Crystalline solids description

In this chapter, general aspects and structural properties of crystalline solid phases are described, and a short introduction is given to modulated and quasicrystal structures (quasi-periodic crystals). Elements of structure systematics with the description of a number of structure types are presented in the subsequent Chapter 7. Finally, both in this chapter and in Chapter 6, dedicated to preparation techniques, characteristic features of typical metastable phases are considered with attention to amorphous and glassy alloys. 

Description White crystalline solid (American Conference of Governmental Industrial Hygienists, 1991)... 

At an earlier point (p. 185) the unit cell of a crystalline structure was described as one of a large number of identical prisms, which, when oriented in the same way and stacked together in three dimensions, form a perfect crystal. The corners of an array of unit cells put together in this way are said to be the points of a space lattice the surroundings about each point of the lattice must be identical to the surroundings about every other point. Additional lattice points may sometimes be put at the face centers or at the body centers of the unit cells in crystalline sodium chloride (Fig. 12-2), for example, chloride ions are located both at the corners and the face centers of the unit cell, and an observer at a corner would have the same surroundings as one at a face center. The description of the structure of a crystalline solid is then a description of the size and shape of the unit cell and of the locations of the atoms within it. 

We have given this primitive modelling of the density to stress the importance of finding localized descriptions of the electron density in a molecule (or solid, especially amorphous materials, e.g. Si, where the periodicity that is so helpful in a crystalline solid no longer is present). 

Some of the discussion of bonding theory will concern distorted crystals or crystals with defects then description in terms of bond orbitals will be essential. Description of electronic states is relatively simple for a perfect crystalline solid, as was shown for CsCl in Chapter 2 for these, use of bond orbitals is not essential and in fact, in the end, is an inconvenience. We shall nevertheless base the formulation of energy bands in crystalline solids on bond orbitals, because this formulation will be needed in other discussions at the point where matrix elements must be dealt with, we shall use the LCAO basis. The detailed discussion of bands in Chapter 6 is done by returning to the bonding and antibonding basis. 

It should be emphasized that the (metastable) colloidal state cannot be described by thermodynamic functions. Consequently, themiodynamic description of a system fails in the presence of colloids. For example, the solubility product is well defined for certain (in general crystalline) solids and takes into account the ions in equilibrium with these solids. Non-ionic and colloidal forms, however, are not taken into account. 

Solid-state chemistry uses the same principles for bonding as those for molecules. The differences from molecular bonding come from the magnitude of the molecules in the solid state. In many cases, a macroscopic crystal can reasonably be described as a single molecule, with molecular orbitals extending throughout. This description leads to significant differences in the molecular orbitals and behavior of solids compared with those of small molecules. There are two major classifications of solid materials crystals and amorphous materials. Our attention in this chapter is on crystalline solids composed of atoms or ions. 

In a few cases, one observes narrow absorption bands caused by the co-excitation of solvent 36) or aqua ligand 85) vibrations. It is worth emphasizing how relatively rare this phenomenon is—again showing a considerable isolation of the individual chromophore. In the case of crystalline solids, the energy band description encounters insurmountable difficulties in the case of fairly localized transitions 50, 53, 57). 

Description It was first synthesized in 1915 by a German chemist Weiland, and then again in 1918 by US chemist Robert Adams who named it adamsite. DM is a yellow-green, odorless crystalline solid that is not very volatile. It is insoluble in water and relatively insoluble in organic solvents... 

The X-ray diffraction technique is the most commonly used experimental method for investigating the crystal structures of crystalline solids. As the underlying theory and methods are detailed in several specific textbooks (e.g.. Ref. [1]), only a brief description of the essential features will be provided at this point. 

Description of the smallest unit Neutral atoms bound together to form a neutral molecule Positive and negative ions bound together in a crystalline solid. 

Section (2) develops a theoretical account of plastic deformation and energy dissipation at the atomic or molecular level. The AFM observations show that plastic deformation of shocked or impacted crystals can significantly deform both the crystal lattice and its molecular components. These molecular and sub-molecular scale processes require a quantum mechanical description and necessarily involve the lattice and molecular potentials of the deforming crystals. A deformed lattice potential is developed which when combined with a quantum mechanical account of plastic flow in crystalline solids will be shown to give reasonably complete and accurate descriptions of the plastic flow and initiation properties of damaged and deformed explosive crystals. The deformed lattice potential allows, for the first time, the damaged state of the crystal lattice to be taken into account when determining crystal response to shock or impact. 

The recent AFM experimental data concerning plastic flow place severe restrictions on possible theoretical accounts of plastic deformation in crystalline solids due to shock or impact. The high spatial resolution of the AFM, = 2 x lO " m, reveals substantial plastic deformation in shocked or impacted crystal lattices and molecules. Understanding how this occurs and its effect on plastic flow requires a quantum mechanical description. The semi-permanent lattice deformation has necessitated the development of a deformed lattice potential which, when combined with a quantum mechanical theory of plastic deformation, makes it possible to describe many of the features found in the AFM records. Both theory and the AFM observations indicate that shock and impact are similar shear driven processes that occur at different shear stress levels and time durations. The role of pressure is to provide an applied shear stress sufficient to cause initiation. 

Band theory provides a picture of electron distribution in crystalline solids. The theory is based on nearly-free-electron models, which distinguish between conductors, insulators and semi-conductors. These models have much in common with the description of electrons confined in compressed atoms. The distinction between different types of condensed matter could, in principle, therefore also be related to quantum potential. This conjecture has never been followed up by theoretical analysis, and further discussion, which follows, is purely speculative. 

---