Unstable matrix defects

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. 

Ti-Beta zeolites and, even more, mesoporous Ti-siUcates can be somewhat unstable to aqueous hydrogen peroxide and to strongly chelating agents. A partial collapse of the lattice and the release of Ti, in the form of Ti02 particles or soluble Ti peroxides, was sometimes observed under these conditions (see also Section 18.4.2). The structural instability grows in parallel with the hydrophiUcity of the surface and the defectiveness of the silica matrix Ti-P < Ti,Al-P Ti-MCM-41 [87-89]. For the same reason, the stability of the catalyst is indirectly related to the method of synthesis, as far as this is able to produce materials with a different content of connectivity defects. 

The distribution of defects in mesophases is often regular, owing to their fluidity, and this introduces pattern repeats. For instance, square polygonal fields are frequent in smectics and cholesteric liquids. Such repeats occur on different scales - at the level of structural units or even at the molecular level. Several types of amphiphilic mesophase can be considered as made of defects . In many examples the defect enters the architecture of a unit cell in a three-dimensional array and the mesophase forms a crystal of defects [119]. Such a situation is found in certain cubic phases in water-lipid systems [120] and in blue phases [121] (see Chap. XII of Vol. 2 of this Handbook). Several blue phases have been modeled as being cubic centred lattices of disclinations in a cholesteric matrix . Mobius disclinations are assumed to join in groups of 4x4 or 8x8, but in nematics or in large-pitch cholesterics such junctions between thin threads are unstable and correspond to brief steps in recombinations. An isotropic droplet or a Ginsburg decrease to zero of the order parameter probably stabilizes these junctions in blue phases. 

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