Diffractometer single-crystal

G.36 A. J. C. Wilson. Elements of X-Ray Crystallography (Reading, Mass. Addison-Wesley, 1970). Powder cameras and diffractometers single-crystal cameras and diffractometers structure determination diffraction by imperfect crystals. 

An x-ray area detector can be used to collect the intensities of many reflections at a time. The crystal must be oriented in many different settings with respect to the incident beam but the detector needs to be positioned at only a few positions to collect all of the data. A charge coupled device (CCD) is used as the area detector on the Siemens SMART single crystal diffractometer system. The SMART detector consists of a flat 6-cm circular phosphorescent screen that converts x-ray photons to visible light photons. The screen is coupled to a tapered fiber optics bundle which is then coupled to a one inch by one inch square CCD chip. The CCD chip has 1024 x 1024 pixels each of which stores an electrical charge proportional to the number of... 

Fig. 12. Siemens SMART Single Crystal X-ray Diffractometer System.

Bragg-Brentano Powder Diffractometer. A powder diffraction experiment differs in several ways from a single-crystal diffraction experiment. The sample, instead of being a single crystal, usually consists of many small single crystals that have many different orientations. It may consist of one or more crystalline phases (components). The size of the crystaUites is usually about 1—50 p.m in diameter. The sample is usually prepared to have a fiat surface. If possible, the experimenter tries to produce a sample that has a random distribution of crystaUite orientations. 

A good diffractometer-spectrograph must function satisfactorily in each role, and it must be built so that the changeover from one to the other is easy, quick, and sure. Unless this is true, two instruments will often be needed where one might have sufficed. The diffractometer should handle either powder samples or single crystals. 

X-ray single crystal analysis Data collection. Siemens SMART IK CCD. Diffractometer. Data collection by SMART (Siemens, 1995) cell refinement SMART data reduction SAINT (Siemens, 1995) program(s) used to solve structure SHELXS97 (Sheldrick, 1997) program(s) used to refine structure SHELXL97 (Sheldrick, 1997) molecular graphics XP in SHELXTL (Sheldrick,... 

Blessing, R.H. (1989) DREADD - data reduction and error analysis for single-crystal diffractometer... 

We have adapted a commercially available x-ray diffractometer normally used for structure determinations on single crystals to operate as a very flexible device for performing x-ray pole figure determinations and related studies on polymeric materials. Descriptions of crystallite orientations, as provided by pole figures, are useful in studying many aspects of the behavior of products made from semicrystalline polymers. This paper describes the software that we have written for our pole figure facility. Except for some vendor-provided routines to drive the hardware Interface all of our software is written in FORTRAN. Menu driven operation is provided to maximize user convenience. 

The three dimensional structure was obtained by means of single crystal X-ray diffraction. CuKa radiation, a graphite monochromator, and a photomultiplier tube were used to collect 1825 total reflections on an automated diffractometer. Of these, 1162 were used for the analysis. Figure 2 shows a computer generated drawing of halcinonide. The position of the chlorine atom was not clear from the Patterson map, but the direct method program "MULTAN" gave its position. 

As we have seen in Chapter 1, we need something near a plane wave in order to see the finest details of the specimen stracture. A single-axis diffractometer utilises a beam that is very far from a plane wave. Thus, single-crystal rocking curves are broadened due to the beam divergence, and the spectral width of the characteristic X-ray lines. 

X-ray diffraction is a widely used tool for structural identification for almost all solids under the right conditions. X-ray diffractometers are generally either single crystal or powder. 

The x-ray powder diffraction pattern of isoxsuprine HCl has been measured using a Philips PW-1710 diffractometer, equipped with a single crystal monochromator and using copper Ka radiation. The pattern thusly obtained is shown in Figure 1, and the table of scattering angles, interplanar d-spacings, and relative intensities are found in Table 1. 

The number of cryo-crystallographic studies on phase transitions is quite large nowadays. It became common with the availability of single crystal diffractometers equipped with digital area detectors that allow very rapid data collections. They are optimal for repeated measurements at variable temperatures of the same crystal in a reasonable amount of time. Thus, the accurate and detailed structural information typically available from a single crystal X-ray diffraction experiment could be... 

Swanson DK, Prewitt CT (1986) A new radiative single crystal diffractometer microfumace incorporating MgO as a high temperature cement and internal temperature calibrant. J Appl Crystallogr 19 1-6... 

Single crystal X-ray diffraction data is nowadays collected using a computer controlled diffractometer, which measures the Bragg angle 6 and the intensity I for each Ai/reflection. Many modern diffractometers employ a flat-plate detector (CCD), so that all the reflections can be collected and measured at the same time. A full data set, which can be thousands of reflections, can be accumulated in hours rather than the days or weeks of earlier times. 

In order to obtain detailed structure, a knowledge of diffraction intensities is essential, the intensities being related to the structure factor. Computer-controlled single-crystal X-ray diffractometers with structure (software) packages have made structure elucidation a routine matter. The availability of synchrotron X-radiation of continuously variable wavelength has made X-ray diffraction a still more powerful structural tool for the study of solids. A technique of great utility to solid state chemists is the Rietveld treatment of powder X-ray diffraction profiles (Rietveld, 1969 Manohar, 1983). Automated structure packages for the determination of unknown structures by this method are now commercially available (see section 2.2.3). In Fig. 2.1, we show a typical set of profile data. 

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