Topics

• Gamma rays are scattered by electrons associated with the atoms in the crystal and, as such, the principles of g-ray diffraction are the same as that of x-ray diffraction. There are, however, important differences between diffraction data recorded with x rays or synchrotron radiation and higher-energy gamma rays concerning potential sources of systematic errror. With the use of a photon energy above 300 keV, the high-energy diffraction case is fully realized (photon energy much higher than electron binding energies), and a structure factor accuracy of 0.1% is achievable, profiting from low absorption and extinction, no dispersion correction, and reasonably large samples making surface effects negligible. The improvement in accuracy is also due to favourable experimental conditions besides energy, such as the perfect stability, homogeneity and spectral line width (Dl/l = 10^-6) of the incident g-ray beam. Single-crystal studies involving phase transitions with a lowering of symmetry need special precaution to keep the specimen untwinned. Domain formation has been suppressed by application of uniaxial pressure or by an external magnetic field. Gamma-ray diffraction is especially suited for electron density work on simple inorganic solids with heavier elements, which demand more than usual data accuracy due to the dominance of the core-electron scattering. Our interest in recent years has been focused on the study of the consequences of magnetic interactions on the electronic structure. Prototypic antiferromagnets including the late 3d transition-metal monoxides and difluorides have been systematically investigated, in most cases both in the paramagnetic and the fully ordered phase. From the experimental model electron density quantitative information is accessible on magnetic ground state properties such as crystal field effects, spin delocalization, spatial extent of d-electrons, spin-orbit coupling, orbital-to-spin angular momentum ratios, etc. Comparisons are made between analyses based on modern neutron and x-ray magnetic measurements providing a stringent validation of results.