Random tetrahedral network

These results show that the HB random tetrahedral network is formed inside the metastable supercooled regime. Note that NHB and PHB are also present in the LDA phase, indicating that the dynamic behavior of LDA is not completely... 

One explanation of this situation might be that the water like density maximum of silica is not in fact the density maximum that is observed experimentally, but Is Instead a thermodynamic feature that exists in the high T regime beyond the range of previous experiments. However, such an explanation then implies that the observed density maximum at 1823 K is a distinct feature associated with a well-structured random tetrahedral network. There Is precedence for such a possibility, in the fact that a low-temperature density maximum occurs in both ice Ih and LDA ice at approximately 50 K [33]. In addition. If there are two density maxima in liquid silica, they must necessarily be separated by a density minimum. Again, the... 

Note added in proof. Earlier in the text it was mentioned that the model used to describe the structure function of low density H20(as) does not describe that of high density H20(as). However, Narten, Venkatesh and Rice 27) do show than an ice I-like network with a near neighbor distance of 2.76 A has the density and distance spectrum of high density H20(as) if one permits 45% of the cavities characteristic of this structure to be occupied by water molecules. These are not ordinary unbonded interstitials. If the cavity molecules are located on the c axis at a distance of 2.76 A from the nearest network molecule each cavity molecule would have second neighbor network molecules at a distance of 3.25 A. Moreover, since occupancy of 45% of the cavities implies that 81% of the water molecules are part of the tetrahedral network and 19% in cavity positions, the average coordination number of nearest neighbors in this model is 4.3, as is found for H20(as) 10 K/10 K. Structure functions calculated for this interstitial variant of a randomized ice I model (the randomization is effected as in the simple ice I... 

Analysis of the HB network based on IS configurations has shown that local PES minima contain both linear bonds (LBs) and bifurcated bonds (BBs) whose fraction is both temperature- and density-dependent. The HB network tends to form a random but nearly tetrahedral network (no bifurcated bonds) on cooling, or on lowering the pressure. 

Zachariasen could successfully explain why certain CNs are favoured for glass formation. Zachariasen random network theory formed the basis of glass formation. During his studies, he observed that the silicate crystals have a tetrahedral network and readily form glass. These tetrahedral networks are symmetrical, non-periodic, and connected with each other at the comers. This leads to the 3-D network extended over aU directions with the isotropic property attributed to the amorphous nature of glasses. He elucidated the following points during his studies. 

Figure 8.2 Shows a random covalent network of tetrahedrally-bonded atoms such as Si or Ge. Note that the coordination of atoms changes. Some regions show the usual six-membered ring strueture found in crystalline materials (rings marked A ). Other areas contain smaller (for example the five-membered ring, marked B , or larger rings. It is rare to find rings larger than eight atoms or smaller than four atoms.

Coordination of atoms in inorganic amorphous semiconductors is usually either two-fold (chain-like structure) or four-fold (tetrahedron based structure). Tetrahedron-based structures consist of randomly-organized networks of tetrahedrally-coordinated atoms. Bonding distortions are primarily in bond angle rather than bond length. 

There are two classes of solids that are not crystalline, that is, p(r) is not periodic. The more familiar one is a glass, for which there are again two models, which may be called the random network and tlie random packing of hard spheres. An example of the first is silica glass or fiised quartz. It consists of tetrahedral SiO groups that are linked at their vertices by Si-O-Si bonds, but, unlike the various crystalline phases of Si02, there is no systematic relation between... 

In summary, pure liquid water consists of HgO molecules held in a random, three-dimensional network that has a local preference for tetrahedral geometry but contains a large number of strained or broken hydrogen bonds. The presence of strain creates a kinetic situation in which HgO molecules can switch H-bond allegiances fluidity ensues. 

For reasons which will become clear, we examine first the case of high temperature H20(as). Two random network models relevant to our hypothesis have been described in the literature. Both are based on distortions from a single locally tetrahedral structure that is like ice Ih. Kell s model 77> is much too small to be very useful. Nevertheless, its successful construction, just as for the case of Ge(as) 78>, Si02(as) 79>, and others, shows the viability of the random network concept. 

The model just described does not conform in detail to the random network model proposed earlier. In particular, the use of a continuum outside the nearest-neighbor tetrahedral structure removes some of the correlations inherent in a continuous network. Nevertheless, given the existence of a broad OOO angular distribution, coupled to an 00 distance distribution much broader than in H20(as), it is unlikely that this assumption introduces any features in serious disagreement with those characteristic of a random network model. 

The special electrical properties of a-AgI inevitably led to a search for other solids exhibiting high ionic conductivity preferably at temperatures lower than 146°C. The partial replacement of Ag by Rb, forms the compound RbAgJs. This compound has an ionic conductivity at room temperature of 25 S m , with an activation energy of only 0.07 eV. The crystal structure is different from that of a-AgI, but similarly the Rb and T ions form a rigid array while the Ag ions are randomly distributed over a network of tetrahedral sites through which they can move. 

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