Irradiation disordering induced

Some materials are very sensitive to disorder [3]. In general, the low-temperature susceptibility follows x a T a (a5=3 0.6 to 0.9). (NMP)(TCNQ) and Qn(TCNQ)2 are examples of this effect, the disorder being intrinsic, attributed to the asymmetry of the cation. (HMTSF)(TNAP) has a similar behavior at low temperature, the disorder being attributed to the TNAP molecule. In (TTT)2I3+6 the disorder results from nonstoichiometry. Similar effects have been obtained when disorder is induced by irradiation... 

Hobbs LW, Sreeram AN, Jesumm CE, Berger, BA (1996) Structural freedom, topological disorder, and the irradiation-induced amorphization of ceramic stiuctures. Nucl Instr Meth Phys Res B116 18-25 Hong, HYP (1976) Crystal stiuctures and crystal chemistiy in the system Nai+xZr2SixP3-xOi2. Mater Res Bull 11 173-182... 

It is instructive to consider reasons for the nonisotropic, irradiation-induced expansion and the implications for the type of disorder produced. Eshelby showed that within the elastic-continuum model of a solid the strain fields caused by distortional defects produce expansion (or contraction) and further that the fractional change in macroscopic dimensions. A///, is equal to the fractional change in lattice parameter, AJ/d, if there is no change in the number of unit cells [156]. The latter was verified experimentally, and information about the radiation-induced disorder was obtained by comparing A/// with Ad/d, and by comparing these quantities with defect concentrations detected by techniques such as optical absorption [150-152,157]. It is possible to distinguish between... 

It is not clear why the earlier workers observed only the yellow or brown color and not the blue color. This may be due to differences between the anhydrous and hydrated materials and/or to differences in the production or stability of the 670 nm band. Since this band was found to be unstable at room temperature [59], it is possible that it was sufficiently unstable in their samples so as to be unobservable. It is also evident that under some irradiation conditions the band is not produced, so that the band may be due to an impurity which is not present in all samples. It is necessary to determine the defect responsible for this band before it is possible to resolve this matter. There is evidence that a band at 565 nm in KN3 is associated with the N2 defect [69], and it is tempting to associate the 670 nm band in Ba(N3)2 with this defect. The observations that after X-irradiation at room temperature or at 78°K the NJ defect is not observed and that the samples are yellow-brown, thus indicating the absence of the 670 nm band, tend to support the speculation. The NJ defect was also observed in Ba(N3)2 H20, but there is no information regarding color or optical properties [37]. There is, in addition, reason to associate the 300-nm band in Ba(N3)2 with the NJ molecular ion. Marinkas found an excitation band for conversion of NJ to N3 in this same region [52]. Correlated studies by magnetic resonance and optical absorption of Ba(N3)2 should further identify and characterize the irradiation-induced disorder. Studies by these techniques and of gas evolution will then undoubtedly enable the mechanism of decomposition to be established. 

Zhang, J., Wang, Y. Q., Tang, M., Valdez, J. A., and Sickafus, K. E. 2010a. Ion irradiation induced order-to-disorder transformations in 5-phase Sc42,Zt3+ (Oi2+ (/2. Nucl. Instrum. Methods B 268(19) 3018-3022. 

More generally, gels can undergo reversible order-disorder transitions, induced by changes either in temperature, irradiation, electric fields, pH (by chemical or electrochemical activation) or solvent properties. Figure 6.95 lists such different types of stimuli enabling a mechanical response of a polyelectrolyte gel. 

H Hanzawa, N Umemura, Y Nisida, H Kanda, M Okada, M Kobayashi. Disorder effecet of nitrogen impurities, irradiation-induced defects, and " C isotope composition on the Raman spectra in synthetic lb diamond. Phys Rev B 54 3793-3799, 1996. 

The most extensive studies of point defects in intermetallic compounds have been performed in ordered alloys. In such alloys both irradiation-induced disordering and irradiation-enhanced ordering can occur, as will be discussed in Section 3 (also see the review by Schulson, 1979). The principal objective of many of these studies was to attempt to elucidate the mechanisms responsible for the irradiation-induced... 

Irradiation-Enhanced Ordering and Irradiation-Induced Disordering of Ordered Alloys... 

The irradiation-induced disordering is expressed by Zee and WUkes (1980) as follows ... 

From measurements on the changes in electrical resistivity, lowering of the transition temperature for superconductivity as well as the magnetic saturation in ferromagnetic materials and decreased intensity in the diffraction intensity in superlattice reflections, it has been shown that irradiation-induced disordering occurs in alloys that were initially ordered. For a general review of some of these aspects, see Schulson (1979). 

Various mechanisms have been proposed for the rearrangement of atoms from their correct positions in an ordered lattice to a random distribution of the atoms as irradiation proceeds. These include thermal spikes by Seitz (1949), replacement collisions by Kinchin and Pease (1955), plastic spikes by Seitz and Koehler (1956), collapse of cascades to vacancy loops by Jenkins and Wilkens (1976), and random recombination by vacancies and interstitials by Carpenter and Schulson (1978). Some examples will now be given of experiments that have been undertaken in an attempt to elucidate some of the mechanisms of irradiation-induced disordering. 

Crystallinity has been studied by x-ray irradiation (85). An initial increase caused by chain scission in the amorphous phase was followed (above 3 kGy or 3 X 10 rad) by a gradual decrease associated with a disordering of the crystallites. The amorphous component showed a maximum of radiation-induced broadening in the nmr at 7 kGy (7 x 10 rad). 

The 355 nm emission is sharp and intense at the start of irradiation, and the intensity decreases with prolonged irradiation time. The 440 nm emission is weak and broad, and the intensity does not change with the irradiation time. Emission spectra of PMPrS obtained at ion fluences of 0.15,0.76, and 1.53 p,C/cm2 shows emission bands at 350 nm and 440 nm. The decrease in the intensity of the main peak indicates that main chain scission (photolysis) occurs under ion beam irradiation. Intense and sharp emission at 340 nm and weak broad emission at 440 nm for PDHS at 354 K are observed at the beginning of the irradiation and decrease on further irradiation. At 313 K and 270 K, sharp intense main emissions at 385 nm are seen. The 340 nm and 385 nm emission bands are assigned to a - a fluorescence. Experimental results have shown the presence of a phase transition at 313 K for PDHS.102,103 Below 313 K, the backbone conformation of PDHS is trans-planar, and above the solid-solid phase change temperature, a disordered conformation is seen. Fluorescent a -a transitions occur at 355 nm for PMPS, 350 nm for PMPrS, and 385 nm and 340 nm for PDHS. Emissions around 440 nm are observed at all temperatures examined and are assigned to defect and network structures induced by ion beams. 

---