Crystalline solid intrinsic properties

Many inorganic solids lend themselves to study by PL, to probe their intrinsic properties and to look at impurities and defects. Such materials include alkali-halides, semiconductors, crystalline ceramics, and glasses. In opaque materials PL is particularly surface sensitive, being restricted by the optical penetration depth and carrier diffusion length to a region of 0.05 to several pm beneath the surface. 

As mentioned before, we shall use small molecules to introduce the fundamentals for more complex molecules, the real core of this book, which will be listed in the next section. Such molecules form solids with remarkable properties (metallicity, superconductivity, ferromagnetism, etc.), some of them at ambient conditions or at much lower hydrostatic pressures than those found for H2 and N2, and some technological applications have been already developed, deserving the name of functional materials. Most of the molecules studied in this book are planar, or nearly planar, which means that the synthesized materials reveal a strong 2D structural character, although the physical properties can be strongly ID, and because of this 2D distribution we shall study surfaces and interfaces in detail. In particular, interfaces play a crucial role in the intrinsic properties of crystalline molecular organic materials and Chapter 4 is devoted to them. 

The host in a clathrate is the entire (usually crystalline) solid and the guest binding cavity does not need to be an intrinsic property of the individual host molecules. 

The crystal structure, i.e., the way atoms are stacked and the nature of the component atoms together determine a compound s intrinsic properties. In this respect crystalline solids do not differ from molecules and have intrinsic properties that depend only on the nature and the arrangement of their component atoms." Familiarity with the intrinsic properties of solid compounds is invaluable for the materials designer. Intrinsic properties cannot be tailored with the preparation... 

In purely geometric terms, a surface has no intrinsic thickness, but in practical terms a surface must comprise one or more layers of atoms or molecules. When dealing with the properties of solids and liquids, the surface and interior are usually considered separately. This is partly because, for any material of significant mass and volume, the number of atoms at the surface is negligible compared to the number of internal atoms. Mostly, however, it reflects the fact that the mathematical treatment of ordered substances is simplified if the structural periodicity is assumed to be infinite. The important point, and the reason that surface analysis is of interest at all, is that the surface differs in its properties from the bulk (internal) material even though it is made of exactly the same atoms. This is because atoms at the surface are relaxed, i.e., not all the bonds are constrained by interactions with neighboring atoms in the same material. Relaxed atoms have free bonds that are available to interact with atoms and molecules on the surface of the adjacent phase, such as the gas or liquid surrounding a crystalline solid. 

A solid surface is intrinsically an imperfection of a crystalline solid by destroying the three-dimensional (3D) periodicity of the structure. That is, the unit cell of a crystal is usually chosen such that two vectors are parallel to the surface and the third vector is normal or oblique to the surface. Since there is no periodicity in the direction normal or oblique on the surface, a surface has a 2D periodicity that is parallel to the surface. By considering the symmetry properties of 2D lattices, one obtains the possible five 2D Bravais lattices shown in Figure 2. The combination of these five Bravais lattices with the 10 possible point groups leads to the possible 17 2D space groups. The symmetry of the surface is described by one of these 17 2D symmetry groups. 

The microstructure is defined as a network of one or more crystalline phases with various types of imperfection. The crystalline phase, which covers the greater part of the whole system, is the origin of the intrinsic properties of a specific material. Imperfections are defects to which most of the processing-dependent properties are attributed. Typical examples of imperfections include solid/vapor and solid/liquid interfaces, grain boundaries, phase boundaries, pores, secondary phases, and so on. Among those listed, the first three defects - that is, the solid/vapor and solid/liquid... 

A convenient description of die crystalline structure of solids is thus seen to consist of successive stages of approximation. First, the mathematically perfect geometrical model is described then departures from diis perfect regularity are permitted. The defonuabilily of solids is allowed for by letting die force constants between adjacent atoms be finite, not infinite. Then, misplacement of atoms is permitted and a variety of crystalline irregularities, called defects, is described. Some of these defects have intrinsic features which affect properties of die crystal other affect the properties by their motion from site to site in the crystal. In spite of their relatively small number, defects are of immense importance. 

Nuclease behaves like a typical globular protein in aqueous solution when examined by classic hydrodynamic methods (40) or by measurements of rotational relaxation times for the dimethylaminonaphth-alene sulfonyl derivative (48)- Its intrinsic viscosity, approximately 0.025 dl/g is also consistent with such a conformation. Measurements of its optical rotatory properties, either by estimation of the Moffitt parameter b , or the mean residue rotation at 233 nin, indicate that approximately 15-18% of the polypeptide backbone is in the -helical conformation (47, 48). A similar value is calculated from circular dichroism measurements (48). These estimations agree very closely with the amount of helix actually observed in the electron density map of nuclease, which is discussed in Chapter 7 by Cotton and Hazen, this volume, and Arnone et al. (49). One can state with some assurance, therefore, that the structure of the average molecule of nuclease in neutral, aqueous solution is at least grossly similar to that in the crystalline state. As will be discussed below, this similarity extends to the unique sensitivity to tryptic digestion of a region of the sequence in the presence of ligands (47, 48), which can easily be seen in the solid state as a rather anomalous protrusion from the body of the molecule (19, 49). 

We will see that it is the interactions between chains or sheets that is responsible for finite temperature transitions. That is the reason for labeling systems of chains or sheet as quasi-one- or two-dimensional. They exhibit one- or two-dimensional behavior until there occurs a crossover to higher dimensionality. Obviously, crystals do exist and as such must be three-dimensional. Binding forces are intrinsically three-dimensional. Let us then take for granted the existence of the crystalline backbone of organic conductors and concentrate on the electronic properties which are of interest to conduction. It is the strong anisotropy in these which is responsible for the stamp of quasi-one- or quasi-two-dimensional solids. 

Crystallinity, like most things, can vary in degree. Even single crystals typically have intrinsic point defects (e.g. lattice site vacancies) and extrinsic point defects (e.g. impurities), as well as extended defects such as dislocations. Defects are critical to the physical properties of crystals and will be extensively covered in later chapters. What we are referring to here with the degree of crystallinity is not the simple presence of defects, but rather the spectmm of crystallinity that encompasses the entire range from crystalline to fully disordered amorphous solids. Table 1.1 lists the various classes. Let s take each of them in the order shown. 

We would like to emphasize that the difference in the structures of LiF and Si02 simulated here, reflects intrinsic ability of substrates to form crystalline and amorphous phases, respectively, and demonstrates the difference in principal properties of solid LiF and Si02- Formation of the SiOz amorphous phase is governed by the nature of the compound, which includes a variety and complexity of the SiOi oligomers and relative stability of the Si-0 bonds. The structure of LiF is naturally crystalline and, thus, an amorphism of LiF may be caused by external forces only, e.g. thermal, pressure, on electric field. 

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