still be viewed as being metallic, prior to the localization of electrons due to disorder. The Omin is given by where I is the mean free path of the electrons. Mott s... 

Filler et al (1969) observed changes in the electroreflectance of Ge-films with increasing disorder in these films. They found that when the film is fully disordered (substrate temperature during evaporation below 200 C) all structure disappears except for the peak corresponding to the transitions from the top of the valence bands (both the highest band and the split-off band) into the conduction band. The peaks move towards smaller energies as disorder increases. This result was interpreted as evidence that the split-off valence band exists in a-Ge (cf Section 4.3), however, some objections have been raised against this interpretation (Fischer (1971)). 

In the critical and insulating regimes, the resistivity ratio and positive MR increase as the extent of disorder increases. The field-induced transitions, from the metallic to the critical and from the critical to the insulating regimes, show that the mobility edge and Fermi level are situated rather close. Hence, due to interchain transport and disorder, conducting polymers are at the metal-insulator boimdary. 

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