Ice-Ih, structure

To illustrate the size effect to the DOS, a series of LD calculations for a number of different size of ice Ih structures based on the same model were made as shown in Fig. 20. The curve (a) is the DOS for the Cmc2 structure. In this structure, all bonds in c-axis are strong and all bonds in the basal plane are weak. Therefore, the peak at 28 meV appears only in the integrated modes associated with vibrations in basal plane and the peak at 37 meV appears only on the c-axis of the hexagonal structure [78], Although the peak positions are correct, the shape of the calculated spectrum does not agree with experimental data - the two peaks are very sharp and well separated from one another. This simulation result resembles the Renker s [49] and Criado et al s [67] results. 

The molecular theory of structural change from low-temperature liquid water to that of ice Ih structure is yet incomplete. Eisenberg and Kauzmann (1969) suggested that the observed temperature dependence of the molar volume of water can be explained qualitatively based on two competing effects (1) continuation of the strengthening of the four-coordination of molecules keeps the ice form ih an open structure (2) the expected decrease in all the anhar-monic vibrations will lead to a decrease in the ice volume. Probably, it is the dominance of the first effect that keeps ice Ih in a low density state. 

In DNA the transition from interfacial to bulk water occurs after ca 1 layer but grana membranes retain interfacial water up to ca 2 layers on each surface. Similarly the water in close contact with gelatine and chitosan did not have the ice Ih structure [43]. 

The oxygen atom pair correlation function for crystalline ice Ih is shown in Fig. 7b. Note that the first peak in the function hoo(R) is completely resolved. The NVR analysis of the large s part of the structure function indicates that this... 

Fig. 7 a. Structure function of polycrystalline ice Ih prepared by vapor deposition at 77 K from X-ray diffraction (from Ref. 27>)... 

Finally, the density of crystalline ice Ih derived by NVR is 0.93 gm cm-3. Given the uncertainty in the determination, which depends on a fitting of the structure function for small R, this can be considered identical to the density of 0.924 gm cm"3 computed from the cell parameters. 

Having called attention to the similarity in the values of vi for H20(as) and ice Ih, we must now call attention to the difference. In the case of the fully coupled OH stretching motion this is 25 cm-1 in the case of the uncoupled OH stretching motion this is also 25 cm-1 in the same direction [n H20(as) >n (ice Ih)]. It is interesting that the uncoupled value of vi in ice II is 3373,3357 and 3323 cm-1, in ice III is 3318 cm-1, in ice V is 3350 cm-1 and in ice VI is 3338 cm-1 27h Each of these ice forms has a more complex crystal structure than ice Ih. In general ices II-VIII have higher density than ice Ih, and have some severely bent... 

To proceed further we require some information about the structures of ices Ih, II and III, and how these structures can be converted, one to the other. In the following we draw heavily upon an excellent discussion published by von Hippel and Farrell 74>. 

Fig. 44. Hexagon structure of Ice Ih (a) reference plane of puckered rings perpendicular to "c (b) hexagon channels viewed parallel to "e (c) fore-shortened channel in "a" direction (from Ref. 74>)...

We are, therefore, led to -propose that low temperatures Hrandom network structure of mixed lattice parentage, i. e. that the structure is characterized by locally random 000 angle deviations from ice Ih or Ic), ice II and ice III type lattices. 

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. 

Fig. 6. The crystal structure of ice (a) the basic unit, showing the tetrahedral arrangement of four oxygen atoms round any one oxygen (b) the open arrangement of these units as they fit together in ordinary ice(Ih) (c) the alternative positions for the hydrogen atoms, which are represented by the small filled circles, two along each hydrogen bond. (The vertical line, marked with 3 in (a), is a threefold symmetry axis all the vertical lines in (b) coincide with trigonal axes of the ice crystal.)...

All common natural gas hydrates belong to the three crystal structures, cubic structure I (si), cubic structure II (sll), or hexagonal structure H (sH) shown in Figure 1.5. This chapter details the structures of these three types of hydrate and compares hydrates with the most common water solid, hexagonal ice Ih. The major contrast is that ice forms as a pure component, while hydrates will not form without guests of the proper size. 

Yet, because all three common hydrate structures consist of about 85% water on a molecular basis, many of the hydrate mechanical properties resemble those of ice Ih. Among the exceptions to this heuristic are yield strength, thermal expansivity, and thermal conductivity. The final portion of this chapter examines mechanical, electrical, and transport properties with emphasis on those properties that differ from ice. 

The most common solid form of water is known as ice Ih (hexagonal ice), with the molecular structure as shown in Figure 2.1 from Durrant and Durrant (1962). In ice each water molecule (shown as a circle) is hydrogen bonded (solid lines) to four others in essentially tetrahedral angles (Lonsdale, 1958). A description of... 

FIGURE 2.1 Basic crystal structure for ice Ih. (Reproduced from Durrant, P.J., Durrant.B., Introduction to Advanced Inorganic Chemistry, John Wiley Sons, Inc., New York (1962). With permission.)... 

Second, the composite hat-curved-harmonic oscillator model provides a good perspective for a spectroscopic investigation of ice I (more precisely, of ice Ih), which is formed at rather low pressure near the freezing point (0°C). The molecular structure of ice I evidently resembles the water structure. Correspondingly, well-known experimental data show a similarity of the FIR spectra (unlike the low-frequency spectra) recorded in liquid water and in ice Ih. This similarity suggests an idea that rotational mobility does not differ much in... 

The density of eHDA is 1.13 g/cm3, which is about 4% less than the density of uHDA [200], The different degree of structural relaxation should be evident when studying the thermal stability at 1 bar. The well-relaxed state is expected to be thermally more stable, and this expectation is confirmed experimentally At 1 bar, eHDA is stable up to 135 K, whereas uHDA is stable merely up to 115 K [190, 194, 196], Earlier, Johari [201] had noticed that HDA produced from LDA ( eHDA ) shows ultrasonic properties that differ from HDA produced from ice Ih ( uHDA ). 

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