Structural properties of liquid alkali-metal--lead alloys: NaPb, KPb, RbPb, and CsPb

The structural properties of liquid equiatomic alkali-metal--lead alloys APb, with A=Na, K, Rb, and Cs, have been studied with neutron diffraction. The experimental data are reduced to the neutron-weighted total structure factors S(Q) and radial distribution functions n(r). In all the systems studied, S(Q) is characterized by a first sharp diffraction peak (FSDP) at Q∼1 A -1 and n(r) by structure in the first coordination shell at r=3--4 A, both features indicating the presence of polyatomic structural units. Two types of structural models have been fitted to the experimental data, both based on consideration of such units: the random packing of structural units model, and the reference interaction site model. The structures of the units are derived from powder-diffraction measurements made with the same instrument on solid polycrystalline samples. Allowing for a slight expansion of the units on melting and using physically reasonable values for the adjustable parameters, both models give satisfactory agreement with the experimental data and are used to give the partial structure factors and pair distribution functions. Inelastic-scattering measurements have been made on one system, liquid KPb, using a spectrometer at the same source. These yield a value for the mean diffusion constant, D=1.2±0.2x10-5 cm2/sec, and a smoothly varying inelastic scattering spectrum with no indication of excitations of a molecular character. The FSDP remains well defined when the inelastic scattering is integrated over small energy transfers, indicating a lifetime of 10-13 sec or longer for the intermediate-range order responsible for this peak