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In a glass, constituents can be either network formers or network modifiers. Network formers are atoms that are directly incorporated into the network, like the Titanium (blue) atoms below. In contrast, network modifiers are unable to be incorporated directly into the network of bonds, and thus force the network to be formed around them. Most rare earth elements, such as the Europium (orange) atoms in this illustration, are too large to be network formers in these glasses, and thus are network modifiers.
Unlike crystals, which have a regular, repeating structure, glasses have no long-term structure. The above illustration depicts a glass. Whereas crystals are usually formed by slowly cooling a liquid to allow it to form a rigid structure, glass can be created by cooling a liquid so rapidly that the atoms do not have time to form the ideal ordered lattice, but instead are trapped in an irregular configuration. This process makes it difficult to uniformly incorporate rare earth metals, which usually start out in the form of powders.
Instead, the sol-gel is dried, removing the water, and finally annealed at 900oC, which is still much lower than the temperatures required to produce melt glass. The high temperatures result in a substance with a similar density to melt glasses. In addition to removing remaining water, the annealing breaks up and eliminates the remaining organic molecules in the sample, yielding a transparent glass. Because the dopants have been dissolved in the solution from the beginning of the process, the resultant glass contains a roughly uniform distribution of the impurity.