User:Shanleyg/Structure of liquids and glasses

Structural Theory of Glass Formation, Zachariasen
While studying glass, Zachariasen began to notice repeating properties in glasses. He postulated rules and patterns that, when atoms followed these rules, they were likely to form glasses. The following rules make up Zachariasen's theory, applying only to oxide glasses.


 * 1) Each oxygen atom in a glass can be bonded to no more than two glass-forming cations
 * 2) The coordination number (CN) of the glass forming cation is 3 or 4
 * 3) The oxygen coordination polyhedra only share corners, not edges or faces
 * 4) At least 3 corners of every polyhedra must be shared, creating a continuous random network.

While these rules only apply to oxide glasses, they were the first rules to establish the idea of a continuous random network for glass structure. He was also the first to classify structural roles for various oxides, some being main glass formers (SiO2, GeO2, P2O5), and some being glass modifiers (Na2O, CaO).

Energy Criterion of K.H Sun
This criterion established a connection between the chemical bond strength and its glass forming tendency. When a material is quenched to form glass, the stronger the bonds, the easier the glass formation.


 * If a bond strength is higher than 80 kcal per bond (high bond strength), it will be Glass Network Forming, meaning it is likely to form a glass.
 * If a bond strength is less than 60 kcal per bond (low bond strength), it will be Glass Network Modifying, since it would only form weak bonds, it would disrupt glass forming networks.
 * If a bond strength is between 60 and 80 kcal per bond ( intermediate bond strength, it will be an intermediate. This means it will not form a glass on it's own, but it partially can while combined with other network forming atoms.

Dietzel's Field Strength Criterion
Dietzel looked at direct Coulombic interactions between atoms. He categorized cations using Field strength where FS=zc/(rc+ra)2, where zc is the charge of the cation, and rc and ra are the radii of the cation and anion respectively. High Field Strength Cations would have a high cation-oxygen bond energy.


 * If FS was greater than 1.3 (small cation with high charge ), it would be a Glass Network Former
 * If FS was less than 0.4 (Large cation with small charge ), it would be a Glass Network Modifier
 * If Fs was between 0.4 and 1.3 (medium sized cation with medium charge ) it would be an intermediate

These three criterion help establish 3 different ways to determine whether or not certain oxides molecules will form glasses, and the likeliness of it.