Unit cell controversy

Iodine heptafluoride exhibits a very similar dimorphism (99), with a high-temperature cubic form, unit-cell dimension a = 6.28 A, at -110°C, and a transition below -120°C to an orthorhombic form with a = 8.74, b = 8.87, andc = 6.14 A, measured at - 145°C. A single-crystal study of the low-temperature phase was reported to show a configuration for the heptafluoride distorted from the expected pentagonal bipyramid (100). There has been some controversy over the interpretation of the X-ray results but the experimental work is very difficult and the distorted model appears valid for the data available. The unit-cell similarity for the high-temperature form, and the low-temperature transition, suggests that rhenium heptafluoride is isostructural with the iodine compound. 

Although the crystal structure of PVA has extensively been studied by many research groups, the detailed structure has not been confirmed. In these investigations there is no conflict for a two-chain monoclinic unit cell with a = 0.783 nm, b 0.252 nm (fiber axis), c = 0.551 nm and )3 % 92° [35, 36]. However, the relative positions of the molecular chains in the 6-projection and the hydrogen bonding are still controversial subjects. 

Two additional developments in zeolite chemistry include the removal of tetrahedral aluminum atoms from the framework by complex-ing agents (53) and the increase in stability of a zeolite by essentially complete removal of the alkali metal cation (3). The latter process— ultrastabilization—is the center of some controversy. Two proposals for explaining the stability of these materials have been advanced one is based upon the removal of tetrahedral aluminum, which results in an increase in the Si/Al ratio of the framework, the formation of additional O—Si—O linkages, and a decrease in the unit cell constant. The other is based upon the complete removal of alkali metal ion, which may act as a catalyst in perturbing the structure at elevated temperatures. Although there may be merit to one or both proposals, probably neither is the sole explanation. 

Polymers can be either amorphous or semicrystalline in structure. The structure of amorphous materials cannot be described in terms of repeating unit cells such as that of crystalline materials because of nonperiodicity, the unit cell of an amorphous material would comprise all atoms. The physics and chemistry of the amorphous state remain poorly understood in many aspects. Although numerous experiments and theoretical studies have been performed, many of the amorphous-state features remain unexplained and others are controversial. One such controversial problem is the nature of glass-liquid transition. 

In addition to selection of the structure of the monomer as the basis for defining the internal coordinates of the repeat unit, the possible structures are usually further constrained by taking advantage of any symmetry possessed by the unit cell. The symmetry is derived from the systematic absence of reflections which are forbidden by the selection rules for a particular space group. In the case of cellulose, the simplification usually introduced is the application of the symmetry of space group P2, which includes a twofold screw axis parallel to the direction of the chains. The validity of this simplification remains the subject of controversy, however, because the reflections which are disallowed under the selection rules of the space group are in fact frequently observed. 

Other workers (19-20) have interpreted these differences in the NMR spectra and other data in alternative ways. They believe that celluloses I and II have the same skeletal conformation but are packed in different lattices. In this theory, the differences within the cellulose I family are derived from the size of the unit cells. Valonia contains a larger 8 chain unit cell, whereas ramie contains a mixture of the 8 chain unit cell and the smaller Meyer and Misch unit cell. Therefore the interpretation of the NMR spectra remains controversial. 

Early work on the solid-state structure of cellulose dates from 1929, when it was proposed that the unit cell has dimensions u = 8.14 A, = 14 A, c = 10.3 A, y = 62°, and contained two cellulose chains. This proposal has generated little controversy. 

A pattern of intensities that are absent for a given unit cell gives important clues about the space group, the description of the symmetry that applies to that crystalline form. For example, the intensities for the meridional reflections on the first, third, fifth, and higher odd layer lines are absent for the Ply space group. The number of chains can be inferred from the dimensions of the unit cell and the density of the sample. Whether there is true symmetry in the cellulose crystallites has always been controversial, with weak odd-order meridional reflections [199] often appearing, thus failing to meet the strict requirement of absence. These... 

The structure of water is a most ubiquitous and controversial concept in the literature concerning liquid water. In the solid, there exists a clear-cut structure due to the periodicity of the geometry of the unit cell. There is no such structure in the liquid, though in some cases, one refers to the local structure of a liquid as revealed by the function g R). For water, there is widespread usage of the concept of structure as a measure of the degree of crystallinity, or the extent to which the structure of ice persists after melting. Having this qualitative idea in mind, we seek to construct a precise definition of the structure which conforms with the... 

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