What Are Crystals

In Secs. 4.3 and 4.4 we discussed the thermodynamics of the crystal -> Uquid transition. This and other famiUar phase equilibria are examples of what are called first-order transitions. There are other less familiar but also well-known... 

The properties described above have important consequences for the way in which these skeletal tissues are subsequently preserved, and hence their usefulness or otherwise as recorders of dietary signals. Several points from the discussion above are relevant here. It is useful to ask what are the most important mechanisms or routes for change in buried bones and teeth One could divide these processes into those with simple addition of new non-apatitic material (various minerals such as pyrites, silicates and simple carbonates) in pores and spaces (Hassan and Ortner 1977), and those related to change within the apatite crystals, usually in the form of recrystallization and crystal growth. The first kind of process has severe implications for alteration of bone and dentine, partly because they are porous materials with high surface area initially and because the approximately 20-30% by volume occupied by collagen is subsequently lost by hydrolysis and/or consumption by bacteria and the void filled by new minerals. Enamel is much denser and contains no pores or Haversian canals and there is very, little organic material to lose and replace with extraneous material. Cracks are the only interstices available for deposition of material. 

What are the types of bonds holding crystals together It is important to... 

The chemical bonding and the possible existence of non-nuclear maxima (NNM) in the EDDs of simple metals has recently been much debated [13,27-31]. The question of NNM in simple metals is a diverse topic, and the research on the topic has basically addressed three issues. First, what are the topological features of simple metals This question is interesting from a purely mathematical point of view because the number and types of critical points in the EDD have to satisfy the constraints of the crystal symmetry [32], In the case of the hexagonal-close-packed (hep) structure, a critical point network has not yet been theoretically established [28]. The second topic of interest is that if NNM exist in metals what do they mean, and are they important for the physical properties of the material The third and most heavily debated issue is about numerical methods used in the experimental determination of EDDs from Bragg X-ray diffraction data. It is in this respect that the presence of NNM in metals has been intimately tied to the reliability of MEM densities. 

Figure 5.18.1 The NaCl crystal structure consisting of two interpenetrating face-centered cubic lattices. The face-centered cubic arrangement of sodium cations (the smaller spheres) is readily apparent with the larger spheres (representing chloride anions) filling what are known as the octahedral holes of the lattice. Calcium oxide also crystallizes in the sodium chloride structure.

Crystals of high purity metals are very soft, while high purity diamond crystals are very hard. Why are they different What features of the atomic (molecular) structures of materials determine how hard any particular crystal, or aggregate of crystals, is Not only are crystals of the chemical elements to be considered, but also compounds and alloys. Glasses can also be quite hard. Is it for similar reasons What about polymeric materials ... 

The Seebeck coefficient for pure LaCo03 is +600 xVK-1. (a) What are the mobile charge carriers (b) Suppose these occur because the crystal contains a trace of an impurity, Co4+, calculate the defect concentration and the formula of the material (data from Robert et al., 2006). 

The energy of formation of defects in PbF2 are anion Frenkel defect, 0.69 eV cation Frenkel defect, 4.53 eV Schottky defect, 1.96 eV. (a) What point defects do these consist of (b) What are (approximately) the relative numbers of these defects in a crystal at 300 K (Data from H. Jiang et al., 2000). 

What are the advantages and disadvantages to using an analyzer crystal in an x-ray fluorescence instrument, as depicted in Figure 10.7 ... 

In addition to providing a novel approach to the preparation of chiral compounds, this type of chemistry may allow one to inquire into the subtle stereochemical details of some crystal-state reactions. For example, what are the approach geometry and the preferred side of attack in the addition of bromine to a chiral olefin (259) What can be learned of the geometry of the labile electronically excited species involved in (2 + 2) photocycloaddition reactions (260) ... 

Not all hot packs use dissolution processes. For example, one kind of hot pack exploits the crystallization of sodium thiosulfate or sodium acetate. Another kind uses the oxidation of iron (rusting). On the Internet, investigate different kinds of hot packs. Are they all used for the same purpose What are the pros and cons of their designs To start your search, go to the web site above and click on Web Links. 

What are the factors determining the position of the Fermi level on the surface of the crystal What agents acting on the crystal can change the position of this level ... 

There is considerable experimental evidence [9-15] for density fluctuations at very early times of crystallization before the full crystallographic features can be detected. What are the molecular origins of structural development in the primordial stage of polymer crystallization ... 

When polymers are crystallized under flow (stirring, extensional, etc.), the ubiquitous morphology [23] is the shish-kebab structure, consisting of central core (shish) surrounded by lamellae (kebabs) attached along the shish. What is the underlying mechanism behind the formation of shish-kebab stmcture ... 

What are the spacings of the 100, 110, and 111 planes in a cubic crystal system of unit cell dimension a In what sequence would you expect to find these reflections in a powder diffraction photograph ... 

Which radiochronometer(s) would be best suited to determine the crystallization age of a basaltic meteorite What are some of the complications that might result in an incorrect age determination ... 

What are the advantages of the radiochemical method compared with other in situ techniques It offers a direct relationship between surface radiation (N ) and surface concentration, which allows a direct measurement of the amount of adsorbed molecules on the electrode, a condition difficult to determine with other in situ techniques. The main limitation of the technique is the availability of radioactive forms of the compound the experimenter wants to study. In this respect, the type of radiation preferred is of the P-type, mainly because of the ease of detection and minimal safety hazards. Typical P-emitters used are H, C, S, Cl, and P, which as constituents of molecules, open a great variability of compounds for study. Figure 6.21 shows some experimental results obtained for the measurement of adsorption on single crystals using this radiochemical method. 

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