Ionic amorphous solids

Hosono, H. 2006. Ionic amorphous oxide semiconductors Material design, carrier transport, and device application. J. Non-Cryst. Solids 352 851-858. 

The same principles that are valid for the surface of crystalline substances hold for the surface of amorphous solids. Crystals can be of the purely ionic type, e.g., NaF, or of the purely covalent type, e.g., diamond. Most substances, however, are somewhere in between these extremes [even in lithium fluoride, a slight tendency towards bond formation between cations and anions has been shown by precise determinations of the electron density distribution (/)]. Mostly, amorphous solids are found with predominantly covalent bonds. As with liquids, there is usually some close-range ordering of the atoms similar to the ordering in the corresponding crystalline structures. Obviously, this is caused by the tendency of the atoms to retain their normal electron configuration, such as the sp hybridization of silicon in silica. Here, too, transitions from crystalline to amorphous do occur. The microcrystalline forms of carbon which are structurally descended from graphite are an example. 

One simple means of causing slow precipitation is to add denaturant to an aqueous solution of protein until the denaturant concentration is just below that required to precipitate the protein. Then water is allowed to evaporate slowly, which gently raises the concentration of both protein and denaturant until precipitation occurs. Whether the protein forms crystals or instead forms a useless amorphous solid depends on many properties of the solution, including protein concentration, temperature, pH, and ionic strength. Finding the exact conditions to produce good crystals of a specific protein often requires many careful trials and is perhaps more art than science. I will examine crystallization methods in Chapter 3. 

There can be slightly different forces holding particles together within a solid. Ionic solids, metallic solids, network atomic solids, molecular solids, and amorphous solids each use a different force or combination of forces to hold molecules or atoms together. 

Classify the bonding in the following amorphous solids as moleculai ionic, metallic, or covalent. 

Amorphous solids including proteins have no crystal cell, but may crystallize and precipitate under appropriate conditions. The repeating crystal cell provides an x-ray diffraction pattern that can provide a detailed 3-dimensional protein structure. To be crystallized, a protein must be pure and in solution. It is then tested for crystal formation by very slow evaporation under a variety of pH and ionic strength conditions. 

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