HIGH ENERGY X-RAY SCATTERING VS. NEUTRON DIFFRACTION. J.R. Schneider, Hasylab at Desy, Notkestr. 85, D-22603 Hamburg

Diffraction experiments with synchrotron radiation of wavelength [[lambda]][[second]]0.15A provide a new probe in condensed matter research because they combine the high penetration power of thermal neutrons with the extreme momentum space resolution obtained in modern X-ray diffraction experiments. After a short presentation of the diffactometers installed at the high field wiggler beamline BW5 at DORIS III and the undulator beamline at the PETRA storage ring at HASYLAB, examples will be discussed which highlight the power of combining the results of such synchrotron radiation experiments with those obtained by neutron scattering. The gain in information achieved this way is due to the different intrinsic cross sections involved and the possibility to perform high resolution inelastic neutron scattering experiments on the identical samples.

The static structure factor of amorphous materials, S(Q), can be measured accurately up to very high momentum transfer both with neutrons and high energy synchrotron radiation. The partial distributions functions calculated from the combined data sets provide more structural informations than each individual measurement yield.

The superstructures of YBCO high T_c superconductors have been studied with a high resolution triple-crystal diffractometer for high energy synchrotron radiation. The intensity gain of 3 orders of magnitude compared to neutron diffraction studies performed on the same sample allowed for novel quenching experiments. Analysis of the diffuse scattering in the vicinity of the four fold degenerate 200/020 reflections provided information on the size of the domains and the domain walls.

Different from neutron scattering, with X-rays two lengths scales have been observed in the critical fluctuations above the second order cubic to tetragonal phase transition in SrTiO₃. In order to find out if this unexpected behaviour is a bulk property or bound to the surface near region probed by X-rays, the critical scattering has been studied with 100 keV synchrotron radiation on the identical sample as used in the neutron studies. The critical scattering from the bulk is well described by a simple Lorentzian and also its temperature dependence fully aggrees with the, neutron data However. a sharp component has been observed in the critical scattering in a surface near layer of 65 um thickness.