The Fddd four-circle diffractometer for single-crystal X-ray studies at temperatures down to 9 K

The Fddd four-circle diffractometer system has been developed to perform X-ray diffraction experiments at temperatures as low as 9 K. The diffractometer consists of: (i) a Siemens rotating-anode generator; (ii) a Huber goniometer with offset X circle; (iii) a Siemens Fast Scintillation Detector; and (iv) an APD `202' Displex cryogenic refrigerator. The belt-driven rotating anode gives an X-ray flux that is far in excess of that obtained from a sealed X-ray tube and leads to faster data collections and the opportunity to study smaller samples. X-ray beam alignment requires precise movements of the 300 kg goniometer and this is achieved using compressed air pads. Steel-braided gas lines that allow the transfer of helium gas between the Displex and the compressor are supported by a counterbalance system. The stress on these lines has been reduced by attaching them to the Displex via high-pressure rotating joints and by passing them through a metal ring that is attached to the χ circle. A compact vacuum gauge has been mounted through one of the four ports on the top of the cryostat and gives valuable information on the pressure within the Displex sample chamber during an experiment. Crystals are mounted on sharpened 0.3 mm graphite pencil leads that are held in a newly designed sample mount. The temperature reported from the second stage of the Displex has been assessed by consideration of temperature standards that include the solid-state phase transitions of benzil (1,2-diphenylethanedione) and terbium vanadate.