Four-Circle Diffractometer (HB-3A)

The Four-Circle Diffractometer goniometer has a full χ circle with a 10-K closed-cycle helium refrigerator. The detector is 3He with a 7-anode array in a honeycomb pattern. The upper 2Θ limit is 100°. A multilayer-[110]-wafer silicon monochromator with the reflection from planes of the <011> zone ensures sharp diffraction peaks in specified ranges of detector angles by control of the horizontal radius of curvature. Any plane from the <011> zone can be set in Bragg position, but only the (155), (133), (022) with (044), and (111) with (333) reflections are of practical interest. For the fixed monochromator angle of 48°, these reflections provide principal incident wavelengths of 0.618, 1.01, 1.56, and 2.55 Å, respectively. A PC-based LabView system provides user-friendly diffractometer control and data acquisition. The beam size is 5 x 5 mm2, and the minimum crystal size is 1 mm3. The maximum crystal dimension is about 4 mm. The flux on the sample is estimated to be greater than 5 × 106 n/cm2/s.

This instrument is suitable for a wide range of small-unit-cell crystallography studies, from structure refinement and solution to charge and nuclear density mapping. Problems from chemistry, physics, materials science, and mineralogy have been addressed. Specific areas of study include hydrogen bonding and weak interactions, organometallics, supramolecular chemistry and crystal engineering, metal hydrides, charge density, pharmaceuticals, and magnetic structures. More general solid-state physics problems in magnetism, diffuse scattering, and ordering phenomena can also be addressed.