The observation of nuclear quadrupole interactions in amorphous solids provides a unique possibility of obtaining information about the angular distribution of local ionic coordinations, complementary to the information about radial distributions deduced from x-ray and neutron diA'raction and from extended x-ray absorption fine structure measurements. In the present paper the relation between ionic coordinations and the distribution of electric field gradients {EFG)is investigated. It is shown that the distribution function P(V",g) of the splitting parameters V" (the electric field gradient) and y (the asymmetry parameter) in general yields zero probability both for V"=0 and for y = 0. For solids which are isotropic on the average, the distribution function of the components V, . I, of the EFG tensor depends only on two variables, the invariant functions of the tensor components [Det(V, ,) and XV, . '"].Expressions for these quantities in terms of the radial coordinates of the ions causing the EFG and of the bond angles between pairs of ions are given. For amorphous solids with random ionic coordination an analytic approximation for the distribution function P(V",y) is derived. This function is strongly dominated by the distribution of ions in the first coordination shell. The results are applied to the analysis of Mossbauer spectra of '"Gd in amorphous Gd-Ni alloys.

We have revisited the epitaxial growth modes of Fe on W(110) and Mo(110), and propose an overview or our contribution to the field. We show that the Stranski-Krastanov growth mode, recognized for a long time in these systems, is in fact characterized by a bimodal distribution of islands for growth temperature in the range ∼ 250 − 700 • C. We observe firstly compact islands whose shape is determined by Wulff-Kaischev's theorem, secondly thin and flat islands that display a preferred height, i.e. independant from nominal thickness and deposition procedure (1.4 nm for Mo, and 5.5 nm for W on the average). We used this effect to fabricate self-organized arrays of nanometers-thick stripes by step decoration. Self-assembled nano-ties are also obtained for nucleation of the flat islands on Mo at fairly high temperature, i.e. ∼ 800 • C. Finally, using interfacial layers and solid solutions we separate two effects on the preferred height, first that of the interfacial energy, second that of the continuouslyvarying lattice parameter of the growth surface.