Four circle diffractometer

FIGURE 2 A four-circle diffractometer is used to obtain highly detailed FIGURE 3 The x-ray diffraction pattern that led to the elucidation of the... 

Most previous attempts to obtain X-ray diffraction data at very low temperatures (< 80 K) have used custom built systems with closed cycle helium refrigerators mounted on large, robust four circle diffractometers. In order to remove the inherent disadvantages of these systems - cost, single application, absorption and scattering of the windows - we have built an open flow system from mainly off-the-shelf components which uses liquid helium as the cryogen. This is not the first open flow helium system [19, 20] but is the first that is mainly off-the-shelf and is mountable on any diffractometer. It is based on an ADP Helitran ESR cryostat with modifications to the nozzle assembly and to the direction of the gas flow. The lowest temperature is estimated to be <30K. At the current price for liquid helium in... 

Prompted by the success of the DAC, opposed-anvil cells equipped with large, normally sapphire, anvils have been used in a number of high-resolution diffraction studies that have used classical four-circle diffractometers [187-189] to perform high quality studies to above 2 GPa. The quality of the data is excellent, particularly if collected using small area detectors which became available in the late 1980s [190], and the use of which is now widespread. 

The cubic space group Pm3m (no systematic absences) was chosen for X-ray diffraction studies for reasons previously cited (9). Preliminary crystallographic experiments and subsequent data collection were performed at 2A°C with an automated, four-circle Syntex PI diffractometer, equipped with a graphite monochromator and a pulse-height analyzer. Molybdenum radiation was used for all experiments (Ka, X 0.70930 a K 2 ... 

For the X-ray crystal structure determination, crystals were grown from methanol solution by slow evaporation at room temperature. Cell parameters and intensity data were derived from measurements on four-circle diffractometer Rigaku AFC5R. Molecular and crystal structures were determined by the direct... 

The structure of etodolac has been established by single crystal X-ray diffraction analysis [9]. Crystals of racemic etodolac were obtained by recrystallization from benzene-petroleum ether. A small brick shaped sample having approximate dimensions 0.2 x 0.2 x 0.3 mm was used for collecting three dimensional intensity data on a computer-controlled Picker FACS-I four circle diffractometer with a graphite monochromator. 

The Peierls169 metal-to-semiconductor phase transition in TTFP TCNQ p was detected in an oscillation camera these streaks became bona fide X-ray spots only below the phase transition temperature of 55 K this transition is incommensurate with the room-temperature crystal structure, due to its partial ionicity p 0.59, and the "freezing" of the concomitant itinerant charge density waves (this effect was missed by four-circle diffractometer experiments, which had been set to interrogate only the intense Bragg peaks of either the commensurate room-temperature metallic structure, or the commensurate low-temperature semiconducting structure). 

Figure 1 Schematic representation of the three main geometries of automated x-ray or neutron single-crystal diffractometers (a) four-circle geometry (b) K-geometry (c) normal-beam geometry. S is the sample position.

Fig. 7) was operated routinely for many years on the neutron diffractometer DIO at the Institut Laue Langevin and was used for studies on, for example, Bechgaard salts at temperatures close to the superconducting transition [87,88]. Since 1990, 1.6 K can be reached routinely with an improved design and a dilution cryostat has been tested successfully in the laboratory which fits the Euierian cradle and is capable of 70 mK (C. M. E. Zeyen and S. Pujol, private communication, 1993). The helium flow cryostat of Argoud and Muller [80] is also well adapted to four-circle geometry and can be installed even on a small Euierian cradle. 

Figure 8 Two-stage Displex CS201 cryocooler mounted on the < >-bearing of a Huber 512 four-circle diffractometer. Helium gas lines and electrical connections are supported from the ceiling by a swinging link suspension. (From Ref. 90.)...

Figure 9 Helium-pressurized cell mounted on the neutron four-circle diffractometer D8 of the Institut Laue Langevin. (From Ref. 36.)...

For structure studies under high pressure at low temperature (HP + LT) the pressure cell must fit in a cryostat and the pressure transmitting medium must not freeze or, if it does, must not produce a large uniaxial additional component. Several combinations of devices described in Sections IV.D.l and IV.D.2 are possible. While four-circle diffractometers have been often used for room-temperature and LT structural studies of organic conductors,... 

The measurements were performed on a four-circle diffractometer, but no detail is given on the diamond anvil cell geometry. 

Alternatively, four-circle diffractometer can have a k (kappa) geometry, in which the / circle is replaced by two arcs, one of which ( > block ) rotates around the vertical > axis and the other ( k block ) around the k shaft, mounted on the (o block (Figure 12). The k and co axes always form the magic angle a = 54°. Note that in either geometry, every... 

Figure 12 Scheme of a four-circle diffractometer (a) conventional geometry, (b) k (kappa) geometry... 

A clear, roughly cubic crystal of edge 0.10-0.15 mm was chosen for data collection. A Picker automatic four-circle diffractometer, equipped with a fine-focus Mo anode tube, was used. High-angle reflections were accurately centered at a takeoff angle of and were used for a least-squares refinement of the ceil parameters. Data were collected and treated as described in a recent article. A complete hemisphere of data was collected for 26 < 55°. Intensities of three standards were collected at intervals of every 200 reflections. A total of 2418 independent intensity data were recorded. 

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