Amorphous anomalies

As a regional observation, arsenic is concentrated in the >1 mm soil fraction, and defines broad-scale anomalies in areas of shallow bedrock or residual soil. It has probably been adsorbed onto amorphous secondary Fe oxides, some of which have formed pisoliths. By contrast, Au is generally concentrated in the claysized fraction of transported soils. 

While considering trends in further investigations, one has to pay special attention to the effect of electroreflection. So far, this effect has been used to obtain information on the structure of the near-the-surface region of a semiconductor, but the electroreflection method makes it possible, in principle, to study electrode reactions, adsorption, and the properties of thin surface layers. Let us note in this respect an important role of objects with semiconducting properties for electrochemistry and photoelectrochemistry as a whole. Here we mean oxide and other films, polylayers of adsorbed organic substances, and other materials on the surface of metallic electrodes. Anomalies in the electrochemical behavior of such systems are frequently explained by their semiconductor nature. Yet, there is a barrier between electrochemistry and photoelectrochemistry of crystalline semiconductors with electronic conductivity, on the one hand, and electrochemistry of oxide films, which usually are amorphous and have appreciable ionic conductivity, on the other hand. To overcome this barrier is the task of further investigations. 

Y. Cao and A.N. Cormack. A structural model for interpretation of an anomaly in alkali aluminosilicate glasses at Al/alkali = 0.2-0.4. In H. Jain and D. Gupta, editors, Diffusion in Amorphous Materials, pages 137-151, Warrendale, PA, 1994. The Minerals, Metals and Materials Society. 

There appear to be no extrinsic field studies on the familiar amorphous chromia gel catalyst although it is known (24) to have only a slight indication of the zero field anomaly shown by a-Cr203 at TN. A gel catalyst aged in hydrogen has a small positive field effect at 323 K and a small negative effect at 273 K. 

The concept of amylopectin forming double helices easily integrates into the currently-accepted cluster model, with the short linear chains of the branches being intertwined into double helices, while the branch points are located in the more amorphous regions between the clusters of double helices. Understanding that parts of amylopectin molecules are capable of forming double helices explains the apparent anomaly that a branched polymer is the source of structural order within granules. 

The electrodeposition of Zn-Co and Zn-Fe alloys in an aqueous bath is classified as an anomalous codeposition [44] because the less noble Zn is preferentially deposited with respect to the more noble metal. This anomaly was attributed to the formation of Zn(OH)+ which adsorbs preferentially on the electrode surface and inhibits the effective deposition of the more noble metal. This anomaly was circumvented by using zinc chloride-n-butylpyridinium chloride ([BP]+C1 / ZnCf ) [27] or [EMIMJ+Ch/ZnCh [28] ionic liquids containing Co(II). The Zn-Co deposits can be varied from Co-rich to Zn-rich by decreasing the deposition potential or increasing the deposition current. XRD measurement reveals the presence of CosZ i in the deposited Zn-Co alloys and that the Co-rich alloys are amorphous and the crystalline nature of the electrodeposits increases as the Zn content of the alloys increases. Addition of propylene carbonate cosolvent to the ionic liquid decreases the melting temperature of the solution and allows the electrodeposition to be performed at a lower temperature. The presence of CoZn alloy is evidenced by the XRD patterns shown in Figure 5.2. 

The question of whether there is a tme glassy nature of amorphous ices is of interest when speculating about possible liquid-liquid transitions in (deeply) supercooled water. For true glasses, the amorphous-amorphous transitions described here can be viewed as the low-temperature extension of liquid-liquid transitions among LDL, HDL, and possibly VHDL. That is, the first-order like LDA <-> HDA transition may map into a first-order LDL HDL transition, and the continuous HDA <-> VHDA transition may map into a smeared HDL VHDL transition. Many possible scenarios are used how to explain water s anomalies [40], which share the feature of a liquid-liquid transition [202, 207-212]. They differ, however, in the details of the nature of the liquid-liquid transition Is it continuous or discontinuous Does it end in a liquid-liquid critical point or at the reentrant gas-liquid spinodal ... 

Temperature dependence of the resistivity in the low temperature grown films is similar to that in disordered and in amorphous materials. An interesting anomaly was noted in MOCVD grown films [18] with very high electron concentrations between 1020 and 1021, where an increase in mobility was observed below 4.2 K. 

The structural correlations are strongly enhanced in the under-cooled state as the temperature is reduced towaids the metastable limit of -40°C (to D2O) and various thermoph ical properties exhibit diverged behaviour [8]. The exact nature of this anomaly is still the subject of some controversy. However, the difiraction pattern indicates that the stmcture is evolving towards that of amorphous ice which is characterised as a continuous random networit of tetrahedral hydrogen-bonds [9]. Recent neutron measurements on amorphous ice [10] have re-infor the earlier conjectures tuid shown that the structure is similar to that of hyper-quenched glassy water produced by rapid cooling of micron-sized water droplets. It can now be realised that the CRN mo l for the disordered phase of ice is effectively the limiting stmcture of water at low temperatures. 

The only anomaly we encountered, apparent in the Table, was the finding that the three-carbon bis-phosphonic acid product, which had a good elemental analysis, did not exhibit any XRD lines. We conjecture that this apparent inability to form a crystalline product results from the fact that there is no obvious way to force a conformation on a three carbon chain so that the terminal phoshonates will be in a parallel configuration, as required by a layered structure. This, then, may be an example of an intrinsically amorphous product. 

Hall effect is another important transport phenomenon and has been extensively studied in amorphous semiconductors. The Hall effect studies also assumed importance because of an anomaly observed between the sign of the charge carriers indicated by Hall coefficient and S in amorphous semiconductors. The Hall coefficient Rh is given by. 

A special feature of amorphous materials is the anomalous behaviour of several properties at very low temperatures, such as in the low temperature specific heats. Most of these anomalies are attributed to the presence of two-level states (TLS) separated by small barriers, which gives rise to tunneling excitations. These excitations are characterized by wide distribution of relaxation times and energies. Several ultrasonic and low temperature specific heat measurements have been performed to characterize the TLS but their physical nature such as their structures, etc. is far from having been understood. These are the ADWP states discussed earlier in Chapter 7 in some detail. 

Upon cooling. These changes are associated with the anomalies of the specific heat at the ferroelectric transition. The results show that the disorder of the high-temperature paraelectric phase (T < Tc) is of dynamical origin. Fluorine-19 spin-lattice relaxation was also investigated. For measurements at 9.14 MHz the observed Ti appears to be dominated by the dynamics of the amorphous phase and exhibits no anomaly through the phase transition. However, from measurements at 20 MHz, well-defined minima in Ti were observed, and associated with the ferroelectric transition. 

The first interpretation of these data was in terms of relaxation via tunneling or disorder modes (Carlos and Taylor, 1980 Reimer a/., 1981c Jeffrey etai, 1981a), which were originally invoked to explain anomalies in the low temperature thermal properties of amorphous solids. Movaghar and... 

So far only the modulus for stretching parallel to the long axis of the sample has been considered. If, however the sample (a) were stretched perpendicular to that direction, equation (11.17) would apply, rather than equation (11.16). It is thus possible to account for the fact that the modulus of a drawn semicrystalline polymer is sometimes found to be greater when it is measured in the direction perpendicular to the draw direction than when it is measured in the direction parallel to the draw direction (see example 11.5). Such an anomaly is likely to disappear at low temperatures when the modulus of the amorphous material rises because it is no longer in the rubbery state. 

Perhaps a sensible procedure is to consider an approach which incorporates both interatomic potentials (classical forces) and fully quantum mechanical methods. One can compute the properties of smaller systems with quantum mechanical approaches and establish the accuracy, or inaccuracy, of interatomic potentials. For example, some elastic anomalies have been reported for a-cristobalite. These elastic anomalies indicated the presence of a negative Poisson ratio in this crystalline form of silica. With the use of interatomic potentials, it is a trivial matter to compute these properties. If the anomalies are confirmed via such calculations, it is likely that the experimental measurements are accurate, and more computationally intense calculations with quantum forces are merited. Another useful role of interatomic potentials is to perform molecular dynamics simulations, e.g., to examine the amorphization of quartz under pressure. One can easily compute the free energy of large systems as a function of both temperature and pressure via interatomic potentials. Sueh calculations can be useful as guides if interpreted in a judicious fashion. 

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