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Single-Crystal X-Ray Diffractometry Using Synchrotron Radiation

100%

Eichhorn K.

Acta Physica Polonica A

1992

vol. 82

issue 1

51-65

The properties of synchrotron radiation relevant to single-crystal X-ray diffractometry are: its high intensity over a wide spectral range, a small source size and a low divergence in the 0.1 mrad range, about 90% linear polarization in the horizontal plane, a pulsed time structure, and a time dependent intensity. The latter property requires monitoring of the primary beam intensity and its polarization state which slightly complicates data collection and needs particular attention in the data reduction stage. The other properties of synchrotron radiation, however, extend the range of X-ray diffractometry to experiments which are not feasable with sealed X-ray tubes. The high source intensity makes data collection possible on crystals down to and below 10 μm diameter. Measurement of weak and very weak ("forbidden") reflections profits from high intensity, low divergence, and a good peak-to-background ratio. Data collection at short wavelengths is useful to decrease both absorption and extinction effects and provides the resolution required for high precision structure analysis. Wavelength tun-ability is frequently used to exploit resonant X-ray scattering ("anomalous dispersion") for structure research. Examples are determination of absolute configuration, contrast variation, and phase determination from both single-and multiple-wavelength measurements ("MAD-phasing"). X-ray dichroism and double refraction are observed in the vicinity of absorption edges, causing an anisotropy and polarization dependence of anomalous scattering. This anisotropy may give rise to a violation of extinction rules for glide-planes and screw-axes, with orientation- and polarization-dependent intensities. More recently, these affects have been successfully used to derive (partial) phase information. Other applications are magnetic X-ray scattering and time-resolved X-ray diffraction, the latter exploiting the time structure of the synchrotron radiation source.

Single-Crystal X-Ray Diffractometry Using Synchrotron Radiation

63%

Eichhorn K., Hümmer K.

The outstanding properties of synchrotron radiation, in particular its high brilliancy over a wide spectral range, its low divergence, its polarization properties, and the pulsed time structure, extend the range of single-crystal X-ray diffractometry to experiments which are not feasible with conventional sources, such as sealed X-ray tubes or rotating anode equipment. Data collection techniques are strongly influenced by the general aims of a diffraction experiment, by the sample quality, its absorption and scattering power, as well as by the reflection profile shape and the instrumental resolution function. Often, the sample properties play a crucial role, and not all samples may be suitable for data collection with synchrotron X-rays. The time-dependence of the primary beam intensity and of its polarization state requires monitoring and normalization to monitor counts, which complicates data collection and data reduction due to sources of both random and systematic errors not known from conventional X-ray sources. There is almost no utilization of X-ray diffraction that cannot profit from the use of synchrotron radiation. X-ray diffraction at a synchrotron radiation source can yield structure factors of an unprecedented quality, provided proper attention is given to sample properties, to data collection strategy and data evaluation procedures. Though little is gained for strong reflections, the improvement is very pronounced for the weaker reflections, including high-order reflections, which can be measured in much shorter time than with conventional X-ray sources. However, synchrotron radiation does not provide a solution to all problems, in some cases conventional laboratory X-ray sources may be more appropriate than synchrotron radiation. Taking into account the limited access to synchrotron radiation sources, X-ray diffraction with synchrotron radiation can only supplement, but not replace conventional X-ray sources and diffraction techniques.

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2 Acta Physica Polonica A

1 1994

1 1992

2 Eichhorn K.

1 Hümmer K.

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Single-Crystal X-Ray Diffractometry Using Synchrotron Radiation

Single-Crystal X-Ray Diffractometry Using Synchrotron Radiation