Single crystal diffractometers

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... 

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

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 diffraction methods (whether based on X-ray or neutron sources) are those that carry the most complete information on the intimate nature of the crystals and therefore provide the most valuable tools for identification, characterization and comparison of polymorphs and pseudo-polymorphs. It is difficult to deny that one of the reasons for the outpouring of new results in the field of crystal engineering is the quantum leap represented by the commercial availability of single-crystal diffractometers equipped with area detectors. These devices have not only reduced the time of data collection by an order of magnitude with... 

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.

In most fields of physical chemistry, the use of digital computers is considered indispensable. Many things are done today that would be impossible without modem computers. These include Hartree-Fock ab initio quantum mechanical calculations, least-squares refinement of x-ray crystal stmctures with hundreds of adjustable parameters and mar r thousands of observational equations, and Monte Carlo calculations of statistical mechanics, to mention only a few. Moreover computers are now commonly used to control commercial instalments such as Fourier transform infrared (FTIR) and nuclear magnetic resonance (FT-NMR) spectrometers, mass spectrometers, and x-ray single-crystal diffractometers, as well as to control specialized devices that are part of an independently designed experimental apparatus. In this role a computer may give all necessary instaic-tions to the apparatus and record and process the experimental data produced, with relatively little human intervention. 

Single crystals, suitable for X-ray crystallography, were obtained directly from the synthesis of the compound (solvent = anhydrous methylene chloride). The crystal structure was determined on a CAD-4 ENRAF-NONIUS PDP 8/M computer-controlled single-crystal diffractometer. Atomic positional parameters, bond lengths and bond angles are published elsewhere 33 >. 

Figure 3 Schematic diagram of the TOPAZ single-crystal diffractometer to be completed at the SNS (spallation neutron source, Oak Ridge, Tennessee) by 2009. (Image downloaded in 2006 from http //neutrons.oml.gov/instrument ystems/beamline 12 topaz/ index.shtml. US Government)...

Peacor DR (1968) A high temperature single crystal diffractometer study of leucite, (K,Na)AlSi206. Z Kristallogr 127 213-224... 

Single-crystal specimens may also be examined in a diffractometer by mounting the crystal on a three-circle goniometer, such as that shown in Fig. 5-7, which will allow independent rotation of the specimen and counter about the diffractometer axis and two other axes passing through the specimen. (Incidentally, independent rotation of the specimen about the diffractometer axis is often called an tu, rather than a 6, rotation.) In fact, special single-crystal diffractometers are available, designed solely for the determination of complex crystal structures. 

An empirical correction that is used by some single-crystal diffractometers equipped with CCD cameras takes the following form ... 

E8.2 If the laboratory single-crystal diffractometer can study crystals that are 50 pm 50 pm x 50 pm, with a new synchrotron source that has a millionfold increase in source intensity, one can reduce the crystal volume by I/IO, thus the size would be 0.5 pm 0.5 pm x 0.5 pm. The opposite happens if the flux is reduced we need bigger crystal. For the 10 times weaker neutron flux we must have a crystal with 10 larger volume—the minimum size is then 500 pm x 500 pm x 500 pm. 

A published crystal structure has been refined by least-squares based on about 8600 reflections measured with an automated three-circle single-crystal diffractometer. A distance least-squares computation (DLS) revealed the space group Am (No. 8) to be the most probable one. The published structure was found to be essentially correct. The weighted R-factor for all reflections used is 9.4%. Further detailed work is in progress. 

The schematic layout of a single crystal diffractometer is given in Fig. 8.39. This system uses an X-ray tube, a sample specimen, and a detector that rotates in an arc described by a Rowland circle. Note that the single crystal sample takes the place of the analyzing crystal... 

Figure 8.41 Schematic of a single crystal diffractometer using X-ray film to record the diffraction pattern. The film is curved along the circumference of the Rowland circle, replacing the movable detector shown in Fig. 8.39.

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