= Crystal structure models of amorphous materials




Structure Models of Amorphous Materials



Models of ordered crystal structures make up the majority of our work for customers, but people occasionally want models that are the exact opposite of these structures. Amorphous materials are characterised by the presence of short-range order but with an absence of long-range order. The materials theselves tend to be glassy, break into irregular fragments - often with curved face - when struck. Because there is no long-range order, x-ray and neutron diffraction/scattering patterns are poorly defined, leading to broad 'humps' in the diffraction patterns, in stark contrast with the sharp, well-defined, peaks displayed by crystalline solids.


Rapid cooling of a molten material favours the formation of amorphous solids as the constituent ions or molecules simply don't have sufficient time to form a crystalline arrangement before they lack thermal energy to rearrange themselves - they are literally frozen into an amorphous struccture. This can be observed in almost any material if cooling of the liquid phase is sufficiently rapid. While some solids are inherently amorphous because efficient packing of their molecules is all but impossible, others are only amorphous under extreme conditions - Aluminium can only be created in an amorphous phase only when the molten metal is cooled at a ridiculous rate in excess of 1013K/s. By contrast cooling liquid silicon dioxide at anything more than a sedate 4K/min results in quartz glass.


What does this mean for a model of an amorphous solid? Well, it means that no two atoms are the same, and the bonds have a huge variety of lengths, which makes our life incredibly difficult. Calculated on the basis of the number of atoms or bonds we can add in any given time, amorphous solids are the most time-consuming models that we are ever asked to make. With a model of crystalline diamond, for example, we simply drill a tetrahedral arrangement of holes in a load of balls and add the same length of bond between each pair of atoms. In amorphous diamond, every ball is drilled uniquely and individually, and can only sit at one specific point in one specific arrangement and in one specific orientation in the model, with unique bonds of specific lengths between the balls. That means that we have to keep track of every hole in every ball and ensure that they are each connected to the correct neighbour with the correct bond length, That all takes a long time.



A crystal structure model of amorphous zirconium dioxide

'Crystal' structure model of amorphous zirconium dioxide



Yet, here we are offering models of amorphous structures for sale. Because of the extra effort that inevitably goes into making these models, they cost more than an equivalent size of a crystalline structure, but for those people who need to be able to demonstrate some of the structural properties of these materials, we offer something that is virtually unique, and certainly not reasonably possible with conventional model kits in most cases.