A NEW HIGH RESOLUTION X-RAY DIFFRACTOMETER FOR SINGLE CRYSTAL INVESTIGATIONS
M. Meven1, St. Mattauch1, G. Heger1, J. Bethke2
1Institut für Kristallographie, RWTH
Aachen, Jägerstraße 17-19, 52056 Aachen, Germany
2Philips Analytical X-Ray, Lelyweg
1, 7602 EA Almelo, Nederlands
In cooperation with Philips Analytical X-Ray, Almelo NL, a prototype of a new single crystal X-ray diffractometer was developed. It is based on a Philips MRD powder diffractometer which was combined with an Eulerian cradle. The flexible modular concept of the diffractometer allows the adjustment to specific experimental demands by selection of different optics (high resolution configuration with Ge[220] 4-crystal monochromator and/or channel cut analyzer, standard configuration with secondary monochromator and other optics).
The high resolution configuration yields rather sharp line profiles with small halfwidths (e.g. a rocking curve with a FWHM near the Darwin width for a Si sample). The high angular resolution allows a very accurate determination of lattice parameters with Dd/d ~ 10-5 and gives the opportunity to analyze the quality of a single crystal by the observation of the reflection profiles.
Thus it is possible, for instance, to analyze the evolution of radiation damages in a single crystal as a function of X-ray irradiation not only by observation of the change of intensities but also by the accurate observation of the change of the shape of reflection profiles. An example for this effect will be presented on the ECM.
Another possibility is the investigation of domain structures/twinning below structural phase transitions (e.g. for ferroic phase transitions).
The standard configuration for quantitative measurement of intensities uses a secondary monochromator of pyrolytic graphite with its diffraction plane perpendicular to the one of the sample. This not only gives a reduction of the measured flourescence but also leads to an angular resolution of the reflections of the sample merely independent from 2Q.
The standard configuration was used to check the quality of the instrument (stability and reproducibility of mechanics, electronics and beam geometry). A complete data set of Bragg reflections was taken from a small spherical silicon sample (0.29 mm diameter) up to sin Q/l=1.22 with MoKa-radiation. The internal R-value was 1.5% after averaging intensities of equivalent reflections and the refinement of the scale factor, extinction and isotropic temperature factor B converged to R(F2)=1.8% with B=0.456(6) Å being in perfect agreement with the literature [1].
Another feature of the system is a closed cycle cryostat for measurements at low temperatures down to 20 K. This, in combination with the high resolution optics, gives the opportunity to observe the evolution of lattice constants as a function of temperature at displacive phase transitions.