Crystal covalency

Besides metallic crystals, covalent crystals also imdergo surface reconstruction as mentioned in Sec. 2.7 and dangling surface atoms reduce the number of their dangling bonds to stabilize the surface energy, thereby forming a reconstructed surface lattice different from the interior lattice. 

Ionic Crystals Molecular Crystals Covalent Crystals Metallic Crystals... 

The four types of crystals and the forces that hold their particles together are ionic crystals, held together by ionic bonding molecular crystals, van der Waals forces and/or hydrogen bonding covalent crystals, covalent bonding and metallic crystals, metallic bonding. 

Covalen t crystals Covalent bonds are formed when atoms share electrons. The formation of covalent crystals results in highly directional bonding and packing, which is far from close-packed. 

Covalent crystals Covalent bonds join the atoms of the crystal (diamond, quartz). 

Ionic crystal, molecular crystal, covalent network solid, metallic crystal... 

Dipole forces, induced dipole forces, and hydrogen bonds are all intermolecular attractions—forces that act between molecules. Induced dipole forces are also known as dispersion forces, London forces, and London dispersion forces. Covalent bonds are the forces that hold atoms together within a molecule. Ionic bonds are the forces that hold ions together within a crystal. Covalent and ionic bonds are much stronger than any of the intermolecular forces. 

The calculation of surface free energy and surface stress data depends critically on the model used for the interatomic interactions. Therefore, calculations for inert gas crystals, ionic crystals, covalent crystals and metals are treated separately as each crystal class is described best by different models of the interactions. 

While in Chaps.2 and 3 a straightforward formalism is developed, the philosophy in Chap.4 is different in this chapter we are concerned with an interpretation of measured phonon dispersion curves and the information they provide for the interatomic forces. It is an important chapter and certainly not an easy one the difficulties are intrinsic and arise from the complicated nature of the different types of chemical bonds. The chapter contains the study of the lattice dynamics of solid inert gases, ionic crystals, covalent solids, molecular crystals and a qualitative discussion of the lattice dynamics of metals. 

The idea of point defects in crystals goes back to Frenkel, who in 1926 proposed the existence of point defects to explain the observed values of ionic conductivity in crystalline solids. In a crystal of composition MX such as a monovalent metal halide or a divalent metal oxide or sulfide, volume ionic conductivity occurs by motion of positive or negative ions in the lattice under the influence of an electric field. If the crystal were perfect, imperfections, such as vacant lattice sites or interstitial atoms, would need to be created for ionic conductivity to occur. A great deal of energy is required to dislodge an ion from its normal lattice position and thus the current in perfect crystals would be very, very small under normal voltages. To get around this difficulty, Frenkel proposed that point defects existed in the lattice prior to the application of the electric field. This, of course, has been substantiated by subsequent work and the concept of point defects in all classes of solids, metals, ionic crystals, covalent crystals, semiconductors, etc., is an important part of the physics and chemistry of crystalline solids, not only with respect to ionic conductivity but also with respect to diffusion, radiation damage, creep, and many other properties. 

As in the case of metals and in contrast to ionic and molecular crystals, covalent crystals yield extended bands in the band model, because of the significant overlap. 

If silicon atoms are substituted for half the carbon atoms in this structure, the resulting structure is that of silicon carbide (carborundum). Both diamond and silicon carbide are extremely hard, and this accounts for their extensive use as abrasives. In fact, diamond is the hardest substance known. To scratch or break diamond or silicon carbide crystals, covalent bonds must be broken. These two materials are also nonconductors of electricity and do not melt or sublime except at very high temperatures. SiC sublimes at 2700 °C, and diamond melts above 3500 °C. 

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