With 39 FiguresThe knowledge of the charge distribution is of basic importance for the detailed understanding of the properties of a solid. In noncubic solids this charge distribution creates an electric field gradient (EFG). The quadrupole hyperfine interaction (QI) between the nuclear quadrupole moment Q and the EFG at the site of the nucleus results in an energy-splitting of the nuclear levels. The investigation of this QI in noncubic metals and alloys can be carried out by a variety of experimental techniques. The application of the perturbed angular correlation methods has been especially fruitful in this field. Its advantages, essential for QI studies in metals, are the ease with which temperature and pressure dependencies can be measured and the applicability to pure metals as well as to systems with extremely diluted impurity probe atoms. Thus, most experimental data have been obtained by this technique during the last few years.The first experiments on the EFG in metals were reported more than two decades ago [S. I-3]. The interest in this field increased drastically after the pioneering work of RAGHAVAN [S.4] who demonstrated the perturbed angular correlation technique to be extremely powerful in this field. The experimental data collected within the following years revealed two remarkable systematic trends: the first is related to the temperature dependence of the quadrupole interaction. It was discovered at Erlangen [S. S,6] that in most cases this temperature variation follows fairly accurately a simple T 3 / 2 relation. This behaviour is found in pure metals as well as in dilute binary alloys and in many cases it holds over the entire temperature range up to the melting pOint.The second important systematics is the correlation between the ionic and the electronic field gradients [S.7,8]. This empirical result was highly unexpected in terms of the conventional ansatz of the EFG based on two independent sources, one from the lattice and the other from the conduction electrons. The "universal correlation" rather implies-besides the dependence on the geometrical array of the lattice atoms -a strong relation of the electronic gradient to the nonsphericity of the probe ion cores, which is described by the Sternheimer anti shielding factor.Stimulated by these recent experimental trends emerging from the large body of available experimental data, significant progress in the theoretical approaches can be recognized. But we are still not able to predict reliable quantitative numbers
