Semi-empirical models for the electric field gradient in metals

A survey over the present state of the empirical investigation of the electric field gradient at the nuclear site of impurity atoms and of matrix atoms in noncubic metal matrices is given. A qualitative explanation in terms of the electronic charge distribution typical for the host's group in the periodic system of elements is discussed. For group IIb hosts, two semi-empirical models which were developed in Bonn are described in more detail, i.e. the ‘naive model’ and the ‘Thomas-Fermi model’.