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SXD

Single crystal diffractometer

This instrument is operational .

This makes SXD especially powerful in applications involving surveys of reciprocal space, such as phase transitions and incommensurate structures, and also in applications where sample orientation may be restricted.

Applications of SXD have included routine structural studies and diffuse scattering. The structure determination programme reflects the strength of single crystal neutron diffraction in studies of non-bonded contacts, specifically those involving hydrogen atoms, thermal parameter analysis, and in the area of charge density studies where neutron diffraction data provide complementary information to high resolution X-ray studies. A recent innovation has resulted from exploitation of the flexible data collection method offered by the time-of-flight Laue technique, which allows data sets to be collected in relatively short periods of time. The ability to measure a structure at more than one temperature can have significant benefits, either to characterise thermal parameter behaviour (methyl group librations in paracetamol) or to study disorder (proton transfer in benzoic acid).

Diffuse scattering caused by both dynamic and static effects can be studied, allowing short range structure, defect structure and local orientational correlations to be probed. A major area of interest is in developing methods to allow the Bragg and diffuse scattering to be used together in an integrated way to give a full picture of the structure under study.

Much of the potential of SXD has been realised since the installation of a second position- sensitive detector, which increased the volume of reciprocal space accessed and hence the data collection rate.

In October 1999, a grant of £1million was awarded by EPSRC to upgrade the SXD detectors. This allowed for the provision of a total of 11 detectors, almost completely surrounding the sample position. 

Applications of SXD include:

• Structure determination (including Hydrogen atom location)
• Diffuse scattering (thermally induced disorder, disorder resulting from defect impurities, or the structure of short range magnetically ordered systems)
• Phase transitions (including changes of symmetry, and superlattice reflections)
• Incommensurate structures
• Fibre diffraction

 

Instrument

Beamline S3

Moderator Ambient water, poisoned at 2 cm

Incident wavelengths 0.2 - 10Å

sin q/l 0.03 - 2.5Å-1

Resolution Dd/d ~ 0.5% (backscattering) - 1% (90 degrees)

Sample position 8.3m from moderator

Beam size at sample < 15 mm diameter

Sample size Dependent on scattering characteristics and unit cell size, at least 1 mm3 and typically 100 mm3

Data collection time Typically 1 - 2 hours per crystal orientation

Position-sensitive detectors Eleven 64 x 64 pixel optically encoded ZnS scintillators each with 192 × 192 mm2 active area and 3 × 3 mm2 resolution

Detector positions (centres) L2 = 0.225m in equatorial plane at longitude = ±37.5°, ±90°, ±142.5°; L2 = 0.27m at latitude = 45°, longitude = 0°, ±90°, 180°; L2 = 0.28m at latitude = -90°, longitude = 0°.

Diffractometer motion w rotation available on vacuum tank, cryostat and furnace. 2-circle (c - f) orienter with CCR

Sample environment Standard sample environments plus the following SXD specific items: Liquid He Orange cryostat (1.5 - 300K) Displex cooler (12 - 300K) Furnace (300 - 1200K) Cryostat with 3He dilution insert: 300 mK – 300 K and omega rotation.

Data acquisition and analysis Windows PC. Labview based instrument control software. Analysis program SXD2001 for initial data visualisation, reduction to structure factors including data corrections, fully corrected volumes in reciprocal space. Supported structure refinement packages: SHELX, GSAS, FULLPROF.