A SMALL-ANGLE NEUTRON-SCATTERING INVESTIGATION OF THE CONCENTRATION FLUCTUATIONS IN THE SOLUTIONS OF POTASSIUM IN MOLTEN POTASSIUM-BROMIDE

A neutron small angle scattering study of the solutions of potassium in potassium bromide over a rather large temperature and concentration range around the liquid-liquid critical point was undertaken in order to investigate the possible interaction between the concentration fluctuations related to the liquid-liquid immiscibility and the non-metal to metal transition observed for the metals in molten salts systems. The gamma and upsilon critical indices were determined with accuracy. All the results consistent with a tridimensional Ising behavior for this sytem, the volumic fraction being the correct order parameter. This is different from the mean field behavior obtained over the same reduced temperature range for the analogous solutions of metals in liquid ammonia. This discrepancy is interpreted with the help of the Ginzburg criterion and the large difference in the value of the characteristic length for the fluctuations, zeta //0 multiplied by (times) zeta //0 is probably dependent on the nature of the metal or the salt chosen. In any case, for K in KBr the interaction between electronic localization processes and concentration fluctuation is likely to be very weak or non-existent.