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Thermal MotionIn practise atoms vibrate about their equilibrium positions: this is called thermal motion. This concept depends not only on temperature but also on the mass of atom and the strengths of the bonds holding it. The higher the temperature the bigger the amplitude of vibrations and the electron density becomes spread out over a larger volume – therefore causing the scattering of the atom to fall off quicker. Thermal motion is a displacement parameter that needs to be considered when calculating (i.e. refining) the final structure of the sample under investigation. Thermal motion is most thoroughly treated as being anisotropic (i.e. different in different directions). Refining a structure:The R-factor gives a measure of the difference between the observed and calculated structure factors and therefore of how well the structure has been refined. R is used to give a guide to the correctness and precision of a structure. In general, the lower the R-value the better the structure determination will be. A good structure determination, as well as having a low R-value, will also have low standard deviations on both the atomic positions and bond lengths calculated from these positions. |
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