THE FDDD 4-CIRCLE DIFFRACTOMETER FOR SINGLE-CRYSTAL X-RAY STUDIES AT TEMPERATURES DOWN TO 9K

The Fddd four-circle diffractometer system has been developed to perfo rm X-ray diffraction experiments at temperatures as low as 9 K. The di ffractometer consists of: (i) a Siemens rotating-anode generator; (ii) a Huber goniometer with offset chi circle; (iii) a Siemens Fast Scint illation Detector; and (iv) an APD '202' Displex cryogenic refrigerato r. The belt-driven rotating anode gives an X-ray flux that is far in e xcess of that obtained from a sealed X-ray tube and leads to faster da ta collections and the opportunity to study smaller samples. X-ray bea m alignment requires precise movements of the 300 kg goniometer and th is is achieved using compressed air pads. Steel-braided gas lines that allow the transfer of helium gas between the Displex and the compress or are supported by a counterbalance system. The stress on these lines has been reduced by attaching them to the Displex via high-pressure r otating joints and by passing them through a metal ring that is attach ed to the chi circle. A compact vacuum gauge has been mounted through one of the four ports on the top of the cryostat and gives valuable in formation on the pressure within the Displex sample chamber during an experiment. Crystals are mounted on sharpened 0.3 mm graphite pencil l eads that are held in a newly designed sample mount. The temperature r eported from the second stage of the Displex has been assessed by cons ideration of temperature standards that include the solid-state phase transitions of benzil (1,2-diphenylethanedione) and terbium vanadate.