MICROCRYSTAL DIFFRACTION, DIFFICULT BUT POSSIBLE. Larry W. Finger, Geophysical Laboratory, 5251 Broad Branch Road, N.W., Washington DC 20015-1305, USA

Since the earliest availability of radiation from synchrotron sources, researchers have been attempting to use the high brightness of such facilities to study the crystal structure of materials that do not form large crystals. Because of the correlation between X-ray scattering and atomic number, the dimensionless scattering power, S = (F₀₀₀/V_U)²V_C³, is a better measure of the strength of the scattered intensity than the volume alone. In the formula, F₀₀₀ is the number of electrons per unit cell, V_U is the volume of the unit cell, V_C is the volume of the crystal, and ( is the wavelength. For SiO₂ (quartz) with a crystal size of 50 (m, and wavelength of 1 Å, S = 8(10¹⁶. This value is near the minimum for laboratory instruments. If the crystal size is reduced to 1 (m, then S = 8(10¹², which is near the lower limit for diffraction from a light-element crystal at present synchrotron sources. Advances in design of insertion devices and focusing optics will reduce the lower limit for the scattering power.

The major difficulties with microcrystal diffraction are those of manipulating and mounting micron-sized crystals, ensuring that backgrounds are minimized, and providing an incident beam that is homogeneous on the scale of the "sphere of confusion" of the diffractometer. Despite these severe problems, recent developments in area detectors, and in beamline optics for insertion device lines have made it possible to obtain reliable intensities from very small crystals.