Crystal structure is how the spatial arrangement of atoms or molecules within a crystalline solid is described. Materials such as gold, quartz and tungsten all have set crystal arrangements. When x-rays are shone upon a crystal with a regular, periodic structure they undergo diffraction, where waves scattered by the individual atoms add coherently to form beams in some directions and cancel each other out in others. This diffraction occurs as a result of the x-rays interacting with the electron cloud of the crystal at varying depths - some x-rays are reflected by the initial plane of electrons, and the remainder continue through, some of which are then reflected by the second plane, and so on. The image of the reflected x-rays then forms a diffraction pattern, with the spacing between atom planes resulting in the phase difference. This is known as Bragg scattering.
The purpose of this object is to replicate this on a larger scale. The ball bearing model has a plastic frame and regularly spaced indents which hold ball bearings. These ball bearings are analogous to the atoms in a crystal. They are regularly spaced out and follow a continuous pattern. This model can be used to visualise the crystalline structure of a given material and the ball bearings can be moved and reordered to portray different structures. The model allows students to appreciate the symmetries and effective planes of atoms and how they can coherently scatter incoming waves, such as X-rays.
This model could also be used to simulate X-ray diffraction from crystals by exposing the model to microwaves. Microwaves used for these demonstrations used a wavelength of 3cm while X-rays have a wavelength of around 10 nanometers, millions of times smaller. By using a scaled up model along with the larger wavelength, the user can observe a larger version of the diffraction effect. Because the model contains relatively few model atoms, the effect observed would be much simpler than what would be observed in real X-ray scattering.
This item is part of the UQ Physics Museum Crystallography Tour
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JDB, SN
Object 1805
