THE STRUCTURE OF MOLTEN K PB, RB PB AND CS PB ALLOYS

We have considered previously published neutron diffraction data, obtained at two different sources, on the molten alloys K(x)Pb1-x, RbPb and CsPb. The difference between results for the same system obtained at the two sources has been shown to be attributable to instrumental resolution. The reverse Monte Carlo method has been used to produce structural models of all these melts. We have demonstrated that there is a range of structures consistent with the experimental data, all of which have in common a tendency for the lead to form clusters, typically of three or four atoms, with 60-degrees Pb-Pb-Pb bond angles. The results appear to be generally consistent with the existence of some tetrahedral Pb4(4)- Zintl ions; however it seems unlikely that all Pb atoms occur in such well defined species since in this case the tetrahedra must approach each other too closely in order to accurately reproduce the experimental data. We have shown that for unconstrained (i.e. not completely tetrahedral) models the peak at a momentum transfer of approximately 1 angstrom-1 observed in the diffraction patterns occurs in the partial structure factor A(PbPb) (Q) and is related to correlations between Pb clusters. The height of this peak varies with Pb concentration in K(x)Pb1-x alloys in the same way as electrical resistivity, and we have postulated that this is due to electron localization associated with the cluster formation. For KPb the unconstrained model contains 25% tetrahedra, in good agreement with thermodynamic data.