Circle diffractometers

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... 

In order to figure out the FWs, the nuclear structure was refined from unpolarized neutron data taken at 30 K, in the paramagnetic state, on a 4-circle diffractometer. Furthermore, a set of 248 flipping ratios was measured with polarized neutrons at 1.6 K, with the spin density long range ordered by a 4.65 T applied magnetic field. 

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. 

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). 

Normal-beam geometry. Since the sample rotates only around the vertical axis, it is very easy to attach a two- or three-stage cryorefrigerator, a standard reservoir-type liquid helium cryostat, or even a compact dilution cryostat, to the to cradle of the instrument. This is the simplest assembly, which also offers the lowest temperatures and, as we shall see later, is also most appropriate for LT + HP studies. Three-circle diffractometers are relatively common in neutron laboratories but a few x-ray diffractometers have also been built (R. Moret, unpublished, 1993 J. Gaultier, unpublished, 1993) and used for measurements on organic conductors. 

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... 

Figure 14 An experimental setting for single-crystal X-ray diffraction study at 30 K, showing a Bruker three-circle diffractometer with a SMART 1000 CCD area detector, a laser device for crystal irradiation and a HehX open flow helium cryostat in action...

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. 

X-Ray M urements. Diffraction data were collected at room temperature (24.5 1.5 ) on a Picker automatic four-circle diffractometer using Mo Ko radiation (X 0.7107 A). Accurate cell dimensions were obtained by measuring the 20 ai e for the Mo Koj peak for the highest angle reflection observable along each of the principal axes. Tlie c axis was oriented along . 

A crystal was mounted on an Enraf-Nonius CAD-4 four-circle diffractometer, and accurate cell dimensions were obtained by a least-squares fit to three sets of eight symmetry-equivalent reflections with 26 between 25 and 29°. The cell dimensions and data collection parameters are summarized in Table I. 

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