Inherent crystal structures

The difference in hydraulic activity of dicalcium silicates arises as a result of the difference in stability of inherent crystal structure. The usual form of dicalcium silicate in Ordinary Portland Cement (OPC) is p-CjS, which reacts more slowly with water and results in the lowest rate of heat... 

Wilmann PG, Petersen J, Pettikiriarachchi A, Buckle AM, Smith SC, Olsen S, Perugini MA, Devenish RJ, Prescott M, Rossjohn J (2005) The 2.1 angstrom crystal structure of the far-red fluorescent protein HcRed Inherent conformational flexibility of the chromophore. J Mol Biol 349 223-237... 

Owing to the strong intermolecular %—% stacking in planar Pcs and the resulting inherent lack of solubility, in particular for the parent unsubstituted Pcs, together with the limited species of unsubstituted Pcs, the development of Pc chemistry faces austere challenges. In addition, these problems hinder deep investigation of the crystal structures of Pcs, which not only makes it difficult to establish the relationship between various physical behaviours and the solid state structures of Pcs, but also has limited, to some extent, their widespread application. 

Apart from these, there are volume defects that cannot conveniently be described in any other terms. The most important of these consist of regions of an impurity phase—precipitates—in the matrix of a material (Fig. 3.39). Precipitates form in a variety of circumstances. Phases that are stable at high temperatures may not be stable at low temperatures, and decreasing the temperature slowly will frequently lead to the formation of precipitates of a new crystal structure within the matrix of the old. Glasses, for example, are inherently unstable, and a glass may slowly recrystallize. In this case precipitates of crystalline material will appear in the noncrystalline matrix. 

The correction of mistakes is an important aspect of crystal nucleation and is achieved by the inherent weakness of most interactions in the typical molecular crystal. Such correction could be facile because only around 15 % of all crystal structures in the CSD are disordered. 

Hr was the first binuclear iron protein for which a three-dimensional structure was determined and is the most extensively studied. Thus, the protein has become the standard of comparison for other binuclear iron proteins, with the inherent hazards of defining a class on the basis of one example. Indeed, recent crystal structures for model compounds and the RNRB2 subunit indicate some surprises and substantial diversity in the class. Nonetheless, the structure of Hr is an appropriate place to begin. 

There is no doubt that a giant step forward has been made in crystal structure prediction by coupling sound theoretical means with massive computer power, but the inherent uncertainties related to randomness and to handling of temperature remain - see above improvement in force fields and in computational procedures, as results demonstrate, are very welcome but are neither indispensable nor sufficient. And there is no hope that this barrier may fall in the future, as it stems from first principles. The next step forward is the inclusion of kinetic energies and temperature in the model. This is already possible, although with great limitations, as described in Sect. 6. 

The principle of maximum symmetry requires that the crystal structure adopted by a given compound be the most symmetric that can satisfy the chemical constraints. We therefore expect to find high-symmetry environments around atoms wherever possible, but such environments are subject to constraints such as the relationship between site symmetry and multiplicity (eqn (10.2)) and the constraint that each atom will inherit certain symmetries from its bonded neighbours. The problems that arise when we try to match the symmetry that is inherent in the bond graph with the symmetry allowed by the different space groups are discussed in Section 11.2.2.4. 

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