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Four-circle Diffractometer (four-circle + diffractometer)

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Chemistry100%

Selected Abstracts

High-pressure single-crystal X-ray diffraction facilities on station 9.8 at the SRS Daresbury Laboratory , hydrogen location in the high-pressure structure of ethanol

JOURNAL OF SYNCHROTRON RADIATION, Issue 1 2001
David R. Allan
A new high-pressure single-crystal diffraction facility has been constructed on station 9.8 at the Synchrotron Radiation Source, Daresbury Laboratory, for a range of studies on a variety of systems of relevance to physics, chemistry and materials science that would otherwise prove intractable with conventional laboratory-based methods. The station has been equipped with a modified Enraf,Nonius CAD4 four-circle diffractometer for high-pressure studies which can be conveniently, and rapidly, interchanged with the Bruker SMART CCD area-detector system when more routine ambient-pressure diffraction work is to be undertaken. This rapid change-over has been achieved by permanently mounting the CAD4 on its own jacking table, formerly used for the station's white-beam diffraction mode, which allows the alignment of the SMART diffractometer to remain undisturbed when the CAD4 is in use. Early results on the test low-melting-point compound ethanol (CH3CH2OH) reveal that excellent refined structures can be obtained, including the location and refinement of the H atoms, demonstrating that one of the original, and major, objectives of the station has been accomplished. [source]

Inversion of 4f -states in CeB6 thermally excited at 430,K

ACTA CRYSTALLOGRAPHICA SECTION B, Issue 5 2007
Ryoko Makita
The 4f states of Ce in a typical Kondo crystal, CeB6, are split into an excited state ,7 and the ground state ,8, with an excitation energy at 560,K. The electron-density distribution of the thermally excited state was measured at 430,K using a four-circle diffractometer equipped with a small furnace. In contrast to the previous results at lower temperature, electrons are transferred from B6 to Ce at 430,K. X-ray atomic-orbital analysis revealed that the 5d -,8 orbitals (the energy level of which is similar to that of the B-2p orbitals) are fully occupied and the 4f -,7 orbitals are more populated than the 4f -,8 orbitals. Fully occupied 5d -,8 makes the 4f -,8 states unstable and the energy levels of 4f -,7 and 4f -,8 are inverted. [source]

A heating stage up to 1173,K for X-ray diffraction studies in the whole orientation space

JOURNAL OF APPLIED CRYSTALLOGRAPHY, Issue 1 2003
R. Resel
A multi-purpose heating attachment designed primarily for X-ray four-circle diffractometers but applicable also for classical powder diffraction is presented. When working in reflection geometry, the air-cooled heating stage allows diffraction studies to be performed on plate-like samples up to 1173,K in the whole orientation space. This paper gives a detailed description of the assembly and important technical specifications for the performance of experiments. The heating characteristics of the heating stage, the displacement of the sample from the goniometer centre as a result of thermal expansion and the influence of the protecting dome on the diffraction experiment are presented. The simple technical construction, the low weight, the small size and good heating performance make this equipment a general purpose heating attachment for X-ray diffraction experiments in reflection geometry. [source]

How precise are measurements of unit-cell ­dimensions from single crystals?

ACTA CRYSTALLOGRAPHICA SECTION B, Issue 4 2000
Frank H. Herbstein
The results of single-site and many-site measurements of cell dimensions from single crystals are compared for Bond and four-circle diffractometers using samples of corundum (essentially pure rhombohedral ,-Al2O3, aluminum oxide) of high diffraction quality, where the effects of small changes in temperature and composition (Cr2O3, chromium oxide, in solid solution) can be taken into account. Similar comparisons are made for four-circle diffractometer measurements on ruby (,-Al2O3, with 0.46 wt % Cr in solid solution). The precisions are some parts in 105. There is partial support for the Taylor,Kennard [Acta Cryst. (1986), B42, 112,120] dictum that standard uncertainties (s.u.s) of cell parameters from routine four-circle diffractometer measurements are less than those for many-site measurements by factors of 5 for cell lengths and 2.5 for cell angles. For organic crystals, independent repetitions of adequate quality for comparison and analysis of routine four-circle diffractometer measurements are available only for ,-oxalic acid dihydrate and anthracene. The experimental standard uncertainties given for these two crystals agree reasonably well with the sample s.u.s at room temperature, but appreciably less well at ,100,K, again giving partial support to the Taylor,Kennard dictum. The relation between specimen characteristics and attainable precision is emphasized; the precisions for routine measurements on good quality organic crystals are some parts in 104. Area-detector measurements of cell dimensions have also been appraised; currently published s.u.s from such measurements appear to be highly unreliable, and this is supported by a recent analysis of the operation of such diffractometers [Paciorek et al. (1999). Acta Cryst. A55, 543,557]. Formulation of a standard protocol for such measurements is badly needed. The dangers inherent in high degrees of replication are illustrated by recounting Kapteyn's Parable of the Chinese Emperor. Attention is drawn to the fact that there has been little improvement in claimed precisions over the past 40,60 years. [source]