The structural properties of liquid equiatomic alkali-metallead 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 systetns studied, S(Q) is characterized by a first sharp diffraction peak (FSDP) at Q -I A ' and n (r) 0 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 ' cm /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 ' sec or longer for the intermediate-range order responsible for this peak.

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The structure factors of liquid Pb, Ko.zsPbo 75r KO 6sPbo 35 and Ko.80Pbo 2o have been measured by means of neutron diffraction. The main peaks of the structure factors of the alloys are preceded by a pre-peak which is most pronounced in Ko,50Pbo This prepeakindicates that there is considerable orderingin the alloys, although there is also evidence for a tendency towards phase separation in at least one of the alloys. From inspection of the radial distribution functions it follows that Pb clusters are formed, the concentrations of which increase when the equi-atomic composition is approached. A simple model of the structure of K,ljoPbo based on the reference interaction site model proves to be in excellent agreement with the measured structure factor.

The structure factors of liquid equiatomic KSn and CsSn alloys have been determined by neutron diffraction. A distinct peak at about 1 A, the first sharp diffraction peak (FSDP), occurs in all measured structure factors. The temperature dependence of the structure has been studied for KSn: apart from the FSDP, whose position shifts toward smaller wave vector and magnitude decreases with increasing temperature, the structure factor remains essentially unaltered. The radial distribution function clearly shows three maxima in the first coordination shell. The simultaneous presence of the FSDP and several maxima in the first coordination shell are indications of formation of polyatomic species in the liquid. Application of two simple structural models, taking into account the presence of structural units, yields reasonable agreement with the measured structure factors.

Enthalpies for equiatomic APb (A=Na, Rb, and Cs) have been measured by high-temperature drop calorimetry over a temperature range covering solid and liquid phases. Thermodynamic information on the solid state transitions and the melting process is derived. With the exception of NaPb, the heat capacities of the liquid alloys show a remarkably strong negative temperature dependence above the melting point. It is suggested that this behavior is a consequence of the dissociation of molecular entities present in the liquid state related to the Zintl structures detected in the corresponding solid compounds. Thermodynamic calculations based on the presence of polyvalently charged anions yield valuable information on the temperature dependence of the concentration of these species.