The Four-Circle Diffractometer at HFIR
The Four-Circle Diffractometer goniometer has a full χ circle with a 4.5–450 K closed-cycle helium refrigerator. The detector is 3He with a 7-anode array in a honeycomb pattern. Currently, only the center anode is used. The upper 2Θ limit is 155°. A multilayer--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 (331), (220) with (440), and (111) with (333) reflections are of practical interest. For the fixed monochromator angle of 48°, these reflections provide principal incident wavelengths of 1.000, 1.536, and 2.540 Å, respectively. A PC-based LabView system provides user-friendly diffractometer control and data acquisition. The beam size is 6 × 8 mm2, and the minimum crystal size is 1 mm3. The maximum crystal dimension is about 5 mm. The flux on the sample can be up to 2.2 × 107 n/cm2/s. The horizontal bending of the monochromator can be changed to optimize the resolution or the intensity depending upon the needs of the measurement. The longer wavelength of 2.540 Å has ~5% λ/3 contamination and the 1.536 Å wavelength has ~5% λ/2 contamination.
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. The instrument is particularly suitable for magnetic structural studies, measuring order parameters, and studying phase transitions. Problems from chemistry, physics, materials science, and mineralogy have been addressed. Recent specific areas of study include magnetic structure and nuclear superstucture in iron pnictide superconductors, phase transitions in multiferroic oxides, temperature dependence of atomic displacement parameters in battery materials and thermoelectric materials, hydrogen bonding in rock-forming minerals, crystallography of novel scintillators, and diffuse scattering.