COMPUTER MODELING STUDIES OF EXPANDED LIQUID KPB

We report on computer modelling studies of the structure of the equiat omic liquid alloy KPb over a temperature range from 873 K up to 1873 K . In the crystalline state, this semiconducting alloy contains tetrahe drally coordinated polyanions Pb-4(4-). Recent neutron diffraction res ults indicate the survival of polyanions in the: expanded liquid alloy even up to the highest temperature measured, as indicated by the pers istence of a prepeak in the static structure factor S(Q) at a momentum transfer of Q approximate to 1 Angstrom(-1) up to 1873 K. Applying th e reverse Monte Carlo technique, we demonstrate that there is a struct ure consistent with the experimental data, which consists of small clu sters with a partially tetrahedral arrangement or an open loose struct ural network of lead atoms, typically with three or fourfold coordinat ion. The calculations show that the peak at approximately 1 Angstrom(- 1) observed in the diffraction patterns occurs predominantly in the pa rtial structure factor S-PbPb(Q), which is related to correlations bet ween Pb clusters, for all temperatures. The Pb-Pb bonds in the cluster s are fluctuating. With increasing temperature the tendency of Pb atom s to form particular clusters becomes a more general tendency to micro phase separation.