Experimental Measurements

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The analysis of a crystal structure by X-ray or neutron diffraction consists of three stages: (1) Data collection. This involves experimental measurement of the directions of scatter of the diffracted beams so that a unit cell can be selected and its dimensions measured. The intensities of as many as possible of the diffracted beams (Bragg reflections) from that same crystal are then recorded. These intensities depend on the nature of the atoms present in the crystal and their relative positions within the unit cell. (2) Finding a “trial structure.” This is the deduction by some method (such as one of those described in Chapters 8 and 9) of a suggested atomic arrangement (a “trial structure”). This is listed as atomic coordinates that have been measured with respect to the unit-cell axes. The intensity of each Bragg reflection corresponding to this trial structure can then be calculated (see Chapter 5) and its value then compared with the corresponding experimentally measured intensity in order to determine whether the trial structure is “good,” meaning that it is essentially correct. (3) Refinement of the trial structure. This involves modification (refinement) of a good trial structure until the calculated and measured intensities agree with each other within the limits of any errors in the observations (see Chapter 11). This is usually done by a leastsquares refinement, although difference electron-density maps may also prove useful. The result of the refinement is information on the three-dimensional atomic coordinates in this particular crystal, together with atomic displacement parameters. This chapter is concerned with the first of these stages, the experimental measurements. This is a rapidly changing area of science as more powerful and precise equipment and detection devices become available. The experimental data that may be derived frommeasurements of an X-ray or neutron diffraction pattern include: (1) The overall appearance of the Bragg reflections at the detection system. Ideally these diffraction maxima should be sharp, well-resolved peaks. Blurred, double spots or arcs may indicate disorder or poor crystal quality. (2) The angles or directions of scattering (including 2Ë, the angular deviation from the direct beam).